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.
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References
[1] Senior notes: Ryan Ho Neurology.pdf (pp. 179–182, Mononeuropathy Multiplex) [2] Senior notes: Maksim Medicine Notes.pdf (p. 269, Peripheral Neuropathy) [3] Senior notes: Ryan Ho Rheumatology.pdf (p. 159, Polyarteritis Nodosa) [4] Senior notes: Block A - Deterioration of eyesight in a diabetic patient_ diabetic complications.pdf (pp. 6–7)
Mononeuropathy Multiplex (Mononeuritis Multiplex)
Mononeuropathy multiplex (also called mononeuritis multiplex) refers to the simultaneous or sequential occurrence of mononeuropathies affecting multiple non-contiguous nerve trunks [1]. The name itself encodes the pathology:
- Mono = single
- neuro = nerve
- pathy = disease
- multiplex = multiple
So it literally means "disease of multiple individual nerves" — but critically, the nerves affected are anatomically unrelated to each other. This distinguishes it from a polyneuropathy (where there is diffuse, symmetric, length-dependent involvement of many nerves) and from a single mononeuropathy (e.g., carpal tunnel syndrome affecting just the median nerve).
Key Concept: The hallmark of mononeuropathy multiplex is asymmetric, multifocal nerve involvement. A patient might have a right common peroneal nerve palsy (foot drop) and a left ulnar neuropathy (claw hand) and a right median neuropathy — nerves in completely different territories, affected at different times or simultaneously. This pattern immediately suggests a systemic process attacking individual nerve trunks rather than a diffuse metabolic insult [1][2].
2. Epidemiology and Risk Factors
Mononeuropathy multiplex is uncommon as an isolated presentation. Precise incidence figures are difficult to establish because it is a clinical pattern rather than a single disease — it always points to an underlying systemic cause. Key epidemiological points:
- Most common cause worldwide: Diabetes mellitus [1][2]. Diabetic mononeuropathy multiplex (sometimes called diabetic mononeuritis multiplex) is by far the most frequently encountered form in clinical practice.
- Most common cause in non-diabetic patients: Systemic vasculitis (especially polyarteritis nodosa, ANCA-associated vasculitis, cryoglobulinaemic vasculitis).
- In the Hong Kong context:
- Diabetes mellitus prevalence is approximately 10% of the adult population, making diabetic-related mononeuropathy multiplex the dominant etiology.
- Hepatitis B virus (HBV) infection, while declining due to universal vaccination (since 1988), remains relevant as a cause of polyarteritis nodosa (PAN)-associated mononeuropathy multiplex in older, unvaccinated cohorts.
- Leprosy is essentially eradicated in Hong Kong but remains relevant for MCQ questions and for understanding the global differential.
- Systemic vasculitis (e.g., granulomatosis with polyangiitis, eosinophilic granulomatosis with polyangiitis) remains an important cause.
| Category | Specific Risk Factors |
|---|---|
| Metabolic | Diabetes mellitus (strongest risk factor), uraemia |
| Autoimmune/Vasculitic | SLE, RA, PAN, GPA (Wegener's), EGPA (Churg-Strauss), cryoglobulinaemia |
| Infectious | HBV, HCV, HIV, leprosy, Lyme disease, syphilis, diphtheria |
| Neoplastic/Paraneoplastic | Lymphoma, carcinomatosis, paraneoplastic syndromes |
| Infiltrative | Amyloidosis (AL or AA), sarcoidosis |
| Other | Hereditary neuropathy with liability to pressure palsies (HNPP) |
3. Anatomy and Function of Peripheral Nerves (Relevant Review)
To understand why mononeuropathy multiplex presents the way it does, one must understand peripheral nerve anatomy:
A peripheral nerve is a complex structure comprising:
- Axons: The "wires" — long extensions of motor neurons (anterior horn cells), sensory neurons (dorsal root ganglia), or autonomic neurons. Each axon carries signals.
- Myelin sheath: Produced by Schwann cells (one Schwann cell per internode in the PNS). Myelin acts as electrical insulation, enabling saltatory conduction — the nerve impulse "jumps" between Nodes of Ranvier, dramatically increasing conduction velocity.
- Endoneurium: Connective tissue around each individual nerve fibre.
- Perineurium: Connective tissue bundling fibres into fascicles. The perineurium forms a blood–nerve barrier.
- Epineurium: The outermost connective tissue sheath surrounding the entire nerve trunk.
This is the key anatomical concept:
- Peripheral nerves are supplied by vasa nervorum (Latin: "vessels of the nerves") — small arterioles (typically 75–300 μm in diameter) that run within the epineurium and perineurium.
- These are end-arteries — they have limited anastomotic connections.
- Because they are end-arteries, occlusion or inflammation of a single vasa nervorum leads to ischaemic infarction of the nerve fascicles it supplies.
- This is precisely what happens in vasculitic mononeuropathy multiplex: the vasculitis targets these small-to-medium sized arteries, causing patchy, multifocal nerve infarction.
Why does vasculitis cause mononeuropathy multiplex and not polyneuropathy? Because the vasa nervorum are individual small arteries supplying individual nerve trunks. Vasculitis affects these vessels in a random, patchy fashion — whichever vasa nervorum happen to be inflamed and occluded determine which nerves are infarcted. Since the process is stochastic and non-length-dependent, the result is asymmetric involvement of unrelated nerves.
| Fibre Type | Diameter | Myelination | Function | Clinical Deficit When Damaged |
|---|---|---|---|---|
| Aα / Aβ (large) | 6–20 μm | Heavily myelinated | Motor, proprioception, light touch, vibration | Weakness, loss of proprioception, absent reflexes |
| Aδ (small) | 1–5 μm | Thinly myelinated | Pain (sharp/acute), temperature | Loss of pinprick, temperature sensation |
| C (small) | 0.2–1.5 μm | Unmyelinated | Pain (dull/chronic), temperature, autonomic | Burning pain, autonomic dysfunction |
In mononeuropathy multiplex, the ischaemic insult to the nerve trunk typically affects all fibre types within the affected fascicles, producing mixed motor-sensory-autonomic deficits in the distribution of each affected nerve.
4. Etiology and Pathophysiology
Classic Mnemonic for Causes of Mononeuritis Multiplex
"WARD SLAP CC" (or the mnemonic from Ryan Ho notes):
- Wegener's granulomatosis (GPA)
- Amyloidosis
- Rheumatoid arthritis
- Diabetes mellitus
- SLE
- Leprosy
- A — (not needed)
- Polyarteritis nodosa
- Carcinomatosis
- Churg-Strauss (EGPA)
The causes can be organized by pathophysiological mechanism:
4.2 Axonal Causes (Nerve Infarction — Most Common Mechanism)
The majority of mononeuropathy multiplex cases are axonal, resulting from nerve infarction due to small-to-medium arterial diseases [1].
-
Mechanism: Diabetic microangiopathy affects the vasa nervorum. Chronic hyperglycaemia leads to:
- Non-enzymatic glycation of basement membrane proteins → thickening of vasa nervorum walls
- Sorbitol accumulation (polyol pathway) → osmotic damage to Schwann cells
- Oxidative stress and advanced glycation end-products (AGEs) → endothelial dysfunction
- Microthrombi formation within vasa nervorum
- The net result is ischaemic infarction of individual nerve trunks
-
Diabetic mononeuropathy presents classically in 4 ways [4]:
- Ptosis and divergent squint → 3rd nerve palsy (characteristically pupil-sparing because parasympathetic fibres run on the periphery of CN III and are supplied by pial vessels — they are relatively spared in microvascular ischaemia but affected in compressive lesions like aneurysm)
- External (lateral) rectus palsy → 6th nerve palsy
- Upper facial and eye pain → trigeminal nerve involvement
- Foot drop → common peroneal nerve palsy
-
"Mix and match" — i.e., a patient can develop a 3rd nerve palsy and a foot drop at a similar time [4].
-
Diabetes-induced microangiopathy → mononeuritis multiplex [4].
High Yield – Diabetic Mononeuropathy
A diabetic patient with an acute, painful pupil-sparing CN III palsy = microvascular ischaemia until proven otherwise. If the pupil IS involved (mydriasis), you must urgently exclude a posterior communicating artery aneurysm compressing CN III.
Vasculitis is the most critical cause to identify because:
- It is treatable with immunosuppression
- If untreated, it progresses relentlessly and can be fatal
- It is the cause in which the pattern "mononeuropathy multiplex" is most classically described
Why does vasculitis cause mononeuropathy multiplex?
- Vasculitis (inflammation and necrosis of vessel walls) targets the vasa nervorum (small-to-medium arteries)
- Inflammatory infiltrates → vessel wall destruction → thrombosis → ischaemic infarction of the nerve trunk
- Since the process is random and multifocal, different nerves are affected at different times → the classic stepwise, asymmetric pattern
Specific Vasculitides:
| Vasculitis | Vessel Size | Key Features | ANCA Status |
|---|---|---|---|
| Polyarteritis nodosa (PAN) | Medium | Mononeuritis multiplex in up to 70% [5], renal involvement, mesenteric ischaemia, HBV-associated in some | Usually ANCA-negative (15% p-ANCA) [5] |
| Granulomatosis with polyangiitis (GPA, Wegener's) | Small-medium | ENT disease, pulmonary nodules/cavities, glomerulonephritis | c-ANCA (anti-PR3) |
| Eosinophilic granulomatosis with polyangiitis (EGPA, Churg-Strauss) | Small-medium | Asthma, eosinophilia, nasal polyps, neuropathy in ~75% | p-ANCA (anti-MPO) in ~40% |
| Microscopic polyangiitis (MPA) | Small | Pulmonary-renal syndrome, mononeuritis multiplex or polyneuropathy | p-ANCA (anti-MPO) |
| Cryoglobulinaemic vasculitis | Small | Associated with HCV, palpable purpura, arthralgia, GN | Negative |
| Rheumatoid vasculitis | Small-medium | Severe, seropositive RA; nodules, nail fold infarcts, digital gangrene | Negative |
PAN deserves special emphasis: neurological involvement (mononeuritis multiplex) occurs in up to 70% of PAN cases [5], making it one of the most common systemic manifestations. The classic teaching is a patient presenting with acute wrist drop or foot drop in the context of systemic illness, weight loss, and renal impairment.
- SLE: Vasculitis of vasa nervorum or antiphospholipid antibody-mediated thrombosis → nerve infarction.
- Rheumatoid Arthritis: Rheumatoid vasculitis occurs in severe, longstanding, seropositive (high-titre RF, anti-CCP positive) RA. It can also cause mononeuropathy via entrapment (e.g., carpal tunnel in inflamed wrist).
- Sjögren syndrome: Small fibre neuropathy or sensory ganglionopathy more common, but mononeuropathy multiplex can occur via vasculitis.
| Infection | Mechanism | Key Points |
|---|---|---|
| Leprosy | Mycobacterium leprae directly invades Schwann cells (tropism for peripheral nerves) → granulomatous inflammation → nerve thickening and destruction | Most common infectious cause worldwide; nerve thickening is characteristic [3]; rare in HK but exam-relevant |
| HIV | Direct viral neurotropism, associated vasculitis, or immune-mediated mechanisms | Can present at any stage; also causes DSPN (distal symmetric polyneuropathy) |
| Hepatitis B / Hepatitis C | HBV → PAN-associated vasculitis; HCV → cryoglobulinaemic vasculitis | Important in HK (HBV) and globally (HCV) |
| Lyme disease | Borrelia burgdorferi → lymphocytic infiltration of nerve, vasculitis | Bilateral facial palsy is a classic feature; rare in HK |
| Syphilis | Vasculitis of vasa nervorum in secondary/tertiary stages | |
| Diphtheria | Diphtheria toxin → demyelination | Rare due to vaccination; pharyngeal/palatal neuropathy early, limb neuropathy late |
- Carcinomatosis / lymphomatous infiltration: Direct tumour infiltration of nerve trunks (neurolymphomatosis).
- Paraneoplastic: Autoantibodies (e.g., anti-Hu/ANNA-1) cause inflammatory damage to nerve trunks. Typically associated with small cell lung cancer.
- Amyloidosis: AL amyloid (from plasma cell dyscrasias like myeloma) deposits in vasa nervorum and endoneurium → ischaemia and direct mechanical compression of nerve fibres. Amyloidosis is listed as a cause of mononeuritis multiplex [1]. Nerve thickening can occur in amyloidosis [3].
4.3 Demyelinating Causes
Demyelinating causes are less common but important because they are potentially treatable [1].
- Definition: An acquired, immune-mediated, purely motor neuropathy affecting multiple individual motor nerves in an asymmetric distribution.
- Pathophysiology: Anti-GM1 ganglioside antibodies → attack on myelin at the Nodes of Ranvier → conduction block (the nerve impulse cannot traverse the demyelinated segment). Importantly, the axon itself is initially preserved — hence the disease is potentially reversible with treatment.
- Why is this classified under mononeuropathy multiplex? Because it affects individual named motor nerves asymmetrically. A patient might have weakness in the right radial nerve distribution and left ulnar nerve distribution.
- Key distinguishing features:
- Pure motor (no sensory involvement)
- No upper motor neuron signs (differentiates from motor neurone disease/ALS)
- Anti-GM1 antibodies positive in ~50%
- NCS shows conduction block (hallmark)
- Responds to IVIg (but NOT to steroids or plasma exchange — this is unique and high-yield)
Important Distinction
Do not confuse MMN with motor neurone disease (MND/ALS). Both present with pure motor, asymmetric weakness. However:
- MMN: LMN signs only, NCS shows conduction block, responds to IVIg
- MND: UMN + LMN signs, NCS shows axonal loss, no effective treatment The distinction is critical because MMN is treatable!
- A variant of CIDP (chronic inflammatory demyelinating polyneuropathy) that presents asymmetrically, mimicking mononeuropathy multiplex.
- Affects both sensory and motor fibres.
- NCS shows demyelinating features (conduction block, temporal dispersion, prolonged distal latencies).
- Responds to IVIg or steroids.
- Autosomal dominant; caused by deletion of PMP22 gene (the same gene duplicated in CMT1A).
- Patients develop recurrent, transient mononeuropathies at sites of compression (e.g., peroneal nerve at fibular head, ulnar nerve at elbow).
