Think of the basal ganglia as a "brake and accelerator" system for movement:

Direct pathway ("GO" pathway):

• Cortex → Striatum (D1 receptors) → GPi/SNr (inhibitory) → Thalamus (disinhibited) → Motor cortex → Movement
• Net effect: facilitates movement (releases the thalamic brake)

Indirect pathway ("STOP" pathway):

• Cortex → Striatum (D2 receptors) → GPe (inhibitory) → STN (disinhibited, excitatory) → GPi/SNr (more inhibition) → Thalamus (more inhibited) → Motor cortex → Less movement
• Net effect: suppresses movement (applies the thalamic brake harder)

Role of Dopamine from SNpc:

• Dopamine from SNpc modulates the balance:
  • D1 receptors (direct pathway): dopamine excites → promotes movement
  • D2 receptors (indirect pathway): dopamine inhibits → reduces suppression of movement
• Therefore, dopamine overall facilitates movement via both pathways

Degeneration of dopaminergic neurons in the SNpc leads to ^[1][2][3]:

1. ↓ D1 stimulation → Direct pathway underactive → Less facilitation of movement
2. ↓ D2 inhibition → Indirect pathway overactive → More suppression of movement
3. Net result: Overactivity of the indirect pathway → excessive inhibition of the thalamus → reduced excitation of motor cortex → bradykinesia, rigidity ^[2]

"60–80% of dopaminergic neurons are lost before motor symptoms emerge" — this is why there is a long prodromal phase with non-motor symptoms ^[2].

The vast majority of iPD cases are sporadic with no identifiable single cause ^[5]. The pathogenesis is considered multifactorial, involving:

1. Genetic susceptibility (polygenic risk)
2. Environmental triggers (pesticides, toxins)
3. Aging-related cellular decline (mitochondrial dysfunction, oxidative stress, impaired protein clearance)

5–10% of PD cases are familial ^[3][5]. Key genes to know:

The key pathological mechanisms converge on dopaminergic neuronal death in the SNpc:

1. α-Synuclein misfolding and aggregation

   • α-Synuclein is a presynaptic protein normally involved in vesicle trafficking
   • In iPD, it misfolds → oligomers → fibrils → Lewy bodies (dense eosinophilic intracytoplasmic inclusions)
   • Misfolded α-synuclein is toxic to neurons and can spread in a prion-like fashion (cell-to-cell transmission), which explains the Braak staging pattern

2. Mitochondrial dysfunction

   • Complex I deficiency in the electron transport chain → ↑ reactive oxygen species (ROS) → oxidative damage
   • PINK1 and Parkin are involved in mitophagy (clearance of damaged mitochondria); mutations → accumulation of dysfunctional mitochondria

3. Ubiquitin-proteasome system (UPS) and autophagy-lysosomal pathway failure

   • Parkin is an E3 ubiquitin ligase; loss of function → impaired protein degradation → protein aggregation
   • GBA encodes glucocerebrosidase (a lysosomal enzyme); mutations → ↓ lysosomal function → ↓ α-synuclein clearance

4. Neuroinflammation

   • Activated microglia release pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) → further neuronal damage
   • Creates a self-perpetuating cycle of degeneration

5. Oxidative stress

   • Dopamine metabolism itself generates ROS (via MAO-B and auto-oxidation)
   • SNpc neurons are particularly vulnerable because they have high metabolic demand, high iron content (Fenton reaction), and relatively low antioxidant defences

• Depigmentation of the substantia nigra (and locus coeruleus): normally dark due to neuromelanin pigment in dopaminergic neurons; loss of neurons → pale appearance on cross-section

• Lewy bodies: eosinophilic intraneuronal inclusion bodies in the basal ganglia (and later cerebral cortex) ^[2][3]
  • Composed primarily of aggregated α-synuclein with ubiquitin
  • Initially found in brainstem → spread rostrally (Braak staging)
• Lewy neurites: dystrophic neurites containing α-synuclein aggregates
• Neuronal loss and gliosis (astrocytic scarring) in the SNpc

This is the framework to think about when you see any patient with parkinsonism:

Causes of Parkinsonism (must know!) ^[2][4]:

Drug-induced parkinsonism from metoclopramide, antipsychotics, or antihistamines is particularly important to identify as it is reversible ^[4].

