Parkinson Disease and Movement Disorders
Watch a person with Parkinson disease reach for a cup and you are watching a very specific machine break down. Not the muscles, not the strength, not the will — the initiation and scaling of movement. Movement disorders are the neurology of the basal ganglia, a set of deep grey-matter nuclei that act less like a motor and more like a gain control and gatekeeper for action. When they fail one way you get too little movement (bradykinesia, rigidity — the hypokinetic disorders, of which Parkinson disease is the archetype). When they fail another way you get too much (chorea, dyskinesia, dystonia — the hyperkinetic disorders). Understanding one axis lets you understand the other.
This page teaches the circuit first, then Parkinson disease and its treatment, because levodopa — one of the great triumphs of twentieth-century pharmacology — only makes sense once you understand what dopamine does in that circuit and why replacing it eventually creates new problems of its own.
Learning Objectives
- Describe the direct and indirect pathways of the basal ganglia and how dopamine modulates each
- Explain the four cardinal features of Parkinson disease and distinguish idiopathic PD from mimics
- Classify tremor and other common movement disorders (chorea, dystonia, myoclonus, tics)
- Explain the rationale, pharmacology, and limitations of levodopa and adjunctive therapies
- Recognise levodopa-induced motor fluctuations and dyskinesias and how they are managed
- Place the history of Parkinson disease and the dopamine discovery in context
Quick Answer
Parkinson disease (PD) is a progressive neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra pars compacta, with intracellular alpha-synuclein aggregates (Lewy bodies) as the pathological hallmark. Dopamine loss unbalances the basal ganglia so that movement becomes hard to initiate and scale down. The four cardinal signs are bradykinesia (required for diagnosis), resting tremor, rigidity, and postural instability (late). Diagnosis is clinical and supported by a clear response to dopaminergic therapy. Levodopa, the immediate metabolic precursor of dopamine, remains the most effective symptomatic treatment; combined with a peripheral decarboxylase inhibitor it dramatically restores mobility, but after years of use it produces motor fluctuations ("wearing off") and involuntary dyskinesias. Movement disorders more broadly split into hypokinetic (too little movement, like PD) and hyperkinetic (too much, like chorea, dystonia, and drug-induced dyskinesia).
Where It Came From
In 1817 a London apothecary-surgeon named James Parkinson published An Essay on the Shaking Palsy, describing six people — some only observed in the street — with "involuntary tremulous motion, with lessened muscular power... with a propensity to bend the trunk forward, and to pass from a walking to a running pace." It was an astonishing act of clinical observation from sparse material. Parkinson called it paralysis agitans. Sixty years later the great French neurologist Jean-Martin Charcot refined the description at the Salpêtrière, separated the rigidity and bradykinesia from the tremor, noted that many patients had little true weakness, and generously renamed the condition maladie de Parkinson — Parkinson disease.
For over a century there was a description but no mechanism and no treatment. The breakthrough came from a different problem: the chemistry of the brain. Dopamine had been considered a mere intermediate in the synthesis of noradrenaline until, in the late 1950s, the Swedish pharmacologist Arvid Carlsson showed that dopamine was itself a neurotransmitter and that most of the brain's dopamine sat in the basal ganglia. He gave animals reserpine (which depletes monoamines), rendered them rigid and akinetic — a Parkinson-like state — and then reversed it with L-DOPA, the precursor that can cross into the brain. This work earned Carlsson the 2000 Nobel Prize. In 1960 Ehringer and Hornykiewicz in Vienna performed the crucial human step: they measured dopamine in the brains of people who had died with Parkinson disease and found it drastically depleted in the striatum. The need had been defined for 140 years; now the deficiency was identified. Oral levodopa in effective, tolerable doses was established as therapy by George Cotzias in the late 1960s. A disease known only by its appearance had become the first neurodegenerative disorder with a rational, molecule-level treatment.