- The recurrent multifocal pattern can mimic mononeuropathy multiplex.
| Feature | Axonal (Vasculitic/Ischaemic) | Demyelinating (Immune-mediated) |
|---|---|---|
| Onset | Often acute, painful | Often subacute/chronic, painless |
| Mechanism | Nerve infarction (vasa nervorum) | Autoimmune demyelination |
| NCS findings | ↓ Amplitude, normal velocity | ↓ Velocity, conduction block, normal/near-normal amplitude |
| Prognosis | Poor if untreated (axonal loss) | Better (potentially reversible) |
| Treatment | Treat underlying cause + immunosuppression | IVIg (MMN), steroids/IVIg (MADSAM/CIDP) |
| Examples | DM, PAN, GPA, EGPA, RA, SLE | MMN, MADSAM, HNPP |
Important: usually only demyelinating neuropathies are susceptible to treatment [1] — this is a key principle in neurology. Axonal loss, once established, is largely irreversible. This is why early diagnosis and treatment of the underlying cause (e.g., vasculitis) is critical to prevent further axonal damage.
5. Classification
Mononeuropathy multiplex can be classified by several schemes:
| Type | Description | Causes |
|---|---|---|
| Axonal (ischaemic/infarction) | Wallerian degeneration distal to site of infarction | DM, vasculitis, infections |
| Demyelinating | Segmental demyelination with conduction block | MMN, MADSAM, HNPP |
| Infiltrative | Direct infiltration of nerve trunk | Amyloidosis, sarcoidosis, lymphoma, leprosy |
| Mixed | Combination of above | Some vasculitides (secondary demyelination after ischaemia) |
| Pattern | Description | Typical Causes |
|---|---|---|
| Acute (hours–days) | Sudden onset of individual nerve palsies | Vasculitis (nerve infarction), DM |
| Subacute (weeks–months) | Progressive stepwise involvement | Vasculitis, paraneoplastic, infections |
| Chronic (months–years) | Slowly progressive | HNPP, amyloidosis, MMN, leprosy |
| Relapsing-remitting | Episodes of nerve palsy with recovery | HNPP, MADSAM |
As the disease evolves, mononeuropathy multiplex shows gradual spread from focal → multifocal → generalized [1]. Over time, if enough individual nerves are affected, the clinical picture can become confluent and mimic a symmetric polyneuropathy — this is called confluent mononeuropathy multiplex. Recognizing this evolution is important because:
- A patient presenting with apparent "polyneuropathy" may actually have confluent mononeuropathy multiplex
- The treatment and prognosis differ dramatically (vasculitis vs metabolic polyneuropathy)
- Clues to the underlying mononeuropathy multiplex pattern: asymmetry, stepwise progression, acute painful onset
6. Clinical Features
6.1 Symptoms
The presentation depends on which specific nerves are affected, but the pattern is characteristic:
- Weakness: Abrupt onset of weakness in the distribution of a specific named nerve, followed (days to weeks later) by weakness in another, anatomically unrelated nerve [1]
- Pathophysiological basis: Ischaemic infarction of motor axons within the affected nerve → Wallerian degeneration → loss of motor innervation to the muscles supplied by that nerve
- Common patterns:
- Foot drop (common peroneal nerve) — inability to dorsiflex the foot
- Wrist drop (radial nerve) — inability to extend the wrist
- Claw hand (ulnar nerve) — inability to abduct/adduct fingers, hyperextension of MCP joints
- Weakness of grip (median nerve) — inability to flex index/middle fingers, opposition of thumb
- The weakness is LMN pattern: flaccid, with wasting, fasciculations, and hyporeflexia in the affected territory [1]
-
Pain: Often the heralding symptom — acute pain followed by focal neuropathy [1]
- Pathophysiological basis: Nerve ischaemia activates nociceptors within the nerve sheath (nervi nervorum) and causes release of inflammatory mediators → severe, burning, dysaesthetic pain in the distribution of the affected nerve
- The pain precedes the motor deficit by hours to days (the ischaemia causes pain before it causes complete axonal death)
- Pain is especially prominent in vasculitic mononeuropathy multiplex
-
Numbness and paraesthesia: In the sensory distribution of the affected nerve
- Pathophysiological basis: Ischaemic damage to sensory axons → loss of afferent input → perceived numbness; irritation of surviving fibres → abnormal spontaneous firing → tingling/paraesthesia
- Unlike polyneuropathy, the sensory loss is not in a glove-and-stocking distribution but rather follows individual nerve territories (e.g., lateral leg and dorsum of foot for common peroneal nerve)
-
Painful neuropathy: Mononeuropathy multiplex is characteristically painful, in contrast to many polyneuropathies. Painful neuropathy is listed as a feature of DM, CTD-related, vasculitis, paraprotein-related, paraneoplastic, amyloidosis [3].
- Autonomic fibres within affected nerve trunks may also be damaged, causing:
- Localised anhidrosis (loss of sweating) in the distribution of the affected nerve
- Vasomotor changes: Colour changes, temperature differences in the affected limb
- In diabetic mononeuropathy multiplex, autonomic involvement may be more widespread: postural hypotension, gastroparesis, erectile dysfunction, bladder dysfunction [4]
- Pathophysiological basis: Ischaemic damage to sympathetic and parasympathetic fibres within the nerve trunk → loss of autonomic regulation
- The cardinal temporal feature: stepwise, sequential onset. A new nerve is affected days to months after the previous one. Patients can often pinpoint "I woke up one morning and couldn't lift my foot, then two weeks later my hand went weak."
6.2 Signs
| Sign | Pathophysiological Basis |
|---|---|
| Weakness in specific named nerve distributions (asymmetric) | Axonal loss → loss of motor innervation |
| Wasting (atrophy) of muscles | Denervation → disuse and trophic factor withdrawal → muscle fibre atrophy. Early wasting is characteristic of axonal but not demyelinating neuropathies [1] |
| Fasciculations | Spontaneous firing of denervated motor units |
| Hyporeflexia / areflexia | Loss of afferent (sensory) and efferent (motor) limbs of the reflex arc |
| No UMN signs (no spasticity, no hyperreflexia, no upgoing plantars) | The lesion is in the peripheral nerve, not the CNS |
| Sign | Pathophysiological Basis |
|---|---|
| Loss of all modalities (pain, temperature, light touch, vibration, proprioception) in the distribution of specific nerves | Ischaemic infarction affects all fibre types within the nerve trunk |
| Asymmetric, non-length-dependent pattern | Random involvement of individual nerve trunks |
| Positive sensory signs: hyperaesthesia, allodynia along affected nerve territory | Irritation and aberrant regeneration of surviving sensory fibres |
| Sign | Pathophysiological Basis |
|---|---|
| Localised anhidrosis | Loss of sympathetic cholinergic innervation to sweat glands |
| Trophic changes: skin atrophy, hair loss, nail changes | Loss of autonomic trophic support |
| Postural hypotension (if widespread, e.g., DM) | Sympathetic vasomotor failure |
This is where the clinical examination becomes critical — looking for clues to the underlying etiology:
| Finding | Suggests |
|---|---|
| Palpable purpura, skin nodules, livedo reticularis, nail-fold infarcts | Vasculitis (PAN, EGPA, RA vasculitis) |
| Malar rash, oral ulcers, alopecia | SLE |
| Swollen, deformed joints (especially hands) | RA |
| Saddle-nose deformity, nasal crusting, haemoptysis | GPA (Wegener's) |
| Asthma, nasal polyps | EGPA (Churg-Strauss) |
| Thickened, hypopigmented skin patches with anaesthetic areas | Leprosy |
| Nerve thickening on palpation (ulnar at elbow, greater auricular in neck) | Leprosy, amyloidosis, CMT, CIDP, Refsum disease, Dejerine-Sottas disease [3] |
| Kaposi sarcoma, oral candidiasis, lymphadenopathy | HIV |
| Hepatomegaly, skin rashes | Hepatitis B/C |
| Weight loss, cachexia, lymphadenopathy | Malignancy, lymphoma |
| Macroglossia, periorbital purpura, carpal tunnel | AL amyloidosis |
| Diabetic retinopathy, nephropathy | Diabetes mellitus |
| Neuropathic ulcers, Charcot joints | Chronic peripheral neuropathy (DM, leprosy) |
| Nerve | Motor Deficit | Sensory Territory | Classic Cause |
|---|---|---|---|
| Common peroneal (fibular head) | Foot drop, weak ankle eversion | Lateral leg, dorsum of foot | DM, vasculitis, compression |
| Ulnar (elbow/wrist) | Claw hand, weak finger abduction/adduction | Medial 1.5 fingers | DM, vasculitis, entrapment |
| Median (wrist) | Weak thumb opposition, thenar wasting | Lateral 3.5 fingers | DM, vasculitis, CTS |
| Radial (spiral groove) | Wrist drop, weak finger extension | Dorsal 1st web space | Vasculitis, compression |
| Posterior tibial | Weak toe flexion, foot inversion | Sole of foot | DM, vasculitis |
| Femoral | Weak knee extension, quadriceps wasting | Anterior thigh, medial leg | DM (diabetic amyotrophy overlap) |
| CN III (oculomotor) | Ptosis, "down and out" eye | — | DM (pupil-sparing) [4] |
| CN VI (abducens) | Lateral rectus palsy | — | DM |
| CN V (trigeminal) | — | Facial pain | DM |
| Lateral cutaneous nerve of thigh | — | Lateral thigh (meralgia paraesthetica) | DM, obesity |
Clinical Pearl – Pattern Recognition
The clinical approach to peripheral neuropathy involves ascertaining the pattern of symptoms and signs [3]:
- Predominantly motor: GBS, CMT, lead toxicity
- Predominantly sensory: DM, alcoholism, vitamin B1/B12 deficiency, uraemia, leprosy
- Autonomic: GBS, porphyria, DM, hereditary neuropathies
- Painful: DM, CTD-related, vasculitis, paraprotein-related, paraneoplastic, amyloidosis
- Nerve thickening: amyloidosis, CMT, CIDP, leprosy, Refsum disease, Dejerine-Sottas disease
For mononeuropathy multiplex specifically, the asymmetric, stepwise, painful presentation with mixed motor-sensory loss in named nerve territories is the defining pattern.
Before confirming mononeuropathy multiplex, these mimics must be excluded [3]:
| Differential | How to Distinguish |
|---|---|
| Polyneuropathy | Symmetric, length-dependent (glove-and-stocking), not in named nerve territories |
| Radiculopathy | Weakness and sensory loss in segmental/dermatomal distribution, not named nerve distribution |
| Plexopathy | Involvement of multiple nerves within one plexus (brachial or lumbosacral), not scattered across different limbs |
| Myopathy | Proximal weakness, preserved reflexes (until late), muscle tenderness, elevated CK, no sensory involvement |
| Myasthenia gravis | Ocular involvement common, fatiguability present, no sensory involvement [3] |
| Myelopathy | Distal paraesthesia precede long tract signs, normal/↑ tendon reflexes [3], upper motor neuron signs |
| Motor neurone disease | UMN + LMN signs coexisting, no sensory involvement, no conduction block on NCS |
| Cause | Pathophysiological Mechanism | Pattern |
|---|---|---|
| DM | Microangiopathy of vasa nervorum → ischaemic infarction | Axonal, painful, cranial nerves common |
| PAN | Necrotising vasculitis of medium arteries (vasa nervorum) → thrombosis → infarction | Axonal, acute, painful, systemic features |
| GPA/EGPA/MPA | Small-medium vessel vasculitis → vasa nervorum inflammation → infarction | Axonal, with organ-specific features |
| SLE | Vasculitis or antiphospholipid-mediated thrombosis → nerve ischaemia | Axonal |
| RA | Rheumatoid vasculitis → vasa nervorum infarction | Axonal, in severe seropositive disease |
| Leprosy | Direct Schwann cell invasion by M. leprae → granulomatous neuritis | Mixed, nerve thickening, sensory-predominant |
| HIV | Vasculitis, direct viral damage, immune-mediated | Variable |
| HBV/HCV | HBV → PAN; HCV → cryoglobulinaemia → vasculitis | Axonal |
| Amyloidosis | Amyloid deposition in endoneurium and vasa nervorum → compression + ischaemia | Axonal, + autonomic, painful |
| Sarcoidosis | Non-caseating granulomas infiltrating nerve trunks | Mixed, cranial nerves (especially CN VII) |
| MMN | Anti-GM1 antibodies → segmental demyelination → conduction block | Demyelinating, pure motor |
| MADSAM | Immune-mediated demyelination (CIDP variant) | Demyelinating, sensorimotor |
| HNPP | PMP22 deletion → mechanically vulnerable myelin → recurrent pressure palsies | Demyelinating, recurrent |
| Paraneoplastic | Anti-neuronal antibodies (anti-Hu) → inflammatory nerve damage | Axonal, subacute |
| Lymphoma | Direct infiltration (neurolymphomatosis) or vasculitis | Variable |
High Yield Summary
Mononeuropathy multiplex = simultaneous or sequential mononeuropathies affecting multiple non-contiguous nerve trunks — the hallmark is asymmetric, multifocal, nerve-territory-specific deficits.
Most common cause: Diabetes mellitus (via microangiopathy of vasa nervorum).
Most important non-diabetic cause: Systemic vasculitis (PAN, GPA, EGPA) — treatable with immunosuppression; PAN causes mononeuritis multiplex in up to 70%.
Mechanism: Mostly axonal due to ischaemic infarction of nerve trunks via vasa nervorum disease. Demyelinating causes (MMN, MADSAM) are less common but potentially reversible.
Clinical pattern: Acute pain followed by focal neuropathy → stepwise addition of new nerve palsies → may become confluent over time.
Key examination: Map affected nerves, look for LMN signs (wasting, weakness, hyporeflexia, fasciculations), look for systemic clues to underlying cause (purpura, joint disease, skin changes).
Only demyelinating neuropathies are usually susceptible to treatment — early distinction between axonal and demyelinating via NCS is essential.
DM mononeuropathy classic presentations: CN III (pupil-sparing ptosis), CN VI, CN V, common peroneal nerve (foot drop) — "mix and match".
Mnemonic for causes: WARD SLAP CC — Wegener's, Amyloidosis, RA, DM, SLE, Leprosy, PAN, Carcinomatosis, Churg-Strauss.
Active Recall - Mononeuropathy Multiplex
[1] Senior notes: Ryan Ho Neurology.pdf (Section 10.2.2 Mononeuropathy Multiplex, pp. 179–180) [2] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 Peripheral neuropathy, p. 269) [3] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 4.4.3 Diseases of Peripheral Nerves, p. 137) [4] Senior notes: Block A - Deterioration of eyesight in a diabetic patient_ diabetic complications.pdf (Diabetic Mononeuropathy section, p. 7) [5] Senior notes: Ryan Ho Rheumatology.pdf (Section 4.7.3 Polyarteritis Nodosa, p. 159) [6] Lecture slides: Neurology - Two cases of lower limb weakness.pdf (pp. 18, 20) [7] Lecture slides: CFB (MED04) Central Nervous System.pdf (p. 3)
Differential Diagnosis of Mononeuropathy Multiplex
The differential diagnosis of mononeuropathy multiplex operates on two levels simultaneously:
- Level 1 — "Is this really mononeuropathy multiplex?" (Anatomical/pattern differential): Confirming that the clinical picture truly represents multiple non-contiguous mononeuropathies rather than a mimic.