The Hoehn and Yahr staging system helps assess disease severity ^[4]:

This is an extremely high-yield comparison for exams ^[2]:

Special tests for PD ^[2]:

Cardinal features of parkinsonism: TRAP — resting Tremor, Rigidity, Akinesia/bradykinesia, Postural instability

Hoehn and Yahr staging: Stage 1 (unilateral) → Stage 5 (wheelchair-bound/bedridden)

Causes: idiopathic PD, drug-induced (metoclopramide, antipsychotics, antihistamines), PSP, MSA, Wilson's disease, NPH, MPTP, CO poisoning

Drug-induced parkinsonism is particularly important to identify as it is reversible. Common culprits: metoclopramide, antipsychotics, antihistamines.

Huntington's disease: autosomal dominant, CAG trinucleotide repeats on chromosome 4p16.3, caudate and putamen atrophy, triad of chorea + psychiatric manifestations + progressive dementia.

PD is one of the most common neurological conditions in older adults. Consider medication review — many drugs can worsen or cause parkinsonism.

Dementia with Lewy Bodies (DLB) shares pathology with PD (Lewy bodies), but DLB has dementia within 1 year of motor symptoms (or before), visual hallucinations, fluctuating cognition, and REM sleep behaviour disorder.

The "1-year rule": If dementia precedes or appears within 1 year of parkinsonism → DLB. If parkinsonism precedes dementia by > 1 year → PD dementia (PDD). Same pathological spectrum, different clinical presentation.

Drug-induced parkinsonism from metoclopramide, antipsychotics, or antihistamines is particularly important to identify as it is reversible ^[4][9].

Why does it happen? These drugs are dopamine D2 receptor antagonists — they block dopamine's action in the striatum, functionally mimicking the dopamine deficiency of iPD. The indirect pathway becomes overactive, just as in iPD, but there is no neuronal death.

How to distinguish from iPD:

Characterised by lower body involvement ^[2][3] — this makes intuitive sense because lacunar infarcts in the basal ganglia (from small vessel disease / lipohyalinosis) tend to affect the motor homunculus representation of the lower limbs preferentially in the putamen/caudate.

Key features:

• Lower limbs > upper limbs ("lower body parkinsonism")
• Stepwise deterioration (reflecting recurrent small vessel strokes) rather than smooth progression ^[3]
• More symmetrical than iPD ^[3]
• Poor response to levodopa ^[3]
• Associated with vascular risk factors (hypertension, diabetes, smoking, hyperlipidaemia)
• Gait: "magnetic gait" — feet stuck to the floor, wide-based, short steps (similar to NPH)
• Pseudobulbar affect and pyramidal signs may coexist (reflecting white matter ischaemia)

Imaging: Multiple lacunar infarcts in basal ganglia and/or subcortical white matter on MRI

Always consider in young patients (< 40–50 years) with parkinsonism, especially if there is liver disease.

Wilson's disease = autosomal recessive disorder of copper metabolism (ATP7B gene) → copper accumulates in the liver, brain (particularly basal ganglia — lenticular nuclei), and cornea.

Wilson's disease can mimic Parkinson's disease — extrapyramidal deposition of copper ^[10].