The Basal Ganglia: A Circuit You Must Know
The basal ganglia are a group of nuclei: the striatum (caudate + putamen), the globus pallidus (external segment GPe and internal segment GPi), the subthalamic nucleus (STN), and the substantia nigra (pars compacta SNc, which makes dopamine, and pars reticulata SNr, an output nucleus like GPi). They form loops with the cortex and thalamus that select and scale voluntary movement while suppressing unwanted movement.
Two pathways run through the striatum to the output nuclei (GPi/SNr):
- Direct pathway ("go"): cortex → striatum → GPi/SNr (inhibitory). It inhibits the inhibitor. GPi/SNr normally clamp down on the thalamus; the direct pathway releases the thalamus, which then excites cortex — facilitating movement.
- Indirect pathway ("stop/brake"): cortex → striatum → GPe → STN → GPi/SNr. The net effect is to increase GPi/SNr output, tightening the clamp on the thalamus and suppressing movement.
Dopamine from the SNc is the tuning knob. Via D1 receptors it excites the direct (go) pathway; via D2 receptors it inhibits the indirect (brake) pathway. So dopamine simultaneously presses the accelerator and eases off the brake — it facilitates movement.
Now the whole of Parkinson disease follows: lose SNc dopamine, and the go pathway is under-driven while the brake pathway is over-active. GPi/SNr fire too much, the thalamus is over-inhibited, cortex is under-facilitated — hence bradykinesia and rigidity. The STN becomes overactive, which is precisely why placing a deep-brain-stimulation electrode in the STN relieves symptoms.
The mirror image explains hyperkinesias. In Huntington disease, indirect-pathway (D2, enkephalin) striatal neurons degenerate first — the brake fails — so movement escapes as chorea. In hemiballismus, a lesion (classically a stroke) of the STN removes drive to GPi, the clamp loosens, and the opposite limbs fling wildly. Levodopa-induced dyskinesia is a pharmacological version of the same over-facilitated state.
Parkinson Disease: Recognising It
PD is defined clinically by bradykinesia plus at least one of rest tremor or rigidity.
- Bradykinesia — slowness and progressive decrement in amplitude with repetition. Test finger-tapping or hand opening/closing: the movements get smaller and slower. This is the obligatory feature. It shows as small handwriting (micrographia), reduced arm swing, and a masked, expressionless face (hypomimia).
- Resting tremor — classically 4–6 Hz, "pill-rolling," worst at rest, damping with action, often starting in one hand. Asymmetry is a strong clue to PD.
- Rigidity — increased tone throughout the range of passive movement ("lead-pipe"), often with a ratchety cogwheel quality when tremor is superimposed. Unlike spasticity, it is not velocity-dependent.
- Postural instability — impaired righting reflexes causing falls; a late feature. If it appears early, doubt the diagnosis.
Two more diagnostic pillars matter for exams: marked and sustained response to dopaminergic therapy, and asymmetric onset. Supportive non-motor features often precede motor signs by years: REM-sleep behaviour disorder (acting out dreams), hyposmia (loss of smell), constipation, and depression — consistent with Braak's proposal that alpha-synuclein pathology ascends from the lower brainstem and olfactory system before reaching the midbrain.
Pathology: loss of pigmented dopaminergic neurons in the SNc (visible as pallor of the normally black substantia nigra at autopsy) and Lewy bodies — intracytoplasmic inclusions of aggregated alpha-synuclein. Symptoms typically appear once roughly 60–80% of nigrostriatal dopamine is already gone, which is why neuroprotection has been so hard to demonstrate: by diagnosis the horse has largely bolted.
Red flags for a PD mimic ("Parkinson-plus" and secondary parkinsonism)
- Early falls + vertical gaze palsy → progressive supranuclear palsy
- Early autonomic failure (fainting, incontinence) + cerebellar signs → multiple system atrophy
- Symmetric parkinsonism + exposure to dopamine-blocking drugs (antipsychotics, metoclopramide) → drug-induced parkinsonism
- Prominent early dementia with fluctuating cognition and visual hallucinations → dementia with Lewy bodies
- Poor or absent response to levodopa → reconsider the diagnosis
Tremor and the Other Hyperkinesias
Tremor is a rhythmic oscillation. The exam skill is classifying it by when it occurs:
- Rest tremor (present at rest, damps on action) → Parkinson disease.
