Polyneuropathy
Polyneuropathy is a diffuse, symmetrical disorder of multiple peripheral nerves, typically presenting with distal sensory loss, weakness, and diminished reflexes in a "stocking-glove" distribution.
Polyneuropathy
Polyneuropathy ("poly" = many, "neuro" = nerve, "pathy" = disease) refers to a diffuse, symmetrical disease process affecting multiple peripheral nerves, typically in a length-dependent manner (i.e., the longest nerves are affected first, so symptoms begin distally and progress proximally — the classic "glove-and-stocking" distribution) [1][2].
Why length-dependent? The longest axons (those supplying the feet) are the most metabolically vulnerable. They have the greatest distance for axonal transport, so any systemic metabolic insult (e.g., hyperglycaemia, toxins, nutritional deficiency) hits them first. This is why polyneuropathy almost always starts in the toes/feet before the fingers/hands.
This distinguishes polyneuropathy from:
- Mononeuropathy — a single nerve affected (e.g., carpal tunnel syndrome)
- Mononeuritis multiplex — multiple non-contiguous individual nerves affected asymmetrically (e.g., vasculitis)
- Radiculopathy — nerve root involvement (dermatomal distribution)
- Plexopathy — nerve plexus involvement
Key Distinction (Exam Favourite)
Polyneuropathy = symmetrical, length-dependent, distal-first involvement of multiple peripheral nerves. If a neuropathy is asymmetrical, think mononeuritis multiplex or radiculopathy, NOT polyneuropathy.
- Prevalence: Approximately 1–3% of the general population; rises to 7–8% in those aged > 55 years [3].
- Most common cause worldwide and in Hong Kong: Diabetic polyneuropathy — affects up to 50% of patients with long-standing diabetes [4][5].
- Among T1DM → 5 to 40%; among T2DM → 1 to 20% [5].
- Second most common cause in developed settings: Alcohol-related polyneuropathy.
- In Hong Kong specifically:
- High prevalence of diabetes (> 10% of adult population) makes diabetic neuropathy the dominant cause.
- Chronic kidney disease (common in HK's ageing population) → uraemic neuropathy.
- Chemotherapy-induced polyneuropathy is increasingly relevant given rising cancer incidence.
- Vitamin B12 deficiency in elderly and vegetarian populations.
- Guillain-Barré syndrome (GBS): incidence 1–2/100,000 per year, with 20% increase in incidence with every 10-year increase in age beyond the first decade [2].
- CIDP: prevalence 0.9–8.9/100,000, more common in older males [2].
Anatomy and Function of Peripheral Nerves
Understanding the structure of a peripheral nerve is essential — it explains why some diseases cause axonal damage while others cause demyelination, and why the clinical features differ.
| Layer | Structure | Function |
|---|---|---|
| Epineurium | Outermost connective tissue sheath | Protects the nerve trunk; contains vasa nervorum (blood supply to the nerve) |
| Perineurium | Surrounds each fascicle | Acts as blood-nerve barrier (analogous to BBB); maintains intrafascicular pressure |
| Endoneurium | Surrounds individual nerve fibres | Contains collagen, fibroblasts, capillaries |
| Axon | The nerve cell's conducting process | Transmits electrical impulses |
| Myelin sheath | Produced by Schwann cells (one Schwann cell per internode) | Insulates axons for saltatory conduction (speeds up signal transmission) |
| Nodes of Ranvier | Gaps between myelin segments | Site of action potential regeneration |
| Fibre Type | Size | Myelination | Function | Clinical Deficit When Damaged |
|---|---|---|---|---|
| Large myelinated (Aα) | Largest | Thick myelin | Motor function, proprioception | Weakness, areflexia, impaired proprioception/vibration |
| Large myelinated (Aβ) | Large | Thick myelin | Light touch, vibration, proprioception | Loss of light touch, vibration, joint position sense |
| Small myelinated (Aδ) | Small | Thin myelin | Sharp/fast pain, temperature, some autonomic | Loss of pinprick, temperature |
| Unmyelinated (C fibres) | Smallest | None | Dull/slow pain, temperature, autonomic | Burning pain, autonomic dysfunction |
Why Does Fibre Type Matter?
Large, myelinated fibres → mostly negative symptoms (loss of touch/proprioception, pins-and-needles) [2]. Small, unmyelinated fibres → mostly positive symptoms (pain) [2]. This is because small fibre damage causes ectopic firing and spontaneous discharge → neuropathic pain, whereas large fibre loss simply removes sensory input.
- Saltatory conduction: Action potentials "jump" between Nodes of Ranvier in myelinated fibres. This is why demyelination slows conduction velocity — the impulse can no longer jump efficiently.
- Axonal transport: Proteins and organelles are synthesised in the cell body and transported down the axon via microtubules. The longest axons (to the feet) are most vulnerable to metabolic disruption — hence length-dependent polyneuropathy.
- Wallerian degeneration: When an axon is severed or severely damaged, the distal segment degenerates. This is irreversible axonal loss and explains why axonal neuropathies recover more slowly (require regrowth at ~1 mm/day) than demyelinating neuropathies (which recover by remyelination).
Aetiology (Focus on Hong Kong)
The causes of polyneuropathy can be broadly categorised by the DANG THERAPIST mnemonic (or similar frameworks), but for a clinical approach, it is most useful to think in terms of:
- Metabolic/Endocrine (most common)
- Toxic/Drug-induced
- Nutritional deficiency
- Inflammatory/Immune-mediated
- Infective
- Hereditary
- Paraneoplastic/Neoplastic
- Others (e.g., critical illness, amyloidosis)
The pathological differentials for lower limb weakness include: Endocrine (Vitamin B12 deficiency resulting in neuropathy/myelopathy, diabetic neuropathy), Trauma/Toxins (neuropathy secondary to chemotherapy), Inflammatory (Guillain-Barré syndrome, vasculitic neuropathy), Autoimmune (multiple sclerosis, neuromyelitis optica), Infection, Vascular, Neoplasm, Degenerative, and Congenital causes. [6]
| Category | Cause | Pathophysiology | HK Relevance |
|---|---|---|---|
| Metabolic | Diabetes mellitus (most common) | Chronic hyperglycaemia → polyol pathway activation (sorbitol accumulation in Schwann cells → osmotic damage) + advanced glycation end-products (AGEs) damage vasa nervorum → microvascular ischaemia + oxidative stress → axonal degeneration ± secondary demyelination | Very high — DM prevalence > 10% in HK |
| Chronic kidney disease / Uraemia | Accumulation of uraemic toxins (e.g., indoxyl sulphate, p-cresyl sulphate) → direct axonal toxicity + impaired axonal transport | High — CKD common in ageing HK population | |
| Hypothyroidism | Mucopolysaccharide deposition in nerve → compression + metabolic dysfunction of Schwann cells | Moderate | |
| Toxic / Drug | Alcohol | Direct neurotoxicity of ethanol + acetaldehyde on axons + associated thiamine (B1) deficiency → axonal degeneration | High — significant alcohol use in HK |
| Chemotherapy (platinum agents, vincristine, taxanes, bortezomib) | Platinum → DNA cross-linking in dorsal root ganglia; Vincristine → disrupts microtubule assembly → impairs axonal transport; Taxanes → stabilise microtubules (paradoxically also impairs transport) | High — rising cancer incidence | |
| Isoniazid | Inhibits pyridoxal phosphokinase → depletes active vitamin B6 → impairs GABA synthesis and nerve function | Relevant — TB still endemic in HK | |
| Ethambutol | Optic neuritis (blurring of vision, scotoma, worsened colour discrimination); peripheral neuropathy (rare) [7] | As above | |
| Amiodarone | Lysosomal accumulation of phospholipids in Schwann cells → demyelination | Used for AF, common in cardiology | |
| Metronidazole | Direct axonal toxicity with prolonged use | Common antibiotic | |
| Heavy metals (lead, arsenic, mercury, thallium) | Lead → motor neuropathy (attacks Schwann cells); Arsenic → axonal degeneration | Occupational exposure | |
| Nutritional | Vitamin B12 deficiency | B12 is essential for methylation of myelin basic protein and for maintaining myelin integrity → demyelination of posterior columns (SCD) + peripheral nerve axonal damage | Can occur in absence of haematological changes as neurones require higher B12 level to function [8]. Common in elderly, vegetarians, pernicious anaemia |
| Vitamin B1 (Thiamine) deficiency | Thiamine → essential cofactor for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in Krebs cycle → energy failure in metabolically active nerve tissue → axonal degeneration | Alcoholics, malnutrition | |
| Vitamin B6 (Pyridoxine) — deficiency OR excess | Deficiency → impaired nerve function; Excess → sensory neuronopathy (dorsal root ganglia toxicity) | Isoniazid causes B6 depletion | |
| Folate deficiency | Impaired DNA synthesis → rarely isolated cause of neuropathy | ||
| Copper deficiency | Posterior column myelopathy + peripheral neuropathy (mimics B12 deficiency) | Post-bariatric surgery, zinc excess | |
| Inflammatory / Immune | Guillain-Barré syndrome (GBS / AIDP) | Trigger: infection (Campylobacter jejuni, Mycoplasma pneumoniae, CMV, EBV), vaccination within 4 weeks → molecular mimicry: organisms share epitopes with antigens on peripheral nerve (GQ1b, GM1) → autoantibody-mediated damage to peripheral nerve [2][9] | Sporadic, worldwide |
| Chronic inflammatory demyelinating polyneuropathy (CIDP) | Autoimmune attack on peripheral nerve myelin; distinguished from GBS by protracted course (> 8 weeks), lack of association with preceding infection, more prominent sensory symptoms, and good response to steroid Tx [2] | ||
| Vasculitic neuropathy | Polyarteritis nodosa: necrotising vasculitis of medium-sized arteries → vasa nervorum ischaemia → mononeuritis multiplex (up to 70%) [10]; also ANCA-associated vasculitis, SLE | ||
| Paraproteinaemic neuropathy | Anti-MAG (myelin-associated glycoprotein) antibodies in IgM paraproteinaemia → demyelination | ||
| Infective | HIV | Direct HIV neurotoxicity + opportunistic infections + antiretroviral drug toxicity (e.g., didanosine, stavudine) → distal sensory polyneuropathy | Relevant in HK |
| Leprosy (Hansen's disease) | Mycobacterium leprae has tropism for Schwann cells → granulomatous inflammation → nerve damage. Palpably thickened nerves [2] | Rare in HK but classic teaching | |
| Hepatitis B/C | Immune complex–mediated vasculitis (HBV-associated PAN) or cryoglobulinaemia (HCV) | HBV endemic in HK | |
| Hereditary | Charcot-Marie-Tooth disease (CMT / HMSN) | Most common inherited neuropathy; CMT1 (demyelinating, AD) — PMP22 duplication → defective myelin; CMT2 (axonal, AD) — various gene mutations | Mostly autosomal dominant [2] |
| Familial amyloid polyneuropathy (FAP) | Transthyretin mutations → amyloid deposition in peripheral nerves → progressive sensorimotor and autonomic neuropathy | ||
| Hereditary neuropathy with liability to pressure palsies (HNPP) | PMP22 deletion → episodic demyelinating neuropathy at compression sites | ||
| Paraneoplastic / Neoplastic | Paraneoplastic sensory neuronopathy | Anti-Hu antibodies attack dorsal root ganglia → pure sensory neuropathy (non-length-dependent) | Associated with small cell lung CA |
| Direct infiltration (lymphoma, leukaemia) | Tumour cells infiltrate peripheral nerves | ||
| POEMS syndrome | Polyneuropathy, Organomegaly, Endocrinopathy, Monoclonal gammopathy, Skin changes [1] — likely VEGF-mediated nerve damage | ||
| Other | Amyloidosis (AL type) | Amyloid fibril deposition in peripheral nerves → autonomic neuropathy, postural hypotension [11] | |
| Critical illness polyneuropathy (CIP) | Systemic inflammation (sepsis/SIRS) → microvascular damage to peripheral nerves → axonal degeneration | Critical illness polyneuropathy listed as complication of MODS in sepsis [12] | |
| Sarcoidosis | Granulomatous infiltration of peripheral nerves | Rare |
HK-Specific High-Yield Causes
For Hong Kong exams, prioritise: (1) Diabetic polyneuropathy, (2) Alcohol-related, (3) Drug-induced (chemotherapy, isoniazid, ethambutol), (4) B12 deficiency, (5) GBS/CIDP, (6) CKD/uraemic neuropathy, (7) Hereditary (CMT). These cover >90% of clinical cases you will encounter.
Pathophysiology: Axonal vs. Demyelinating — The Two Fundamental Mechanisms
This distinction is the single most important concept in polyneuropathy because it determines the electrophysiological pattern, the differential diagnosis, and often the treatability.
- Mechanism: Damage to the axon itself (the "wire"). The myelin sheath is initially intact.
- Pathology: Wallerian degeneration distally → the axon dies back from the distal end.
- Why length-dependent? The longest axons have the most metabolic demand and are first to fail.
- NCS pattern: Normal conduction velocity (myelin is intact), ↓ amplitude (fewer functioning axons) [1][2].
- Clinical pattern: Early wasting is characteristic of axonal but not demyelinating neuropathies [2].
- Recovery: Slow — requires axonal regrowth (~1 mm/day = ~1 inch/month).
- Common causes: Diabetes, alcohol, uraemia, drugs/toxins, nutritional deficiency, most hereditary neuropathies.
- Treatability: Majority not directly treatable → management largely by ↓ exposure to toxins and treatment of underlying disease process [2].
- Mechanism: Damage to the Schwann cells or their myelin sheath. The axon is initially preserved.
- Pathology: Segmental demyelination → loss of insulation → slowed or blocked conduction.
- NCS pattern: ↓ Conduction velocity (myelin is damaged → impulse travels slowly), normal/near-normal amplitude (axons intact) [1][2]. Also: conduction block, temporal dispersion, prolonged distal latency, prolonged F-wave latency.
- Recovery: Faster — remyelination can occur over weeks to months.
- Common causes: GBS, CIDP, paraproteinaemic neuropathy, CMT1, anti-MAG neuropathy.
- Treatability: Usually immunosuppressive treatment (e.g., IVIg, pulse steroids, plasmapheresis) — this is why distinguishing demyelinating from axonal is so clinically important [2].
Exam Pearl: Why Does NCS Distinction Matter?
Important because usually only demyelinating neuropathies are susceptible to treatment [2]. If you identify a demyelinating pattern on NCS, you must actively search for a treatable cause (GBS, CIDP, paraproteinaemia). Axonal neuropathies are generally managed by treating the underlying cause.
In practice, many chronic polyneuropathies have elements of both axonal loss and secondary demyelination (e.g., chronic diabetic neuropathy). The "primary" pathology determines the classification.