- Level 2 — "What is the underlying cause?" (Etiological differential): Once the pattern is confirmed, determining which systemic disease is responsible.
This two-tier approach is fundamental because the management hinges entirely on the underlying cause — there is no "generic" treatment for mononeuropathy multiplex itself.
Level 1: Pattern Differential — Is It Really Mononeuropathy Multiplex?
Before jumping to etiological differentials, you must first exclude conditions that mimic the asymmetric multifocal weakness pattern. This is the "Where is the lesion?" question [6][8].
High Yield – GC Lecture Slide
Where is the lesion (anatomical differentials)? Consider: cerebrum, brainstem, spinal cord, anterior horn cells, nerve root, brachial/lumbosacral plexus, peripheral nerve, neuromuscular junction, muscle, bone and joints, metabolic, functional [8]. This systematic localization framework is the starting point for all neurological presentations.
| Mimic | Key Distinguishing Features | Why It Can Be Confused |
|---|---|---|
| Polyneuropathy | Symmetric, length-dependent (glove-and-stocking), not in named nerve territories; gradual onset | Confluent mononeuropathy multiplex can look symmetric once many nerves are involved |
| Multiple entrapment neuropathies | Nerves affected at typical entrapment sites (carpal tunnel, cubital tunnel, fibular head); predisposing conditions (DM, hypothyroidism, pregnancy, acromegaly) [9] | Multiple nerves are involved, but each at a known compression point — this is not true mononeuropathy multiplex |
| Radiculopathy (single or multiple) | Weakness and sensory loss in segmental/dermatomal distribution, not named peripheral nerve distribution [3]; positive nerve root tension signs (Spurling, SLR) | Can cause asymmetric limb weakness, but the pattern follows dermatomes/myotomes rather than peripheral nerve territories |
| Plexopathy (brachial or lumbosacral) | Multiple nerves affected but all within one plexus; usually one limb affected | Looks like multiple mononeuropathies but confined to one limb; causes include trauma, radiation, neoplastic infiltration, diabetic amyotrophy |
| Motor neurone disease (MND/ALS) | UMN + LMN signs coexisting, no sensory involvement, progressive course; NCS shows axonal loss without conduction block | Asymmetric limb weakness with wasting can superficially resemble mononeuropathy multiplex |
| Myopathy | Proximal weakness, preserved reflexes (until late), muscle tenderness, elevated CK [3]; no sensory involvement | Proximal pattern differs from the distal/mixed pattern of mononeuropathy multiplex |
| Myasthenia gravis | Ocular involvement common, fatiguability present, no sensory involvement [3]; fluctuating weakness | Asymmetric weakness can occur, but fatiguability and ocular features distinguish it |
| Myelopathy | Distal paraesthesia precede long tract signs, normal/↑ tendon reflexes [3]; UMN signs (spasticity, hyperreflexia, upgoing plantars), sensory level | Bilateral limb weakness can occur, but UMN signs and sensory level differentiate |
| Multifocal CNS lesions (e.g., MS, multiple strokes) | UMN signs, cranial nerve palsies with central features, MRI shows CNS lesions | Multiple CNS lesions can cause asymmetric weakness, but the examination reveals UMN not LMN pattern |
| Functional neurological disorder | Fluctuating, variable symptoms not conforming to neuroanatomical principles [3]; Hoover sign, give-way weakness | Can mimic any pattern but lacks anatomical consistency |
Critical Exam Tip
The most common mistake students make is calling multiple entrapment neuropathies in a diabetic patient "mononeuropathy multiplex." While both involve multiple nerves, true mononeuropathy multiplex implies a systemic process (e.g., vasculitis, microangiopathy) attacking nerve trunks at non-entrapment sites. Multiple entrapments suggest mechanical vulnerability of nerves (often due to generalised soft tissue swelling in DM, hypothyroidism, acromegaly). The distinction matters because the workup and management differ — entrapment neuropathies may need surgical decompression, while vasculitic mononeuropathy multiplex needs immunosuppression.
| Feature | Points Towards Mononeuropathy Multiplex | Points Away |
|---|---|---|
| Named peripheral nerve territories affected | ✓ | — |
| Asymmetric, non-length-dependent | ✓ | Length-dependent = polyneuropathy |
| Stepwise, sequential onset | ✓ | Simultaneous onset of all limbs = GBS or polyneuropathy |
| LMN signs only (wasting, hyporeflexia) | ✓ | UMN signs = myelopathy or MND |
| Sensory involvement in nerve territory | ✓ | No sensory = myopathy, NMJ, or MND |
| Acute pain heralding each new deficit | ✓ (vasculitic) | Pain absent in most myopathies |
| Systemic features (fever, weight loss, rash) | ✓ (suggests systemic cause) | — |
Level 2: Etiological Differential — What Is Causing the Mononeuropathy Multiplex?
Once the pattern is confirmed, the etiological differential is the critical next step. The causes can be systematically categorised using the pathological differentials framework from the lecture slides [6][8]:
Vascular, Infection, Neoplasm, Degenerative, Inflammatory, Congenital, Autoimmune, Trauma/Toxins, Endocrine [6]
| Category | Cause | Key Distinguishing Clues | Pathophysiological Basis |
|---|---|---|---|
| Metabolic | Diabetes mellitus (most common) [1][2][4] | Known DM; pupil-sparing CN III palsy, CN VI palsy, foot drop; other diabetic complications (retinopathy, nephropathy) present [4] | Diabetes-induced microangiopathy → vasa nervorum ischaemia [4] |
| Primary Vasculitis | PAN | ANCA-negative; medium-vessel disease; mononeuritis multiplex in up to 70% [5]; renal infarctions (not GN), mesenteric ischaemia, livedo reticularis, skin nodules; ± HBV association | Necrotising vasculitis of medium arteries including vasa nervorum → thrombosis → nerve infarction |
| GPA (Wegener's) | c-ANCA (anti-PR3); ENT disease (saddle nose, nasal crusting, sinusitis), pulmonary nodules/cavities, RPGN | Small-to-medium vessel granulomatous vasculitis | |
| EGPA (Churg-Strauss) | p-ANCA (anti-MPO) in ~40%; triad: asthma + eosinophilia + vasculitis; neuropathy in ~75% | Eosinophilic granulomatous vasculitis | |
| MPA | p-ANCA (anti-MPO); pulmonary-renal syndrome; no granulomas | Small vessel necrotising vasculitis | |
| Cryoglobulinaemic vasculitis | HCV association; palpable purpura, arthralgia, GN; cryoglobulins positive, low C4 | Immune complex deposition in small vessels | |
| Secondary Vasculitis / CTD | RA vasculitis | Long-standing, severe, seropositive RA (high RF, anti-CCP); nail-fold infarcts, digital gangrene, skin ulcers | Vasculitis complicating severe RA |
| SLE | Young female; malar rash, oral ulcers, arthritis, serositis; ANA, anti-dsDNA positive | Vasculitis of vasa nervorum or antiphospholipid-mediated thrombosis | |
| Sjögren syndrome | Sicca symptoms (dry eyes, dry mouth); anti-Ro/La positive | Vasculitis or cryoglobulinaemia | |
| Infectious | Leprosy | Travel/endemic area; hypopigmented anaesthetic skin patches; nerve thickening [3]; acid-fast bacilli on skin smear | Direct Schwann cell invasion by M. leprae |
| HIV | Risk factors; CD4 count low; may present at any stage | Direct viral neurotropism, vasculitis, immune-mediated | |
| Lyme disease | Tick bite history; erythema migrans; bilateral facial palsy; endemic area (not HK) | Borrelia → lymphocytic infiltration of nerve | |
| HBV/HCV | Known carrier; LFT derangement; HBV → PAN; HCV → cryoglobulinaemia | Virus-associated vasculitis | |
| Syphilis | Risk factors for STI; positive treponemal serology | Meningovascular syphilis → vasculitis of vasa nervorum | |
| Infiltrative | Amyloidosis | Macroglossia, periorbital purpura, carpal tunnel, nephrotic syndrome, cardiac involvement; nerve thickening [3]; serum/urine paraprotein; Congo red stain positive | AL amyloid deposition in endoneurium and vasa nervorum |
| Sarcoidosis | Bilateral hilar lymphadenopathy, erythema nodosum; elevated ACE; CN VII palsy common; non-caseating granulomas on biopsy | Granulomatous infiltration of nerve trunks | |
| Neurolymphomatosis | Known lymphoma/leukaemia; progressive painful neuropathy; MRI neurography shows nerve enhancement | Direct lymphomatous infiltration of nerve | |
| Paraneoplastic | Anti-Hu (ANNA-1) neuropathy | Smoking history; subacute sensory neuropathy or sensorimotor neuropathy; search for SCLC | Autoantibodies cross-react with neuronal antigens |
| Demyelinating | MMN | Pure motor, asymmetric; anti-GM1 antibodies in ~50%; NCS shows conduction block; responds to IVIg; does NOT respond to steroids | Immune-mediated segmental demyelination at Nodes of Ranvier |
| MADSAM (Lewis-Sumner) | Sensorimotor, asymmetric; CIDP variant; NCS shows demyelinating features; responds to IVIg or steroids | Immune-mediated demyelination (CIDP variant) | |
| HNPP | Family history; recurrent pressure palsies at typical entrapment sites; AD inheritance (PMP22 deletion) | Mechanically vulnerable myelin → episodic demyelination | |
| Endocrine | Hypothyroidism | Generalised soft tissue swelling → multiple entrapments; also associated with true polyneuropathy; check TSH | Myxoedematous infiltration → nerve compression |
High Yield – Lecture Slide Pathological Differentials
The pathological differentials framework from the GC lecture slides systematically covers: Vascular (stroke, spinal cord infarction), Infection, Neoplasm, Degenerative, Inflammatory (Guillain-Barré syndrome, vasculitic neuropathy), Congenital, Autoimmune, Trauma/Toxins, Endocrine (vitamin B12 deficiency, diabetic neuropathy) [6]. For mononeuropathy multiplex, the most relevant categories are vascular/inflammatory (vasculitis), infectious (leprosy, HIV), autoimmune (CTDs), and endocrine (DM).
Approach to Narrowing the Etiological Differential
The clinical approach to narrowing the differential follows a logical sequence:
- Yes: DM is the most common cause. However, do not stop here — diabetics can also develop vasculitis, and other causes must still be considered, especially if:
- The onset is unusually acute and painful
- There are systemic features (fever, weight loss, rash, elevated inflammatory markers)
- The patient has other autoimmune conditions
- The neuropathy progresses rapidly despite glycaemic control
- No: Proceed to search for other causes
| Feature | What It Suggests |
|---|---|
| Constitutional symptoms (fever, weight loss, fatigue) | Systemic vasculitis, malignancy, infection |
| Skin lesions (purpura, nodules, livedo, ulcers, nail-fold infarcts) | Vasculitis (PAN, EGPA, RA vasculitis) |
| Renal involvement (proteinuria, haematuria, rising creatinine) | GPA, MPA, PAN, SLE, cryoglobulinaemia |
| Pulmonary involvement (haemoptysis, nodules, infiltrates) | GPA, MPA, EGPA |
| ENT disease (sinusitis, nasal crusting, epistaxis) | GPA |
| Asthma + eosinophilia | EGPA |
| Joint disease | RA, SLE |
| Sicca symptoms | Sjögren syndrome |
| Raynaud phenomenon | Scleroderma, SLE, MCTD |
| Feature | Infection to Consider |
|---|---|
| Travel to endemic area, hypopigmented patches, nerve thickening | Leprosy |
| Tick bite, erythema migrans | Lyme disease |
| HIV risk factors | HIV neuropathy |
| Known HBV/HCV carrier | PAN (HBV), cryoglobulinaemia (HCV) |
| High-risk sexual behaviour, genital ulcers | Syphilis |
| Feature | Consider |
|---|---|
| Weight loss, lymphadenopathy, splenomegaly | Lymphoma, carcinoma |
| Macroglossia, periorbital purpura, heart failure, renal failure | AL amyloidosis |
| Monoclonal band on serum protein electrophoresis | Myeloma, POEMS, amyloidosis |
| Smoking history, subacute sensory > motor neuropathy | Paraneoplastic (SCLC) |
This is the single most important investigation for narrowing the differential:
| NCS Pattern | Interpretation | Causes |
|---|---|---|
| Axonal (↓amplitude, normal velocity) [1][2] | Nerve fibre death (Wallerian degeneration) | DM, vasculitis, infections, amyloidosis, paraneoplastic |
| Demyelinating (↓velocity, conduction block, normal amplitude) [1][2] | Myelin damage with preserved axons | MMN, MADSAM, HNPP |
Distinction between axonal and demyelinating is based on electrophysiological testing and is important because usually only demyelinating neuropathies are susceptible to treatment [1]. This principle drives the urgency of NCS in the workup.
| Pattern | Suggests |
|---|---|
| Pure motor | MMN (demyelinating, conduction block, anti-GM1); consider MND as differential |
| Sensorimotor | Vasculitis, DM, infections, amyloidosis, MADSAM |
| Sensory-predominant | Paraneoplastic (anti-Hu), Sjögren ganglionopathy |
Must-Not-Miss Differentials in Mononeuropathy Multiplex
- Systemic vasculitis (PAN, GPA, EGPA, MPA): Treatable but fatal if missed. Look for systemic features, check ANCA, ESR/CRP.
- Diabetes mellitus: Most common cause overall. Check HbA1c, fasting glucose. But don't be falsely reassured — DM patients can have vasculitis too.
- Connective tissue disease (RA, SLE): Look for joint/skin/renal features, check ANA, RF, anti-CCP.
- Infection (HIV, HBV, HCV, leprosy): Especially in Hong Kong, check HBV status in older patients; check HIV in at-risk populations.
- Multifocal motor neuropathy (MMN): Treatable with IVIg. Pure motor, conduction block on NCS, anti-GM1 antibodies. Must differentiate from MND.
- Amyloidosis: Infiltrative cause with autonomic features, macroglossia, renal/cardiac involvement. Check serum/urine paraprotein, tissue biopsy with Congo red stain.
- Malignancy/paraneoplastic: Weight loss, smoking history, subacute course. Screen for SCLC, check anti-Hu.
