Parkinson Disease

• Definition: A progressive neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra pars compacta, defined clinically by bradykinesia plus rest tremor and/or rigidity, and pathologically by intraneuronal α-synuclein inclusions (Lewy bodies).
• Why it matters: It is a leading cause of late-life disability. Motor decline drives falls, fractures, and immobility, while dysphagia predisposes to aspiration pneumonia — the usual proximate cause of death. Non-motor features (dementia, psychosis, orthostatic hypotension, depression) often determine nursing-home placement more than tremor does.
• Therapeutic significance: It is one of the few neurodegenerative diseases with genuinely effective symptomatic therapy — dopamine replacement — but no agent has been shown to be disease-modifying, so counseling centers on symptom control and complication avoidance.

Epidemiology worth recalling

• Frequency: The second most common neurodegenerative disease after Alzheimer disease and the most common movement disorder of aging; prevalence is on the order of 1% of adults over age 60 and rises steeply with each subsequent decade.
• Age: Typical onset in the sixth to eighth decades. Onset before age 50 is termed young-onset PD and should prompt consideration of monogenic causes (PARKIN, PINK1, DJ-1).
• Sex: Men are affected roughly 1.5 times as often as women.
• Course: Insidious, asymmetric onset with gradual progression over years to decades; a prodromal phase marked by hyposmia, constipation, and REM sleep behavior disorder may precede motor onset by a decade or more.
• Distinguishing point: Parkinsonism (the syndrome) is far broader than Parkinson disease (the idiopathic Lewy-body illness); drug-induced parkinsonism is the most common non-degenerative mimic and is potentially reversible.

Mechanistic categories

• Idiopathic (sporadic) PD: The large majority of cases. Presumed multifactorial interaction of aging, genetic susceptibility, and environmental exposure converging on α-synuclein misfolding, mitochondrial complex I dysfunction, oxidative stress, and impaired lysosomal/proteasomal clearance.
• Monogenic PD: Roughly 5–10% familial. LRRK2 (G2019S) is the most common autosomal dominant form, enriched in Ashkenazi Jewish and North African Berber populations and clinically nearly indistinguishable from sporadic disease. SNCA duplication/triplication causes dose-dependent early aggressive disease. Autosomal recessive PARKIN, PINK1, and DJ-1 mutations impair mitophagy and cause young-onset, slowly progressive, levodopa-responsive disease.
• Genetic risk modifier: Heterozygous GBA (glucocerebrosidase) variants are the most common genetic risk factor for PD and predict earlier cognitive decline — the same gene that is biallelically mutated in Gaucher disease.
• Toxic/secondary parkinsonism: MPTP (a meperidine-analog contaminant) is oxidized by MAO-B to MPP+, which poisons complex I in nigral neurons; rotenone and paraquat act similarly. Manganese, carbon monoxide, and repetitive head trauma produce parkinsonism with atypical features. Dopamine-blocking drugs (typical antipsychotics, risperidone, metoclopramide, prochlorperazine) and dopamine-depleting agents (reserpine, tetrabenazine) cause drug-induced parkinsonism, classically symmetric and reversible over weeks to months.

Non-modifiable risk factors

• Advancing age — the dominant risk factor.
• Male sex; family history in a first-degree relative; specific ethnic backgrounds for LRRK2 founder mutations.

Modifiable/environmental risk factors examiners plant in the stem

• Pesticide and herbicide exposure, rural residence, well-water use, and farming occupation.
• Traumatic brain injury, particularly repetitive.
• Inverse associations: cigarette smoking and caffeine intake are consistently associated with lower risk — an epidemiologic curiosity, not a recommendation.

• Initiating lesion: Misfolded, phosphorylated α-synuclein aggregates into oligomers and fibrils that seed further misfolding in a prion-like manner, overwhelming lysosomal and proteasomal clearance. Aggregates accumulate as Lewy bodies and Lewy neurites. Mitochondrial complex I dysfunction and oxidative stress amplify the injury — dopamine metabolism itself generates reactive quinones, which may explain nigral vulnerability.
• Anatomic target: Pigmented dopaminergic neurons of the substantia nigra pars compacta degenerate, producing gross depigmentation of the substantia nigra (and locus coeruleus) at autopsy — the classic neuropathology image.
• Threshold effect: Compensatory upregulation of surviving terminals means motor signs emerge only after a substantial majority of striatal dopamine is lost; this latency explains the long prodrome and why disease is already advanced at diagnosis.