Classification
Polyneuropathy can be classified along multiple axes. This is how you systematically narrow the differential:
| Time Course | Duration | Examples |
|---|---|---|
| Acute | Days to 4 weeks | GBS, porphyria, toxins (e.g., thallium), vasculitis |
| Subacute | 4–8 weeks | Nutritional, drug-induced, paraneoplastic |
| Chronic | > 8 weeks | Diabetes, alcohol, CIDP, hereditary (CMT), uraemia |
| Relapsing-remitting | Fluctuating | CIDP (1/3 of cases), porphyria |
| Distribution | Description | Examples |
|---|---|---|
| Symmetric, length-dependent | Classic glove-and-stocking | Most metabolic, toxic, nutritional causes |
| Symmetric, non-length-dependent | Proximal + distal equally | CIDP, GBS |
| Asymmetric | Not fitting a single nerve territory | Mononeuritis multiplex (vasculitis, DM), multifocal motor neuropathy |
| Type | Clinical Features | Examples |
|---|---|---|
| Motor-predominant | Weakness, wasting, foot drop | GBS, CIDP, CMT, multifocal motor neuropathy, lead poisoning |
| Sensory-predominant | Numbness, tingling, pain | DM (early), B12 deficiency, paraneoplastic, alcohol |
| Sensorimotor (mixed) | Both motor and sensory | Most common pattern — DM, alcohol, CIDP, CMT |
| Autonomic-predominant | Postural hypotension, GI/GU dysfunction | DM autonomic neuropathy, amyloidosis, GBS (can occur) |
| Small fibre | Pain, temperature loss, autonomic | Early DM, amyloidosis, Fabry disease |
| Type | NCS Findings | Examples |
|---|---|---|
| Axonal | ↓ Amplitude, normal/near-normal velocity | DM, alcohol, toxins, uraemia, B12 def |
| Demyelinating | ↓ Velocity, conduction block, prolonged distal latency | GBS, CIDP, CMT1, anti-MAG |
| Mixed | Features of both | Chronic DM, some toxic neuropathies |
Clinical Features
A. Symptoms (What the Patient Tells You)
History will reveal the diagnosis and direct our neurological examination [13][14].
Key presenting symptoms to ask about: Sensory (pain, numbness, paraesthesia, modality involved), Motor (weakness, movement disorders), Distribution, Time of onset and duration [2][15].
| Symptom | Mechanism | Fibre Type |
|---|---|---|
| Numbness ("my feet feel dead/wooden") | Loss of sensory input from damaged sensory axons → negative symptom | Large fibre (Aβ) |
| Tingling / "Pins and needles" (paraesthesia) | Ectopic impulse generation from injured/regenerating nerve fibres → positive symptom | Large fibre |
| Burning pain | Spontaneous firing of damaged nociceptive C-fibres and Aδ fibres → positive symptom; also central sensitisation in dorsal horn | Small fibre (C, Aδ) |
| Lancinating / "Electric shock" pain | Ectopic discharges from demyelinated axons → ephaptic cross-talk between fibres | Mixed |
| Band-like tightness | Distorted sensory processing from partial denervation | Mixed |
| Allodynia (pain from light touch) | Aβ fibres (normally non-nociceptive) sprout into nociceptive laminae of dorsal horn after C-fibre loss → light touch now perceived as pain (central sensitisation) | Central mechanism |
| Loss of balance / unsteadiness (sensory ataxia) | Proprioceptive loss → brain doesn't know where limbs are in space → worse in dark (no visual compensation) → positive Romberg sign | Large fibre (Aα, Aβ) |
| "Walking on cotton wool" | Loss of plantar sensation → altered proprioceptive feedback | Large fibre |
Distribution: Glove-and-stocking pattern — starts distally in feet, ascends proximally; hands involved later when symptoms reach mid-calf level [2][5].
| Symptom | Mechanism |
|---|---|
| Difficulty tip-toeing | Distal LL weakness — ankle plantarflexion (S1–S2, tibial nerve) |
| Foot drop (tripping over feet, slapping gait) | Distal LL weakness — ankle dorsiflexion (L4–L5, common peroneal nerve); most characteristic early motor symptom of polyneuropathy |
| Difficulty with fine hand movements (buttons, keys) | Distal UL weakness — intrinsic hand muscles |
| Difficulty climbing stairs, standing from chair, combing hair | Proximal weakness — suggests GBS, CIDP, or myopathy rather than typical distal polyneuropathy [2] |
| Muscle cramps | Denervation → motor unit instability → involuntary contractions |
| Symptom | Mechanism |
|---|---|
| Postural dizziness / lightheadedness | Orthostatic hypotension due to central/peripheral sympathetic denervation [5] |
| Resting tachycardia | Impaired cardiac output control (early feature of autonomic neuropathy) [5] |
| Dry skin (anhidrosis) distally, compensatory sweating proximally | Distal hypohidrosis with compensatory proximal hyperhidrosis — sudomotor denervation [5] |
| Gastroparesis (nausea, early satiety, bloating) | Vagal autonomic neuropathy → loss of interstitial cells of Cajal coordination [16] |
| Constipation or diarrhoea (esp. nocturnal) | Painless watery nocturnal diarrhoea — enteric nervous system dysfunction [5] |
| Erectile dysfunction | Autonomic denervation of pelvic vasculature |
| Bladder dysfunction (incomplete emptying, overflow incontinence) | ↓ Ability to sense full bladder, incomplete emptying, recurrent UTI [5] |
- Family history of similar problem → ?genetic disease (CMT, FAP)
- Past medical history: Previous trauma, previous neurological diseases
- Drug history: Chemotherapy, metronidazole, amiodarone → neuropathy; Alcohol abuse → neuropathy; Steroids → proximal muscle weakness (important differential)
- Travel history
- Other/prior neurological symptoms; other systemic symptoms
- Past medical and surgical history; vaccination history; social history
B. Signs (What You Find on Examination)
Physical examination is guided by history and helps with localisation and further narrowing differential diagnosis [6].
Examine: Distribution and character of weakness (unilateral/bilateral, upper and/or lower limbs, proximal and/or distal, myotome/nerve distribution, UMN and/or LMN pattern); any sensory impairment (distribution, nature); cranial nerve and cerebellar signs; sphincter disturbances (anal tone, palpable bladder) [6].
| Sign | Description | Mechanism |
|---|---|---|
| Wasting (muscle atrophy) | Visible loss of muscle bulk, especially in intrinsic foot muscles (extensor digitorum brevis), then hand intrinsics | Denervation → loss of trophic support → muscle fibre atrophy. Early wasting is characteristic of axonal but not demyelinating neuropathies [2] |
| Fasciculation | Irregular, non-rhythmical contraction of muscle; increased by tapping over bulk of large muscles; indicative of LMN lesion [15] | Spontaneous depolarisation of denervated motor units |
| Weakness | Typically distal > proximal; foot dorsiflexion (foot drop) often earliest motor sign | Loss of motor axons |
| Hypotonia / ↓ Tone | ↓ Tone → LMN lesion [15] | Loss of lower motor neurone input to muscle spindle |
| Hyporeflexia / Areflexia | Diminished or absent deep tendon reflexes, ankle jerk lost first (longest reflex arc) | Interruption of the monosynaptic reflex arc (sensory afferent OR motor efferent limb) |
| Foot drop / Steppage gait | High-stepping gait to clear the dropped foot | Weakness of ankle dorsiflexors (tibialis anterior) |
Classical Reflex Pattern in B12 Deficiency
Peripheral neuropathy classically: ↑ knee jerk (from subacute combined degeneration of cord — UMN) + ↓ ankle jerk (from peripheral neuropathy — LMN) [8]. This mixed UMN + LMN picture is a classic exam question. The explanation: B12 deficiency causes BOTH myelopathy (posterior + lateral column degeneration → UMN signs) AND peripheral neuropathy (→ LMN signs). The knee jerk is mediated by L3-4 (shorter nerve, less affected by PN, and the UMN lesion predominates), while the ankle jerk (S1-2, longest reflex arc) is lost due to peripheral neuropathy.
| Sign | Description | Mechanism |
|---|---|---|
| Glove-and-stocking sensory loss | Distal symmetrical loss of sensation, ascending proximally | Length-dependent axonal degeneration |
| Impaired vibration sense (128 Hz tuning fork) | Lost at toes first, then ankles, then knees | Large myelinated fibre loss (dorsal column-medial lemniscal pathway) |
| Impaired proprioception (joint position sense) | Cannot tell direction of passive toe/finger movement | Large fibre loss → sensory ataxia |
| Impaired pinprick/temperature | Lost in stocking distribution | Small fibre loss (spinothalamic pathway) |
| Positive Romberg sign | Falls/sways with eyes closed (worse than eyes open) | Proprioceptive loss — with eyes open, vision compensates; eyes closed removes this compensation → unsteadiness |
| Pseudoathetosis | Slow writhing finger movements when eyes closed and arms outstretched | Severe proprioceptive loss → brain cannot track finger position |
| Sensory level | Horizontal level on trunk where sensation changes | More suggestive of myelopathy, but can occur in severe polyneuropathy |
| Sign | Mechanism |
|---|---|
| Pes cavus (high-arched foot) | Chronic denervation of intrinsic foot muscles → imbalance between long extensors/flexors and intrinsic muscles → arch elevation. Classic of hereditary neuropathy (CMT) [1][2] |
| Claw hand | Denervation of lumbricals/interossei → MCP hyperextension + IP flexion [1][2] |
| Kyphoscoliosis | Chronic paraspinal muscle denervation (hereditary neuropathies) [1][2] |
| Trophic ulcers (esp. plantar) | Loss of protective pain sensation → repeated unrecognised trauma → ulceration |
| Charcot joints (neuropathic arthropathy) | Loss of proprioception + pain → joint subjected to abnormal stresses → destruction. Charcot arthropathy is a classic complication of diabetic neuropathy [5] |
| Disuse atrophy, hair loss, brittle nails | Denervation of trophic nerve supply to skin appendages [2] |
Palpably thickened nerves (e.g., great auricular, ulnar at elbow, common peroneal at fibular head) — found in [2]:
- Leprosy
- Acromegaly
- HMSN (CMT)
- CIDP
- Neurofibromatosis
- Amyloidosis
- Refsum disease
This is a classic short-case exam finding — if you palpate thickened nerves, the differential is narrow.
| Sign | Mechanism |
|---|---|
| Orthostatic hypotension (> 20 mmHg systolic drop on standing) | Sympathetic vasomotor failure → inability to vasoconstrict on standing |
| Resting tachycardia | Vagal denervation → unopposed sympathetic tone |
| Dry, cracked skin distally | Sudomotor denervation → anhidrosis |
| Warm foot with bounding pulses but absent sensation | Autonomic neuropathy → arteriolar vasodilation (neuropathic foot, as opposed to the cold, pulseless ischaemic foot) |
| Dilated, sluggish pupils | Parasympathetic pupillary denervation |
The 5 Questions to Ask Yourself
- Is it really polyneuropathy? (vs. myelopathy, myopathy, mononeuritis multiplex)
- What is the time course? (acute → GBS; chronic → DM, CMT, CIDP)
- What fibres are involved? (motor, sensory, autonomic, mixed)
- Is it axonal or demyelinating? (NCS is key — determines treatability)
- What is the underlying cause? (metabolic screen, autoimmune, genetic)
Special Focus: Key Aetiologies and Their Pathophysiology
Most common form of diabetic neuropathy is sensory polyneuropathy: glove-and-stocking sensory loss of all modalities or paraesthesia [5].
Pathophysiology (multiple interacting mechanisms):
- Polyol pathway: Hyperglycaemia → aldose reductase converts glucose to sorbitol → accumulates in Schwann cells (cannot cross cell membrane) → osmotic stress → cell swelling and damage + depletes NADPH (needed for glutathione regeneration → ↑ oxidative stress)
- Advanced glycation end-products (AGEs): Glucose non-enzymatically glycates proteins → AGEs cross-link extracellular matrix proteins, activate RAGE receptors → inflammation, endothelial dysfunction
- Microvascular ischaemia: AGE damage to vasa nervorum → endoneurial hypoxia → axonal injury
- Oxidative stress: All of the above pathways generate reactive oxygen species
- Protein kinase C activation: Hyperglycaemia → diacylglycerol → PKC activation → ↑ vascular permeability, ↓ blood flow
- Net result: Axonal degeneration (length-dependent) ± secondary demyelination
Screening: by symptoms and monofilament (small fibre) and tuning fork (large fibre) tests [5].
Pathophysiology: Dual mechanism:
- Direct toxic effect of ethanol and its metabolite acetaldehyde on peripheral nerve axons
- Nutritional deficiency — particularly thiamine (B1), but also B6, B12, folate, niacin — from poor dietary intake + impaired absorption + increased metabolic demand
- Net result: Axonal degeneration, sensory-predominant
Definition: Acute inflammatory demyelinating polyneuropathy (AIDP) affecting nerve roots [1].
Trigger → Molecular mimicry: the organisms share epitopes with an antigen on peripheral nerve (GQ1b, GM1) → autoantibody-mediated damage to peripheral nerve [1][2].
Variants:
| Variant | Antibody | Target | Features |
|---|---|---|---|
| AIDP (most common in West) | Various | Schwann cell/myelin | Classic ascending weakness + areflexia |
| AMAN (acute motor axonal neuropathy — common in Asia) | Anti-GM1, anti-GD1a | Motor axolemma | Pure motor, rapid onset, axonal pattern |
| AMSAN | Anti-GM1 | Motor + sensory axolemma | Severe, poor prognosis |
| Miller Fisher | Anti-GQ1b | Oculomotor nerves, cerebellar pathways | Ophthalmoplegia, ataxia, areflexia |
Similar to AIDP in terms of clinical features, CSF findings, pathology, NCS findings; distinguished by protracted course (> 8 weeks) with relapses, lack of association with preceding infection, more prominent sensory symptoms, and good response to steroid treatment [2].
Nerve biopsy: segmental demyelination and remyelination, "onion-bulb" appearance [2] — this "onion-bulb" represents repeated cycles of demyelination and remyelination, with concentric layers of Schwann cell processes accumulating around the axon.
Neuropsychiatric features: ?related to ↓ methylation of myelin proteins [8]:
- Subacute combined degeneration of cord: affects dorsal (DC-ML) and lateral columns (CST)
- Sensory: impaired vibration/proprioception esp bilateral LL, +ve Romberg
- Motor: paraparesis ± incontinence if severe
- Peripheral neuropathy
- Other: depression, irritability, cognitive slowing, dementia, psychosis
Listed as a neurological complication of multiple organ dysfunction syndrome (MODS) in sepsis [12].