The lecture slides teach a systematic approach using "What is the lesion (pathological differentials)?" [6]:
| Category | Relevant Differentials for Mononeuropathy Multiplex |
|---|---|
| Vascular | Vasculitic neuropathy (PAN, GPA, EGPA, MPA); DM microangiopathy |
| Infection | HIV, leprosy, Lyme disease, HBV (→PAN), HCV (→cryoglobulinaemia), syphilis |
| Neoplasm | Paraneoplastic (SCLC, anti-Hu), neurolymphomatosis |
| Degenerative | HNPP (hereditary) |
| Inflammatory | Vasculitic neuropathy, MMN, MADSAM |
| Congenital | HNPP |
| Autoimmune | SLE, RA, Sjögren, MCTD |
| Trauma/Toxins | Multiple entrapment neuropathies (not true mononeuropathy multiplex, but a mimic) |
| Endocrine | Diabetic neuropathy (mononeuropathy multiplex pattern), hypothyroidism (entrapment) |
Active Recall - Differential Diagnosis of Mononeuropathy Multiplex
References
[1] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 Disease of the Peripheral Nerves, pp. 179–181) [2] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 Peripheral neuropathy, p. 269) [3] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 4.4.3 Diseases of Peripheral Nerves, p. 137) [4] Senior notes: Block A - Deterioration of eyesight in a diabetic patient_ diabetic complications.pdf (Diabetic Mononeuropathy section, p. 7) [5] Senior notes: Ryan Ho Rheumatology.pdf (Section 4.7.3 Polyarteritis Nodosa, p. 159) [6] Lecture slides: Neurology - Two cases of lower limb weakness.pdf (pp. 18, 20) [8] Lecture slides: CFB_Neuro clinical skills demonstration_01.08.22_file to students.pdf (pp. 7–8) [9] Senior notes: Maksim Surgery Notes.pdf (Section 5.2 Compression neuropathy, pp. 243–245)
Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities
1. Diagnostic Criteria
Mononeuropathy multiplex is a clinical pattern rather than a single disease entity — therefore, there are no universally codified "diagnostic criteria" in the way that, say, SLE has the SLICC/ACR criteria. Instead, the diagnosis is established through a combination of:
- Clinical recognition of the pattern: Asymmetric, multifocal involvement of ≥ 2 non-contiguous named peripheral nerve trunks
- Electrodiagnostic confirmation: NCS/EMG confirming multiple mononeuropathies in anatomically unrelated territories
- Etiological identification: Determining the underlying systemic cause
That said, there are practical diagnostic requirements that must be satisfied:
| Requirement | Explanation |
|---|---|
| ≥ 2 named peripheral nerves involved | Single nerve = mononeuropathy, not multiplex |
| Non-contiguous territories | If all nerves belong to one plexus → plexopathy; if all within one root distribution → radiculopathy |
| Asymmetric distribution | Symmetric involvement → think polyneuropathy instead |
| Not length-dependent | Glove-and-stocking = polyneuropathy; mononeuropathy multiplex can affect proximal nerves (e.g., femoral) alongside distal nerves (e.g., common peroneal) |
| LMN pattern in each affected territory | Wasting, weakness, hyporeflexia in the distribution of each affected nerve. If UMN signs → consider myelopathy or CNS lesion |
| Confirmed by NCS/EMG | This is the definitive step — electrodiagnostic testing confirms the distribution of neuropathy, involvement of sensory/motor/autonomic fibres, and pattern (axonal vs demyelinating) [2] |
High Yield – When Is the Pattern 'Confirmed'?
The diagnosis of mononeuropathy multiplex is considered electrodiagnostically confirmed when NCS/EMG demonstrates abnormalities (reduced amplitudes or conduction velocities, or conduction block) localised to ≥ 2 anatomically separate nerve trunks that cannot be explained by a single root, plexus, or CNS lesion. NCS confirms distribution of neuropathy, involvement of sensory/motor/autonomic fibres, and pattern: axonal (normal velocity, ↓amplitude) vs demyelinating (↓velocity, normal amplitude) [2].
Once the pattern is confirmed, the underlying cause has its own diagnostic criteria. The most important include:
| Cause | Key Diagnostic Criteria / Features |
|---|---|
| DM | Known DM or HbA1c ≥ 6.5% (48 mmol/mol); other microvascular complications present; NCS shows axonal pattern |
| PAN | Clinical + compatible underlying cause + biopsy showing necrotizing medium-sized vasculitis [5]; ANCA usually negative |
| GPA | ACR/EULAR 2022 criteria: nasal/oral involvement, pulmonary involvement, c-ANCA/anti-PR3, glomerulonephritis, biopsy showing granulomatous inflammation |
| EGPA | Asthma + eosinophilia ( > 1.5 × 10⁹/L) + ≥ 2 organ involvement (neuropathy, pulmonary infiltrates, sinus abnormalities); p-ANCA/anti-MPO in ~40% |
| SLE | SLICC or EULAR/ACR 2019 classification criteria; ANA + anti-dsDNA; renal/haematological/neurological involvement |
| MMN | EFNS/PNS 2010 criteria: definite motor conduction block on NCS in ≥ 2 nerves outside common entrapment sites; pure motor involvement; anti-GM1 antibodies support but not required |
| Amyloidosis | Tissue biopsy with Congo red stain showing salmon pink colour [10]; typing of amyloid (AL vs AA vs ATTR); serum/urine paraprotein |
The diagnostic workup follows a logical, stepwise approach — moving from less invasive to more invasive investigations, and from clinical pattern recognition to etiological confirmation. This mirrors the general principle taught in the endocrine workup: history and PE → baseline bloods → screening tests → confirmatory tests → imaging → invasive tests [11].
High Yield – GC Lecture Slide
Neurological investigation is performed to confirm the clinical suspicion of "What is the lesion?" with caveat of PAN-investigation: false positive or false negative investigative findings are possible [12]. This means: always interpret investigations in clinical context — a "normal" NCS does not exclude early neuropathy, and incidental findings (e.g., white matter changes on MRI) should not be over-interpreted.
Stepwise Diagnostic Algorithm (Flowchart)
3. Investigation Modalities — Detailed Breakdown
This is not merely a formality — a thorough history and examination will correctly identify the pattern and narrow the differential in the majority of cases.
History points specific to mononeuropathy multiplex:
- Onset and temporal sequence: Which nerve was affected first? How quickly did the next one appear?
- Pain: Was there acute pain preceding each new deficit? (Suggests vasculitic nerve infarction)
- Systemic features: Fever, weight loss, night sweats, rash, joint pain, sicca symptoms, haemoptysis
- DM history: Known diabetic? Glycaemic control?
- Drug history: Any neurotoxic drugs? (Vincristine, isoniazid, etc.)
- Infection risk: Travel, sexual history, tick exposure, IV drug use
- Family history: Recurrent pressure palsies in family → HNPP
Examination points:
- Map each affected nerve: motor (weakness + wasting in specific muscles), sensory (loss in specific nerve territory), reflexes (reduced in affected segments)
- Look for nerve thickening on palpation: ulnar nerve at elbow, common peroneal at fibular head, great auricular nerve in neck — thickening suggests leprosy, amyloidosis, CMT, CIDP, Refsum disease, Dejerine-Sottas [3]
- Systemic examination: skin (purpura, livedo, nodules, ulcers, nail-fold infarcts), joints (synovitis, deformities), eyes (scleritis, retinal vasculitis), lungs (crackles), kidneys (oedema, hypertension)
- Postural BP for all neuropathy to detect autonomic involvement [1]
Baseline bloods: CBC, LRFT, glucose, B12/folate, TFT, ESR/CRP, autoimmune (ANA, RF, ANCA), SPE Ig [2]
This is the initial "screening net" to catch the common causes and assess for systemic involvement.
| Investigation | What It Tells You | Key Findings |
|---|---|---|
| CBC | Anaemia, leukocytosis, eosinophilia, thrombocytosis/penia | NcNc anaemia → chronic disease (vasculitis, malignancy); eosinophilia → EGPA; leukopenia → SLE; thrombocytopenia → SLE, DIC |
| ESR / CRP | Systemic inflammation | Markedly elevated ESR (often > 60–100 mm/hr) in vasculitis; ESR characteristically very high in GCA/PAN [5]; elevated CRP in active vasculitis or infection |
| L/RFT | Hepatic and renal function | ↑Cr → renal involvement (GPA, MPA, PAN, SLE, DM nephropathy, amyloidosis); deranged LFT → HBV/HCV, amyloidosis |
| Fasting glucose / HbA1c | Diabetes screening | HbA1c ≥ 6.5% confirms DM |
| TFT | Hypothyroidism | ↑TSH → hypothyroidism can predispose to entrapment neuropathies; exclude as contributing factor |
| B12 / Folate | Nutritional deficiency | Low B12 → subacute combined degeneration; can coexist but typically causes polyneuropathy rather than mononeuropathy multiplex |
| Urinalysis | Renal involvement | Proteinuria + haematuria → glomerulonephritis (GPA, MPA, SLE); proteinuria alone → amyloidosis, DM nephropathy |
Step 3: Electrodiagnostics — NCS and EMG
This is the cornerstone investigation for mononeuropathy multiplex. It serves three critical purposes:
- Confirms the pattern: Multiple individual nerve trunk abnormalities in non-contiguous territories
- Distinguishes axonal from demyelinating: This dictates the etiological differential and treatment approach
- Quantifies severity: Determines degree of axonal loss vs reversible conduction block
High Yield – Electrodiagnostic Principle
NCS/EMG confirms distribution of neuropathy, involvement of sensory/motor/autonomic fibres, and pattern: axonal (normal velocity, ↓amplitude) vs demyelinating (↓velocity, normal amplitude) [2]. This distinction is important because usually only demyelinating neuropathies are susceptible to treatment [1].
NCS assesses the speed and amplitude of electrical signals along peripheral nerves. Understanding the physics helps you interpret the results:
- Conduction velocity (CV): Measures how fast the signal travels along the nerve. This depends on myelination — myelin enables saltatory conduction. If myelin is damaged (demyelinating), the signal must travel continuously along the axon → CV is reduced.
- Amplitude: Measures the number of functioning axons. If axons have died (axonal degeneration/Wallerian degeneration), fewer axons contribute to the signal → amplitude is reduced.
- Conduction block: The amplitude of the compound muscle action potential (CMAP) is significantly lower when stimulating proximal to a lesion compared to distal stimulation. This means the nerve impulse "gets stuck" at the demyelinated segment. The axon is intact but the signal cannot traverse the damaged myelin.
| Parameter | Axonal Pattern | Demyelinating Pattern |
|---|---|---|
| Motor CV | Normal or mildly reduced | Significantly reduced ( < 70-80% of lower limit of normal) |
| CMAP amplitude | Reduced | Normal or mildly reduced (unless secondary axonal loss) |
| SNAP amplitude | Reduced | Normal or mildly reduced |
| Conduction block | Absent | Present (hallmark of acquired demyelination, e.g., MMN, MADSAM) |
| Temporal dispersion | Absent | Present (demyelination causes variable slowing across fibres → the waveform "spreads out") |
| Distal motor latency | Normal | Prolonged |
| F-wave latency | Normal | Prolonged or absent |
Pattern in mononeuropathy multiplex specifically:
- NCS abnormalities are restricted to individual named nerves rather than diffusely affecting all nerves
- The abnormalities are asymmetric — affected nerves show markedly abnormal parameters while uninvolved nerves are normal
- In vasculitic mononeuropathy multiplex: axonal pattern (reduced amplitudes, preserved velocities) in the distribution of specific nerves
- In MMN: demyelinating pattern with conduction block in motor nerves, with normal sensory studies
EMG records the electrical activity of muscle fibres, which indirectly tells you about the innervating nerve:
| Finding | Interpretation |
|---|---|
| Fibrillation potentials and positive sharp waves at rest | Evidence of active denervation — muscle fibres that have lost their nerve supply fire spontaneously |
| Large, polyphasic motor unit potentials (MUPs) | Evidence of chronic denervation with reinnervation — surviving motor neurons sprout collateral axons to reinnervate orphaned muscle fibres, creating larger motor units |
| Reduced recruitment | Fewer functional motor units → fewer MUPs fire during voluntary contraction |
| Myopathic pattern (small, short-duration, polyphasic MUPs, early recruitment) | Would point to myopathy rather than neuropathy → helps exclude this mimic |
In mononeuropathy multiplex: EMG shows denervation changes limited to muscles innervated by the affected nerves, with normal findings in muscles innervated by unaffected nerves. This patchy, asymmetric pattern on EMG is the electrodiagnostic fingerprint of mononeuropathy multiplex.
Common Exam Pitfall
NCS may show false negative findings in early disease where structural changes are not obvious [12]. If NCS is performed within the first 7–10 days of acute nerve infarction, Wallerian degeneration may not yet be complete, and the amplitude may still be relatively preserved. Repeat NCS in 2–3 weeks if clinical suspicion remains high.