Circuit consequences — why the motor picture looks the way it does

• Direct pathway hypoactivity: Striatal D1 receptors are excitatory; dopamine loss removes drive from the movement-facilitating direct pathway.
• Indirect pathway hyperactivity: Striatal D2 receptors are inhibitory; dopamine loss disinhibits the indirect pathway, increasing subthalamic nucleus drive.
• Net effect: Both changes increase inhibitory GPi/SNr output to the thalamus, so thalamocortical excitation of the supplementary motor area falls. The result is hypokinesia — poverty and slowness of movement with difficulty initiating it. Rigidity reflects loss of normal descending modulation of tone; tremor arises from pathologic oscillatory synchronization within basal ganglia–thalamocortical loops.
• Relative cholinergic excess in the striatum accounts for the benefit of anticholinergics on tremor.

Beyond the nigra

• Braak staging: Synuclein pathology is proposed to ascend from the olfactory bulb and dorsal motor nucleus of the vagus, through pontine and midbrain nuclei, and finally to limbic and neocortical regions — accounting for anosmia, constipation, and REM sleep behavior disorder preceding tremor, and for dementia appearing late.
• Nondopaminergic degeneration (cholinergic nucleus basalis, noradrenergic locus coeruleus, serotonergic raphe) explains the levodopa-unresponsive features: freezing, postural instability, dysautonomia, depression, and cognitive decline.

Cardinal motor features (asymmetric onset is essential)

• Bradykinesia: Required for diagnosis. Slowness plus decrementing amplitude on repetitive finger tapping — the sequence effect — reflecting failed thalamocortical facilitation of movement. Manifests as micrographia, hypomimia (masked facies), reduced blink rate, hypophonic monotone speech, and difficulty rising from a chair.
• Rest tremor: 4–6 Hz pill-rolling tremor that appears when the limb is fully supported, dampens with voluntary action, and re-emerges after a latency when arms are held out. Worsens with mental stress (serial sevens) and disappears in sleep.
• Rigidity: Velocity-independent increased tone throughout range; cogwheeling when superimposed tremor interrupts the resistance. Contrast with spasticity's velocity-dependent clasp-knife pattern.
• Postural instability: A later feature; positive pull test with retropulsion. Its presence early is a red flag for progressive supranuclear palsy.

Gait and posture

• Shuffling, festinating gait with stooped posture, reduced arm swing, en-bloc turning, and freezing of gait at doorways — freezing is largely nondopaminergic and responds poorly to levodopa.

Non-motor features, often preceding motor onset by years

• Hyposmia/anosmia (olfactory bulb synucleinopathy) and constipation (enteric and dorsal vagal involvement).
• REM sleep behavior disorder: loss of REM atonia from pontine involvement; dream enactment is highly predictive of future synucleinopathy.
• Autonomic: orthostatic hypotension, urinary urgency, erectile dysfunction, seborrheic dermatitis, and sialorrhea from reduced spontaneous swallowing rather than excess saliva production.
• Neuropsychiatric: depression, anxiety, apathy, and late executive-predominant dementia with visual hallucinations.
• Exam sign: Myerson sign — failure to habituate to repeated glabellar tapping.

• Diagnosis is clinical. There is no routine confirmatory biomarker; the initial step is a careful history and neurologic exam, plus a medication review to exclude dopamine-blocking drugs.

Criteria used by name

• MDS Clinical Diagnostic Criteria (2015): Establish parkinsonism = bradykinesia plus at least one of rest tremor or rigidity. Then classify as clinically established or clinically probable PD using supportive criteria (dramatic and sustained levodopa response, levodopa-induced dyskinesia, rest tremor, documented hyposmia or cardiac MIBG denervation), absolute exclusion criteria (cerebellar signs, downward vertical supranuclear gaze palsy, cortical sensory loss, exposure to a dopamine blocker, absent response to high-dose levodopa), and red flags (early falls, early severe dysautonomia, rapid gait decline, symmetric non-progressive course).
• UK Parkinson's Disease Society Brain Bank criteria are the older framework still cited on exams.
• Severity/staging: Hoehn and Yahr stages (unilateral → bilateral → postural instability → assisted → wheelchair/bed-bound) and the MDS-UPDRS for longitudinal tracking.

