Parkinson's Disease - Clinical Features, Levodopa, and Non-Motor Symptoms

Definition / Overview

• Parkinson's disease (PD) is a progressive neurodegenerative disorder characterised by the loss of dopaminergic neurons in the substantia nigra pars compacta, resulting in the classic triad of bradykinesia, rigidity, and resting tremor.
• It is the second most common neurodegenerative disease after Alzheimer's disease, with prevalence rising sharply after age 60.

Key pathological hallmarks:

• Degeneration of nigrostriatal dopaminergic neurons
• Accumulation of misfolded $\alpha$-synuclein protein aggregates forming Lewy bodies within surviving neurons
• By the time motor symptoms emerge, approximately 60-80% of dopaminergic neurons have already been lost

PD is primarily a clinical diagnosis, there is no definitive confirmatory blood test or imaging study in routine practice.

Pathophysiology

Nigrostriatal Pathway

• The substantia nigra projects to the striatum (caudate and putamen) via the nigrostriatal pathway, providing tonic dopaminergic input that facilitates voluntary movement.
• Dopamine acts on both D1 receptors (excitatory, direct pathway) and D2 receptors (inhibitory, indirect pathway) to modulate the thalamo-cortical motor loop.

When dopamine is depleted:

• The direct pathway (facilitating movement) is underactive
• The indirect pathway (inhibiting movement) is overactive
• Net result: excessive inhibition of the thalamus → reduced cortical motor activation → bradykinesia and rigidity

Braak Staging

PD pathology spreads in a caudal-to-rostral pattern (Braak's hypothesis):

• Stages 1-2: Olfactory bulb, dorsal motor nucleus of vagus → explains prodromal symptoms (anosmia, constipation, REM sleep behaviour disorder)
• Stages 3-4: Substantia nigra → motor symptoms emerge
• Stages 5-6: Neocortex → cognitive and neuropsychiatric features

Clinical Features

Motor Symptoms, The Cardinal Four

Additional motor features:

• Micrographia: handwriting becomes progressively smaller
• Hypomimia: reduced facial expression ("masked facies")
• Hypophonia: soft, monotonous speech
• Festinant gait: short, shuffling steps with forward trunk flexion; reduced arm swing
• Freezing of gait: sudden inability to initiate or continue walking
• Camptocormia: abnormal forward flexion of the trunk
• Key point: Idiopathic PD almost always presents asymmetrically at onset.
• Symmetric onset or early falls should raise suspicion for an atypical parkinsonian syndrome.

Non-Motor Symptoms

• Non-motor features often precede motor symptoms by years and significantly impair quality of life.
• They are frequently under-recognised.

Prodromal / Pre-motor Features

• Anosmia or hyposmia: present in up to 90%; one of the earliest features
• Constipation: may predate motor symptoms by many years
• REM sleep behaviour disorder (RBD): acting out dreams; strong predictor of synucleinopathy
• Depression and anxiety: may be part of the disease, not just a reaction to it
• Restless legs syndrome

Autonomic Dysfunction

• Orthostatic hypotension (worsened by dopaminergic medications)
• Urinary urgency and frequency
• Erectile dysfunction
• Excessive sweating or seborrhoea

Neuropsychiatric

• Depression: occurs in ~50%; treat with SSRIs
• Anxiety and apathy
• Psychosis: visual hallucinations (often well-formed); may be medication-induced or disease-related
• Dementia: Parkinson's disease dementia (PDD) develops in a significant proportion after many years of motor disease; distinguish from Dementia with Lewy bodies (DLB) where dementia precedes or occurs concurrently with parkinsonism

Sleep Disturbances

• REM sleep behaviour disorder
• Excessive daytime sleepiness (often exacerbated by dopaminergic medications)
• Insomnia

Sensory Symptoms

• Pain (central or musculoskeletal in origin)
• Numbness and tingling

Diagnosis

PD is a clinical diagnosis based on the UK Brain Bank criteria (in practice: bradykinesia plus at least one of resting tremor, rigidity, or postural instability), with supportive features and exclusion of red flags.