Step 4: Targeted Etiological Workup
Once electrodiagnostics confirm the pattern (and distinguish axonal vs demyelinating), targeted investigations are pursued based on the clinical context.
| Investigation | Rationale | Key Findings |
|---|---|---|
| ANCA panel (c-ANCA/PR3, p-ANCA/MPO) | Screen for ANCA-associated vasculitis | c-ANCA/PR3 → GPA; p-ANCA/MPO → MPA, EGPA |
| ANA, anti-dsDNA, anti-ENA panel | Screen for CTD (SLE, Sjögren, MCTD) | ANA + anti-dsDNA → SLE; ANA + anti-Ro/La → Sjögren |
| RF, anti-CCP | Screen for RA | High-titre RF + anti-CCP in severe RA → risk of rheumatoid vasculitis |
| Complement levels (C3, C4) | Distinguish complement-consuming processes | ↓C3/C4 → immune complex-mediated: SLE, cryoglobulinaemia, HCV; Normal C3/C4 → PAN, GPA, EGPA [13][14] |
| Cryoglobulins | Screen for cryoglobulinaemic vasculitis | Positive → usually HCV-associated; type II or III cryoglobulinaemia |
| HBV serology (HBsAg, anti-HBs, anti-HBc) | HBV-associated PAN | HBsAg positive → consider PAN |
| HCV serology | HCV-associated cryoglobulinaemia | Anti-HCV positive → check cryoglobulins |
| HIV test | HIV neuropathy | HIV positive → can cause vasculitic or direct viral neuropathy |
| Syphilis serology (RPR/VDRL, TPHA) | Meningovascular syphilis | Positive treponemal test → consider tertiary syphilis vasculitis |
| Serum protein electrophoresis (SPE) and serum free light chains (FLC) | Screen for paraprotein / amyloidosis / myeloma | Presence of paraprotein ± immunoparesis → AL amyloidosis, myeloma, POEMS [10][15] |
| Urine protein electrophoresis (UPE) | Detect Bence Jones proteinuria | Monoclonal light chains in urine → myeloma, AL amyloidosis |
| ACE level | Screen for sarcoidosis | Elevated → sarcoidosis (though low sensitivity and specificity) |
| CXR / CT thorax | Pulmonary involvement | Nodules/cavities → GPA; infiltrates → EGPA; hilar lymphadenopathy → sarcoidosis; mass → malignancy |
| Urinalysis (dipstick + microscopy) | Renal involvement | Dysmorphic RBCs, RBC casts → glomerular haematuria [14]; proteinuria → renal disease |
| Anti-neuronal antibodies (anti-Hu/ANNA-1, anti-CV2) | Paraneoplastic neuropathy | Positive → search for underlying malignancy (SCLC, thymoma) |
| CT TAP | Malignancy screen | Indicated if paraneoplastic or lymphoma suspected |
| Echocardiography | Cardiac amyloidosis or vasculitis-related cardiac disease | Restrictive pattern with sparkling myocardium → cardiac amyloidosis |
High Yield – SPE Interpretation
SPE is indicated for investigation of unexplained neuropathy [15]. The four patterns to recognise: 1) Normal immunoglobulin, no paraprotein → normal; 2) Pan-immunoparesis → immunodeficiency or light chain/non-secretory myeloma; 3) Raised immunoglobulin, no paraprotein → polyclonal response (reactive); 4) Presence of paraprotein ± immunoparesis → plasma cell dyscrasia (myeloma, WM, amyloidosis) [15].
| Investigation | Rationale | Key Findings |
|---|---|---|
| Anti-GM1 antibodies | MMN | Positive in ~50% of MMN; supports diagnosis alongside conduction block on NCS |
| Anti-MAG antibodies | Paraprotein-associated demyelinating neuropathy | Positive → associated with IgM paraprotein and distal demyelinating neuropathy (usually symmetric rather than multiplex, but can overlap) |
| SPE and serum FLC | Paraproteinaemic neuropathy | IgM paraprotein → consider Waldenström macroglobulinaemia or POEMS |
| Genetic testing for PMP22 deletion | HNPP | Heterozygous deletion of PMP22 on chromosome 17 → HNPP (AD inheritance) |
| CSF analysis | CIDP / MADSAM variant | Albumino-cytological dissociation (elevated protein with normal cells) supports inflammatory demyelinating neuropathy [3] |
Nerve biopsy is the last resort for inflammatory, infective, or infiltrative disorders [2].
When to biopsy:
- Cause remains unclear despite thorough non-invasive workup
- Vasculitis is suspected but serology is negative (ANCA-negative vasculitis, non-systemic vasculitic neuropathy)
- Amyloidosis or sarcoidosis is suspected
- Lymphomatous infiltration is considered
Site:
- Sural nerve is the most commonly biopsied nerve (sensory nerve at the ankle — accessible, and the resulting sensory loss is limited to a small area of the lateral foot)
- Alternatively, a superficial peroneal nerve can be biopsied (combined nerve and muscle biopsy of peroneus brevis is popular as it allows simultaneous assessment of nerve and adjacent muscle for vasculitis)
Why sural nerve? It is a purely sensory nerve, superficially located, and easily accessible at the ankle. Biopsying a motor nerve would cause permanent motor deficit, which is unacceptable. The sural nerve biopsy provides enough tissue for light microscopy, immunohistochemistry, electron microscopy, and teased fibre analysis.
Histopathological findings and their significance:
| Finding | Diagnosis Suggested |
|---|---|
| Necrotising vasculitis of epineurial/perineurial arterioles with fibrinoid necrosis, transmural inflammatory infiltrate | Vasculitic neuropathy (PAN, GPA, EGPA, non-systemic vasculitic neuropathy) |
| Axonal degeneration in a "patchy" distribution (some fascicles severely affected, adjacent fascicles normal) | Vasculitic neuropathy (ischaemic infarction pattern) |
| Congo red stain → salmon pink colour under polarised light (apple-green birefringence) | Amyloidosis [10] |
| Non-branching fibrils on electron microscopy in extracellular compartment | Amyloidosis [10] |
| Non-caseating granulomas | Sarcoidosis |
| Acid-fast bacilli within Schwann cells; granulomatous inflammation | Leprosy |
| Lymphomatous infiltration of nerve fascicles | Neurolymphomatosis |
| Segmental demyelination and remyelination (onion bulbs) | Chronic demyelinating neuropathy (CIDP, MADSAM) |
| Investigation | When to Use | Key Information |
|---|---|---|
| MRI neurography | Suspected neurolymphomatosis; localisation of nerve lesions for biopsy guidance | Shows nerve enlargement and enhancement; can identify skip lesions |
| PET-CT | Suspected malignancy, vasculitis of large vessels | FDG-avid nerve trunks in neurolymphomatosis; aortic uptake in large vessel vasculitis |
| Temporal artery biopsy or USG | If GCA coexists with mononeuropathy multiplex (rare but described) | Panarteritis with mixed infiltrates, fragmentation of intima, necrosis of media ± giant cells [5] |
| Skin biopsy | Vasculitic skin lesions (purpura, nodules, ulcers) | Can confirm vasculitis histologically without need for nerve biopsy |
| Fat pad aspirate | Suspected amyloidosis (less invasive than nerve biopsy) | Congo red positive → amyloid deposition; type by mass spectrometry |
| Bone marrow biopsy | Suspected plasma cell dyscrasia (myeloma, WM, amyloidosis) | Clonal plasma cells; immunophenotyping |
| Lumbar puncture / CSF | Suspected CIDP, MADSAM, GBS, sarcoidosis, CNS lymphoma | Elevated protein with normal cells → inflammatory demyelinating; lymphocytic pleocytosis → infection, sarcoidosis, lymphoma |
The following table summarises how to integrate investigation results:
| Clinical Scenario | NCS Pattern | Key Blood Tests | Nerve Biopsy | Diagnosis |
|---|---|---|---|---|
| Diabetic with CN III + foot drop | Axonal, multifocal | HbA1c ≥ 6.5%, normal ANCA/ANA | Not usually needed | Diabetic mononeuropathy multiplex |
| Weight loss, purpura, renal impairment, ANCA-negative | Axonal, multifocal | ↑ESR/CRP, ±HBsAg+, ANCA negative | Necrotising vasculitis of medium arteries | PAN |
| Sinusitis, haemoptysis, foot drop | Axonal, multifocal | c-ANCA/PR3+, ↑Cr, active urine sediment | Granulomatous vasculitis | GPA |
| Asthma, eosinophilia, wrist drop | Axonal, multifocal | Eosinophils > 1.5, p-ANCA/MPO+ | Eosinophilic vasculitis | EGPA |
| Young female, malar rash, arthritis, foot drop | Axonal, multifocal | ANA+, anti-dsDNA+, ↓C3/C4 | Vasculitis or immune complex deposition | SLE |
| Pure motor, asymmetric UL weakness, no sensory | Demyelinating, conduction block | Anti-GM1+ in ~50% | Not usually needed | MMN |
| Macroglossia, carpal tunnel, foot drop, proteinuria | Axonal, multifocal | Paraprotein on SPE, abnormal FLC | Congo red + amyloid deposits | AL Amyloidosis |
| Recurrent pressure palsies, family history | Demyelinating at entrapment sites | Normal | Tomaculous neuropathy (sausage-like myelin swellings) | HNPP |
| Hypopigmented patches, nerve thickening, foot drop | Mixed axonal/demyelinating | Skin smear for AFB | Granulomatous neuritis with AFB | Leprosy |
High Yield Summary — Investigations
The investigation approach to mononeuropathy multiplex follows these steps:
- History and examination — map affected nerves, look for systemic clues, check postural BP
- Baseline bloods — CBC, LRFT, glucose, B12/folate, TFT, ESR/CRP, autoimmune markers (ANA, RF, ANCA), SPE Ig [2]
- NCS/EMG — the definitive investigation: confirms distribution (multiple non-contiguous mononeuropathies), fibre type involvement, and pattern (axonal vs demyelinating). Axonal = ↓amplitude, normal velocity; Demyelinating = ↓velocity, normal amplitude [2]
- Targeted etiological workup — guided by NCS pattern and clinical context (ANCA, complement, cryoglobulins, HBV/HCV/HIV, SPE/FLC, anti-GM1, etc.)
- Nerve biopsy — last resort for inflammatory/infective/infiltrative disorders [2]; sural nerve most common; look for necrotising vasculitis, amyloid deposition, granulomas, or lymphomatous infiltration
Key principle: Usually only demyelinating neuropathies are susceptible to treatment [1] — this makes the axonal vs demyelinating distinction on NCS the single most impactful investigation finding.
False negatives can occur in early disease where structural changes have not yet developed [12] — repeat NCS in 2–3 weeks if clinical suspicion remains high.
Active Recall - Diagnosis and Investigations of Mononeuropathy Multiplex
References
[1] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 Disease of the Peripheral Nerves, pp. 179–181) [2] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 Peripheral neuropathy, p. 269) [3] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 4.4.3 Diseases of Peripheral Nerves, pp. 134–138) [5] Senior notes: Ryan Ho Rheumatology.pdf (Section 4.7.3 Polyarteritis Nodosa, p. 159) [10] Senior notes: Block A - Hematology Data Interpretation.pdf (Amyloidosis section, p. 1) [11] Senior notes: Block A - Introduction to Endocrine investigations.pdf (Principle 1: Sequence of investigations, p. 3) [12] Lecture slides: GCBA_Fundamentals_Neuro_Introduction to Neurological Investigations and Emergencies_Prof KC Teo.pdf (p. 9) [13] Senior notes: Ryan Ho Fundamentals.pdf (Evaluation of nephritic syndrome, p. 360) [14] Senior notes: Ryan Ho Urogenital.pdf (Evaluation of nephritic syndrome, p. 63) [15] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Serum protein electrophoresis, p. 27)
Management of Mononeuropathy Multiplex
The management of mononeuropathy multiplex is fundamentally different from managing a simple mononeuropathy (where you might simply decompress an entrapped nerve). Here, the neuropathy is a manifestation of a systemic disease, so management must address three simultaneous goals:
- Treat the underlying cause (disease-modifying therapy) — this is the most important step
- Prevent further nerve damage — early, aggressive treatment to halt ongoing nerve infarction/demyelination before irreversible axonal loss occurs
- Manage symptoms — neuropathic pain, weakness, rehabilitation
The key principle from the teaching notes: Axonal neuropathies are largely not directly treatable — management is by reducing exposure to toxins and treatment of the underlying disease process. Demyelinating neuropathies usually respond to immunosuppressive treatment (e.g., IVIg, pulse steroids, plasmapheresis) [1].
This principle is worth internalising from first principles:
- Axonal damage = the nerve fibre itself has died (Wallerian degeneration). Dead axons cannot be resurrected — they can only slowly regenerate at ~1 mm/day from the site of injury, and recovery is incomplete. Therefore, the priority is to stop the disease process that is killing the axons (e.g., treat the vasculitis, control the diabetes).
- Demyelinating damage = the myelin sheath is damaged but the axon is intact. Remyelination can occur if the inflammatory process is halted. Therefore, these conditions respond to immunomodulation (IVIg, steroids, plasmapheresis), and recovery is often better.
Treatment Modalities by Underlying Cause
Pathophysiology recap: Microangiopathy of vasa nervorum → ischaemic nerve infarction → axonal loss. The damage is typically self-limiting (the acute ischaemic event resolves), but recurrence is common if glycaemic control remains poor.
Management:
| Intervention | Details | Rationale |
|---|---|---|
| Glycaemic control | Aim HbA1c < 7–8% [16]; intensify oral hypoglycaemics or insulin as needed; prefer SGLT2i and GLP-1 agonists [16] | Hyperglycaemia drives microangiopathy → reducing glucose slows vasa nervorum damage and reduces risk of recurrence |
| Supportive management | Generally supportive but may require MRI to rule out stroke [16] | Acute mononeuropathies (e.g., CN III palsy) in DM are self-limiting; must exclude structural causes (e.g., posterior communicating artery aneurysm for CN III, stroke for lateralising signs) |
| Neuropathic pain management | Gabapentin / pregabalin / amitriptyline for neuropathic pain [1][16] | See below for detailed neuropathic pain pharmacology |
| CVD risk factor control | Stop smoking, BP control (esp by ACEI/ARB), treat hyperlipidaemia (statins or fibrates) [16] | DM patients have accelerated atherosclerosis; vascular risk factor control reduces overall micro- and macrovascular complications |
| Rehabilitation | Physiotherapy, ankle-foot orthosis for foot drop, occupational therapy for hand weakness | Preserves function during recovery; prevents contractures |
| Prognosis | 60% with good functional recovery in 12–24 months but mild residual weakness may remain [16]; usually transient [16] | Axonal regeneration occurs at ~1 mm/day; cranial nerve palsies typically recover in 3–6 months |
High Yield – Diabetic Mononeuropathy Management
Management directed to controlling underlying hyperglycaemia [16]. Most diabetic mononeuropathies are self-limiting. The main management decision is ruling out other causes (stroke, aneurysm) and optimising glycaemic control to prevent recurrence.
B. Vasculitic Mononeuropathy Multiplex (Most Important — Treatable Emergency)
This is the most critical cause to manage aggressively because:
- Untreated vasculitis causes progressive, irreversible nerve infarction
- Systemic vasculitis can be fatal (renal failure, mesenteric infarction, cardiac ischaemia)
- Treatment with immunosuppression is highly effective in halting disease progression
General Principles of Vasculitis Management:
- Immunosuppression is the cornerstone [5]
- Treatment follows a two-phase model: induction (aggressive therapy to achieve remission) followed by maintenance (less toxic therapy to prevent relapse)
Management of PAN [5]:
| Severity | Induction | Maintenance | Notes |
|---|---|---|---|
| Mild disease | Oral steroid (prednisolone 1 mg/kg/day) ± azathioprine / methotrexate [5] | Taper steroids over months; continue steroid-sparing agent | Mild = no organ-threatening features |
| Moderate/severe disease | Oral steroid + cyclophosphamide [5] | Azathioprine or methotrexate after remission achieved | Severe = renal, GI, cardiac, or progressive neurological involvement |
| Severe/refractory | Course of IV pulse steroid [5] (e.g., methylprednisolone 500–1000 mg IV daily × 3 days) | Escalate to rituximab if refractory to cyclophosphamide | |
| HBV-associated PAN | Antivirals (entecavir or tenofovir) + short course of steroids ± plasma exchange [5] | Continue antivirals; avoid prolonged immunosuppression | Prolonged immunosuppression can worsen HBV viraemia; antivirals are the primary treatment |
| Hypertension | ACEI/ARB if HTN [5] | Renal artery involvement can cause renovascular hypertension |
Prognosis of PAN: Poor (13% 5-year survival) if untreated; fair (80% 5-year survival) if treated [5]. Major source of mortality: renal failure, mesenteric/cardiac/cerebral infarction [5]. Relapse not uncommon (9.2% at 1 year, 24% at 5 years) [5].