Key clues:

• Young age (typically < 40 years)
• Kayser-Fleischer rings (copper deposition in Descemet's membrane of the cornea — seen on slit lamp)
• Liver disease (hepatitis, cirrhosis, fulminant hepatic failure)
• Coombs-negative haemolytic anaemia ^[10] (copper directly damages red cell membranes)
• Movement disorder can include tremor, rigidity, dystonia, dysarthria ("wing-beating tremor" — a coarse postural/intention tremor is more characteristic than rest tremor)
• Psychiatric features (personality change, depression, psychosis)

Investigations: ↓ serum ceruloplasmin, ↑ 24-hour urinary copper, ↑ hepatic copper on biopsy, KF rings on slit lamp, genetic testing (ATP7B — though genetic test does not have to be positive to reach a diagnosis since the hereditary pathway is very heterogeneous ^[10])

The classic triad: "Wet, Wacky, Wobbly" — urinary incontinence, dementia, gait apraxia ^[3][7].

Why it mimics parkinsonism: The dilated ventricles compress the periventricular white matter and corona radiata fibres supplying the legs, producing a "lower body parkinsonism." There is NO true basal ganglia pathology.

Imaging: ALL ventricles enlarged disproportionate to sulcal effacement ^[7] (i.e. ventriculomegaly out of proportion to cortical atrophy — this distinguishes NPH from ex-vacuo hydrocephalus of Alzheimer's)

Key distinction from iPD:

• NPH: no resting tremor, gait apraxia (not festinating), urinary incontinence, no psychiatric symptoms, sleep disorder, or dysautonomic features ^[7]
• Treatable with VP shunt (CSF diversion) — therefore crucial not to miss

Autosomal dominant condition caused by CAG trinucleotide repeats on chromosome 4p16.3, leading to atrophy of the caudate and putamen ^[4].

Triad: chorea + psychiatric manifestations (depression, psychosis) + progressive dementia ^[4].

Why it enters the parkinsonism differential:

• The juvenile-onset (Westphal variant) of Huntington's disease can present with an akinetic-rigid phenotype rather than chorea — so it can look like parkinsonism ^[3]
• Juvenile-onset cases inherited from fathers tend to be more severe due to anticipation (expansion of CAG repeats during paternal meiosis) ^[4]

Key clues: Family history (AD), chorea, psychiatric features, caudate atrophy on MRI ("box-car" ventricles)

• Historically associated with encephalitis lethargica (von Economo's encephalitis, 1917–1928 pandemic) — now extremely rare ^[3]
• Any viral encephalitis affecting the basal ganglia can cause parkinsonism (e.g. Japanese encephalitis, HSV, HIV)
• May present years after the acute encephalitis
• Oculogyric crises (sustained upward gaze deviation) are a characteristic feature of post-encephalitic parkinsonism

Essential tremor is NOT parkinsonism — it is a pure tremor disorder without bradykinesia, rigidity, or postural instability. However, it is the most common movement disorder and the most frequent reason patients are referred "? PD."

The detailed comparison table was presented in the prior section ^[2], but the critical points for the differential are:

Presence of Parkinsonism: bradykinesia + rest tremor OR rigidity ^[2]

• Bradykinesia is mandatory — you cannot diagnose parkinsonism without it
• It must be combined with at least one of: resting tremor or rigidity
• Why bradykinesia is mandatory: it is the most specific feature of dopaminergic deficit in the nigrostriatal pathway. Tremor alone could be essential tremor; rigidity alone could be other causes. Bradykinesia with decrement is highly specific for basal ganglia dysfunction.

These are "investigations" you can do at the bedside — and they are first-line.

Routine blood tests do not diagnose iPD but are essential to exclude metabolic and secondary causes:

These are newer developments that are beginning to enter clinical practice:

The gold standard and most effective drug for motor symptoms of iPD.