Supportive Features

• Asymmetric onset
• Resting tremor
• Excellent and sustained response to levodopa
• Progressive course

Red Flags, Consider Alternative Diagnosis

Investigations

Routine investigations are primarily to exclude other causes and assess comorbidities:

• MRI brain: to exclude structural causes (normal in idiopathic PD, or shows incidental age-related changes); midbrain atrophy ("hummingbird sign") in PSP
• Bloods: TFTs, B12, LFTs, renal function (baseline before medications)
• DaTscan (dopamine transporter SPECT): can confirm nigrostriatal degeneration; useful when diagnosis is uncertain; does not differentiate PD from atypical parkinsonism
• Levodopa challenge: a robust, sustained response strongly supports idiopathic PD

Management

General Principles

• PD is incurable; treatment is symptomatic, aimed at maintaining function and quality of life
• No medication has been proven to slow or halt disease progression
• Management requires a multidisciplinary team (MDT): neurologist, GP, physiotherapist, occupational therapist, speech pathologist, psychologist, social worker
• Timing to initiate treatment depends on functional impact, not a fixed threshold
• Exercise has robust evidence for symptomatic benefit and should be encouraged throughout the disease course

Pharmacological Management

Levodopa + Carbidopa (Co-careldopa)

Levodopa is the most effective symptomatic treatment for PD.

• Mechanism: Levodopa is a dopamine precursor that crosses the blood-brain barrier (dopamine itself cannot). In the surviving nigrostriatal neurons, it is decarboxylated to dopamine, restoring dopaminergic tone
• Carbidopa is a peripheral DOPA decarboxylase inhibitor, it reduces conversion of levodopa to dopamine outside the CNS, thereby:
  • Reducing peripheral side effects (nausea, vomiting, orthostatic hypotension)
  • Reducing the required levodopa dose (approximately 4-5 fold)
  • Increasing CNS bioavailability

In early PD: The response is consistent and lasts throughout the day, a "long-duration response."

Clinical considerations:

• Absorbed from the small intestine; high-protein meals can compete with absorption (large neutral amino acids use the same transporter)
• Half-life of levodopa is short (~1-2 hours), contributing to later motor fluctuations
• Available as immediate-release and extended-release formulations

Motor Complications of Long-Term Levodopa Therapy

Within 2-5 years of treatment, up to 50% of patients develop motor complications:

Managing motor fluctuations:

• Add a COMT inhibitor (entacapone, opicapone) to extend levodopa effect by blocking peripheral breakdown of levodopa
• Add or switch to a MAO-B inhibitor (selegiline, rasagiline, safinamide) to reduce dopamine catabolism in the CNS
• Add a dopamine agonist
• Adjust levodopa dosing intervals (smaller, more frequent doses)
• Consider continuous drug delivery: levodopa/carbidopa intestinal gel (Duodopa) via percutaneous jejunostomy, or subcutaneous apomorphine infusion for advanced disease

Dopamine Agonists

Non-ergot dopamine agonists (pramipexole, ropinirole, rotigotine patch) act directly on dopamine receptors, no metabolic conversion required.

Advantages:

• Longer half-life than levodopa → lower risk of early motor fluctuations
• Often used as first-line in younger patients (age <60-65) to delay levodopa introduction

Disadvantages:

• Less effective than levodopa for motor symptoms
• More neuropsychiatric side effects: impulse control disorders (gambling, hypersexuality, compulsive eating, shopping), warn all patients; hallucinations; excessive daytime somnolence; oedema
• Older ergot-derived agonists (bromocriptine, cabergoline) are largely abandoned due to cardiac valvulopathy and pulmonary fibrosis

