Parkinson's Disease — Causes, Symptoms & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Parkinson's disease (PD) is a progressive neurodegenerative disorder caused by the loss of dopaminergic neurons in the substantia nigra pars compacta — a region of the midbrain critical for motor control. The pathological hallmark is Lewy bodies — intracellular aggregates of misfolded alpha-synuclein protein. PD affects approximately 10 million people worldwide, making it the second most common neurodegenerative disease after Alzheimer's disease. Its incidence increases sharply with age, doubling with each decade over 60 years. The classical motor triad of tremor, rigidity, and bradykinesia, combined with postural instability, defines the disease clinically. Non-motor features (constipation, hyposmia, REM sleep behaviour disorder, depression) often precede motor symptoms by years. Over 80% of dopaminergic neurons in the substantia nigra are lost before motor symptoms appear, highlighting the brain's compensatory capacity and the importance of neuroprotective research. GLP-1 receptor agonists are under investigation as potentially disease-modifying agents in PD.

PD results from a combination of genetic predisposition and environmental factors leading to progressive dopaminergic neurodegeneration. Approximately 10-15% of PD cases are familial — caused by mutations in LRRK2 (most common genetic cause in some populations), SNCA, PRKN, PINK1, DJ-1, and GBA1. GBA1 (Gaucher disease gene) mutations are the most common genetic risk factor, conferring 5-10x elevated risk. Environmental risk factors include pesticide exposure (paraquat, rotenone), manganese exposure (occupational), rural living, and well water use. Protective factors include coffee consumption, smoking (paradoxically — nicotine is neuroprotective), and physical exercise. Age is the single strongest risk factor. The Braak staging model proposes that pathology begins in the peripheral nervous system and olfactory bulb before reaching the substantia nigra.

Motor features: Resting tremor (3-5 Hz, 'pill-rolling' thumb tremor, typically unilateral onset, suppressed with voluntary movement), Rigidity (lead-pipe or cogwheel resistance on passive joint movement), Bradykinesia (slowness of movement with progressive fatigue of repetitive movements — assessed by finger tapping, hand movements), and Postural Instability (late feature — impaired righting reflexes, pull test positive — leading to falls). Gait: shuffling small steps, reduced arm swing, festination (accelerating forward), freezing of gait (sudden inability to initiate or continue walking). Micrographia (small cramped handwriting). Non-motor features (often more disabling): cognitive impairment and dementia (Parkinson's disease dementia in 80% after 20 years), depression (50%), anxiety, psychosis (hallucinations, delusions — often drug-induced), autonomic dysfunction (orthostatic hypotension, constipation, urinary urgency, seborrhoea, sweating), sleep disturbance (REM sleep behaviour disorder, insomnia, excessive daytime sleepiness), hyposmia (reduced smell — prodromal feature), pain, and fatigue.

PD remains a clinical diagnosis based on UK Parkinson's Disease Society Brain Bank Criteria: bradykinesia plus at least one of tremor, rigidity, or postural instability, with exclusion of other causes and supportive criteria (unilateral onset, rest tremor, progressive course, good response to levodopa). There is no definitive diagnostic blood test or imaging for typical PD. DaTscan (123I-FP-CIT SPECT imaging) demonstrates reduced dopamine transporter uptake in the striatum — confirms dopaminergic deficit but cannot distinguish PD from other parkinsonian syndromes (MSA, PSP, DLB). MRI brain is performed to exclude structural causes (normal in PD). Autonomic tests, olfactory testing, and cognitive assessment support diagnosis and severity evaluation. Genetic testing is offered to patients with young-onset PD or strong family history. Response to levodopa (50% or greater UPDRS improvement) supports PD diagnosis.

Levodopa (combined with carbidopa/benserazide to prevent peripheral conversion): the most effective symptomatic treatment, producing dramatic initial improvement. Long-term complications include dyskinesias (involuntary movements) and motor fluctuations (wearing-off between doses) after 5-10 years. Dopamine agonists (pramipexole, ropinirole, rotigotine patch): used as initial therapy in younger patients to delay levodopa motor complications, or in combination. Risk of impulse control disorders (gambling, hypersexuality, binge eating). MAO-B inhibitors (rasagiline, selegiline): modest symptomatic benefit, neuroprotective properties being studied; often used as initial or add-on therapy. COMT inhibitors (entacapone, opicapone): extend levodopa duration; used for wearing-off. Amantadine: reduces dyskinesias in advanced PD. Advanced therapies: Duodopa (levodopa-carbidopa intestinal gel via PEG-J pump) for severe motor fluctuations; Apomorphine (subcutaneous infusion or pen injection) for rescue of off episodes; Deep Brain Stimulation (DBS) — implanted electrodes in subthalamic nucleus or globus pallidus — highly effective for motor fluctuations and dyskinesias in carefully selected patients. Physiotherapy, speech therapy, and occupational therapy are essential throughout.

Advanced Parkinson's disease causes severe motor disability — gait freezing (sudden inability to initiate movement), severe postural instability with falls (40-60% of patients fall at least once yearly), dysphagia causing aspiration pneumonia (the leading cause of PD-related death), and dysarthria. Non-motor complications include Parkinson's disease dementia (80% after 20 years), hallucinations and psychosis from dopaminergic medications, autonomic dysfunction (orthostatic hypotension, constipation, urinary retention), depression and anxiety (30-40%), and REM sleep behaviour disorder. Levodopa-induced motor complications (dyskinesias, wearing-off phenomena) develop in 50% after 5 years of treatment. Deep brain stimulation of the subthalamic nucleus reduces dyskinesias by 60-80% in carefully selected patients.

No proven preventive strategy exists. Regular aerobic exercise is the strongest evidence-based intervention that may slow PD progression — exercise promotes neuroplasticity, improves motor function, cognition, and mood. Aim for 150 minutes weekly. Physiotherapy (including treadmill, dance, boxing, and Tai Chi) improves balance, gait, and fall prevention. Speech and language therapy (Lee Silverman Voice Treatment — LSVT LOUD) addresses hypophonia. Occupational therapy for daily living adaptations, fall prevention, and home safety assessment. Maintain adequate nutrition — dysphagia develops in advanced disease; dietitian input is essential. Parkinson's UK and local support organizations provide education, support groups, and exercise programs. Consult a movement disorder specialist (neurologist with PD subspecialty) for optimal medication management as disease progresses.

Consult a neurologist promptly if you notice a new resting tremor (particularly asymmetric, in one hand), unexplained stiffness and slowing of movement, or significant changes in handwriting, facial expression, or gait. Non-motor prodromal symptoms — loss of sense of smell, REM sleep behaviour disorder (acting out dreams violently), and chronic constipation — can precede PD by years and warrant neurological review. Established PD patients should seek urgent medical attention for sudden worsening of motor symptoms (consider dopaminergic medication interaction or infection), high fever and muscle rigidity (neuroleptic malignant syndrome or dopaminergic drug withdrawal), hallucinations, falls causing injuries, or significant dysphagia with aspiration risk.