- Pathophysiology: Systemic inflammatory response → cytokine-mediated microvascular damage to vasa nervorum → endoneurial oedema → axonal degeneration
- Presents as difficulty weaning from ventilator + limb weakness in ICU patients
- NCS: axonal pattern
- Management: treat underlying sepsis; no specific treatment; slow recovery
| Drug | Mechanism | Type | Prevention |
|---|---|---|---|
| Isoniazid | Depletes pyridoxine (B6) → impairs nerve function | Axonal, sensory | Vitamin B6 for those with risk factors (pregnancy, DM, renal disease) [7] |
| Ethambutol | Arabinosyltransferase inhibition; optic neuritis + peripheral neuropathy (rare) [7] | Axonal | Baseline visual acuity, monthly monitoring |
| Vincristine | Disrupts microtubule assembly → impairs axonal transport | Axonal, motor > sensory | Dose reduction |
| Cisplatin / Oxaliplatin | DNA damage in dorsal root ganglia → sensory neuronopathy | Axonal, sensory | Dose-dependent; magnesium supplementation |
| Taxanes (paclitaxel, docetaxel) | Microtubule stabilisation → impaired axonal transport | Axonal, sensory | Dose reduction |
| Amiodarone | Phospholipid accumulation in Schwann cells | Demyelinating | Monitor; consider alternative antiarrhythmic |
| Metronidazole | Direct axonal toxicity | Axonal, sensory | Avoid prolonged courses |
| Thalidomide | Antiangiogenic → vasa nervorum ischaemia | Axonal, sensory | |
| Bortezomib | Proteasome inhibition → DRG neurotoxicity | Axonal, sensory | Switch to subcutaneous route |
| Nitrofurantoin | Direct axonal toxicity | Axonal, sensorimotor | Avoid in renal impairment |
High Yield Summary
Definition: Polyneuropathy = symmetric, length-dependent disease of multiple peripheral nerves (distal → proximal, "glove-and-stocking").
Most common cause: Diabetes mellitus (HK and worldwide).
Two fundamental mechanisms: Axonal (↓ amplitude, normal velocity on NCS; slow recovery; treat underlying cause) vs. Demyelinating (↓ velocity, conduction block; treatable with immunotherapy).
Clinical features:
- Sensory: numbness, tingling, burning pain (small fibre), loss of proprioception/vibration (large fibre)
- Motor: distal weakness (foot drop), wasting, fasciculation, areflexia (LMN signs)
- Autonomic: postural hypotension, resting tachycardia, gastroparesis, bladder dysfunction
- Trophic: pes cavus, claw hand, trophic ulcers, Charcot joints
- Nerve thickening: leprosy, CMT, CIDP, acromegaly, neurofibromatosis
Key teaching points:
- Only demyelinating neuropathies are usually susceptible to treatment — this is why NCS is essential.
- B12 deficiency → ↑ knee jerk (myelopathy/UMN) + ↓ ankle jerk (neuropathy/LMN).
- GBS: molecular mimicry (GQ1b, GM1) → acute demyelinating polyneuropathy; CSF: albuminocytologic dissociation.
- CIDP: distinguished from GBS by > 8 weeks duration, relapsing course, steroid-responsive.
- Isoniazid neuropathy prevented by vitamin B6 supplementation.
- Critical illness polyneuropathy: axonal, occurs in sepsis/MODS.
Systematic approach: Time course → Fibre type → Distribution → NCS (axonal vs demyelinating) → Targeted investigations.
Active Recall - Polyneuropathy Overview
[1] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 — Peripheral neuropathy) [2] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 — Disease of the Peripheral Nerves, pp. 179–184) [3] Epidemiological data — general medical literature [4] Senior notes: Ryan Ho Endocrine.pdf (p. 98 — Diabetic peripheral neuropathy) [5] Senior notes: Ryan Ho Endocrine.pdf (pp. 98–99 — Diabetic peripheral and autonomic neuropathy) [6] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (pp. 18, 20) [7] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p. 41 — Anti-TB drug side effects) [8] Senior notes: Ryan Ho Haemtology.pdf (p. 29 — B12 deficiency, SCD, peripheral neuropathy) [9] Lecture slides: Learning_Points_All_Lectures.txt (Neurology section — GBS) [10] Senior notes: Ryan Ho Rheumatology.pdf (p. 159 — PAN) [11] Senior notes: Block A - Hematology Data Interpretation.pdf (p. 1 — Amyloidosis, autonomic neuropathy) [12] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p. 36 — MODS, critical illness polyneuropathy) [13] Lecture slides: CFB (MED04) Central Nervous System.pdf (p. 3 — History taking) [14] Lecture slides: GC 094. Where is the lesion I.pdf (p. 4 — History structure) [15] Senior notes: Ryan Ho Fundamentals.pdf (p. 102 — Neurological examination of PNS) [16] Senior notes: Block A - Indigestion and 'heartburn'.pdf (p. 25 — Gastroparesis pathophysiology)
The differential diagnosis of polyneuropathy operates on two levels simultaneously:
- "Is this actually polyneuropathy, or is it something else mimicking it?" — i.e., anatomical differential diagnosis (localisation).
- "If it IS polyneuropathy, what is the underlying cause?" — i.e., aetiological differential diagnosis.
Both are essential. Missing the first level means you might treat a myelopathy as a neuropathy (dangerous). Missing the second means you fail to identify a treatable cause.
Level 1: Anatomical Differential Diagnosis — "Is It Really Polyneuropathy?"
The patient presents with bilateral distal weakness, sensory loss, or both. Before you commit to "polyneuropathy," you must systematically consider every anatomical level from cortex to muscle that could produce similar symptoms.
Where is the lesion (anatomical differentials)? [17][18]:
- Cerebrum (primary motor cortex, corona radiata, internal capsule)
- Brainstem (cerebral peduncle, anterior pons, medulla oblongata)
- Extrapyramidal system
- Spinal cord (anterior horn cells, cervical cord)
- Nerve root
- Brachial plexus
- Peripheral nerve
- Neuromuscular junction
- Muscle
- Bone and joints
- Metabolic
- Functional
What is the lesion (pathological differentials)? [17][18]:
- Vascular (stroke, spinal cord infarction)
- Infection (encephalitis, myelitis, old polio)
- Neoplasm (brain tumour, Pancoast tumour with brachial plexus compression/infiltration)
- Degenerative (Parkinson's disease, cervical myelopathy/radiculopathy, carpal tunnel syndrome)
- Inflammatory (multiple sclerosis, myelitis, Guillain-Barré syndrome, vasculitic neuropathy)
- Congenital (cerebral palsy)
- Autoimmune (encephalitis, neuromyelitis optica, myasthenia gravis)
- Trauma/Toxins (subdural/epidural haematoma, fracture; neuropathy secondary to chemotherapy)
- Endocrine (B12 deficiency resulting in neuropathy/myelopathy, diabetic neuropathy)
| Condition | Key Distinguishing Features | Why It Mimics Polyneuropathy |
|---|---|---|
| Myelopathy (e.g., cervical spondylotic, MS, B12 deficiency) | UMN signs: hyperreflexia, spasticity, upgoing plantars, sensory level, sphincter disturbance early [2]. Distal paraesthesia can precede long tract signs. Normal or ↑ tendon reflexes (vs. ↓ in polyneuropathy) | Bilateral distal paraesthesia in hands/feet can be the earliest symptom of cord compression. But the reflexes are brisk, not absent — that's the giveaway. |
| Mononeuritis multiplex | Involvement of entirely unrelated nerves, relative preservation of certain nerves [2]. Asymmetrical. Individual nerve territories identifiable. | When many nerves are affected, it can superficially look symmetrical. But careful examination reveals asymmetry and non-length-dependent pattern. |
| Bilateral radiculopathy / Cauda equina syndrome | Weakness and sensory loss in segmental (dermatomal) distribution [2]. Cauda equina: saddle anaesthesia, urinary retention with overflow incontinence, bilateral radicular pain, LMN signs [19]. | Bilateral L5/S1 radiculopathy can cause bilateral foot drop and distal sensory loss, mimicking polyneuropathy. But the pattern follows dermatomes, not a stocking distribution. |
| Myopathy (e.g., polymyositis, muscular dystrophy, steroid myopathy) | Preserved reflexes (until very late), muscle tenderness, proximal weakness (difficulty standing from chair, climbing stairs, combing hair), CK markedly elevated, EMG diagnostic [2]. No sensory involvement. | Proximal weakness can be confused with a motor-predominant neuropathy like GBS. But myopathy has NO sensory loss and reflexes are preserved. |
| Myasthenia gravis | Ocular involvement common (ptosis, diplopia), fatigability present, no sensory involvement [2]. Fluctuating weakness that worsens with repeated use. | Generalised weakness can mimic polyneuropathy. But MG has fatigability, ocular involvement, and NO sensory loss. |
| Lambert-Eaton myasthenic syndrome (LEMS) | May present with distal paraesthesia and hyporeflexia in addition to weakness → RNS diagnostic [2]. Reflexes improve after exercise (facilitation). Associated with small cell lung cancer. | LEMS is tricky because it can have hyporeflexia AND paraesthesia — very similar to polyneuropathy. The key is that weakness improves with repeated use (opposite of MG), and repetitive nerve stimulation shows incremental response. |
| Motor neuron disease (ALS) | Combination of UMN + LMN signs without sensory involvement [20]. Fasciculation prominent. NCS: sensory and motor nerve conduction studies are NORMAL [20]. EMG shows denervation. | The LMN component (wasting, fasciculation, weakness) can resemble motor neuropathy. But ALS has UMN signs (brisk reflexes, upgoing plantars) AND LMN signs in the SAME limb — this mixed picture doesn't occur in polyneuropathy. |
| Psychogenic / Functional weakness | Fluctuating, variable symptoms not conforming to neuroanatomical principles [2]. Hoover sign positive. Give-way weakness. Normal reflexes, normal NCS. | Can mimic any neurological presentation. The inconsistency on examination is the clue. |
Critical Exam Trap: Myelopathy vs. Polyneuropathy
The most dangerous misdiagnosis is confusing cervical myelopathy for polyneuropathy. Both can present with bilateral hand numbness and clumsiness. The key differentiator: reflexes. In polyneuropathy, reflexes are reduced or absent. In myelopathy, reflexes are brisk with UMN signs (Hoffmann's sign, upgoing plantars, clonus). A sensory level on the trunk should immediately make you think myelopathy, NOT polyneuropathy [2]. Miss a cord compression and the patient may become permanently paraplegic.
Level 2: Aetiological Differential Diagnosis — "What Is Causing the Polyneuropathy?"
Once you've confirmed it is polyneuropathy, the next step is determining the underlying cause. This is driven by the clinical characterisation (time course, fibre type, NCS pattern).
Differential diagnosis of peripheral neuropathy (polyneuropathy) — need to know! [1]:
- Metabolic: DM, alcohol, B12 (sensory), hypothyroidism, uraemia, chronic liver disease
- Inflammatory: GBS (AIDP) / CIDP, vasculitides
- Malignancy: paraneoplastic, paraproteinaemia
- Hereditary: CMT (HMSN)
- Drugs: isoniazid, vincristine, cisplatin, metronidazole (sensory), colchicine (sensory)
- Infective: HIV, Lyme's disease
- Infiltrative: amyloidosis, sarcoidosis
High-Yield Exam List
This list from the senior notes [1] is the core differential for any polyneuropathy question. Memorise it. In an exam, if asked "list causes of polyneuropathy," these 7 categories with their specific examples will score full marks.
The tempo of onset is one of the most powerful discriminators:
| Time Course | Duration | Key Differentials | Why This Helps |
|---|---|---|---|
| Acute (days to 4 weeks) | < 4 weeks | GBS, porphyria, toxins (arsenic, thallium), vasculitis, diphtheria, critical illness polyneuropathy | Acute onset polyneuropathy = GBS until proven otherwise. GBS reaches nadir within 4 weeks [21][22]. |
| Subacute (4–8 weeks) | 4–8 weeks | Drug-induced, nutritional (B1, B12), paraneoplastic, vasculitis | Drugs and deficiencies typically accumulate over weeks. |
| Chronic (> 8 weeks) | > 8 weeks | DM, alcohol, CIDP, CMT, uraemia, hypothyroidism, paraproteinaemia, amyloidosis | CIDP continues to progress or has relapses for more than 8 weeks [2][21][22]. Most metabolic/toxic causes are chronic. |
| Relapsing-remitting | Fluctuating | CIDP (1/3), porphyria, HNPP | Relapsing course should trigger consideration of CIDP and porphyria. |
| NCS Pattern | Key Differentials | Clinical Reasoning |
|---|---|---|
| Axonal (↓ amplitude, normal velocity) | DM, alcohol, drugs/toxins, B12 deficiency, uraemia, paraneoplastic, amyloidosis, critical illness, CMT2 | Most common pattern. Usually metabolic/toxic aetiology. Majority not directly treatable → management by ↓ exposure to toxins and treatment of underlying disease [2]. |
| Demyelinating (↓ velocity, conduction block) | GBS, CIDP, paraproteinaemia (anti-MAG), CMT1 | Usually only demyelinating neuropathies are susceptible to treatment [2]. Actively search for treatable cause. |
| Mixed | Chronic DM, CIDP with secondary axonal loss | Long-standing demyelinating process → secondary axonal degeneration. |
| Predominant Involvement | Key Differentials | Why |
|---|---|---|
| Motor-predominant | GBS, CIDP, multifocal motor neuropathy, CMT, lead poisoning, porphyria, diphtheria | Motor nerves selectively targeted by anti-ganglioside antibodies (GBS) or genetic defects (CMT). |
| Sensory-predominant | DM (early), B12 deficiency, alcohol, paraneoplastic (anti-Hu), drugs (cisplatin, metronidazole), amyloidosis (small fibre) | Dorsal root ganglia or distal sensory terminals are the primary target. |
| Sensorimotor | DM (advanced), alcohol (advanced), CIDP, uraemia, CMT | Most polyneuropathies eventually affect both fibre types. |
| Autonomic-predominant | DM, amyloidosis, GBS (can occur), pure autonomic failure, Fabry disease | Unmyelinated autonomic fibres selectively damaged. |
| Small fibre (pain + temperature + autonomic) | DM (early), amyloidosis, Fabry disease, HIV, sarcoidosis, coeliac disease | Small fibres have less metabolic reserve. Standard NCS may be NORMAL (tests only large fibres). Skin biopsy for intraepidermal nerve fibre density is diagnostic. |
When a patient presents with acute flaccid paralysis, the differential is particularly important and commonly examined:
Differential diagnosis of GBS [1][21][22]:
| Category | Conditions | Key Distinguishing Feature |
|---|---|---|
| Other acute polyneuropathies | Vasculitis, Lyme disease, porphyria, sarcoidosis, paraneoplastic disease, critical illness, severe vitamin B1 (thiamine) deficiency, acute arsenic poisoning | Vasculitis → asymmetric; Porphyria → abdominal pain + psychiatric features + dark urine; Critical illness → ICU setting; Arsenic → GI symptoms, Mees' lines |
| Spinal cord disease | Spinal cord compression, acute transverse myelitis | UMN signs (brisk reflexes, upgoing plantars), sensory level, sphincter disturbance early |
| Motor neuron disease | Poliomyelitis, ALS, progressive spinal muscular atrophy | Polio → asymmetric, fever; ALS → mixed UMN + LMN, no sensory involvement; SMA → pure LMN, no sensory involvement |
| NMJ disease | Botulism, myasthenia gravis, Lambert-Eaton myasthenic syndrome | Botulism → descending paralysis (opposite to GBS), pupil involvement, food history; MG → fatigability, ocular; LEMS → incremental response on RNS |
| Muscle disease | Acute polymyositis | Proximal weakness, muscle tenderness, markedly ↑ CK, preserved reflexes |
DDx for GBS also includes: Poliomyelitis, other causes of polyneuropathy (e.g., toxins, vasculitis, lymphomatous infiltration, critical illness polyneuropathy), NMJ disorders (e.g., MG, botulism) [1].