The treatment approach for ANCA-associated vasculitis follows current (2024–2026) guidelines (EULAR/ACR recommendations):
| Phase | Severe/Organ-Threatening | Non-Severe |
|---|---|---|
| Induction | Glucocorticoids (IV pulse then oral taper) + rituximab (preferred over cyclophosphamide in 2024+ guidelines) OR cyclophosphamide (IV pulse or oral) | Glucocorticoids ± methotrexate or mycophenolate |
| Maintenance | Rituximab (500 mg IV every 6 months) or azathioprine (2 mg/kg/day) for ≥ 24 months | Azathioprine or methotrexate |
| Adjunct | Trimethoprim-sulfamethoxazole (for GPA — reduces nasal relapse and Pneumocystis prophylaxis); ACEI/ARB for renal protection |
Why rituximab? Rituximab is a monoclonal antibody against CD20 on B cells ("ritux" → "rit" = rituximab targets). B cells are the precursors of plasma cells that produce ANCA antibodies. By depleting B cells, you stop the production of the pathogenic ANCA autoantibodies. Multiple RCTs (RAVE, RITUXVAS) have shown rituximab is non-inferior or superior to cyclophosphamide for induction, with fewer side effects (less infertility, less bladder toxicity).
Why cyclophosphamide for severe disease? Cyclophosphamide ("cyclo" = cycle, "phosph" = phosphorus-containing, "amide" = amide group) is an alkylating agent that crosslinks DNA → kills rapidly dividing cells including lymphocytes. It is highly effective but toxic: haemorrhagic cystitis (due to acrolein metabolite → prevented by mesna), infertility, bone marrow suppression, increased malignancy risk (especially bladder cancer).
Special considerations for EGPA:
- Mepolizumab (anti-IL-5 monoclonal antibody) — approved for relapsing/refractory EGPA. IL-5 is the key cytokine for eosinophil survival and maturation. Blocking IL-5 reduces eosinophil-driven tissue damage.
- Conventional immunosuppression remains first-line for severe organ involvement.
| Cause | Treatment Approach |
|---|---|
| RA vasculitis | High-dose glucocorticoids + cyclophosphamide or rituximab; optimise RA disease-modifying therapy (methotrexate, biologics) |
| SLE vasculitis | Glucocorticoids + cyclophosphamide (or mycophenolate mofetil); hydroxychloroquine as baseline therapy; treat lupus nephritis per ISN/RPS class if present |
| Cryoglobulinaemic vasculitis | Treat underlying HCV (direct-acting antivirals — sofosbuvir/velpatasvir or glecaprevir/pibrentasvir); rituximab for severe vasculitis; plasma exchange for severe hyperviscosity |
This is vasculitis confined to the peripheral nerves without systemic involvement. It is diagnosed by nerve biopsy showing vasculitis in the absence of systemic disease.
- Mild: Glucocorticoids alone (prednisolone 1 mg/kg/day, taper over months)
- Severe or progressive: Glucocorticoids + cyclophosphamide or azathioprine
- Prognosis is generally better than systemic vasculitis
| Infection | Treatment | Key Points |
|---|---|---|
| Leprosy | WHO multi-drug therapy: dapsone + rifampicin ± clofazimine (duration depends on paucibacillary vs multibacillary: 6 vs 12 months) | Nerve damage may progress despite treatment due to immune-mediated reactions (type 1 reversal reaction, type 2 ENL) → treat with prednisolone |
| HIV | Antiretroviral therapy (ART); treat any opportunistic infections; for vasculitic neuropathy → short course glucocorticoids ± IVIg | Immune reconstitution inflammatory syndrome (IRIS) can paradoxically worsen neuropathy after ART initiation |
| Lyme disease | IV ceftriaxone (2 g daily × 14–28 days) for neurological Lyme; oral doxycycline for early disease | |
| Syphilis | IV benzylpenicillin (18–24 MU/day × 10–14 days) for neurosyphilis | |
| HBV-associated PAN | Antivirals (entecavir/tenofovir) + short course steroids ± plasma exchange | See PAN section above |
| HCV-associated cryoglobulinaemia | Direct-acting antivirals (DAAs) ± rituximab for severe vasculitis | Achieving sustained virological response (SVR) often resolves the cryoglobulinaemic vasculitis |
D. Demyelinating Causes
This is a critical management scenario because MMN is treatable but only with the right drug:
| Treatment | Details | Notes |
|---|---|---|
| IVIg (first-line and only proven treatment) | 2 g/kg over 2–5 days (induction); then maintenance 1 g/kg every 2–4 weeks or 2 g/kg every 4–8 weeks, titrated to response | ~80% of patients respond. IVIg modulates the immune system — it provides anti-idiotypic antibodies that neutralise pathogenic anti-GM1, saturates Fc receptors on macrophages reducing antibody-mediated damage, and inhibits complement activation |
| Subcutaneous Ig (SCIg) | Alternative to IVIg for maintenance; equivalent efficacy with fewer systemic side effects | More convenient for patients — can be self-administered at home |
Critical Management Point
MMN does NOT respond to steroids or plasma exchange — this is unique among immune-mediated neuropathies and a very high-yield exam point. In fact, steroids can worsen MMN. The mechanism is unclear but may relate to the fact that steroids enhance antibody-mediated complement activation rather than suppressing it in this specific condition. If you give steroids to a patient with MMN thinking it is CIDP, the patient will get worse — this is why the distinction matters.
Cyclophosphamide has been used in refractory cases but is reserved as a last resort due to toxicity.
| Treatment | Details |
|---|---|
| IVIg | First-line (same dosing as CIDP: 2 g/kg induction, then maintenance) |
| Glucocorticoids | Can be effective (unlike MMN); oral prednisolone 1 mg/kg/day then taper |
| Plasma exchange | Alternative for acute exacerbations |
| Steroid-sparing agents | Azathioprine, mycophenolate, or rituximab for maintenance |
| Management | Details |
|---|---|
| Avoidance of nerve compression | Educate patient to avoid prolonged pressure on nerves (don't cross legs, avoid leaning on elbows, use wrist rests) |
| No specific pharmacological treatment | The underlying genetic defect (PMP22 deletion) cannot be corrected |
| Physiotherapy and rehabilitation | During acute episodes; most episodes recover spontaneously |
| Genetic counselling | Autosomal dominant — 50% chance of transmission to offspring |
| Cause | Treatment |
|---|---|
| AL Amyloidosis | Chemotherapy targeting the clonal plasma cell population: bortezomib + dexamethasone ± cyclophosphamide (VCd); consider autologous stem cell transplant (ASCT) in eligible patients; daratumumab-based regimens for relapsed disease |
| Sarcoidosis | Glucocorticoids (prednisolone 0.5–1 mg/kg/day); methotrexate or mycophenolate for steroid-sparing; infliximab for refractory neurosarcoidosis |
| Neurolymphomatosis | Systemic chemotherapy ± intrathecal chemotherapy ± radiotherapy directed at the underlying lymphoma |
Treatment is directed at the underlying malignancy. Unfortunately, paraneoplastic neuropathies often respond poorly to immunosuppression alone because the immune response is directed at antigens shared between the tumour and nerve tissue. Removing the tumour source may halt progression but established neurological deficits often do not recover.
Symptomatic Management (Applies to All Causes)
Regardless of the underlying etiology, symptomatic management runs in parallel with disease-modifying therapy:
Gabapentin / pregabalin / amitriptyline for neuropathic pain [1][16]
| Drug | Class | Mechanism | Dose | Key Side Effects | Contraindications |
|---|---|---|---|---|---|
| Gabapentin | α2δ ligand (anti-epileptic) | Binds α2δ subunit of voltage-gated Ca²⁺ channels → ↓ release of excitatory neurotransmitters (glutamate, substance P, norepinephrine) from presynaptic terminals in the dorsal horn → reduces central sensitisation | Start 300 mg/day, titrate to 900–3600 mg/day in divided doses | Sedation, dizziness, peripheral oedema, weight gain | Caution in renal impairment (renally excreted — dose adjustment required) |
| Pregabalin | α2δ ligand (anti-epileptic) | Same as gabapentin but more potent, more predictable pharmacokinetics (linear absorption) | Start 75 mg BD, titrate to 150–300 mg BD | Same as gabapentin; may also cause blurred vision | Same as gabapentin |
| Amitriptyline | Tricyclic antidepressant (TCA) | Inhibits reuptake of serotonin and norepinephrine → ↑descending inhibitory pain pathways from brainstem (periaqueductal grey → raphe nuclei → dorsal horn); also blocks Na⁺ channels | Start 10–25 mg nocte, titrate to 50–75 mg nocte | Anticholinergic: dry mouth, constipation, urinary retention, blurred vision; cardiac: prolonged QT, arrhythmias; sedation | Avoid in elderly (falls risk, anticholinergic burden); avoid with recent MI or heart block; avoid with MAOIs |
| Duloxetine | SNRI | Inhibits serotonin and norepinephrine reuptake → ↑descending inhibitory pathways | Start 30 mg/day, target 60 mg/day | Nausea, dizziness, insomnia | Avoid with MAOIs; caution in hepatic impairment |
| Carbamazepine | Na⁺ channel blocker (anti-epileptic) | Stabilises inactivated state of voltage-gated Na⁺ channels → ↓ abnormal ectopic discharges from damaged nerves | 200–1200 mg/day | Hyponatraemia (SIADH), agranulocytosis, hepatotoxicity, Stevens-Johnson syndrome; HLA-B*1502 screening required in Han Chinese (high prevalence in HK) | Contraindicated with HLA-B*1502 (risk of SJS/TEN); hepatic porphyria; AV block |
| Capsaicin cream | Topical TRPV1 agonist | Depletes substance P from sensory nerve endings → desensitisation of nociceptors | Apply 3–4 times daily | Burning sensation at application site | Broken skin |
Hong Kong-Specific Point
Before prescribing carbamazepine to any patient in Hong Kong, HLA-B1502 genotyping* is mandatory. The prevalence of this allele is ~8% in Han Chinese, and carriers have a dramatically elevated risk of Stevens-Johnson syndrome and toxic epidermal necrolysis with carbamazepine/oxcarbazepine. This is a pharmacogenomics principle directly relevant to HKU teaching.
| Intervention | Indication | Details |
|---|---|---|
| Physiotherapy | All patients with motor deficits | Strengthening exercises, gait training, balance training, prevention of contractures |
| Ankle-foot orthosis (AFO) | Foot drop (common peroneal palsy) | Prevents foot from catching on the ground during swing phase of gait; reduces falls |
| Wrist splint | Wrist drop (radial nerve palsy) | Keeps wrist in extension to allow functional use of hand |
| Occupational therapy | Hand weakness (ulnar/median nerve palsy) | Adaptive devices, grip aids, vocational rehabilitation |
| Symptom | Management |
|---|---|
| Postural hypotension | Fludrocortisone (0.1–0.3 mg/day), midodrine (2.5–10 mg TDS); compression stockings; slow postural changes; adequate salt and fluid intake |
| Gastroparesis | Prokinetics (metoclopramide, domperidone); small frequent meals; erythromycin (motilin receptor agonist) |
| Erectile dysfunction | PDE5 inhibitors (sildenafil); vacuum devices; intracavernosal injection |
| Bladder dysfunction | Intermittent self-catheterisation; anticholinergics (oxybutynin) for overactive bladder |
Special Treatment Considerations
In practice, if a patient presents with rapidly progressive mononeuropathy multiplex and systemic features strongly suggesting vasculitis, empiric immunosuppression with high-dose glucocorticoids should be initiated BEFORE biopsy results are available. The rationale:
- Each day of untreated vasculitis = more nerve infarction = irreversible axonal loss
- Delaying treatment by 2–3 weeks for biopsy processing can result in devastating permanent deficits
- Glucocorticoids will reduce inflammation and may partially preserve nerve function
However, ideally obtain the biopsy sample BEFORE starting steroids (even if you start steroids immediately after the biopsy), because steroids can suppress the inflammatory infiltrate and make the biopsy non-diagnostic.