- Postural/action tremor (worst holding a posture or reaching) → essential tremor, the commonest movement disorder. It is typically bilateral, involves the hands and often the head/voice, has a family history, and characteristically improves with a small amount of alcohol and with propranolol or primidone. Do not confuse it with PD: essential tremor has no bradykinesia and no rigidity.
- Intention tremor (worsens as the finger approaches a target) → cerebellar disease.
Other hyperkinetic disorders worth knowing:
| Disorder | Character | Classic cause |
|---|---|---|
| Chorea | Brief, random, flowing, dance-like | Huntington disease, Sydenham (post-strep), pregnancy, lupus |
| Dystonia | Sustained co-contraction, twisting postures | Genetic, drug-induced, focal (writer's cramp, cervical) |
| Myoclonus | Sudden, brief, shock-like jerks | Metabolic, epileptic, post-hypoxic |
| Tics | Stereotyped, suppressible, with an urge | Tourette syndrome |
| Hemiballismus | Large-amplitude flinging of one side | Subthalamic nucleus lesion |
Dyskinesia is a general term for abnormal involuntary movement; in PD clinics it almost always refers to levodopa-induced dyskinesia — writhing, choreiform movements that appear as a side effect of chronic dopamine replacement (see below).
Levodopa and the Pharmacology of Replacement
Dopamine itself cannot be given as a drug — it does not cross the blood–brain barrier. Levodopa (L-DOPA) does, using the large-neutral-amino-acid transporter, and is then decarboxylated to dopamine inside surviving neurons. Given alone, most of an oral dose is converted to dopamine in the periphery by DOPA decarboxylase, causing nausea, vomiting, and hypotension while little reaches the brain. The solution, and the standard of care, is to combine levodopa with a peripheral decarboxylase inhibitor — carbidopa (as co-careldopa/Sinemet) or benserazide (co-beneldopa/Madopar) — which does not itself cross into the brain. This lets far more levodopa reach the CNS, cuts the required dose by about 75%, and greatly reduces peripheral side effects.
Other drug classes act on the same system and are used to spare or supplement levodopa:
- Dopamine agonists (pramipexole, ropinirole, rotigotine patch) stimulate receptors directly; useful early or as add-ons; watch for somnolence and impulse-control disorders (pathological gambling, hypersexuality).
- MAO-B inhibitors (selegiline, rasagiline) block central dopamine breakdown; modest benefit, useful early.
- COMT inhibitors (entacapone, opicapone) block peripheral levodopa breakdown, extending each dose's "on" time — used for wearing-off.
- Amantadine — the one drug that specifically reduces established dyskinesia (NMDA antagonism).
- Anticholinergics (trihexyphenidyl) — mainly for tremor in younger patients; avoid in the elderly (confusion).
The long-term problem: motor fluctuations and dyskinesia
Levodopa works beautifully for years — the so-called honeymoon period. Over time, as dopaminergic neurons continue to die, the brain loses its ability to buffer and store dopamine, so the patient's motor state starts to track the blood level of the drug. Two problems emerge:
- Wearing off: benefit fades before the next dose ("off" periods of return of stiffness and slowness). Managed by more frequent dosing, COMT or MAO-B inhibitors, agonists, or extended-release formulations.
- Peak-dose dyskinesia: involuntary choreiform movements when the dopamine level is high, reflecting pulsatile, non-physiological receptor stimulation of a sensitised striatum.