Exam Pearl: GBS vs. Transverse Myelitis vs. Myasthenia Gravis
All three can cause acute weakness. Distinguish by:
- GBS: Ascending, areflexia, NO sensory level, CSF albuminocytologic dissociation
- Transverse myelitis: Sensory level, UMN signs below level, sphincter disturbance early, hyperreflexia
- MG: Fatigability, ocular involvement, NO sensory loss, NO reflex changes
For a patient with slowly progressive distal sensory loss ± weakness over months to years:
| Cause | Key Clue on History/Exam |
|---|---|
| Diabetes mellitus | Known DM, HbA1c elevated, microvascular complications (retinopathy, nephropathy) |
| Alcohol | Social history, liver stigmata, macrocytosis, ↓ thiamine |
| B12 deficiency | Macrocytic anaemia, hypersegmented neutrophils, ↑ knee jerk + ↓ ankle jerk (SCD + PN), glossitis, early greying [8][23] |
| Chronic kidney disease | Known CKD, ↑ creatinine/urea, anaemia |
| Hypothyroidism | Fatigue, weight gain, constipation, ↑ TSH |
| CIDP | Relapsing course, proximal AND distal weakness, thickened nerves, responds to steroids |
| CMT / HMSN | Family history, pes cavus, distal wasting (inverted champagne bottle legs), onset in childhood/adolescence, mostly autosomal dominant [2] |
| Paraproteinaemia | Serum protein electrophoresis (SPE) abnormality, anti-MAG antibodies |
| Amyloidosis | Autonomic neuropathy, postural hypotension, organomegaly, Congo red stain salmon-pink [24] |
| Paraneoplastic | Subacute onset, non-length-dependent, known malignancy or weight loss, anti-Hu antibodies |
| Drug-induced | Temporal relationship to drug initiation, reversible on discontinuation |
D/dx of CIDP [2]:
- Chronic axonal neuropathy: typically affects distal muscles sparing knee reflex
- Hereditary demyelinating neuropathies: uniform homogenous slowing of NCV without conduction block
- Acquired demyelinating neuropathies: HIV-related, SLE, paraproteinaemia-related
Why the NCS pattern matters here: CIDP shows multifocal, non-uniform conduction slowing WITH conduction block — this non-uniformity proves it is acquired (the immune attack is patchy). Hereditary demyelinating neuropathies (CMT1) show uniform slowing WITHOUT block because every Schwann cell is genetically affected equally.
This integrates the two levels of differential diagnosis into a stepwise clinical approach:
| Feature | DM | Alcohol | GBS | CIDP | CMT | B12 Deficiency | Amyloidosis |
|---|---|---|---|---|---|---|---|
| Onset | Chronic | Chronic | Acute | Chronic / relapsing | Childhood | Subacute-chronic | Chronic |
| Distribution | Distal, symmetric | Distal, symmetric | Proximal + distal | Proximal + distal | Distal, symmetric | Distal, symmetric | Distal, symmetric |
| Motor vs Sensory | Sensory > motor | Sensory > motor | Motor > sensory | Sensorimotor | Sensorimotor | Sensory ± myelopathy | Small fibre + autonomic |
| Autonomic | Prominent | Mild | Can occur | Mild | Rare | Rare | Prominent |
| NCS | Axonal | Axonal | Demyelinating | Demyelinating | Demyelinating (CMT1) or Axonal (CMT2) | Axonal | Axonal |
| Key clue | HbA1c, retinopathy | Liver stigmata | Post-infective, ascending | > 8 weeks, steroid-responsive | Pes cavus, FHx, inverted champagne bottle | Macrocytosis, ↑ knee jerk + ↓ ankle jerk | Postural hypotension, organomegaly, Congo red |
High Yield Summary
Two-level approach: (1) Confirm it IS polyneuropathy (exclude myelopathy, myopathy, NMJ disorders, radiculopathy); (2) Identify the underlying cause.
The most dangerous mimic: Myelopathy — distinguished by brisk reflexes, UMN signs, sensory level.
Must-know aetiological differential (from senior notes): DM, alcohol, B12, hypothyroidism, uraemia, chronic liver disease, GBS/CIDP, vasculitides, paraneoplastic, paraproteinaemia, CMT, drugs (isoniazid, vincristine, cisplatin, metronidazole, colchicine), HIV, Lyme disease, amyloidosis, sarcoidosis.
NCS-based stratification: Axonal → metabolic/toxic (treat underlying cause); Demyelinating → immune-mediated or hereditary (potentially treatable with immunotherapy).
Acute weakness differential (GBS): Must distinguish from spinal cord compression (UMN signs, sensory level), botulism (descending), MG (fatigability, ocular), poliomyelitis (asymmetric, fever), acute polymyositis (proximal, ↑CK, preserved reflexes).
CIDP vs GBS: CIDP > 8 weeks, relapsing, no preceding infection, more sensory, steroid-responsive.
CIDP vs CMT: CIDP has non-uniform conduction slowing WITH conduction block; CMT has uniform slowing WITHOUT block.
Active Recall - Polyneuropathy Differential Diagnosis
[1] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 — Peripheral neuropathy, differential diagnosis list) [2] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 — Disease of the Peripheral Nerves, pp. 179–184) [8] Senior notes: Ryan Ho Haemtology.pdf (p. 29 — B12 deficiency, SCD, peripheral neuropathy) [17] Lecture slides: CFB_Neuro clinical skills demonstration_01.08.22_file to students.pdf (pp. 7–8) [18] Lecture slides: Neurology- Two cases of lower limb weakness.pdf (pp. 18, 20) [19] Senior notes: Maksim Surgery Notes.pdf (p. 222–223 — Cauda equina syndrome) [20] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (pp. 1286–1291 — ALS diagnosis) [21] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (pp. 1282–1285 — GBS differential and diagnostic criteria) [22] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (pp. 547–548 — GBS differential and diagnostic criteria) [23] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (p. 18 — Pernicious anaemia) [24] Senior notes: Block A - Hematology Data Interpretation.pdf (p. 1 — Amyloidosis, autonomic neuropathy)
Diagnostic Criteria
Unlike conditions such as GBS or multiple sclerosis, there is no single universal set of "diagnostic criteria" for polyneuropathy as a whole. This is because polyneuropathy is a syndrome (a clinical pattern), not a single disease. The diagnostic process therefore involves:
- Confirming the syndrome — demonstrating that the clinical picture is consistent with polyneuropathy (symmetric, length-dependent, LMN ± sensory pattern).
- Characterising the syndrome — time course, fibre type, axonal vs. demyelinating.
- Identifying the underlying aetiology — targeted investigations based on the characterisation.
However, specific subtypes of polyneuropathy DO have formal diagnostic criteria:
Required features of GBS [21][22]:
- Progressive weakness of arms and legs (sometimes initially only in legs) — ranging from minimal weakness to paralysis of four limbs, trunk, bulbar and facial muscles with external ophthalmoplegia
- Areflexia and hyporeflexia in weak limbs
Supportive features of GBS [21][22]:
- Progression of symptoms over days to 4 weeks
- Recovery starting 2–4 weeks after progression halts
- Relative symmetry
- Pain
- No fever at onset
- CN involvement especially bilateral facial nerve weakness
- Mild sensory signs and symptoms
- Autonomic dysfunction
- Elevated protein in CSF with a normal cell count
- Electrodiagnostic abnormalities consistent with GBS
Features rendering diagnosis of GBS doubtful [21][22]:
- Marked persistent asymmetry of weakness
- Decrement or loss of sensation below a spinal cord root level
- Severe sensory signs with little or no limb weakness at onset
- Bowel and bladder dysfunction at onset
- Severe and persistent bowel and bladder dysfunction
- Severe pulmonary dysfunction with little or no limb weakness at onset
- Fever at onset
- CSF pleocytosis with a white cell count > 50/mm³
Exam Pearl: Features That Make You Doubt GBS
If a patient with "ascending weakness" has a sensory level, bladder dysfunction at onset, fever at onset, or marked asymmetry, you should doubt GBS and think myelopathy, cauda equina, or poliomyelitis instead. CSF pleocytosis (WCC > 50/mm³) should raise concern for infective causes (HIV polyradiculopathy, Lyme disease, CMV) [21][22].
Dx: clinical + documented demyelination [2]:
- NCS: multifocal conduction block, differential ↓NCV in different nerve segments
- CSF: albuminocytologic dissociation
- Nerve Bx: segmental demyelination and remyelination, "onion-bulb" appearance
The formal EFNS/PNS criteria require:
- Clinical: Progressive or relapsing symmetrical proximal AND distal weakness + sensory dysfunction for > 8 weeks
- Electrodiagnostic: At least one of — motor NCV reduction, prolonged distal motor latency, prolonged or absent F-waves, conduction block, temporal dispersion — in at least 2 nerves
- Supportive: CSF protein elevation, MRI showing gadolinium enhancement/hypertrophy of nerve roots or plexus, nerve biopsy with demyelination/remyelination, clinical improvement with immunotherapy
Although not formalised as a single criteria set, the clinical-electrodiagnostic confirmation follows this framework:
| Step | Requirement | Purpose |
|---|---|---|
| 1. Clinical | Symmetric, distal-predominant sensory ± motor ± autonomic symptoms + signs (hyporeflexia, stocking sensory loss, distal weakness) | Establish the syndrome clinically |
| 2. Electrodiagnostic | NCS/EMG confirms neuropathic pattern (not myopathic, not NMJ) and defines axonal vs. demyelinating | Confirm the clinical suspicion [25]. Distinguish from mimics. |
| 3. Aetiological | Targeted blood/CSF/genetic tests identify underlying cause | Guide treatment |
Neurological investigation: to confirm the clinical suspicion. What is the lesion? Caveat of PAN-investigation: false +ve or false -ve investigative findings [25].
Important Principle
False positive: White matter change / silent lacunar infarct. False negative: 'Normal limits' NCV, Early disease where structural changes are not obvious [25]. This means a normal NCS does NOT exclude polyneuropathy — particularly small fibre neuropathy, where standard NCS (which tests only large myelinated fibres) can be completely normal.
Diagnostic Algorithm
The diagnostic approach to polyneuropathy is a systematic, stepwise process. Think of it in 4 stages:
History will reveal the diagnosis and direct our neurological examination [13].
| What to Determine | How | Why It Matters |
|---|---|---|
| Is it polyneuropathy? | Symmetric, distal, LMN signs, ± sensory loss in glove-and-stocking | Excludes myelopathy (UMN), myopathy (proximal, preserved reflexes), NMJ (fatigability) |
| Time course | Acute ( < 4w), subacute (4–8w), chronic ( > 8w), relapsing | Acute → GBS; Chronic → DM, CMT, CIDP |
| Fibre type | Motor, sensory, autonomic, mixed; large vs. small fibre | Narrows differential |
| Associated features | Systemic disease, family history, drugs, alcohol, travel | Identifies likely aetiology |
Investigations for peripheral neuropathy on history/exam [26]:
- Sensory symptoms, pain in extremities or position-dependent
- Ocular-sparing
- Autonomic involvement
- Symmetrical distal weakness, early wasting, a-/hyporeflexia, non-dissociated sensory involvement, trophic changes, steppage gait
This is the pivotal investigation — it answers the question that determines treatment.
Stage 3: Baseline Blood Screen — Identify the Cause
Investigation Modalities: Detailed Breakdown
1. Electrodiagnostic Studies (NCS and EMG)
This is the single most important investigation in polyneuropathy. It serves three purposes:
- Confirms the presence and distribution of neuropathy
- Distinguishes axonal from demyelinating pathology
- Excludes mimics (myopathy, NMJ disorder, radiculopathy)
Investigations for peripheral nerve lesions: Nerve conduction study, CK level, EMG [26].
NCS works by stimulating a nerve at one point and recording the electrical response downstream. It measures:
| Parameter | What It Tells You | Normal Mechanism |
|---|---|---|
| Conduction velocity (CV) | Speed of signal propagation | Saltatory conduction along myelinated fibres |
| Distal motor latency (DML) | Time from distal stimulation to muscle response | Reflects conduction through the most distal nerve segment |
| Compound muscle action potential (CMAP) amplitude | Number of functioning motor axons | Each intact axon contributes to the summed electrical response |
| Sensory nerve action potential (SNAP) amplitude | Number of functioning sensory axons | As above for sensory fibres |
| F-wave latency | Conduction along the entire length of motor nerve (including proximal segments and roots) | Antidromic impulse travels to anterior horn cell, returns orthodromically |
| Conduction block | Focal loss of conduction across a nerve segment | Normally, all impulses propagate through |
| Pattern | CV | Amplitude | Other Features | Interpretation |
|---|---|---|---|---|
| Axonal | Normal or mildly ↓ | ↓ Amplitude | Early denervation on EMG | Axon itself is damaged; myelin intact |
| Demyelinating | ↓↓ Velocity ( < 70% LLN) | Normal or mildly ↓ | Conduction block, temporal dispersion, prolonged DML, prolonged F-wave latency | Myelin is damaged; axon initially intact |
| Uniform demyelinating | Uniformly ↓ across ALL nerves | Normal | NO conduction block | Hereditary demyelinating neuropathy (CMT1): uniform homogeneous slowing without conduction block [2] — because every Schwann cell is genetically identical, so the defect is uniform |
| Non-uniform demyelinating | Variably ↓, different segments | ± ↓ | Conduction block present, differential slowing | Acquired demyelinating neuropathy (GBS, CIDP): multifocal conduction block, differential ↓NCV in different nerve segments [2] — because the immune attack is patchy |
Exam Pearl: Conduction Block
Conduction block = significant drop in CMAP amplitude between proximal and distal stimulation sites (typically > 50% drop). It means the impulse is "blocked" at a focal point — the axon is alive but the myelin at one spot is too damaged to conduct. This is the hallmark of acquired demyelinating disease and does NOT occur in hereditary demyelinating neuropathies. Finding conduction block = acquired, potentially treatable disease.