MAO-B Inhibitors

• Selegiline, rasagiline, safinamide
• Inhibit monoamine oxidase type B → reduce dopamine catabolism
• Mild symptomatic benefit; used as monotherapy in early disease or adjuncts in later disease
• Avoid with serotonergic agents (SSRIs, SNRIs, tramadol), risk of serotonin syndrome (less risk with MAO-B selective agents but still caution advised)

Anticholinergics

• Benztropine, trihexyphenidyl
• Primarily for tremor-predominant disease in younger patients
• Largely avoided in older patients, risk of confusion, urinary retention, constipation, and cognitive impairment
• Contraindicated in patients with cognitive impairment

Amantadine

• NMDA receptor antagonist with mild dopaminergic effects
• Useful for reducing peak-dose dyskinesias in later disease
• Also has mild antiparkinsonian effect

Summary Drug Table

Surgical Management

Deep brain stimulation (DBS):

• Most common surgical option; electrodes placed in subthalamic nucleus (STN) or globus pallidus interna (GPi)
• Modulates abnormal basal ganglia circuitry
• Best candidates: Younger patients, good response to levodopa, significant motor fluctuations/dyskinesias, intact cognition
• Not indicated in atypical parkinsonism, significant dementia, or poor levodopa response

Complications & Special Considerations

Managing Non-Motor Symptoms

Drug-Induced Parkinsonism

• Caused by dopamine-blocking or dopamine-depleting agents
• Common culprits: antipsychotics (all typical; most atypical except quetiapine/clozapine), metoclopramide, prochlorperazine, haloperidol
• Usually symmetric onset, no resting tremor, resolves on drug cessation (may take months)
• Never use these agents in a patient with PD

Atypical Parkinsonian Syndromes (Parkinson-Plus)

Palliative and End-Stage Considerations

• As disease progresses, dysphagia becomes a significant concern, early speech pathology referral
• Aspiration pneumonia is a leading cause of death in advanced PD
• Medications may need to be converted to dispersible or patch formulations when swallowing fails
• Subcutaneous apomorphine infusion can maintain motor control when oral medications are no longer feasible
• Address advance care planning early, involve the patient while capacity is preserved

Long-Term Care and Referral in Australian Practice

When to Refer

• New diagnosis → Neurologist for confirmation, workup, and initiation of therapy
• Ongoing shared care with GP is appropriate for stable disease
• Physiotherapy: Gait training, falls prevention, LSVT BIG program
• Speech pathology: Hypophonia (LSVT LOUD), dysphagia assessment
• Occupational therapy: Home safety, adaptive equipment, driving assessment
• Psychologist/psychiatrist: Depression, anxiety, impulse control disorders

Driving

• PD can impair safe driving; in Australia, all medical practitioners have a duty to advise patients about fitness to drive
• Referral to occupational therapist for on-road driving assessment as disease progresses
• Notify relevant state/territory roads authority if the patient lacks capacity to self-restrict

Carer Support

• Significant carer burden, refer carers to Parkinson's Australia and state-based support organisations
• Consider NDIS (if under 65) or aged care pathways (My Aged Care) for home support services
• Advance care planning should involve the patient and family early

Key Exam Points Summary

• Bradykinesia is the cardinal and required feature for diagnosis
• Levodopa + carbidopa is the most effective symptomatic treatment: carbidopa reduces peripheral side effects and increases CNS levodopa availability
• Motor complications (wearing-off, dyskinesias) develop in ~50% within 2-5 years, younger age at onset is a risk factor
• Dopamine agonists preferred in younger patients to delay levodopa; watch for impulse control disorders
• Avoid dopamine antagonists (typical antipsychotics, metoclopramide) in PD, use quetiapine or clozapine if antipsychotic is necessary
• Non-motor symptoms (depression, autonomic dysfunction, cognitive impairment, sleep disorders) are common and impair quality of life
• Atypical features (early falls, symmetric onset, poor levodopa response, autonomic failure, vertical gaze palsy) → suspect Parkinson-plus syndrome, refer to neurology