| Drug | Key Contraindications / Precautions |
|---|---|
| Cyclophosphamide | Pregnancy (teratogenic); active infection; severe bone marrow suppression. Monitor: CBC weekly, urinalysis for haematuria (haemorrhagic cystitis). Always give with mesna (binds acrolein in bladder) and adequate hydration |
| Rituximab | Active hepatitis B (can cause fatal HBV reactivation — check HBsAg, anti-HBs, anti-HBc before use; give prophylactic entecavir if anti-HBc+); live vaccines contraindicated; progressive multifocal leukoencephalopathy (PML — rare but serious) |
| Methotrexate | Pregnancy (teratogenic — "folic acid antagonist"); significant renal impairment (renally excreted → accumulates); hepatic disease; concurrent trimethoprim (both inhibit dihydrofolate reductase → pancytopenia). Always co-prescribe folic acid 5 mg weekly (not on the same day as MTX) |
| Azathioprine | Check TPMT/NUDT15 genotype before starting — deficiency causes life-threatening myelosuppression. Do not combine with allopurinol (inhibits xanthine oxidase which metabolises azathioprine → accumulation → severe bone marrow toxicity) |
| Mycophenolate | Pregnancy (teratogenic); active GI disease (diarrhoea common); concurrent antacids reduce absorption |
| IVIg | IgA deficiency with anti-IgA antibodies (anaphylaxis risk — rare); caution in renal impairment (sucrose-containing preparations can cause osmotic nephrotoxicity); thrombotic risk (stroke, PE) |
| Prednisolone (long-term) | DM (worsens glycaemic control); osteoporosis (always co-prescribe calcium/vitamin D ± bisphosphonate); peptic ulcer (co-prescribe PPI); infections (reactivation of TB — check CXR and IGRA/Mantoux); hypertension; cataracts; adrenal suppression on withdrawal |
| Parameter | Frequency | Purpose |
|---|---|---|
| Serial neurological examination | Every clinic visit (monthly initially, then 3–6 monthly) | Monitor for new nerve involvement, recovery of existing deficits |
| NCS/EMG | Baseline, then every 6–12 months or if clinical change | Objective measurement of nerve function; detects subclinical progression or improvement |
| Inflammatory markers (ESR, CRP) | Monthly during induction, then 3-monthly | Monitor disease activity in vasculitis |
| ANCA titre | 3–6 monthly (in ANCA vasculitis) | Rising titre may predict relapse (though treat the patient, not the number) |
| CBC, LRFT | Every 2–4 weeks during immunosuppression | Monitor for drug toxicity (bone marrow suppression, hepatotoxicity, nephrotoxicity) |
| Urinalysis | Monthly during cyclophosphamide; regularly in vasculitis | Detect haemorrhagic cystitis (cyclophosphamide) or renal relapse (vasculitis) |
| HbA1c | 3-monthly in DM | Monitor glycaemic control |
| Bone density | Baseline then annually if on long-term steroids | Screen for steroid-induced osteoporosis |
| Cause | Prognosis |
|---|---|
| DM | Usually transient; 60% good functional recovery in 12–24 months [16]; recurrence common if glycaemic control poor |
| PAN | Poor (13% 5-year survival) if untreated; fair (80% 5-year survival) if treated [5] |
| GPA/MPA | Good with treatment (5-year survival > 80%); relapse common (50% at 5 years for GPA) |
| EGPA | Generally better prognosis than GPA/MPA; Five Factor Score (FFS) predicts outcomes |
| SLE | Depends on overall disease activity and renal involvement |
| MMN | Good with IVIg; chronic condition requiring long-term maintenance; slow progression if undertreated |
| Amyloidosis | Poor without treatment; improving with modern chemotherapy; median survival ~4 years with treatment for AL amyloidosis |
| Leprosy | Nerve damage often irreversible despite treatment; early MDT prevents progression |
| NSVN | Generally good; most respond to steroids alone |
High Yield Summary — Management
Principle: Axonal neuropathies (majority of mononeuropathy multiplex) are not directly treatable — manage by treating the underlying disease. Demyelinating neuropathies usually respond to immunosuppressive treatment [1].
Diabetic: Glycaemic control (HbA1c < 7–8%), supportive management, neuropathic pain relief (gabapentin/pregabalin/amitriptyline). Usually transient with 60% good recovery in 12–24 months [16].
Vasculitic (most important treatable cause):
- PAN: Steroids ± azathioprine/MTX (mild); Steroids + cyclophosphamide (moderate/severe); IV pulse steroids (severe/refractory). HBV-PAN: antivirals first [5].
- ANCA vasculitis: Steroids + rituximab (preferred 2024+) or cyclophosphamide for induction; rituximab or azathioprine for maintenance.
- Prognosis of PAN: 13% 5-year survival untreated vs 80% treated [5].
MMN: IVIg is the ONLY effective treatment. Does NOT respond to steroids or plasma exchange — this is unique and high-yield.
Symptomatic: Gabapentin / pregabalin / amitriptyline for neuropathic pain [1][16]; physiotherapy; AFO for foot drop; occupational therapy.
Key drug safety in HK: HLA-B*1502 screening before carbamazepine; TPMT/NUDT15 genotyping before azathioprine; HBV screening before rituximab.
Active Recall - Management of Mononeuropathy Multiplex
References
[1] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 Disease of the Peripheral Nerves, pp. 179–182) [2] Senior notes: Maksim Medicine Notes.pdf (Section 11.12 Peripheral neuropathy, p. 269) [5] Senior notes: Ryan Ho Rheumatology.pdf (Section 4.7.3 Polyarteritis Nodosa, p. 159) [16] Senior notes: Ryan Ho Endocrine.pdf (Section C: Diabetic Neuropathy, p. 97)
Complications of Mononeuropathy Multiplex
Complications of mononeuropathy multiplex arise from three sources: (1) the neuropathy itself (direct consequences of nerve damage), (2) the underlying systemic disease (e.g., vasculitis causing organ failure), and (3) the treatment (iatrogenic complications of immunosuppression). A comprehensive understanding of all three is essential because exam questions frequently test complications across these domains.
1. Complications of the Neuropathy Itself
These are the direct consequences of peripheral nerve damage — the downstream effects of losing motor, sensory, and autonomic function in the territories of the affected nerves.
| Complication | Mechanism | Clinical Significance |
|---|---|---|
| Persistent weakness and disability | Axonal loss from nerve infarction is largely irreversible. Regeneration occurs at ~1 mm/day but is often incomplete, especially if the distance to the target muscle is long or if the endoneurial architecture is disrupted | 60% achieve good functional recovery in 12–24 months but mild residual weakness may remain [16]; severe axonal loss → permanent deficit |
| Muscle wasting (atrophy) | Denervated muscle fibres lose trophic signals from their motor neurons → progressive fibre atrophy → eventual replacement by fibrosis and fat. Early wasting is characteristic of axonal but not demyelinating neuropathies [1] | Wasted muscles cannot recover even if reinnervation occurs late, because the muscle fibres have been replaced by fibrous tissue |
| Contractures | Unopposed action of antagonist muscles when agonists are paralysed (e.g., foot drop → Achilles tendon shortens due to unopposed plantar flexion) combined with prolonged immobility | Contractures are preventable with early physiotherapy and splinting; once established, they require surgical release |
| Falls and trauma | Foot drop (common peroneal palsy) → the foot catches on the ground during the swing phase of gait → tripping and falling. Wrist drop → inability to grip objects → dropping things | Falls → fractures (especially in elderly or osteoporotic patients); head injuries; loss of independence |
| Gait abnormalities | Steppage gait: patient lifts the knee excessively high to clear the dropped foot from the ground. If bilateral, the gait appears clumsy and high-stepping | Impaired mobility → reduced exercise capacity → deconditioning → further disability (vicious cycle) |
| Complication | Mechanism | Clinical Significance |
|---|---|---|
| Chronic neuropathic pain | Damaged sensory fibres develop abnormal spontaneous activity (ectopic discharges from sites of axonal injury); central sensitisation occurs in the dorsal horn (wind-up phenomenon) → chronic pain that persists even after the original cause is treated | The most debilitating complication for many patients; can be refractory to standard analgesics; requires specific neuropathic pain agents (gabapentin, pregabalin, amitriptyline, duloxetine) |
| Neuropathic ulcers (trophic ulcers) | Loss of protective pain sensation → patient cannot feel pressure, friction, or thermal injury → repeated unnoticed microtrauma → tissue breakdown → ulceration. Particularly affects weight-bearing surfaces (soles of feet) and pressure points | Trophic changes: disuse atrophy, hair loss, brittle nails, trophic ulcers, Charcot joints [1]; ulcers are prone to secondary infection → osteomyelitis → amputation |
| Charcot joint (neuropathic arthropathy) | Loss of proprioception + pain sensation → joint is subjected to repeated undetected trauma → progressive destruction of bone and cartilage → joint deformity and instability | Charcot joints are listed as a trophic complication of peripheral neuropathy [1]; most commonly affects the foot and ankle in diabetic patients; painless swelling with bony deformity |
| Burns and injuries | Loss of pain and temperature sensation → patient cannot detect hot surfaces, sharp objects, or chemical irritants | Particularly problematic in diabetic patients with mononeuropathy multiplex superimposed on distal symmetric polyneuropathy (double insult) |
Why Do Neuropathic Ulcers Develop?
Think of it from first principles: the purpose of pain is protective — it tells you to move away from something harmful. When you lose pain sensation in your foot, you don't shift your weight off pressure points, you don't notice the pebble in your shoe, and you don't feel the hot pavement in summer. Each of these undetected microtraumas causes tissue damage that accumulates. Add impaired blood supply (from DM or vasculitis) and impaired autonomic function (reduced sweating → dry, cracked skin), and you have the perfect storm for ulceration. This is why foot inspection is a critical part of annual microvascular screening in diabetes [17].
If autonomic fibres within the affected nerve trunks are damaged (particularly common in DM-related and amyloidosis-related mononeuropathy multiplex):
| Complication | Mechanism | Clinical Significance |
|---|---|---|
| Postural hypotension | Loss of sympathetic vasoconstrictor innervation to splanchnic and peripheral vessels → failure to increase systemic vascular resistance on standing → blood pools in lower limbs → ↓venous return → ↓cardiac output → ↓BP → cerebral hypoperfusion | Autonomic disturbances: postural hypotension, gastroparesis, constipation, nocturnal diarrhoea, impotence, bladder dysfunction [4]; postural hypotension → syncope → falls → fractures |
| Gastroparesis | Vagal neuropathy → impaired gastric motility → delayed gastric emptying | Nausea, vomiting, bloating, early satiety; erratic absorption of oral medications (including oral hypoglycaemics and insulin) → unpredictable glycaemic control |
| Neurogenic bladder | Autonomic denervation of detrusor muscle and/or urethral sphincter → urinary retention with overflow incontinence or detrusor overactivity | Recurrent UTIs due to incomplete bladder emptying; overflow incontinence; hydronephrosis → renal impairment |
| Erectile dysfunction | Autonomic neuropathy of pelvic parasympathetic nerves → impaired penile erection | Impotence in men [4]; significant impact on quality of life |
| Cardiac autonomic neuropathy | Loss of vagal tone → resting tachycardia; impaired heart rate variability; loss of pain perception during myocardial ischaemia → "silent MI" | ↑ cardiovascular mortality; silent MI → delayed presentation → worse outcomes |
2. Complications from the Underlying Systemic Disease
Since mononeuropathy multiplex is nearly always a manifestation of a systemic disease, the complications of that disease are inextricable from the overall picture. The neuropathy may be the presenting feature that leads to diagnosis of a life-threatening systemic condition.
Major source of mortality in PAN: renal failure, mesenteric/cardiac/cerebral infarction [5].
| Organ System | Complication | Mechanism | Notes |
|---|---|---|---|
| Renal | Renal infarction, renal failure, renovascular hypertension (PAN); crescentic glomerulonephritis (GPA, MPA) | PAN: medium artery thrombosis → renal parenchymal infarction; GPA/MPA: small vessel vasculitis → necrotising GN | Renal failure is a leading cause of death in untreated vasculitis; may require dialysis or transplant |
| GI | Mesenteric ischaemia, bowel perforation, GI haemorrhage | Mesenteric artery vasculitis → ischaemia of bowel wall; transmural necrosis → perforation | Acute abdomen in a vasculitis patient → surgical emergency |
| Cardiac | Myocardial ischaemia, coronary arteritis, pericarditis, cardiomyopathy | Coronary artery vasculitis or accelerated atherosclerosis | Cardiac involvement = poor prognosis |
| Pulmonary | Pulmonary haemorrhage (GPA, MPA), pulmonary infiltrates (EGPA) | Capillaritis → alveolar haemorrhage; eosinophilic infiltration in EGPA | Massive haemoptysis → respiratory failure → death |
| CNS | Stroke, cranial nerve palsies, cerebral vasculitis | Vasculitis of cerebral arteries or vasa nervorum of cranial nerves | CNS involvement occurs in 5–10% of PAN [5] |
| Skin | Digital gangrene, ulceration, tissue loss | Small vessel vasculitis → cutaneous infarction | Nail-fold infarcts, digital tip necrosis |
Gradual spread from focal → multifocal → generalised [1]. Over time, if the underlying disease is not controlled, the stepwise involvement of individual nerves eventually leads to confluent mononeuropathy multiplex — a state that clinically resembles a symmetric polyneuropathy. This is a complication in the sense that:
- The pattern becomes harder to distinguish from polyneuropathy (may delay correct diagnosis)
- The cumulative nerve damage becomes extensive and functionally devastating
- Recovery potential diminishes as more and more nerve trunks are irreversibly damaged
| Underlying Cause | Key Systemic Complications Beyond the Neuropathy |
|---|---|
| DM | Microvascular: retinopathy, nephropathy. Macrovascular: IHD (accounts for 70% deaths in DM), peripheral vascular disease, cerebrovascular disease [4][17] |
| SLE | Lupus nephritis, haematological cytopenias, serositis, antiphospholipid syndrome (thrombosis, pregnancy loss) |
| RA | Cervical myelopathy (atlantoaxial subluxation), interstitial lung disease, amyloidosis |
| Amyloidosis | Cardiac failure (restrictive cardiomyopathy), nephrotic syndrome, macroglossia, bleeding diathesis |
| Lymphoma | Bone marrow failure, tumour lysis syndrome, secondary infections |
3. Complications of Treatment (Iatrogenic)
The treatments used for mononeuropathy multiplex — particularly immunosuppressive agents — carry their own significant complications. These are especially important because patients often require prolonged courses of potent immunosuppression.