EMG involves inserting a needle electrode into muscle to assess electrical activity. It provides complementary information:
| Finding | Meaning | Mechanism |
|---|---|---|
| Fibrillation potentials | Spontaneous activity of individual denervated muscle fibres | After denervation, muscle fibres become hypersensitive to acetylcholine and fire spontaneously |
| Positive sharp waves | Same significance as fibrillations | Another morphology of spontaneous denervation activity |
| Large, polyphasic motor unit potentials (MUPs) | Chronic denervation with reinnervation | Surviving motor neurons sprout collateral branches to reinnervate orphaned muscle fibres → larger, more complex motor units |
| Reduced recruitment | Fewer motor units firing for a given effort | Loss of motor axons → fewer units available |
| Fasciculation potentials | Spontaneous firing of a whole motor unit | Unstable motor unit from partial denervation |
EMG is most useful in distinguishing myopathic causes of weakness from neuropathic causes [27]:
- Neuropathic pattern: fibrillations, positive sharp waves, large amplitude/long duration MUPs, reduced recruitment (neurogenic pattern)
- Myopathic pattern: Low amplitude, short duration, polyphasic potentials with early (full) recruitment [27]
- ALS: Combine features of acute and chronic denervation and reinnervation. NCS: Sensory and motor nerve conduction studies are NORMAL [20] — this helps distinguish ALS from polyneuropathy
2. Blood Tests — The Systematic Screen
Baseline bloods: CBC, LRFT, glucose, B12/folate, TFT, ESR/CRP, autoimmune (ANA, RF, ANCA), SPE Ig [1].
Workup for peripheral nerve lesions: Glc/A1c, CBC, L/RFT, TFT, vit B12/B9, ESR/CRP, CXR [26].
| Test | What You're Looking For | Interpretation |
|---|---|---|
| Fasting glucose / HbA1c | Diabetes mellitus | HbA1c ≥ 6.5% or FPG ≥ 7.0 mmol/L diagnostic of DM. HbA1c 5.7–6.4% = prediabetes (can still cause neuropathy) |
| Vitamin B12 / Holotranscobalamin | B12 deficiency | Serum holotranscobalamin = the active fraction, what we use nowadays [28]. Low B12 with macrocytosis and hypersegmented neutrophils is classic, but can occur without haematological changes [8] |
| Folate | Folate deficiency | Rarely sole cause of neuropathy, but may co-exist |
| CBC | Anaemia (macrocytic → B12/folate; normocytic → chronic disease), pancytopenia (B12, myeloma) | Macrocytic anaemia with polyneuropathy = think B12. MCV > 120 fL generally only has 2 DDx: chemotherapy or pernicious anaemia [23] |
| RFT (Cr, urea, eGFR) | Uraemic neuropathy (CKD) | Uraemic toxins cause axonal neuropathy |
| LFT | Chronic liver disease (alcoholic, hepatitis) | Hepatic neuropathy, or pointer to alcohol use |
| TFT (TSH, fT4) | Hypothyroidism | Causes mixed demyelinating/axonal neuropathy + may cause CTS |
| ESR / CRP | Systemic inflammation | Elevated → vasculitis, connective tissue disease, infection, malignancy |
| Serum protein electrophoresis (SPE) + immunoglobulins | Paraproteinaemia, myeloma | Indication for SPE: investigation of unexplained neuropathy [29]. Look for M-band (monoclonal spike). If paraprotein present with immunoparesis → consider myeloma [29] |
| Test | Indication | What You're Looking For |
|---|---|---|
| Serum free light chains + ratio | Suspected paraproteinaemia / amyloidosis | Abnormal kappa:lambda ratio → clonal plasma cell disorder |
| Anti-ganglioside antibodies | Suspected GBS | GQ1b antibodies: Miller-Fisher, Bickerstaff encephalitis; GM1 and GD1a antibodies: AMAN, AMSAN [21][22] |
| Anti-MAG antibodies | Demyelinating neuropathy with IgM paraprotein | Anti-MAG in ~50% of Waldenström-associated neuropathy → demyelinating, sensory > motor [30] |
| ANA, anti-ENA, RF, ANCA | Suspected vasculitic / connective tissue disease neuropathy | Vasculitis (ANCA), SLE (ANA, anti-dsDNA), RA (RF) |
| HIV serology | Risk factors for HIV | HIV can cause distal sensory polyneuropathy directly or via antiretrovirals |
| Lyme disease serology | Travel to endemic area, tick exposure | Lyme can cause polyradiculopathy or mononeuritis multiplex |
| Urine porphyrins (ALA, PBG) | Acute motor neuropathy + abdominal pain + psychiatric features | Acute intermittent porphyria |
| Heavy metal screen (lead, arsenic, mercury) | Occupational exposure, unusual presentation | Lead → motor neuropathy; Arsenic → sensorimotor + Mees' lines |
| Anti-Hu / Anti-Yo antibodies | Suspected paraneoplastic neuropathy | Paraneoplastic — anti-Hu = sensory neuronopathy (small cell lung CA) [2] |
| Genetic testing | Family history, pes cavus, young onset | CMT (PMP22 duplication for CMT1A), FAP (TTR mutation) |
| CXR / CT thorax | All patients with unexplained neuropathy | Sarcoidosis, lung malignancy (paraneoplastic), lymphoma |
SPE Is Essential in Unexplained Polyneuropathy
Serum protein electrophoresis is indicated in the investigation of unexplained neuropathy, heavy proteinuria, or renal impairment [29]. Never omit SPE in your workup — paraproteinaemic neuropathies are treatable, and missing them is a preventable error. An IgM M-band with anti-MAG positivity → treat with rituximab. An IgG/IgA M-band → investigate for myeloma or POEMS.
When to perform LP: Suspected GBS, CIDP, or other inflammatory/infective causes of polyneuropathy.
CSF analysis by lumbar puncture [21][22]:
| CSF Finding | GBS | CIDP | Normal | Infective Polyradiculopathy |
|---|---|---|---|---|
| Protein | ↑ (1st week: normal; 2nd week: ~80% abnormal; 3rd–4th week: Peak) | ↑ | Normal ( < 0.45 g/L) | ↑ |
| Cell count | NO increase (WBC < 50 cells/μL) | Normal or mildly ↑ ( < 10) | < 5 cells/μL | ↑↑ (often > 50) |
| Glucose | Normal | Normal | Normal | ↓ if bacterial/TB |
| Pattern name | Cytoalbuminologic (albuminocytologic) dissociation | Albuminocytologic dissociation | — | Pleocytosis with ↑ protein |
Why does protein rise in GBS? Due to increased permeability of the blood-brain barrier at the level of proximal nerve roots [21][22] — the inflammation at the nerve roots disrupts the blood-nerve barrier, allowing plasma proteins to leak into the CSF.
When: Last resort — reserved for inflammatory, infective, or infiltrative disorders where less invasive tests are inconclusive.
Site: Sural nerve (sensory nerve at the ankle) — chosen because:
- It is a pure sensory nerve (no motor deficit from biopsy)
- Easily accessible
- Commonly affected in polyneuropathy
Key histological findings:
| Finding | Condition |
|---|---|
| Segmental demyelination and remyelination, "onion-bulb" appearance | CIDP [2] — concentric Schwann cell layers from repeated demyelination-remyelination cycles |
| Necrotising vasculitis of vasa nervorum | Vasculitic neuropathy (PAN, ANCA-vasculitis) |
| Congo red stain, salmon-pink colour → indicative of amyloid deposition; electron microscopy showing non-branching fibrils | AL amyloidosis [24] |
| Granulomas | Sarcoidosis, leprosy |
| Axonal degeneration with loss of myelinated fibres | Non-specific — seen in most axonal neuropathies |
When: Patient has burning pain, temperature loss, autonomic symptoms, but NCS is normal (NCS only tests large myelinated fibres).
Technique: 3mm punch biopsy from distal leg (lateral calf 10cm above lateral malleolus) ± proximal thigh.
Finding: Reduced intraepidermal nerve fibre density (IENFD) — counted using PGP9.5 immunostaining. Compared to age- and sex-matched normative data.
This is the gold standard for diagnosing small fibre neuropathy.
| Investigation | Indication | Key Findings |
|---|---|---|
| Fat pad / rectal biopsy | Suspected amyloidosis | Congo red positive, apple-green birefringence under polarised light |
| Quantitative sensory testing (QST) | Small fibre assessment | Elevated thermal detection thresholds |
| Autonomic function tests | Suspected autonomic neuropathy | Tilt-table test (orthostatic hypotension), heart rate variability, sudomotor testing (QSART) |
| MRI with gadolinium (nerve roots/plexus) | CIDP, nerve root/plexus infiltration | Gadolinium enhancement of thickened nerve roots (CIDP), mass lesions |
| Genetic testing | Suspected hereditary neuropathy | PMP22 duplication (CMT1A), TTR mutation (FAP), connexin-32 (CMT1X) |
| Repetitive nerve stimulation (RNS) | If NMJ disorder suspected as mimic | Decremental response → MG; Incremental response → LEMS |
| Whole-body PET-CT | Paraneoplastic neuropathy, POEMS, lymphoma | Identifies occult malignancy |
| Clinical Scenario | First-Line Investigations | Second-Line Investigations |
|---|---|---|
| Chronic, symmetric, distal sensorimotor | NCS/EMG, HbA1c, B12, TFT, RFT, LFT, CBC, ESR, SPE | HIV, autoimmune panel, heavy metals, genetic test, nerve Bx |
| Acute ascending weakness (GBS suspected) | NCS/EMG, CSF (LP), FVC (respiratory monitoring!) | Anti-ganglioside antibodies (GM1, GQ1b) [21][22], MRI spine (exclude cord compression) |
| Chronic relapsing proximal + distal weakness | NCS/EMG (look for non-uniform demyelination), CSF, SPE | MRI nerve roots (enhancement), nerve Bx (onion-bulbs), trial of steroids |
| Burning feet, normal NCS | Skin biopsy (IENFD), QST, autonomic function tests | HbA1c, amyloid workup, Fabry enzyme assay |
| Neuropathy with M-band on SPE | Immunofixation, serum free light chains, anti-MAG, β2-microglobulin | Bone marrow aspirate/trephine, skeletal survey or PET-CT, fat pad biopsy for amyloid |
| Young patient with pes cavus + family history | NCS (uniform vs. non-uniform slowing), genetic testing | CMT gene panel |
Workup order: not necessary if mild symptoms with clear cause (e.g., DM neuropathy). Systemic screen: DM (fasting glucose, HbA1c); Metabolic (CBC, L/RFT, vit B12/folate, thyroid, porphyria); Autoimmune (ESR/CRP, anti-ganglioside Ab, anti-MAG, anti-ENA, RF, ANCA); Neoplastic (SPE, CXR, CT or PET-CT, anti-Hu/anti-Yo); Infective (HIV, Lyme disease serology). LP for cell count, protein and glucose. Electrophysiological study: EMG/NCS for demyelinating vs axonal. Sural nerve biopsy (seldom done). Genetic test if FHx +ve [2].
High Yield Summary
No single diagnostic criteria for polyneuropathy as a syndrome — diagnosis is clinical + electrophysiological. Specific criteria exist for GBS (Asbury-Cornblath) and CIDP (EFNS/PNS).
GBS diagnostic criteria: Progressive weakness + areflexia + progression over days to 4 weeks + CSF albuminocytologic dissociation + electrodiagnostic abnormalities. Doubt GBS if: sensory level, bladder dysfunction at onset, fever, marked asymmetry, CSF pleocytosis > 50.
NCS/EMG is the pivotal investigation: Axonal (↓ amplitude, normal velocity) vs. Demyelinating (↓ velocity, conduction block). Uniform demyelination without conduction block = hereditary (CMT). Non-uniform with conduction block = acquired (GBS/CIDP) → treatable.
Baseline blood screen for ALL polyneuropathy: HbA1c, B12, TFT, CBC, L/RFT, ESR/CRP, SPE with immunoglobulins.
CSF in GBS: Albuminocytologic dissociation — ↑ protein, normal cell count. May be normal in first week.
Normal NCS does NOT exclude polyneuropathy — small fibre neuropathy requires skin biopsy for diagnosis.
SPE must be performed in all cases of unexplained polyneuropathy to exclude paraproteinaemia.
Active Recall - Polyneuropathy Diagnosis and Investigations
[1] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 — Peripheral neuropathy) [2] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 — Disease of the Peripheral Nerves, pp. 179–184) [8] Senior notes: Ryan Ho Haemtology.pdf (p. 29 — B12 deficiency, SCD, peripheral neuropathy) [13] Lecture slides: CFB (MED04) Central Nervous System.pdf (p. 3 — History taking) [20] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (pp. 1289–1291 — ALS diagnosis, EMG, NCS) [21] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (pp. 1282–1285 — GBS diagnostic criteria) [22] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (pp. 547–548 — GBS diagnostic criteria) [23] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (p. 18 — MCV > 120) [24] Senior notes: Block A - Hematology Data Interpretation.pdf (p. 1 — Amyloidosis, Congo red stain) [25] Lecture slides: GCBA_Fundamentals_Neuro_Introduction to Neurological Investigations and Emergencies_Prof KC Teo.pdf (p. 9) [26] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 134 — Approach to generalised weakness table) [27] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p. 1760 — EMG in myopathy vs neuropathy) [28] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (p. 19 — Holotranscobalamin) [29] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (p. 27 — SPE indications) [30] Senior notes: Ryan Ho Haemtology.pdf (p. 109 — Waldenström, anti-MAG neuropathy)
The management of polyneuropathy rests on three pillars:
- Treat the underlying cause (disease-modifying therapy)
- Symptomatic treatment (especially neuropathic pain and autonomic dysfunction)
- Supportive and rehabilitative care (prevent complications, restore function)
The specific approach depends entirely on whether the neuropathy is axonal (treat the underlying cause, remove the offending agent) or demyelinating (often immunotherapy-responsive).
Management approach [2]:
- Disease-modifying Tx: Axonal — majority not directly treatable → Mx largely by ↓ exposure to toxins and treatment of underlying disease process
- Demyelinating — usually immunosuppressive Tx (e.g., IVIg, pulse steroids, plasmapheresis)
- Symptomatic Tx: gabapentin and TCA for management of neuropathic pain
Pillar 1: Disease-Modifying Therapy (Treat the Cause)
A. Metabolic and Toxic Causes (Axonal Neuropathies)
This is the most common polyneuropathy you will manage. The mainstay is optimising glycaemic control — you cannot reverse existing nerve damage, but you can slow or halt progression.