Worked example
A 68-year-old man has had asymmetric right-hand rest tremor, micrographia, and reduced right arm swing for two years. Examination shows cogwheel rigidity and decrementing finger-taps on the right; gait is slow with reduced arm swing but no falls. This is textbook idiopathic PD. He is started on co-careldopa three times daily and improves markedly — the response supports the diagnosis. Four years later he reports that by late morning, before his midday dose, the tremor and stiffness return, and about an hour after each dose he develops writhing movements of his trunk. This is the classic combination of wearing-off (an "off" phenomenon, treated by shortening the dose interval or adding entacapone) and peak-dose dyskinesia (an "on" phenomenon, helped by amantadine or smoothing the dopamine level). When medication management is exhausted, deep brain stimulation of the STN or GPi can dramatically reduce both fluctuations and dyskinesia.
Real-World Applications
- Bedside diagnosis without a scanner: PD is a clinical diagnosis. Recognising asymmetric rest tremor plus decrementing bradykinesia, and confirming a levodopa response, is the core skill — imaging is used mainly to exclude mimics.
- Medication timing on the ward: in-patients with PD must get their levodopa on time, every time. A missed or delayed dose can precipitate severe "off" immobility, aspiration risk, and rarely a dangerous akinetic crisis. This is a genuine patient-safety issue that junior doctors routinely mishandle.
- Avoid dopamine blockers: never give metoclopramide or typical antipsychotics (e.g. haloperidol) to a PD patient for nausea or delirium — they block the very receptors you are trying to stimulate and can cause a crisis. Use domperidone for nausea and quetiapine or clozapine for psychosis.
- Public health / iatrogenic disease: drug-induced parkinsonism from antipsychotics is common and reversible — always review the medication list before diagnosing PD.
Common Mistakes
- Calling any tremor "Parkinson's." Wrong because the commonest tremor is essential tremor, which is an action tremor with no bradykinesia. Correction: classify tremor by when it occurs (rest vs posture vs intention) and look for the accompanying bradykinesia and rigidity that define PD.
- Thinking rigidity equals spasticity. Wrong because they are different signs from different circuits: spasticity (upper motor neuron) is velocity-dependent and worse in one direction (clasp-knife); rigidity (basal ganglia) is uniform through the range (lead-pipe/cogwheel). Correction: test tone at different speeds and note the pattern.
- Delaying or omitting levodopa doses in hospital. Wrong because PD patients depend on strict timing; a two-hour delay can mean the patient is frozen and cannot swallow. Correction: prescribe exact times, allow self-administration where safe, and never substitute a dopamine-blocking antiemetic.
- Assuming dyskinesia means "too much disease." Wrong — dyskinesia is a sign of treatment (high dopamine levels), not of worsening pathology. Correction: reduce or smooth the dopaminergic load, or add amantadine, rather than escalating the dose.
Comparison and Connections
| Feature | Parkinson disease | Essential tremor |
|---|---|---|
| Tremor type | Rest, 4–6 Hz, pill-rolling | Action/postural, faster |
| Bradykinesia / rigidity | Present | Absent |
| Symmetry | Asymmetric onset | Usually bilateral |
| Alcohol response | No change | Often improves |
| First-line drug | Levodopa | Propranolol / primidone |
| Axis | Hypokinetic | Hyperkinetic |
|---|---|---|
| Problem | Too little movement | Too much movement |
| Basal ganglia state | Excess GPi output | Reduced GPi output |
| Example | Parkinson disease | Chorea, hemiballismus, dyskinesia |
| Dopamine | Deficient | Relative excess / brake failure |
The basal ganglia connect naturally to the study of the nervous system's physiology and to pharmacology for the dopaminergic drug classes. Within neurology, contrast the rhythmic movement of tremor with the paroxysmal electrical disturbances covered under epilepsy, and the neurodegeneration theme with dementia.
Practice Questions
Recall
Q: What is the pathological hallmark of Parkinson disease and which neurotransmitter-producing neurons are lost? A: Lewy bodies (aggregated alpha-synuclein) inside neurons; loss of dopaminergic neurons in the substantia nigra pars compacta.
Understanding
Q: Why is carbidopa given together with levodopa, and why does carbidopa not cause its own central effects? A: Carbidopa inhibits peripheral DOPA decarboxylase, so less levodopa is converted to dopamine outside the brain — increasing CNS delivery and reducing nausea and hypotension. Carbidopa cannot cross the blood–brain barrier, so it does not block the needed central conversion.