Glucocorticoids are used in virtually all vasculitic causes. Long-term use produces predictable complications:
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| Osteoporosis and fractures | Glucocorticoids inhibit osteoblast function, enhance osteoclast activity, reduce intestinal calcium absorption, increase renal calcium excretion → net bone loss | Calcium + vitamin D supplementation for all patients on > 3 months steroids; bisphosphonate (alendronate) if high fracture risk; DEXA scan baseline and annually |
| Steroid-induced diabetes / worsened glycaemic control | Glucocorticoids increase hepatic gluconeogenesis, cause peripheral insulin resistance, and reduce glucose uptake by muscle | Monitor fasting glucose and HbA1c regularly; may need to start or intensify hypoglycaemic therapy; important in diabetic patients with mononeuropathy multiplex who are then given steroids for vasculitis |
| Opportunistic infections | Immunosuppression → impaired cell-mediated and humoral immunity → reactivation of latent infections (TB, HBV, Pneumocystis, fungal) | Screen for latent TB (CXR + IGRA) before starting high-dose steroids; HBV screening; TMP-SMX prophylaxis for Pneumocystis if co-prescribed with another immunosuppressant |
| Avascular necrosis (osteonecrosis) | Glucocorticoids cause fat hypertrophy within bone → ↑intraosseous pressure → ↓blood flow to subchondral bone → ischaemic necrosis; most commonly affects femoral head | Monitor for hip pain; MRI if suspected; may require joint replacement |
| Cushingoid features | Redistribution of body fat (moon face, buffalo hump, truncal obesity), skin thinning, striae, easy bruising | Minimise steroid dose and duration; use steroid-sparing agents early |
| Peptic ulcer disease | Glucocorticoids reduce prostaglandin synthesis → ↓protective gastric mucus and ↓mucosal blood flow | Co-prescribe PPI if concomitant NSAID use or history of PUD |
| Adrenal suppression | Exogenous glucocorticoids suppress the HPA axis (negative feedback on ACTH) → adrenal gland atrophy; abrupt withdrawal → adrenal crisis (hypotension, shock, hypoglycaemia) | Never stop steroids abruptly after prolonged use; taper gradually; educate patients about sick-day rules (double the dose during intercurrent illness) |
| Cataracts and glaucoma | Posterior subcapsular cataracts (glucocorticoids alter lens protein metabolism); open-angle glaucoma (↑IOP from ↑aqueous humour production) | Annual ophthalmological review |
| Myopathy (steroid myopathy) | Glucocorticoids cause type II muscle fibre atrophy → proximal weakness | Can be confused with active myositis or disease progression; CK is normal in steroid myopathy (↑ in inflammatory myopathy) |
| Psychiatric effects | Insomnia, agitation, psychosis, depression | Dose-related; monitor mental health |
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| Haemorrhagic cystitis | Acrolein (hepatic metabolite of cyclophosphamide) is directly toxic to bladder urothelium → haemorrhagic inflammation | Co-administer mesna (2-mercaptoethane sulfonate — binds acrolein in urine); adequate hydration (≥ 2L/day); morning dosing (to ensure daytime voiding) |
| Bone marrow suppression | Alkylating agent → kills rapidly dividing haematopoietic cells → leucopenia, anaemia, thrombocytopenia | Monitor CBC weekly during treatment; dose-adjust for leucopenia (keep WCC > 3.0) |
| Infertility | Toxic to gonads (oocytes and spermatogonia) → premature ovarian failure, azoospermia | Discuss fertility preservation (sperm banking, oocyte cryopreservation) BEFORE starting treatment; particularly important in young patients |
| Secondary malignancy | Alkylation of DNA → mutagenesis → ↑risk of bladder cancer (up to 33× with cumulative dose), myelodysplasia, acute leukaemia | Cumulative dose limit (ideally < 25 g lifetime); urinalysis monitoring for haematuria; regular haematological follow-up |
| Infections | Profound immunosuppression → opportunistic infections | Pneumocystis prophylaxis (TMP-SMX); avoid live vaccines; monitor for fever |
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| HBV reactivation | B-cell depletion removes immune surveillance against HBV → viral replication resumes → fulminant hepatitis | Screen HBV status (HBsAg, anti-HBs, anti-HBc) BEFORE treatment; prophylactic antivirals (entecavir) if anti-HBc positive; this is especially important in Hong Kong given high HBV prevalence |
| Hypogammaglobulinaemia | Prolonged B-cell depletion → reduced immunoglobulin production → recurrent infections | Monitor IgG levels; consider IVIg replacement if IgG < 4 g/L with recurrent infections |
| Progressive multifocal leukoencephalopathy (PML) | JC virus reactivation due to immunosuppression → demyelination of CNS white matter | Rare but devastating; suspect if new neurological symptoms during rituximab treatment; MRI brain; no effective treatment |
| Infusion reactions | Cytokine release during first infusion (especially if high tumour/B-cell burden) → fever, rigours, hypotension | Pre-medicate with paracetamol + antihistamine + hydrocortisone; slow infusion rate |
| Late-onset neutropenia | Immune-mediated neutropenia weeks to months after rituximab | Monitor CBC; usually self-limiting |
| Complication | Mechanism | Prevention/Management |
|---|---|---|
| Thromboembolic events | IVIg increases blood viscosity + contains procoagulant factors → DVT, PE, stroke, MI | Caution in elderly, immobilised, or cardiovascularly compromised patients; adequate hydration; slow infusion rate |
| Renal toxicity | Sucrose-containing IVIg preparations → osmotic nephrotoxicity (sucrose is filtered at the glomerulus but not reabsorbed → osmotic injury to proximal tubular cells) | Use sucrose-free preparations in patients with renal impairment; monitor RFT |
| Aseptic meningitis | Mechanism unclear; possibly immune complex-mediated meningeal inflammation | Severe headache, neck stiffness, photophobia within 24–48 hours of infusion; CSF shows neutrophilic pleocytosis but negative cultures; self-limiting |
| Anaphylaxis in IgA deficiency | Patients with complete IgA deficiency may have anti-IgA antibodies → anaphylaxis when exposed to IgA in donor IVIg | Screen for IgA deficiency before first IVIg; use IgA-depleted preparations if indicated |
| Haemolytic anaemia | Anti-A/anti-B isoagglutinins in pooled donor IVIg → haemolysis of recipient's red cells (particularly non-group O recipients) | Monitor for anaemia post-infusion; check DAT if haemolysis suspected |
| Drug | Key Complications |
|---|---|
| Azathioprine | Bone marrow suppression (especially if TPMT/NUDT15 deficient — genotype BEFORE starting); hepatotoxicity; pancreatitis; ↑risk of lymphoma with long-term use; severe toxicity if co-prescribed with allopurinol |
| Methotrexate | Hepatic fibrosis/cirrhosis (cumulative dose-dependent); bone marrow suppression; pneumonitis (hypersensitivity); oral ulcers; teratogenic (contraception essential); potentiated by renal impairment and concurrent trimethoprim |
| Mycophenolate | GI intolerance (diarrhoea, nausea — most common); bone marrow suppression; teratogenic; ↑infection risk |
These are frequently underappreciated but have a profound impact on patient outcomes:
| Complication | Details |
|---|---|
| Depression and anxiety | Chronic pain, disability, loss of independence, uncertainty about prognosis → high rates of depression (up to 30–50% in chronic neuropathy); depression worsens pain perception (negative pain-mood cycle) |
| Loss of employment and financial strain | Hand weakness (ulnar/median neuropathy) → inability to perform manual work; foot drop → inability to stand for prolonged periods; chronic fatigue from systemic disease |
| Social isolation | Mobility impairment, chronic pain, visible disability (muscle wasting, gait abnormality) → reduced social participation |
| Sleep disturbance | Neuropathic pain characteristically worsens at night (reduced distraction, altered sensory processing) → insomnia → daytime fatigue → further functional decline |
Gradual spread from focal → multifocal → generalised [1]:
| Stage | Complication | Explanation |
|---|---|---|
| Early (focal) | Single nerve deficit (e.g., foot drop alone) | May be dismissed as entrapment or mechanical injury → delayed diagnosis of underlying systemic disease |
| Intermediate (multifocal) | Cumulative functional disability; diagnostic complexity | Multiple nerves involved → progressive loss of limb function; may still be misdiagnosed as polyneuropathy |
| Late (confluent/generalised) | Severe disability; pattern mimics polyneuropathy | Extensive axonal loss → wheelchair dependence; difficulty distinguishing from primary polyneuropathy → may delay recognition of treatable underlying cause (e.g., vasculitis) |
| Respiratory involvement | Phrenic nerve involvement → diaphragmatic weakness → respiratory failure | Rare in mononeuropathy multiplex (more common in GBS/CIDP) but can occur if the phrenic nerve is among those affected; requires monitoring with FVC |
High Yield Summary — Complications
Complications of the neuropathy itself:
- Motor: persistent weakness, muscle wasting, contractures, falls, gait abnormality (steppage gait)
- Sensory: chronic neuropathic pain, trophic ulcers, Charcot joints [1], burns and injuries from loss of protective sensation
- Autonomic: postural hypotension, gastroparesis, erectile dysfunction, neurogenic bladder, cardiac autonomic neuropathy [4]
Complications of the underlying disease:
- Vasculitis: renal failure, mesenteric/cardiac/cerebral infarction [5] — these are the major causes of mortality
- DM: retinopathy, nephropathy, IHD (70% of DM deaths), PVD, CVD [4][17]
- Pattern progression: focal → multifocal → generalised [1], making diagnosis harder and disability worse
Complications of treatment:
- Steroids: osteoporosis, DM, infections, avascular necrosis, adrenal suppression, Cushing
- Cyclophosphamide: haemorrhagic cystitis (prevent with mesna), infertility, secondary malignancy, myelosuppression
- Rituximab: HBV reactivation (screen before use — critical in HK), hypogammaglobulinaemia, PML
- IVIg: thromboembolism, renal toxicity, aseptic meningitis, anaphylaxis in IgA deficiency
- Azathioprine: myelosuppression (check TPMT/NUDT15), interaction with allopurinol
Psychological: Depression, loss of employment, social isolation, sleep disturbance — screen for and address proactively.
Active Recall - Complications of Mononeuropathy Multiplex
References
[1] Senior notes: Ryan Ho Neurology.pdf (Section 10.2 Disease of the Peripheral Nerves, pp. 179–182) [4] Senior notes: Block A - Deterioration of eyesight in a diabetic patient_ diabetic complications.pdf (Diabetic Neuropathy and Complications sections, pp. 6–7) [5] Senior notes: Ryan Ho Rheumatology.pdf (Section 4.7.3 Polyarteritis Nodosa, p. 159) [16] Senior notes: Ryan Ho Endocrine.pdf (Section C: Diabetic Neuropathy, p. 97) [17] Senior notes: Ryan Ho Endocrine.pdf (Section 4.1.4.2: Chronic Diabetic Complications, p. 94)
High Yield Summary
Mononeuropathy multiplex = simultaneous or sequential mononeuropathies affecting multiple non-contiguous nerve trunks — the hallmark is asymmetric, multifocal, nerve-territory-specific deficits.
Most common cause: Diabetes mellitus (via microangiopathy of vasa nervorum).
Most important non-diabetic cause: Systemic vasculitis (PAN, GPA, EGPA) — treatable with immunosuppression; PAN causes mononeuritis multiplex in up to 70%.
Mechanism: Mostly axonal due to ischaemic infarction of nerve trunks via vasa nervorum disease. Demyelinating causes (MMN, MADSAM) are less common but potentially reversible.
Clinical pattern: Acute pain followed by focal neuropathy → stepwise addition of new nerve palsies → may become confluent over time.
Key examination: Map affected nerves, look for LMN signs (wasting, weakness, hyporeflexia, fasciculations), look for systemic clues to underlying cause (purpura, joint disease, skin changes).
Only demyelinating neuropathies are usually susceptible to treatment — early distinction between axonal and demyelinating via NCS is essential.
DM mononeuropathy classic presentations: CN III (pupil-sparing ptosis), CN VI, CN V, common peroneal nerve (foot drop) — "mix and match".
Mnemonic for causes: WARD SLAP CC — Wegener's, Amyloidosis, RA, DM, SLE, Leprosy, PAN, Carcinomatosis, Churg-Strauss.
High Yield Summary — Investigations
The investigation approach to mononeuropathy multiplex follows these steps:
- History and examination — map affected nerves, look for systemic clues, check postural BP
- Baseline bloods — CBC, LRFT, glucose, B12/folate, TFT, ESR/CRP, autoimmune markers (ANA, RF, ANCA), SPE Ig [2]
- NCS/EMG — the definitive investigation: confirms distribution (multiple non-contiguous mononeuropathies), fibre type involvement, and pattern (axonal vs demyelinating). Axonal = ↓amplitude, normal velocity; Demyelinating = ↓velocity, normal amplitude [2]
- Targeted etiological workup — guided by NCS pattern and clinical context (ANCA, complement, cryoglobulins, HBV/HCV/HIV, SPE/FLC, anti-GM1, etc.)
- Nerve biopsy — last resort for inflammatory/infective/infiltrative disorders [2]; sural nerve most common; look for necrotising vasculitis, amyloid deposition, granulomas, or lymphomatous infiltration
Key principle: Usually only demyelinating neuropathies are susceptible to treatment [1] — this makes the axonal vs demyelinating distinction on NCS the single most impactful investigation finding.
False negatives can occur in early disease where structural changes have not yet developed [12] — repeat NCS in 2–3 weeks if clinical suspicion remains high.
High Yield Summary — Management
Principle: Axonal neuropathies (majority of mononeuropathy multiplex) are not directly treatable — manage by treating the underlying disease. Demyelinating neuropathies usually respond to immunosuppressive treatment [1].
Diabetic: Glycaemic control (HbA1c < 7–8%), supportive management, neuropathic pain relief (gabapentin/pregabalin/amitriptyline). Usually transient with 60% good recovery in 12–24 months [16].
Vasculitic (most important treatable cause):
- PAN: Steroids ± azathioprine/MTX (mild); Steroids + cyclophosphamide (moderate/severe); IV pulse steroids (severe/refractory). HBV-PAN: antivirals first [5].
- ANCA vasculitis: Steroids + rituximab (preferred 2024+) or cyclophosphamide for induction; rituximab or azathioprine for maintenance.
- Prognosis of PAN: 13% 5-year survival untreated vs 80% treated [5].
MMN: IVIg is the ONLY effective treatment. Does NOT respond to steroids or plasma exchange — this is unique and high-yield.
Symptomatic: Gabapentin / pregabalin / amitriptyline for neuropathic pain [1][16]; physiotherapy; AFO for foot drop; occupational therapy.
Key drug safety in HK: HLA-B*1502 screening before carbamazepine; TPMT/NUDT15 genotyping before azathioprine; HBV screening before rituximab.
High Yield Summary — Complications
Complications of the neuropathy itself:
- Motor: persistent weakness, muscle wasting, contractures, falls, gait abnormality (steppage gait)
- Sensory: chronic neuropathic pain, trophic ulcers, Charcot joints [1], burns and injuries from loss of protective sensation
- Autonomic: postural hypotension, gastroparesis, erectile dysfunction, neurogenic bladder, cardiac autonomic neuropathy [4]
Complications of the underlying disease:
- Vasculitis: renal failure, mesenteric/cardiac/cerebral infarction [5] — these are the major causes of mortality
- DM: retinopathy, nephropathy, IHD (70% of DM deaths), PVD, CVD [4][17]
- Pattern progression: focal → multifocal → generalised [1], making diagnosis harder and disability worse
Complications of treatment:
- Steroids: osteoporosis, DM, infections, avascular necrosis, adrenal suppression, Cushing
- Cyclophosphamide: haemorrhagic cystitis (prevent with mesna), infertility, secondary malignancy, myelosuppression
- Rituximab: HBV reactivation (screen before use — critical in HK), hypogammaglobulinaemia, PML
- IVIg: thromboembolism, renal toxicity, aseptic meningitis, anaphylaxis in IgA deficiency
- Azathioprine: myelosuppression (check TPMT/NUDT15), interaction with allopurinol
Psychological: Depression, loss of employment, social isolation, sleep disturbance — screen for and address proactively.
Mononeuropathy
Mononeuropathy is the damage or dysfunction of a single peripheral nerve, resulting in sensory, motor, or mixed deficits in the distribution of that nerve.
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.