- Optimise glycaemic control as mainstay
- CVD risk factor control: stop smoking
- Glycaemic control: aim HbA1c < 7–8%
- Prefer SGLT2i and GLP1a — these agents have additional cardiovascular and renal benefits beyond glucose lowering [4]
- BP control (especially by ACEI/ARB) and treat hyperlipidaemia (by statins or fibrates) [4]
- Gabapentinoids (gabapentin/pregabalin) and antidepressants (amitriptyline) for pain [5]
Why does glycaemic control help? By reducing chronic hyperglycaemia, you decrease the drivers of neuropathy: polyol pathway activation, AGE formation, oxidative stress, and microvascular ischaemia. The landmark DCCT/EDIC trial showed that intensive glycaemic control in T1DM reduced neuropathy incidence by ~60%. In T2DM the evidence is less dramatic but still supports glycaemic optimisation.
| Intervention | Target | Mechanism of Benefit |
|---|---|---|
| HbA1c control | < 7% (individualized) | Reduces all pathogenic pathways (polyol, AGE, oxidative stress) |
| BP control (ACEI/ARB) | < 130/80 mmHg | Protects vasa nervorum; also renoprotective |
| Statin therapy | LDL < 1.8 mmol/L if established CVD | Reduces atherosclerotic burden → protects macrovascular supply to nerves |
| Smoking cessation | Complete cessation | Smoking worsens microvascular disease |
| Foot care programme | Annual screening + patient education | Prevents ulceration → prevents amputation |
Diabetic Foot Care — High Yield
Screening by monofilament (small fibre) and tuning fork (large fibre) tests [5]. RFs of foot ulcer development: previous foot ulceration (most important), neuropathy (80%), foot deformity, concomitant vascular disease [5]. Every diabetic patient with neuropathy needs structured foot care: daily foot inspection, appropriate footwear, podiatry referral, and prompt treatment of any wound.
| Intervention | Rationale |
|---|---|
| Alcohol cessation | Remove the direct neurotoxin (ethanol + acetaldehyde) |
| Thiamine (B1) replacement | IV thiamine initially (Pabrinex) to replete stores rapidly, then oral maintenance. Thiamine is a cofactor for pyruvate dehydrogenase — without it, nerves cannot generate ATP via aerobic metabolism |
| Balanced nutrition | Correct concurrent deficiencies (B6, B12, folate, niacin) |
| Neuropathic pain management | As per symptomatic treatment below |
Why IV before oral? Alcoholics have impaired GI absorption of thiamine. Oral thiamine is absorbed via a saturable transporter, so only ~5 mg can be absorbed per dose. IV bypasses this limitation.
The principle is simple: stop the offending drug (if possible) or reduce the dose.
| Drug | Action | Notes |
|---|---|---|
| Isoniazid | Vitamin B6 (pyridoxine) supplementation for those with risk factors (pregnancy, DM, renal disease) [7]. If neuropathy develops → stop isoniazid if severe, or add high-dose B6 if mild | Prevention is better than cure — always co-prescribe B6 with isoniazid in at-risk patients |
| Vincristine | Dose reduction or switch to less neurotoxic agent | Axonal transport disruption is dose-dependent |
| Cisplatin/Oxaliplatin | Dose reduction; consider magnesium supplementation for oxaliplatin | Dorsal root ganglia neurotoxicity is cumulative |
| Taxanes | Dose reduction | Cumulative sensory neuropathy |
| Amiodarone | Consider switching to alternative antiarrhythmic | Demyelinating neuropathy from phospholipid accumulation in Schwann cells |
| Metronidazole | Avoid prolonged courses ( > 4 weeks high dose) | Direct axonal toxicity |
| Treatment | Regimen | Rationale |
|---|---|---|
| IM hydroxocobalamin | Loading: 1 mg IM on alternate days for 2 weeks (6 doses). Maintenance: 1 mg IM every 2–3 months for life (if pernicious anaemia) | Bypasses GI absorption issues (pernicious anaemia, gastrectomy, terminal ileum disease). IM route ensures reliable delivery |
| Oral cyanocobalamin | 1–2 mg daily if dietary deficiency only (not pernicious anaemia) | ~1% of oral B12 is absorbed by passive diffusion even without intrinsic factor — but this is only sufficient if the cause is dietary, not malabsorption |
Can occur in absence of haematological changes as neurones require higher B12 level to function [8] — this means you should treat neurological B12 deficiency even if the CBC is normal.
Key point: Neurological damage from B12 deficiency may be irreversible if treatment is delayed. The window for recovery narrows with duration of symptoms. Start treatment immediately on clinical suspicion; do not wait for confirmatory tests.
- Optimise renal replacement therapy — dialysis improves but may not fully reverse neuropathy
- Renal transplantation — the most effective treatment; can lead to significant improvement or resolution
- Ensure adequate dialysis dose (Kt/V target ≥ 1.4 for haemodialysis)
- Levothyroxine replacement — correct the hypothyroid state
- Neuropathy gradually improves over months with normalisation of TSH
B. Inflammatory/Immune-Mediated Causes (Demyelinating Neuropathies)
These are the treatable polyneuropathies — the ones where correctly identifying the demyelinating pattern on NCS changes the patient's outcome.
Management has two components: specific immunotherapy and general supportive care.
Mx [31]:
- General supportive: monitor respiratory (FVC, ABG), autonomic (BP, ECG) and bulbar function ± intubation
- Mechanical ventilation if ↑CO₂ ↓O₂, FVC < 15 mL/kg BW, inefficient cough, dysphagia, atelectasis
- General care for immobility (e.g., support stockings) and nutritional support
- Treatment of complications, e.g., respiratory failure, cardiac arrhythmia, DVT
- Specific immunotherapy for moderately severe or progressive disease
- Plasma exchange: start plasmapheresis for 5 exchanges over 2 weeks
- High dose IVIg: 0.4 g/kg/day for 5 days
- Note: combination of plasmapheresis + IVIg or steroids has no benefit over either alone
Early IVIg or plasmapheresis reduces disability and duration of illness [9].
Why IVIg works: IVIg contains pooled polyclonal immunoglobulins from thousands of donors. It modulates the immune system by: (1) blocking Fc receptors on macrophages (reducing antibody-mediated nerve damage), (2) neutralising pathogenic autoantibodies, (3) modulating complement activation, (4) providing anti-idiotypic antibodies.
Why plasmapheresis works: It physically removes circulating autoantibodies, complement components, and immune complexes from the blood. Each exchange replaces approximately one plasma volume with albumin.
IVIg vs. Plasmapheresis — how to choose:
| Factor | IVIg | Plasmapheresis |
|---|---|---|
| Efficacy | Equivalent | Equivalent |
| Ease of administration | Easier (peripheral IV) | Requires large-bore venous access + specialised equipment |
| Availability | More widely available | Requires apheresis unit |
| Contraindications | IgA deficiency (anaphylaxis risk), severe renal impairment, thromboembolic risk | Haemodynamic instability, sepsis, coagulopathy |
| Side effects | Headache, aseptic meningitis, thrombosis, renal impairment | Hypotension, hypocalcaemia (from citrate), line-related infection |
| In practice | Usually first choice in HK/most centres | Preferred if IVIg contraindicated or unavailable |
Key Points for GBS Management
- Corticosteroids have NO role in GBS — this is different from CIDP. Do not give steroids for GBS.
- Combination of plasmapheresis + IVIg has no benefit over either alone [31] — do not combine.
- Respiratory monitoring is paramount — GBS kills through respiratory failure. FVC must be monitored serially. Intubate if FVC < 15 mL/kg, or if declining by > 30% from baseline, or if NIF (negative inspiratory force) < -20 cmH₂O.
- Autonomic instability can cause fatal arrhythmia — continuous cardiac monitoring in ICU.
Mx [2]:
- IVIg, plasmapheresis, pulse steroids for severe/fulminant disease
- Steroid + steroid-sparing agents for milder disease
- Prognosis: 2/3 responds to standard therapy, majority ambulatory following treatment
Unlike GBS, corticosteroids DO work in CIDP. This is a key distinguishing feature in management.
| Treatment | Regimen | Indication | Notes |
|---|---|---|---|
| IVIg | Loading: 2 g/kg over 2–5 days. Maintenance: 0.4–1 g/kg every 3–4 weeks | Severe/fulminant disease, or patient preference | Often first-line; rapid onset of action (days to weeks) |
| Corticosteroids | Prednisolone 1 mg/kg/day then taper, OR IV methylprednisolone pulse | Milder disease, or to reduce IVIg dependency | Effective but significant long-term side effects (osteoporosis, DM, infections, cataracts, myopathy) |
| Plasmapheresis | 5 exchanges over 2 weeks then taper frequency | Severe/refractory, or when IVIg/steroids fail | Short-lived effect → often needs repeated courses |
| Steroid-sparing agents | Azathioprine, mycophenolate, cyclosporine | To reduce steroid dose in long-term maintenance | Slow onset (weeks to months); monitor for myelosuppression/hepatotoxicity |
| Rituximab | Anti-CD20 monoclonal antibody | Refractory CIDP not responding to above | Depletes B cells; useful in anti-MAG neuropathy |
Why steroids work in CIDP but NOT GBS: This is not fully understood, but the prevailing theory relates to the different immunopathology. GBS is an acute monophasic illness driven by molecular mimicry → once the autoimmune attack is initiated, steroids cannot reverse the damage (and may even worsen outcomes by impairing axonal repair). CIDP is a chronic immune-mediated process with ongoing inflammation → steroids suppress this chronic inflammatory activity effectively.
| Subtype | Treatment |
|---|---|
| IgM with anti-MAG | Rituximab (anti-CD20 depletes the B-cell clone producing anti-MAG) |
| POEMS syndrome | Radiation (if solitary plasmacytoma) or chemotherapy (if disseminated) — targets the underlying clonal plasma cell disorder |
| AL Amyloidosis | Chemotherapy (bortezomib-based regimens) ± autologous stem cell transplant |
| Multiple Myeloma | Treat the myeloma (chemotherapy, stem cell transplant) |
- Corticosteroids (prednisolone 1 mg/kg/day) ± cyclophosphamide for moderate/severe disease
- Treat the underlying vasculitis (PAN, ANCA-vasculitis, SLE)
- In HBV-associated PAN: antivirals (entecavir/tenofovir) + short-course steroids
Charcot-Marie-Tooth Disease (CMT/HMSN)
- No disease-modifying treatment currently available
- Management is entirely supportive:
- Physiotherapy and occupational therapy
- Ankle-foot orthoses (AFOs) for foot drop
- Orthopaedic surgery for severe foot deformities (pes cavus, claw toes)
- Genetic counselling for family planning
- Avoid neurotoxic drugs (vincristine absolutely contraindicated in CMT — can cause fulminant neuropathy)
Pillar 2: Symptomatic Treatment
Neuropathic pain is the most distressing symptom for many patients. It results from ectopic firing of damaged nerve fibres, central sensitisation in the dorsal horn, and disinhibition of descending pain-modulating pathways. Standard analgesics (paracetamol, NSAIDs) are largely ineffective because they target nociceptive pain, not neuropathic mechanisms.
Symptomatic Tx: gabapentin and TCA for management of neuropathic pain [2].
Gabapentinoids (gabapentin/pregabalin) and antidepressants (amitriptyline) for pain [5].
Gabapentin/pregabalin/amitriptyline for neuropathic pain [4].
| Drug Class | Examples | Mechanism of Action | Key Side Effects | Contraindications / Cautions |
|---|---|---|---|---|
| Gabapentinoids | Gabapentin (start 100–300 mg TDS, max 3600 mg/day); Pregabalin (start 75 mg BD, max 600 mg/day) | Bind to α2δ subunit of voltage-gated calcium channels in dorsal horn → ↓ excitatory neurotransmitter release (glutamate, substance P) → ↓ central sensitisation | Drowsiness, dizziness, weight gain, peripheral oedema | Renal impairment (dose adjust — both are renally excreted). Caution in elderly (falls risk). Pregabalin is a controlled substance (abuse potential) |
| Tricyclic antidepressants | Amitriptyline (start 10–25 mg nocte, max 75–150 mg/day); Nortriptyline | Block reuptake of serotonin and noradrenaline in descending pain-inhibitory pathways → enhance endogenous pain modulation. Also block sodium channels on injured nerve fibres → ↓ ectopic firing | Anticholinergic effects (dry mouth, constipation, urinary retention, blurred vision), drowsiness, weight gain, cardiac conduction abnormalities (QT prolongation) | Contraindicated: recent MI, heart block, arrhythmias, narrow-angle glaucoma, concurrent MAOIs. Caution: elderly (anticholinergic burden, falls), prostatism |
| SNRIs | Duloxetine (start 30 mg daily, max 120 mg/day); Venlafaxine | Serotonin + noradrenaline reuptake inhibition → enhance descending inhibitory pathways | Nausea, insomnia, dizziness, hypertension (at high doses) | Hepatic impairment (duloxetine); uncontrolled hypertension (venlafaxine) |
| Topical agents | Capsaicin 8% patch; Lidocaine 5% plaster | Capsaicin: depletes substance P from C-fibre terminals → defunctionalises nociceptors. Lidocaine: blocks sodium channels locally | Capsaicin: initial burning at application site. Lidocaine: minimal systemic absorption | Capsaicin: avoid on broken skin, mucous membranes. Applied by trained personnel only |
| Opioids | Tramadol; tapentadol; as last resort: oxycodone | μ-opioid receptor agonism ± monoamine reuptake inhibition | Constipation, nausea, drowsiness, dependency, respiratory depression | Avoid long-term use due to tolerance, hyperalgesia, addiction. Use only as 3rd-line or for severe exacerbations |
Stepwise Approach to Neuropathic Pain (Current Guidelines)
First-line: Gabapentin/pregabalin OR amitriptyline/nortriptyline OR duloxetine — choose based on patient comorbidities and side effect profile.
Second-line: Combination of two first-line agents from different classes (e.g., pregabalin + duloxetine).
Third-line: Tramadol or topical agents (capsaicin patch, lidocaine plaster).
Last resort: Strong opioids — short-term only, with clear goals and exit strategy.