Pharmacology — explained from first principles:

• Levodopa (L-DOPA) = L-3,4-dihydroxyphenylalanine, the natural biosynthetic precursor of dopamine
• Dopamine itself cannot cross the blood-brain barrier (BBB), but levodopa can (it is an amino acid transported by the large neutral amino acid transporter)
• Once levodopa crosses the BBB → it is converted to dopamine by aromatic L-amino acid decarboxylase (AADC) (also called DOPA decarboxylase) in the remaining nigrostriatal terminals and other brain cells
• Problem: AADC also exists peripherally (gut, liver, blood vessels). If you give levodopa alone, ~95% is decarboxylated peripherally → dopamine in the periphery causes nausea, vomiting, orthostatic hypotension, and cardiac arrhythmias, and very little reaches the brain
• Solution: Always co-administer with a peripheral decarboxylase inhibitor (DDI) that does NOT cross the BBB → blocks peripheral conversion → more levodopa reaches the brain, fewer peripheral side effects

The choice between Madopar and Sinemet is largely institutional preference. In Hong Kong, Madopar is more commonly used.

Why levodopa is the gold standard:

• Most potent symptomatic relief of motor symptoms
• Clear and dramatic beneficial response to dopaminergic therapy is itself an MDS supportive criterion for diagnosing iPD ^[2]
• All patients with iPD will eventually need levodopa

Side effects:

Long-term motor complications (these are the major management challenges — discussed in detail in Section 4):

• Wearing-off (end-of-dose deterioration)
• On-off fluctuations
• Levodopa-induced dyskinesia (peak-dose, diphasic, or off-state dystonia)

These are synthetic drugs that directly stimulate dopamine receptors in the striatum, bypassing the need for presynaptic dopaminergic neurons.

Why use them?

• They have a longer half-life than levodopa → more stable dopaminergic stimulation → theoretically less pulsatile receptor stimulation → lower risk of motor fluctuations and dyskinesias than levodopa (this is the rationale for using them first in younger patients)
• Can be used as monotherapy in early disease or as adjunct to levodopa in advanced disease

Side effects (class effects):

MAO-B = Monoamine Oxidase type B. This enzyme is the major metabolic pathway for dopamine in the brain. Inhibiting MAO-B → ↓ dopamine breakdown → ↑ dopamine availability in the synaptic cleft.

Indications:

• Early monotherapy: Mild symptom relief; suitable as first agent in young patients with mild symptoms
• Adjunct to levodopa: Reduces "off" time by extending dopamine action; smooths out motor fluctuations

Side effects: Nausea, headache, insomnia (selegiline), orthostatic hypotension

Important drug interaction:

• MAO-B inhibitors + pethidine (meperidine) → serotonin syndrome (excitation, hyperthermia, rigidity, death). Avoid this combination absolutely.
• MAO-B inhibitors + SSRIs/SNRIs → theoretical serotonin syndrome risk (though rare at selective MAO-B doses). Use with caution, monitor.

COMT = Catechol-O-Methyltransferase. This enzyme methylates levodopa peripherally (to 3-O-methyldopa) and centrally (dopamine to 3-methoxytyramine). Inhibiting COMT → more levodopa available for brain delivery → prolongs levodopa's action.

Indication: Adjunct to levodopa in patients with wearing-off motor fluctuations — by inhibiting peripheral COMT, you increase the bioavailability and half-life of levodopa, extending its "on" time

Side effects:

• Dyskinesias (because you are effectively increasing levodopa exposure — may need to reduce levodopa dose when adding a COMT inhibitor)
• Diarrhoea (especially entacapone)
• Orange discolouration of urine (entacapone) — harmless but warn patients
• Hepatotoxicity (tolcapone — monitor LFTs)

Rationale from first principles: In the normal striatum, there is a balance between dopaminergic (inhibitory) and cholinergic (excitatory) interneuron activity. When dopamine is depleted, the cholinergic system is relatively overactive → this contributes especially to tremor. Anticholinergics restore the balance by reducing cholinergic excess.

