Parkinson's Disease is a progressive neurodegenerative disorder primarily affecting movement but also producing a wide range of non-motor symptoms. The principal motor features are bradykinesia, rigidity and tremor, although not every person develops all three to the same degree. The disease is associated with progressive loss of dopamine-producing neurons in the substantia nigra, reducing dopaminergic signalling within circuits involved in movement. Symptoms usually develop gradually and initially may be relatively mild. Non-motor manifestations can include depression, anxiety, sleep disturbance, loss of smell, constipation, autonomic dysfunction and cognitive changes. The NHS describes Parkinson's disease as progressive brain damage involving loss of nerve cells in the substantia nigra and reduced dopamine availability.
The motor syndrome of Parkinson's Disease involves characteristic changes in basal-ganglia circuitry. Dopamine from the substantia nigra pars compacta modulates pathways that facilitate and inhibit movement, and loss of dopaminergic input disrupts this balance. Bradykinesia is central to the clinical diagnosis and manifests as slowness and reduced amplitude of movement. Rigidity produces increased resistance to passive movement, while resting tremor often begins asymmetrically and may diminish during voluntary movement. Postural instability can emerge later and increases fall risk. Parkinsonism is the broader clinical syndrome of bradykinesia combined with other characteristic movement abnormalities, whereas Parkinson's Disease is one cause of parkinsonism. This distinction matters because several other neurological disorders and medications can produce parkinsonian features without having the same underlying pathology.
Non-motor symptoms are a major part of Parkinson's Disease and can precede obvious motor abnormalities. Loss of smell can occur early, while constipation, REM sleep behaviour disorder, depression, anxiety and autonomic dysfunction can also occur. Later cognitive changes may include impaired executive function, slowed thinking and visuospatial difficulties. Some people eventually develop Parkinson's disease dementia, characterised by more substantial cognitive impairment affecting everyday functioning. Psychosis can occur, sometimes related to disease progression and sometimes influenced by medication or other factors. Recognising these non-motor features is important because they can significantly affect quality of life even when movement symptoms are relatively controlled. Parkinson's Disease should therefore not be conceptualised solely as a tremor disorder.
The cause of Parkinson's Disease is usually multifactorial and remains incompletely understood. Most cases are considered sporadic, involving interactions between genetic susceptibility, ageing and environmental influences. A smaller proportion are associated with identifiable genetic variants, and family history can increase risk. Research has implicated mitochondrial dysfunction, impaired protein handling, neuroinflammation, oxidative stress and other cellular mechanisms, but no single mechanism explains every case.
The abnormal protein alpha-synuclein is closely associated with many forms of Parkinson's pathology, particularly through Lewy bodies and related aggregates, although the relationship between alpha-synuclein accumulation and neuronal death is complex. The existence of multiple biological pathways helps explain clinical heterogeneity and the difficulty of developing one universally effective disease-modifying treatment.
Diagnosis of Parkinson's Disease is primarily clinical. There is no single routine test that conclusively establishes the diagnosis, so specialists consider the history, neurological examination and pattern of symptoms. Brain imaging may be used to exclude other causes, while specialised dopamine-transporter imaging can sometimes provide additional evidence in uncertain cases but does not independently distinguish every form of parkinsonism. Response to dopaminergic treatment can also contribute to clinical reasoning, although it is not a standalone diagnostic test. Early diagnosis can be difficult because symptoms may initially be subtle, and tremor or slowness can arise from other conditions. The NHS notes that diagnosis is based on symptoms, medical history and detailed physical examination, with specialist referral when Parkinson's disease is suspected.
Treatment aims to control symptoms, preserve function and maintain quality of life because there is currently no established cure that reverses the underlying neurodegeneration. Levodopa is the most effective medicine for many motor symptoms and is commonly combined with a peripheral dopa-decarboxylase inhibitor. Dopamine agonists and monoamine oxidase-B inhibitors can also be used in appropriate circumstances. Medication effects can change over years, with some people developing wearing-off fluctuations or dyskinesias associated with long-term levodopa exposure. Physiotherapy, occupational therapy and speech and language therapy are important components of multidisciplinary care. Advanced disease may be treated with approaches such as deep-brain stimulation in selected individuals. Treatment therefore requires continual adjustment as motor, cognitive, psychiatric and autonomic symptoms evolve.
Parkinson's Disease can substantially affect walking, balance, handwriting, speech, swallowing, facial expression, dressing, eating and other everyday activities. Falls become an important concern when postural instability develops, while swallowing problems can contribute to malnutrition, dehydration and aspiration. Speech therapy can address reduced vocal volume and communication difficulties, while occupational therapy can adapt tasks and environments to preserve independence.
Exercise and physical activity are commonly incorporated into management because maintaining strength, mobility and cardiovascular fitness can support function, although exercise is not a cure. Social and psychological support is also important because depression, anxiety, sleep problems and changing independence can be as disruptive as the motor symptoms. The course varies considerably between individuals, so care must be personalised rather than based on a fixed timetable.
Parkinson's Disease should be distinguished from essential tremor, drug-induced parkinsonism, atypical parkinsonian disorders and dementia with Lewy bodies. Essential tremor usually produces action or postural tremor rather than the classic resting tremor pattern, while drug-induced parkinsonism can follow exposure to dopamine-blocking medicines. Dementia with Lewy bodies shares pathological features with Parkinson's disease dementia, and the conventional clinical distinction often depends on the timing of cognitive and motor symptoms: dementia arising before or around the onset of parkinsonism is generally classified differently from dementia developing after established Parkinson's disease. NICE material recognises the close relationship between Parkinson's disease dementia and dementia with Lewy bodies and notes that they may represent a continuum of related pathology.