In practice: amitriptyline is cheap, effective, and helps with sleep (useful since neuropathic pain is often worse at night). Start low (10 mg nocte), titrate slowly. In elderly or cardiac patients, use gabapentin instead.
| Symptom | Treatment | Mechanism |
|---|---|---|
| Orthostatic hypotension | Non-pharmacological first: adequate hydration (2–3 L/day), increased dietary salt, compression stockings, rise slowly from bed. Pharmacological: fludrocortisone (0.1–0.3 mg daily), midodrine (α1-agonist, 5–10 mg TDS) | Fludrocortisone: mineralocorticoid → Na/water retention → ↑ plasma volume. Midodrine: direct vasoconstriction → ↑ peripheral resistance |
| Gastroparesis | Small frequent meals, prokinetics (metoclopramide 10 mg TDS before meals, domperidone), erythromycin (motilin agonist) at low dose | Prokinetics enhance gastric emptying by ↑ acetylcholine release (metoclopramide) or stimulating motilin receptors (erythromycin) |
| Bladder dysfunction | Timed voiding, intermittent self-catheterisation if large residuals, anticholinergics (oxybutynin) for overactive bladder, bethanechol for atonic bladder | Tailored to whether the problem is overactivity or retention |
| Erectile dysfunction | PDE5 inhibitors (sildenafil, tadalafil) | Inhibit breakdown of cGMP → sustained smooth muscle relaxation → penile erection |
| Nocturnal diarrhoea | Loperamide, codeine phosphate; if suspected bacterial overgrowth → empirical antibiotics (rifaximin) | Loperamide: μ-opioid agonist in gut wall → ↓ motility and secretion |
| Anhidrosis | Avoid overheating, humidity control | No specific pharmacological treatment |
| Problem | Intervention | Rationale |
|---|---|---|
| Foot drop | Ankle-foot orthosis (AFO) | Splints the ankle in neutral → prevents dragging toes, reduces falls |
| Distal weakness | Physiotherapy: strengthening exercises for preserved muscles, stretching to prevent contractures | Maintains function and prevents deformity |
| Falls | OT home assessment, walking aids (stick, frame), balance training | Proprioceptive loss + distal weakness = high falls risk |
| Hand dexterity loss | OT: adaptive devices (button hooks, large-handled utensils, modified keyboards) | Compensates for intrinsic hand muscle weakness |
| Domain | Interventions |
|---|---|
| Physiotherapy | Strengthening, stretching, balance training, gait retraining |
| Occupational therapy | Adaptive devices, workplace modifications, energy conservation |
| Podiatry | Nail care, callus management, custom footwear — especially in diabetic neuropathy |
| Pain team | Multidisciplinary pain management for refractory neuropathic pain |
| Psychology | Chronic pain and disability can cause depression/anxiety; CBT, counselling |
| DVT prophylaxis | In immobilised patients (GBS, severe polyneuropathy): LMWH + compression stockings |
| Nutritional support | Nasogastric feeding if bulbar dysfunction (GBS); correct nutritional deficiencies |
| Genetic counselling | For hereditary neuropathies (CMT, FAP, HNPP) |
Because GBS is the neurological emergency you are most likely to encounter in polyneuropathy, here is a more detailed protocol:
Prognosis: mortality 2–12%, 20% permanent disability, 10% severe disability [31].
| Aetiology | Disease-Modifying Treatment | First-Line Pain Management | Key Supportive Measures |
|---|---|---|---|
| Diabetic | Glycaemic control (HbA1c < 7%), ACEI/ARB, statin, SGLT2i/GLP1a preferred [4][5] | Gabapentin/pregabalin or amitriptyline [5] | Foot care programme, podiatry, annual screening |
| Alcoholic | Alcohol cessation + IV thiamine → oral maintenance | Amitriptyline or gabapentin | Nutritional rehabilitation, liver assessment |
| B12 deficiency | IM hydroxocobalamin (loading then maintenance) | If needed: gabapentin | Treat underlying cause (pernicious anaemia, diet) |
| Drug-induced | Stop/switch offending drug; B6 for isoniazid [7] | As needed | Monitor for recovery after drug cessation |
| Uraemic | Optimise dialysis; renal transplant if eligible | Gabapentin (dose-adjusted for renal function!) | Avoid nephrotoxins |
| Hypothyroid | Levothyroxine | As needed | Monitor TSH |
| GBS | IVIg 0.4 g/kg/day × 5 days OR plasmapheresis × 5 over 2 weeks [31]. No steroids | Gabapentin, opioids for severe pain | FVC monitoring, ICU if respiratory failure, DVT prophylaxis [31] |
| CIDP | IVIg, plasmapheresis, or pulse steroids ± steroid-sparing agents [2] | As needed | Long-term follow-up, assess for relapse |
| CMT | None — no disease-modifying therapy | As needed | AFOs, physiotherapy, orthopaedic surgery, genetic counselling, avoid vincristine |
| Vasculitic | Steroids ± cyclophosphamide | As needed | Treat underlying vasculitis |
| Paraneoplastic | Treat underlying malignancy | As needed | Oncology referral |
High Yield Summary
Three pillars of polyneuropathy management: (1) Treat the underlying cause, (2) Symptomatic treatment (especially neuropathic pain), (3) Supportive and rehabilitative care.
Axonal neuropathies: Mostly treated by addressing the underlying cause (glycaemic control for DM, B12 replacement, stop offending drugs, dialysis for uraemia). No direct nerve repair therapy.
Demyelinating neuropathies: Often immunotherapy-responsive → IVIg, plasmapheresis, corticosteroids (CIDP only — NOT GBS).
GBS management: (1) Monitor FVC (intubate if < 15 mL/kg), (2) IVIg OR plasmapheresis (NOT both, NOT steroids), (3) DVT prophylaxis, pain management, nutrition, physiotherapy.
CIDP management: IVIg, plasmapheresis, or steroids all effective. Steroids ± steroid-sparing agents for milder/chronic disease. 2/3 respond to standard therapy.
Neuropathic pain: First-line = gabapentin/pregabalin OR amitriptyline OR duloxetine. Combine if monotherapy insufficient. Avoid long-term opioids.
Diabetic neuropathy: Glycaemic optimisation is mainstay. Foot care programme is essential. SGLT2i and GLP1a preferred oral agents.
CMT: No disease-modifying treatment. Supportive care only. Never give vincristine to CMT patients.
Active Recall - Polyneuropathy Management
[2] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 — Disease of the Peripheral Nerves, pp. 179–184) [4] Senior notes: Ryan Ho Endocrine.pdf (pp. 97–98 — Diabetic neuropathy management) [5] Senior notes: Ryan Ho Endocrine.pdf (pp. 98–99 — Diabetic peripheral neuropathy, screening, foot care) [7] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p. 41 — Isoniazid, B6 prevention) [8] Senior notes: Ryan Ho Haemtology.pdf (p. 29 — B12 deficiency, neurological features without haematological changes) [9] Lecture slides: Learning_Points_All_Lectures.txt (Neurology section — GBS, early IVIg/plasmapheresis) [31] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 139 — GBS management, supportive care, immunotherapy, prognosis)
Complications of polyneuropathy arise from the loss of motor, sensory, and autonomic nerve function. The key insight is that peripheral nerves don't just transmit sensation and motor commands — they protect tissues, maintain homeostasis, and enable adaptation to the environment. When these functions fail, the consequences cascade through multiple organ systems.
The complications can be organised into:
- Complications of sensory loss (the most clinically important group)
- Complications of motor loss
- Complications of autonomic dysfunction
- Complications of acute polyneuropathy (GBS-specific)
- Complications of treatment
- Psychosocial complications
1. Complications of Sensory Loss
This is the single most important complication to understand. It is the leading cause of non-traumatic lower limb amputation worldwide.
Foot ulcers: 25% lifetime risk, annual risk 2%/year [5].
RFs of foot ulcer development [5]:
- Previous foot ulceration (most important)
- Neuropathy (80%): loss of monofilament sensation, neuropathy disability score
- Foot deformity
- Concomitant vascular disease
Pathophysiology of the neuropathic ulcer — why does loss of sensation cause an ulcer?
Think of it from first principles. Normal feet are protected by a feedback loop: pressure on the skin → pain signal → brain → shift weight off that spot. When sensory nerves are damaged:
- Loss of protective pain sensation → Patient doesn't feel abnormal pressure, friction, or injury → repeated unrecognised microtrauma to skin
- Motor neuropathy → Intrinsic foot muscle denervation → muscle imbalance → altered foot biomechanics → claw toes, prominent metatarsal heads → abnormal pressure distribution → focal high-pressure areas
- Autonomic neuropathy → Loss of sweating → dry, cracked skin (portal of entry for infection) + arteriolar vasodilation → warm foot with bounding pulses (the "neuropathic foot")
- Result: Painless ulcer, typically at pressure points (metatarsal heads, heel, tips of deformed toes)
Prevention of drastic consequences — foot care [32].
Overt complications: clinical neuropathy → symptomatic pain relief; prevention of drastic consequences — foot care [32].
| Stage | What Happens | Management |
|---|---|---|
| Intact skin, neuropathy present | High-risk foot — patient cannot feel injuries | Screening by monofilament and tuning fork tests [5]. Patient education: daily foot inspection, appropriate footwear, no walking barefoot, podiatry referral |
| Neuropathic ulcer | Painless ulcer at pressure point; may have callus rim | Offloading (total contact cast or therapeutic shoe), wound debridement, infection control |
| Infected ulcer / Cellulitis | Erythema, warmth, purulent discharge; may be painless due to neuropathy | Broad-spectrum antibiotics (cover Gram-positives + Gram-negatives + anaerobes); deep tissue culture; vascular assessment |
| Osteomyelitis | Deep infection eroding into bone (often metatarsals) | Prolonged IV antibiotics (6–8 weeks); surgical debridement; MRI for diagnosis (probe-to-bone test is clinical screening) |
| Gangrene | Tissue necrosis — wet (infected) or dry (ischaemic) | Amputation (toe, ray, transmetatarsal, or major — below/above knee depending on vascular supply) |
Neuropathic vs. Ischaemic Foot — Must Know for Exams
| Feature | Neuropathic Foot | Ischaemic Foot |
|---|---|---|
| Temperature | Warm | Cold |
| Pulses | Present / bounding | Absent / diminished |
| Skin | Dry, cracked (anhidrosis) | Shiny, thin, hairless |
| Ulcer location | Plantar (pressure points) | Tips of toes, between toes, lateral border |
| Pain | Painless | Painful |
| Deformity | Claw toes, pes cavus, Charcot | None specific |
| ABI | Normal or falsely elevated (calcified vessels) | Low ( < 0.9) |
Many diabetic feet are neuroischaemic (combination of both) — this carries the worst prognosis.
Charcot arthropathy [5] — a devastating complication of severe sensory neuropathy.
Pathophysiology: Loss of proprioception and pain → joint subjected to abnormal mechanical stresses without protective reflexes → repeated unrecognised micro-fractures → bone and joint destruction → collapse and deformity.
- Most common site: Midfoot (tarsometatarsal joints — Lisfranc area)
- Presentation: Swollen, warm, erythematous foot (often mistaken for cellulitis or gout). Often painless or only mildly painful despite severe bony destruction
- Complications of Charcot: "Rocker-bottom" foot deformity → new pressure points → ulceration → infection → amputation
- Diagnosis: X-ray (fragmentation, new bone, joint destruction); MRI for early Charcot (bone marrow oedema before structural changes)
- Management: Immediate offloading with total contact cast until acute phase resolves (may take 3–6 months); then custom moulded footwear
- Mechanism: Proprioceptive loss (large fibre) → sensory ataxia → unsteady gait especially in dark; distal motor weakness → foot drop → tripping
- Consequences: Fractures (especially hip fractures in elderly → significant morbidity and mortality), soft tissue injuries, subdural haematoma
- Prevention: Walking aids, AFOs for foot drop, occupational therapy home assessment (grab rails, adequate lighting, remove trip hazards), balance training
- Loss of temperature and pain sensation → patients may sustain burns from hot water, radiators, or hot objects without awareness
- Similarly, patients may not notice cuts, blisters, or foreign bodies in shoes
2. Complications of Motor Loss
- Mechanism: Weakness of ankle dorsiflexors (tibialis anterior, L4–L5) → foot slaps the ground during walking → compensatory high-stepping (steppage) gait
- Consequences: Tripping and falls, impaired mobility, difficulty with stairs, reduced independence
- Management: Ankle-foot orthosis (AFO), physiotherapy for strengthening of preserved muscles
These develop from chronic imbalance between long extrinsic and short intrinsic foot/hand muscles:
| Deformity | Mechanism | Consequences |
|---|---|---|
| Pes cavus (high arch) | Intrinsic foot muscle denervation → long extensors/flexors unopposed → arch elevation | Abnormal pressure distribution → calluses → ulceration |
| Claw toes | Loss of lumbricals/interossei → MCP hyperextension + IP flexion | Dorsal IP pressure from shoes → ulceration; prominent metatarsal heads → plantar ulcers |
| Claw hand | Loss of lumbricals/interossei in hand → MCP hyperextension + IP flexion | Loss of hand dexterity, difficulty grasping objects |
| Kyphoscoliosis | Paraspinal muscle denervation (in hereditary neuropathies) | Restrictive lung disease, chronic back pain |
- Prolonged weakness → disuse → muscle fibre atrophy beyond that caused by denervation alone
- Without physiotherapy → fixed joint contractures → further immobility → vicious cycle
- Prevention: Regular physiotherapy, passive range-of-motion exercises (especially important in GBS/ICU patients)
- Mechanism: Motor neuropathy involving phrenic nerve (C3–C5) and intercostal nerves → diaphragmatic and intercostal muscle weakness → hypoventilation → type 2 respiratory failure
- Mechanical ventilation if ↑CO₂ ↓O₂, FVC < 15 mL/kg BW, inefficient cough, dysphagia, atelectasis [2][31]
- Consequences: Aspiration pneumonia, atelectasis, ventilator-associated pneumonia, prolonged ICU stay
- Mortality of GBS: 2–12% [31] — respiratory failure is the leading cause of death
Autonomic neuropathy affects multiple organ systems simultaneously. It is most prominent in diabetic neuropathy and amyloidosis.
Diabetic autonomic neuropathy manifestations [5]:
| System | Complication | Mechanism | Clinical Impact |
|---|---|---|---|
| Cardiovascular | Resting tachycardia (early), failure of exercise-induced ↑HR resulting in exercise intolerance [5] | Vagal denervation → unopposed sympathetic tone | Increased myocardial oxygen demand; exercise limitation |
| Orthostatic hypotension due to central/peripheral sympathetic denervation [5] | Failure to vasoconstrict splanchnic/peripheral beds on standing | Syncope, falls → injuries | |
| Silent myocardial ischaemia | Cardiac afferent denervation → patient cannot feel angina | MI may present with heart failure or arrhythmia instead of chest pain — delayed recognition → worse outcomes | |
| QT prolongation | Cardiac autonomic denervation → electrical instability | Risk of sudden cardiac death (5-year mortality of symptomatic cardiac autonomic neuropathy is ~50%) | |
| GI | Gastroparesis: N/V, early satiety, bloating, upper abdominal pain [5] | Vagal denervation → loss of ICC coordination → delayed gastric emptying | Erratic glycaemic control (food absorption unpredictable); malnutrition; weight loss |
| Diarrhoea: painless watery nocturnal diarrhoea [5] | Enteric nervous system dysfunction ± bacterial overgrowth from dysmotility | Dehydration, electrolyte disturbance, social embarrassment | |
| Constipation | Colonic dysmotility | May alternate with diarrhoea | |
| Genitourinary | Bladder dysfunction: ↓ability to sense full bladder, incomplete emptying, recurrent UTI, overflow incontinence [5] | Detrusor denervation → atonic bladder with ↑ residual volume | Recurrent UTIs (stasis → bacterial growth); hydronephrosis → CKD |
| Ejaculatory dysfunction: retrograde ejaculation, erectile dysfunction [5] | Sympathetic denervation of internal urethral sphincter (retrograde ejaculation); parasympathetic denervation of cavernous arteries (ED) | Infertility; significant psychological impact | |
| Sudomotor | Distal hypohidrosis with compensatory proximal hyperhidrosis [5] | Sympathetic sudomotor denervation (cholinergic fibres to sweat glands) | Dry, cracked feet → skin breakdown → ulceration; compensatory drenching sweating of trunk/face |
| Thermoregulatory impairment and hyperthermia [5] | Loss of sweating → cannot dissipate heat | Heat stroke risk in hot environments (relevant in HK summers) | |
| Pupillomotor | Impaired pupillary constriction | Parasympathetic denervation of pupillary sphincter | Poor dark adaptation; sluggish light reflex |
Silent MI in Autonomic Neuropathy — A Killer Complication
Cardiac autonomic neuropathy denervates cardiac pain afferents. This means diabetic patients with autonomic neuropathy may have a heart attack without chest pain. They may present only with breathlessness, nausea, or confusion. This is why "atypical" MI presentations are so common in diabetics — and why autonomic neuropathy carries a markedly increased cardiovascular mortality.