Indications:

• Tremor-dominant iPD, especially in younger patients — anticholinergics are most effective for tremor, less so for bradykinesia/rigidity
• Drug-induced parkinsonism (acute dystonic reactions from antipsychotics)

Contraindications / Caution:

• Elderly patients (> 65): avoid due to cognitive side effects — anticholinergics worsen memory and can precipitate delirium/dementia
• Closed-angle glaucoma (mydriasis → ↑ IOP)
• Prostatic hypertrophy (urinary retention)
• Cognitive impairment/dementia

Side effects (predictable from anticholinergic mechanism):

• Dry mouth, blurred vision, constipation, urinary retention, cognitive impairment, confusion, hallucinations

Mnemonic for anticholinergic side effects: "Blind as a bat, dry as a bone, red as a beet, mad as a hatter, hot as a hare, full as a flask" — mydriasis, dry mouth/skin, flushing, delirium, hyperthermia, urinary retention.

Mechanism: Multiple mechanisms — NMDA receptor antagonist (reduces glutamatergic excitotoxicity in the indirect pathway), enhances dopamine release, blocks dopamine reuptake, and has mild anticholinergic properties.

Key indications:

• Levodopa-induced dyskinesia — amantadine is the only oral medication with proven efficacy for reducing peak-dose dyskinesias (via NMDA antagonism — reduces excessive glutamatergic signalling from the overactive STN)
• Can provide mild symptomatic benefit in early PD (modest anti-parkinsonian effect)

Side effects:

• Livedo reticularis (mottled purplish skin discolouration, typically on legs)
• Ankle oedema
• Insomnia, hallucinations, confusion (especially in elderly)
• Renally excreted — reduce dose in CKD

The pathophysiology relates to the interplay between progressive nigrostriatal denervation and pulsatile dopamine receptor stimulation:

1. Early disease: Enough dopaminergic neurons survive to "buffer" exogenous levodopa — they take up levodopa, convert it to dopamine, store it, and release it steadily. This provides a smooth, sustained therapeutic effect.
2. Advanced disease: Most dopaminergic terminals are lost → levodopa is now converted to dopamine by non-dopaminergic cells (serotonergic neurons, glia) that lack storage capacity → dopamine levels in the striatum mirror plasma levodopa levels → pulsatile stimulation of postsynaptic dopamine receptors → receptors undergo plastic changes (sensitisation) → dyskinesias and fluctuations.

DBS is FDA-approved for essential tremor, Parkinson's disease, dystonia, and OCD ^[2].

Principle: High-frequency electrical stimulation of a deep brain target disrupts pathological neuronal firing patterns. In iPD, stimulation of the STN or GPi modulates the overactive indirect pathway, mimicking the effect of dopamine on basal ganglia circuitry.

Patient selection criteria ^[2]:

Contraindications ^[2]:

Sites of implantation ^[2]:

Procedure ^[2]:

• Pre-operative: Madopar challenge test (confirms levodopa responsiveness), MMSE, assessment by psychiatrist
• Target under LA: stereotactic frame guidance, implant microelectrode and pulse generator
• Post-op: DBS can be started after a few days, adjust settings 6–8 weeks later

Principle: Continuous jejunal infusion of levodopa-carbidopa gel via a PEG-J tube. Bypasses erratic gastric emptying and provides continuous dopaminergic stimulation → markedly reduces motor fluctuations and dyskinesias.

Indication: Advanced iPD with severe motor fluctuations refractory to optimised oral therapy; alternative to DBS (especially for patients who are not DBS candidates)

Complications: PEG-J site infection, tube dislodgement, peritonitis, polyneuropathy (possibly from vitamin B6/B12 deficiency due to high-dose levodopa)

Principle: Apomorphine (potent D1/D2 agonist) delivered via a subcutaneous pump → continuous dopaminergic stimulation

Indication: Advanced iPD with severe motor fluctuations; alternative to DBS/LCIG

Complications: Subcutaneous nodules at injection sites, nausea (co-prescribe domperidone), somnolence, hallucinations

• Postural instability is a late feature of iPD ^[2] and is least responsive to dopaminergic therapy ^[2]
• Why? Postural control depends on non-dopaminergic systems (cholinergic pedunculopontine nucleus, noradrenergic locus coeruleus, serotonergic raphe nuclei) that are also degenerating but are NOT addressed by dopamine replacement
• Falls are the single most important cause of morbidity in advanced iPD — leading to hip fractures, subdural haematomas, and hospitalisation
• Compounded by orthostatic hypotension (both disease-related autonomic dysfunction and medication-related)

Carbidopa/levodopa can cause falls through orthostatic hypotension ^[13] — always consider medication contribution to falls in elderly PD patients.

