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

Medical Student Learning Objective: MS_NEU_012

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