GBS has unique complications due to its acute, potentially fulminant course:
Treatment of complications, e.g., respiratory failure, cardiac arrhythmia, DVT [2][31].
| Complication | Mechanism | Management |
|---|---|---|
| Respiratory failure | Phrenic and intercostal nerve involvement → diaphragmatic weakness | Serial FVC monitoring; intubate early (FVC < 15 mL/kg); mechanical ventilation |
| Cardiac arrhythmia | Autonomic neuropathy → sympathetic/parasympathetic imbalance → sinus tachycardia, bradycardia, asystole | Continuous ECG monitoring; atropine for symptomatic bradycardia; temporary pacing if needed |
| Labile blood pressure | Autonomic instability → alternating hypertension and hypotension | Avoid triggers (suctioning, position changes); short-acting IV antihypertensives for crises |
| Deep vein thrombosis / PE | Immobility + dehydration + ICU setting | Support stockings [31]; LMWH prophylaxis |
| Aspiration pneumonia | Bulbar weakness → impaired swallow and cough | Swallow assessment; NG feeding if unsafe swallow; chest physiotherapy |
| Paralytic ileus | Autonomic involvement of GI tract | NPO, NG decompression, prokinetics |
| Urinary retention | Autonomic involvement of bladder | Indwelling catheter; trial of void when improving |
| Neuropathic pain | Nerve root inflammation + ectopic firing | Gabapentin, pregabalin, opioids for severe pain |
| ICU-acquired weakness | Prolonged immobility + critical illness myopathy ± corticosteroid exposure | Early physiotherapy; avoid unnecessary steroids |
| Psychological | Sudden paralysis → anxiety, depression, PTSD | Psychological support; clear communication about prognosis |
| Residual disability | Incomplete recovery of axonal damage | 20% permanent disability, 10% severe disability [31]; rehabilitation programme |
5. Complications of Treatment
| Complication | Mechanism |
|---|---|
| Headache / aseptic meningitis | IgG entering CSF → meningeal irritation |
| Thromboembolism (DVT, PE, MI, stroke) | ↑ serum viscosity + procoagulant factors in IVIg |
| Renal impairment (osmotic nephropathy) | Sucrose-containing IVIg preparations → osmotic injury to renal tubular cells |
| Anaphylaxis in IgA-deficient patients | Anti-IgA antibodies → type I hypersensitivity to IgA in IVIg product |
| Haemolytic anaemia | Anti-A or anti-B isoagglutinins in pooled IVIg |
| Complication | Mechanism |
|---|---|
| Osteoporosis / avascular necrosis | ↓ Osteoblast activity + ↑ osteoclast activity |
| Steroid-induced diabetes | Gluconeogenesis ↑ + insulin resistance |
| Steroid myopathy (proximal weakness) | Type II muscle fibre atrophy — can confound assessment of CIDP recovery |
| Cushingoid features | Redistribution of fat, skin thinning, striae |
| Immunosuppression → infections | T-cell suppression → opportunistic infections (PJP, TB reactivation) |
| Cataracts | Posterior subcapsular cataract from altered lens protein metabolism |
Steroid Myopathy vs. CIDP Relapse
A patient on long-term steroids for CIDP who develops new proximal weakness — is it CIDP relapse or steroid-induced myopathy? Steroid myopathy: proximal weakness WITHOUT sensory loss, normal reflexes, normal CK, EMG shows myopathic pattern. CIDP relapse: sensory + motor involvement, ↓ reflexes, NCS changes. Getting this distinction right avoids increasing steroids when you should actually be stopping them.
As covered in the management section, drugs themselves can cause or worsen neuropathy:
- Isoniazid: peripheral neuropathy (numbness of fingers/toes) — prevention: Vitamin B6 for those with risk factors (pregnancy, DM, renal disease) [7]
- Ethambutol: optic neuritis (blurring of vision, scotoma, worsened colour discrimination); peripheral neuropathy (numbness of fingers/toes) → rare [7]
- Chemotherapy agents (vincristine, cisplatin, taxanes) — dose-dependent cumulative toxicity
| Complication | Mechanism | Impact |
|---|---|---|
| Depression | Chronic pain, disability, loss of independence, body image changes | Prevalence 20–30% in chronic polyneuropathy; worsens pain perception (bidirectional relationship) |
| Anxiety | Fear of progression, falls, amputation | Avoidance behaviour → deconditioning → worsening disability |
| Sleep disturbance | Neuropathic pain worse at night; restless legs-like symptoms | Fatigue → reduced function → reduced quality of life |
| Social isolation | Mobility limitation, foot drop (embarrassment), bladder/sexual dysfunction | Withdrawal from social activities |
| Reduced employment | Hand dexterity loss, fatigue, pain, cognitive effects of analgesics | Financial burden, loss of identity/purpose |
| Relationship strain | Erectile dysfunction, dependency on carers | Marital stress, caregiver burnout |
| Nerve Function Lost | Complication | Worst-Case Outcome |
|---|---|---|
| Sensory | Neuropathic ulcers, Charcot joints, burns, falls, unrecognised injuries | Amputation (diabetic foot) |
| Motor | Foot drop, skeletal deformities, respiratory failure (GBS), contractures | Death (respiratory failure in GBS) |
| Autonomic | Orthostatic hypotension, gastroparesis, bladder dysfunction, erectile dysfunction, silent MI | Sudden cardiac death (QT prolongation, silent MI) |
| Treatment-related | IVIg thrombosis/renal injury, steroid myopathy/osteoporosis, drug-induced worsening | Treatment toxicity compounding disability |
| Psychosocial | Depression, anxiety, social isolation, employment loss | Suicide; complete loss of independence |
High Yield Summary
Most important complication: Diabetic foot disease — neuropathy accounts for 80% of foot ulcer risk factors [5]. Previous foot ulceration is the most important risk factor [5]. Prevention through structured foot care programmes and patient education is essential. Prevention of drastic consequences — foot care [32].
Charcot arthropathy: Painless joint destruction from loss of proprioception. Often misdiagnosed as cellulitis. Midfoot most common. Offloading is key.
Autonomic complications: Cardiac autonomic neuropathy carries highest mortality (silent MI, QT prolongation → sudden death). Gastroparesis causes erratic glycaemic control. Bladder atony → recurrent UTIs → CKD.
GBS-specific: Respiratory failure is the leading cause of death (mortality 2–12% [31]). Serial FVC monitoring is mandatory. Also DVT, arrhythmia, aspiration.
Treatment complications: IVIg → thrombosis, renal injury, anaphylaxis in IgA deficiency. Steroids → myopathy (mimics CIDP relapse), osteoporosis, DM, infections.
Psychosocial: Depression affects 20–30% of patients with chronic polyneuropathy. Must be actively screened and managed.
Active Recall - Complications of Polyneuropathy
[2] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 — Disease of the Peripheral Nerves, pp. 179–184) [5] Senior notes: Ryan Ho Endocrine.pdf (pp. 98–99 — Diabetic peripheral and autonomic neuropathy, foot ulcer risk factors) [7] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p. 41 — Isoniazid and ethambutol side effects) [31] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 139 — GBS management, complications, prognosis) [32] Lecture slides: GC 042. Deterioration of eyesight in a diabetic patient diabetic complications [Update 2025].pdf (p. 15 — Treatment of chronic complications, foot care)
High Yield Summary
Definition: Polyneuropathy = symmetric, length-dependent disease of multiple peripheral nerves (distal → proximal, "glove-and-stocking").
Most common cause: Diabetes mellitus (HK and worldwide).
Two fundamental mechanisms: Axonal (↓ amplitude, normal velocity on NCS; slow recovery; treat underlying cause) vs. Demyelinating (↓ velocity, conduction block; treatable with immunotherapy).
Clinical features:
- Sensory: numbness, tingling, burning pain (small fibre), loss of proprioception/vibration (large fibre)
- Motor: distal weakness (foot drop), wasting, fasciculation, areflexia (LMN signs)
- Autonomic: postural hypotension, resting tachycardia, gastroparesis, bladder dysfunction
- Trophic: pes cavus, claw hand, trophic ulcers, Charcot joints
- Nerve thickening: leprosy, CMT, CIDP, acromegaly, neurofibromatosis
Key teaching points:
- Only demyelinating neuropathies are usually susceptible to treatment — this is why NCS is essential.
- B12 deficiency → ↑ knee jerk (myelopathy/UMN) + ↓ ankle jerk (neuropathy/LMN).
- GBS: molecular mimicry (GQ1b, GM1) → acute demyelinating polyneuropathy; CSF: albuminocytologic dissociation.
- CIDP: distinguished from GBS by > 8 weeks duration, relapsing course, steroid-responsive.
- Isoniazid neuropathy prevented by vitamin B6 supplementation.
- Critical illness polyneuropathy: axonal, occurs in sepsis/MODS.
Systematic approach: Time course → Fibre type → Distribution → NCS (axonal vs demyelinating) → Targeted investigations.
High Yield Summary
Two-level approach: (1) Confirm it IS polyneuropathy (exclude myelopathy, myopathy, NMJ disorders, radiculopathy); (2) Identify the underlying cause.
The most dangerous mimic: Myelopathy — distinguished by brisk reflexes, UMN signs, sensory level.
Must-know aetiological differential (from senior notes): DM, alcohol, B12, hypothyroidism, uraemia, chronic liver disease, GBS/CIDP, vasculitides, paraneoplastic, paraproteinaemia, CMT, drugs (isoniazid, vincristine, cisplatin, metronidazole, colchicine), HIV, Lyme disease, amyloidosis, sarcoidosis.
NCS-based stratification: Axonal → metabolic/toxic (treat underlying cause); Demyelinating → immune-mediated or hereditary (potentially treatable with immunotherapy).
Acute weakness differential (GBS): Must distinguish from spinal cord compression (UMN signs, sensory level), botulism (descending), MG (fatigability, ocular), poliomyelitis (asymmetric, fever), acute polymyositis (proximal, ↑CK, preserved reflexes).
CIDP vs GBS: CIDP > 8 weeks, relapsing, no preceding infection, more sensory, steroid-responsive.
CIDP vs CMT: CIDP has non-uniform conduction slowing WITH conduction block; CMT has uniform slowing WITHOUT block.
High Yield Summary
No single diagnostic criteria for polyneuropathy as a syndrome — diagnosis is clinical + electrophysiological. Specific criteria exist for GBS (Asbury-Cornblath) and CIDP (EFNS/PNS).
GBS diagnostic criteria: Progressive weakness + areflexia + progression over days to 4 weeks + CSF albuminocytologic dissociation + electrodiagnostic abnormalities. Doubt GBS if: sensory level, bladder dysfunction at onset, fever, marked asymmetry, CSF pleocytosis > 50.
NCS/EMG is the pivotal investigation: Axonal (↓ amplitude, normal velocity) vs. Demyelinating (↓ velocity, conduction block). Uniform demyelination without conduction block = hereditary (CMT). Non-uniform with conduction block = acquired (GBS/CIDP) → treatable.
Baseline blood screen for ALL polyneuropathy: HbA1c, B12, TFT, CBC, L/RFT, ESR/CRP, SPE with immunoglobulins.
CSF in GBS: Albuminocytologic dissociation — ↑ protein, normal cell count. May be normal in first week.
Normal NCS does NOT exclude polyneuropathy — small fibre neuropathy requires skin biopsy for diagnosis.
SPE must be performed in all cases of unexplained polyneuropathy to exclude paraproteinaemia.
High Yield Summary
Three pillars of polyneuropathy management: (1) Treat the underlying cause, (2) Symptomatic treatment (especially neuropathic pain), (3) Supportive and rehabilitative care.
Axonal neuropathies: Mostly treated by addressing the underlying cause (glycaemic control for DM, B12 replacement, stop offending drugs, dialysis for uraemia). No direct nerve repair therapy.
Demyelinating neuropathies: Often immunotherapy-responsive → IVIg, plasmapheresis, corticosteroids (CIDP only — NOT GBS).
GBS management: (1) Monitor FVC (intubate if < 15 mL/kg), (2) IVIg OR plasmapheresis (NOT both, NOT steroids), (3) DVT prophylaxis, pain management, nutrition, physiotherapy.
CIDP management: IVIg, plasmapheresis, or steroids all effective. Steroids ± steroid-sparing agents for milder/chronic disease. 2/3 respond to standard therapy.
Neuropathic pain: First-line = gabapentin/pregabalin OR amitriptyline OR duloxetine. Combine if monotherapy insufficient. Avoid long-term opioids.
Diabetic neuropathy: Glycaemic optimisation is mainstay. Foot care programme is essential. SGLT2i and GLP1a preferred oral agents.
CMT: No disease-modifying treatment. Supportive care only. Never give vincristine to CMT patients.
High Yield Summary
Most important complication: Diabetic foot disease — neuropathy accounts for 80% of foot ulcer risk factors [5]. Previous foot ulceration is the most important risk factor [5]. Prevention through structured foot care programmes and patient education is essential. Prevention of drastic consequences — foot care [32].
Charcot arthropathy: Painless joint destruction from loss of proprioception. Often misdiagnosed as cellulitis. Midfoot most common. Offloading is key.
Autonomic complications: Cardiac autonomic neuropathy carries highest mortality (silent MI, QT prolongation → sudden death). Gastroparesis causes erratic glycaemic control. Bladder atony → recurrent UTIs → CKD.
GBS-specific: Respiratory failure is the leading cause of death (mortality 2–12% [31]). Serial FVC monitoring is mandatory. Also DVT, arrhythmia, aspiration.
Treatment complications: IVIg → thrombosis, renal injury, anaphylaxis in IgA deficiency. Steroids → myopathy (mimics CIDP relapse), osteoporosis, DM, infections.
Psychosocial: Depression affects 20–30% of patients with chronic polyneuropathy. Must be actively screened and managed.
Mononeuropathy Multiplex
Mononeuropathy multiplex is the simultaneous or sequential involvement of two or more individual, non-contiguous peripheral nerves, often due to vasculitis, diabetes, or other systemic conditions.
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is a severe X-linked recessive neuromuscular disorder caused by mutations in the dystrophin gene, leading to progressive skeletal muscle degeneration and weakness, typically presenting in early childhood.