Consequences of falls:

• Fractures (especially hip — #NOF)
• Head injury → subdural haematoma (these patients are often frail and may be on anticoagulants)
• Fear of falling → activity restriction → deconditioning → worsening disability (vicious cycle)
• Loss of independence, institutionalisation

• Sudden, transient inability to move the feet, typically at doorways, turns, narrow spaces, or when approaching a target
• Affects ~50% of patients after 5 years and ~80% after 10 years
• Pathophysiology: Disruption of supplementary motor area and basal ganglia locomotor circuits, involving both dopaminergic and non-dopaminergic pathways — hence often resistant to levodopa adjustment
• FOG is a major precipitant of falls
• Management: Visual cueing (laser pointer, lines on floor), auditory cueing (metronome, rhythmic music), physiotherapy, optimising dopaminergic therapy

• Oropharyngeal bradykinesia and rigidity → impaired coordination of the swallowing mechanism → silent aspiration, choking
• Develops in ~80% of patients by the advanced stage
• Aspiration pneumonia is the leading cause of death in iPD ^[3]

The most common cause of death in DLB/PD spectrum is failure to thrive, followed by pneumonia and swallowing difficulties, other medical conditions, and complications from a fall ^[14].

• Why aspiration pneumonia kills: Weakened cough reflex (brainstem degeneration) + impaired airway protection (laryngeal rigidity/bradykinesia) + reduced mobility (poor chest expansion, secretion clearance) → recurrent aspiration → pneumonia → sepsis
• Management: SALT assessment, modified diet textures, postural techniques during eating, PEG tube in advanced disease (controversial — does not clearly improve survival), chest physiotherapy

• Low-volume, monotonous, tremulous speech (microphonia) ^[2] worsens progressively
• Palilalia (involuntary repetition of syllables/words) may develop
• Impact: social isolation, depression, reduced quality of life, difficulty communicating needs (especially important in institutional care)
• Management: Lee Silverman Voice Treatment (LSVT LOUD); communication aids in advanced stages

• Motor symptoms re-emerge before the next dose of levodopa is due
• Pathophysiology: As more dopaminergic terminals are lost, the remaining terminals cannot store and release dopamine steadily. The therapeutic effect becomes directly dependent on the plasma half-life of levodopa (~90 minutes). The "on" period shortens progressively.
• Earliest motor complication; affects ~50% at 5 years, ~80% at 10 years
• Can also involve non-motor wearing-off: anxiety, pain, dysphoria, sweating returning before next dose

• Unpredictable, sudden switching between "on" (mobile, functional) and "off" (akinetic, rigid, immobile) states, not clearly related to dosing schedule
• Pathophysiology: Severely depleted dopaminergic terminals + erratic levodopa absorption + postsynaptic receptor changes → unpredictable dopamine levels in the striatum
• Very distressing — patients can "freeze" suddenly mid-activity

• Risk factors for earlier LID development: younger age of onset (more years of therapy, more plastic receptors), higher levodopa dose, lower body weight, female sex
• LID can become as disabling as the parkinsonism itself — some patients prefer to be "off" rather than dyskinetic

• PD-dementia affects 10–15% of PD patients at any time point ^[3], but cumulative risk reaches up to 80% after 20 years of disease
• Pathophysiology: Cortical Lewy body spread (Braak stages 5–6) + degeneration of the cholinergic nucleus basalis of Meynert + mesocortical dopamine depletion
• Profile: Subcortical dementia — executive dysfunction, slowed processing, visuospatial impairment, attention deficits. Memory retrieval is impaired but recognition may be relatively preserved (unlike AD where encoding itself is lost)
• Drugs that may worsen cognitive function: anticholinergic drugs and dopaminergic drugs for motor symptoms of PD ^[6]
• PD-dementia is worsened by anticholinergics ^[3] — this is why anticholinergics are absolutely contraindicated in elderly PD patients with cognitive impairment