Application
Q: A PD patient on levodopa develops choreiform movements about an hour after each dose but becomes stiff and slow before the next dose is due. Name both phenomena and one management step for each. A: Peak-dose dyskinesia (add amantadine or smooth/lower the dopamine peak) and wearing-off (shorten the dose interval or add a COMT inhibitor such as entacapone).
Analysis
Q: Using the direct/indirect pathway model, explain why both a subthalamic nucleus stroke and chronic levodopa can produce excess movement, whereas nigral degeneration produces too little. A: Nigral dopamine loss under-drives the direct (go) pathway and over-drives the indirect (brake) pathway, raising GPi/SNr output and over-inhibiting the thalamus — hypokinesia. An STN lesion removes excitatory drive to GPi, lowering its output and releasing the thalamus → hemiballismus. Chronic pulsatile levodopa over-facilitates a sensitised striatum, similarly lowering effective GPi restraint → dyskinesia. Both hyperkinetic states share reduced GPi output.
FAQ
Is Parkinson disease hereditary? Mostly not — the great majority is sporadic. A minority (often younger onset) carries mutations such as LRRK2, PARKIN, or GBA, and having a first-degree relative modestly raises risk. It is best thought of as gene–environment interaction.
Does levodopa "stop working" or damage the brain? It does not damage the brain, and the drug itself keeps working. What changes is the disease: as more neurons die, the brain can no longer buffer dopamine, so the smooth all-day benefit gives way to fluctuations. This is why timing and adjuncts become important, not because levodopa is "used up."
Should levodopa be delayed to "save it for later"? The old fear that early levodopa hastens decline is not supported by evidence. Current practice treats symptoms that affect quality of life; levodopa remains the most effective drug. In younger patients an agonist may be started first mainly to delay dyskinesia, not because levodopa is harmful.
Why do PD patients sometimes lose their sense of smell or act out dreams years earlier? Alpha-synuclein pathology appears to begin in the olfactory bulb and lower brainstem before reaching the midbrain (Braak staging). Hyposmia and REM-sleep behaviour disorder are among the earliest markers and can precede motor signs by a decade.
What is deep brain stimulation and who is it for? A neurosurgical implant delivers high-frequency stimulation to the STN or GPi, effectively "quieting" the overactive circuit. It is offered to patients with good levodopa responsiveness but disabling fluctuations or dyskinesia, and can markedly reduce both. It treats symptoms, not the underlying degeneration.
Can nausea from PD drugs be treated normally? No — avoid metoclopramide and prochlorperazine, which block dopamine receptors and can worsen parkinsonism. Domperidone is preferred because it works largely outside the blood–brain barrier.
Quick Revision
- PD = loss of SNc dopamine neurons + Lewy bodies (alpha-synuclein); symptoms once ~60–80% depleted.
- Cardinal signs: bradykinesia (required) + rest tremor / rigidity; postural instability is late; onset is asymmetric.
- Dopamine excites the direct (go, D1) pathway and inhibits the indirect (brake, D2) pathway; losing it → hypokinesia.
- Classify tremor: rest = PD; action = essential tremor; intention = cerebellar.
- Levodopa + carbidopa/benserazide is first-line; the DDC inhibitor keeps dopamine formation out of the periphery.
- Long-term: wearing-off (off state) and peak-dose dyskinesia (on state); amantadine helps dyskinesia; DBS for refractory cases.
- Never give dopamine-blocking antiemetics/antipsychotics; use domperidone / quetiapine.
- History: James Parkinson 1817; Charcot named it; Carlsson (dopamine as transmitter), Hornykiewicz (striatal depletion), Cotzias (oral levodopa).
Related Topics
Prerequisites
Related Topics
Next Topics
- Dementia and neurodegeneration (Alzheimer and Lewy body disease)
- Stroke and vascular neurology (including basal ganglia strokes and hemiballismus)