Management of PD dementia: Cholinesterase inhibitors (e.g. rivastigmine, donepezil), memantine ^[6]. No therapies have been shown to modify the course of the disease or influence prognosis ^[6] — all treatment is symptomatic.

• PD-psychosis is common in late stages, can be due to levodopa/DA agonists and the disease itself ^[3]
• Spectrum: frightening nightmares → vivid dreams → formed visual illusions → visual hallucinations (often benign "passage hallucinations" initially) → paranoid delusions → delirium
• Pathophysiology: Multifactorial — cortical Lewy body pathology (serotonergic/cholinergic deficit) + dopaminergic medication overstimulation of mesolimbic/mesocortical pathways + advanced age + cognitive impairment
• Manifestations: frightening nightmares, vivid dreams/illusions, overt mania, visual hallucinations, delusions ^[3]

Management of PD psychosis: Reduce dose or even stop PD meds (monitor motor functions). Start antipsychotics if inadequate response (e.g. pimavanserin) ^[6].

• Remember the step-wise drug reduction hierarchy: ↓ amantadine/selegiline → ↓ DA agonist → ↓ levodopa (levodopa is least likely to cause psychosis) → add clozapine or pimavanserin ^[2]

• Depression affects > 50% of iPD patients ^[3] — making it one of the most common non-motor complications
• Pathophysiology: Degeneration of serotonergic (raphe nuclei), noradrenergic (locus coeruleus), and dopaminergic (mesolimbic) systems — this is a true neurobiological depression, not merely "reactive" to disability
• Associated with serotonergic deficiency, can consider SSRIs ^[3]
• Anxiety is also extremely common and may be part of non-motor wearing-off
• Under-recognised and under-treated — significantly impacts quality of life

• Pathological gambling, hypersexuality, compulsive shopping, binge eating, punding (repetitive purposeless behaviours)
• Prevalence: ~15–20% of patients on dopamine agonists
• Pathophysiology: D3 receptor stimulation in the mesolimbic "reward" pathway by dopamine agonists → abnormal reward-seeking behaviour
• Can be devastating — financial ruin, relationship breakdown, legal consequences
• Management: Reduce or stop the DA agonist; this is the most effective intervention. Screen actively at every follow-up.

• A specific subtype of ICD where patients compulsively take excessive levodopa despite worsening dyskinesias
• Essentially a "dopamine addiction" — patients escalate doses far beyond therapeutic need because of the euphoria associated with the "on" state
• More common in young-onset PD, male, history of mood disorders or substance use
• Management: Structured drug dosing, reduce total dopaminergic load, psychiatric support

• Weight loss — multifactorial: dyskinesias increase energy expenditure; dysphagia reduces oral intake; gastroparesis causes nausea/early satiety; depression reduces appetite
• Malnutrition → sarcopenia → further weakness and fall risk → vicious cycle
• Vitamin B12/B6 deficiency — associated with high-dose levodopa therapy (levodopa metabolism consumes these vitamins); may contribute to peripheral neuropathy

• Clinical course: insidious onset with gradual progression over 10–15 years ^[3]
• Untreated → ↓ UPDRS motor score 8–9% per year ^[3]
• Mortality rate 2–5× general population matched for age ^[3]
• Average survival from onset = 13 years ^[3]
• Often die from complications e.g. pneumonia ^[3] (aspiration pneumonia is the leading cause of death)
• Better prognosis: tremor-dominant subtype, younger onset, female sex
• Worse prognosis: PIGD subtype (akinetic-rigid), older onset, early cognitive impairment, early autonomic failure