Ancillary testing — used to exclude mimics, not to confirm PD

• Brain MRI: Normal in PD. Obtain it to exclude vascular parkinsonism, normal-pressure hydrocephalus, and structural lesions; look for the hummingbird sign of PSP or the hot cross bun sign of MSA.
• DaTscan (¹²³I-ioflupane SPECT): Shows reduced striatal presynaptic dopamine transporter binding. It distinguishes degenerative parkinsonism from essential tremor, drug-induced parkinsonism, and psychogenic tremor — but it is abnormal in PSP, MSA, and CBD too, so it cannot separate PD from atypical parkinsonism.
• Levodopa challenge: A robust, sustained response strongly supports PD; failure of an adequate trial argues for an atypical syndrome.
• Selected settings: ceruloplasmin and serum copper in a patient under 40 to exclude Wilson disease; genetic testing in young-onset or strongly familial cases. CSF α-synuclein seed amplification assays are an emerging research tool.
• Gold standard: postmortem demonstration of nigral neuron loss with Lewy bodies.

• No stabilization is needed at diagnosis, but two situations are urgent: abrupt withdrawal of dopaminergic therapy (parkinsonism-hyperpyrexia syndrome) and aspiration from dysphagia. Never stop levodopa abruptly, including perioperatively.

When to start and with what

• Timing: Treat when symptoms interfere with function; there is no evidence that delaying levodopa slows disease.
• Levodopa/carbidopa (dopamine precursor plus peripheral AADC inhibitor): the most effective symptomatic agent. The American Academy of Neurology 2021 practice guideline concludes that levodopa provides superior motor benefit to dopamine agonists in early PD and recommends it as initial therapy for most patients, while counseling that earlier levodopa carries higher risk of dyskinesia and dopamine agonists carry higher risk of impulse-control disorders, hallucinations, somnolence, and edema — a trade-off that favors agonists or MAO-B inhibitors mainly in younger patients with mild symptoms.
• Dopamine agonists (pramipexole, ropinirole, rotigotine patch) and MAO-B inhibitors (rasagiline, selegiline) are reasonable alternatives in young patients seeking to delay motor complications. AAN specifically advises against dopamine agonists in patients over 70 and in those with cognitive impairment or prior impulse-control problems.

Escalation for motor fluctuations and dyskinesia

• Wearing-off: fractionate levodopa dosing, or add a COMT inhibitor (entacapone, opicapone) or MAO-B inhibitor to extend levodopa half-life; istradefylline (adenosine A2A antagonist) is an add-on option.
• Peak-dose dyskinesia: amantadine (NMDA antagonist), including the extended-release formulation.
• Sudden off episodes: subcutaneous or sublingual apomorphine rescue.
• Advanced disease: continuous carbidopa/levodopa enteral suspension via PEG-J.

Device/surgical therapy

• Deep brain stimulation of the subthalamic nucleus or globus pallidus interna for levodopa-responsive motor fluctuations, dyskinesia, or refractory tremor with intact cognition and no active psychosis; MRI-guided focused ultrasound thalamotomy is an ablative alternative for tremor.

Contraindicated or hazardous

• Typical antipsychotics, risperidone, and metoclopramide — D2 blockade worsens parkinsonism. For PD psychosis use quetiapine, clozapine, or pimavanserin.
• Nonselective MAOIs with meperidine; anticholinergics (trihexyphenidyl, benztropine) in older adults per the AGS Beers Criteria.
• Rivastigmine is the cholinesterase inhibitor approved for PD dementia.

Disease-related

• Falls and hip fracture: from postural instability, freezing, and orthostasis; largely levodopa-unresponsive because the underlying circuits are nondopaminergic.
• Dysphagia and aspiration pneumonia: bradykinetic oropharyngeal musculature; signaled by coughing with meals, weight loss, or recurrent lower-lobe infiltrates. This is the usual immediate cause of death and is an emergency when it presents as hypoxic pneumonia.
• PD dementia: executive and visuospatial decline appearing years after motor onset from cortical Lewy pathology and cholinergic denervation. If cognitive decline precedes or begins within about a year of parkinsonism, the diagnosis is dementia with Lewy bodies instead.
• Neuropsychiatric: depression, anxiety, apathy, and visual hallucinations (well-formed, of people or animals, initially with retained insight).
• Autonomic failure: neurogenic orthostatic hypotension (droxidopa, midodrine, fludrocortisone), urinary retention, and constipation progressing to obstipation or ileus.

Treatment-related

• Motor fluctuations: as the nigrostriatal terminal pool shrinks, dopamine buffering is lost and plasma levodopa levels dictate the clinical state — producing wearing-off, unpredictable on–off phenomena, and early-morning or off-period dystonia.
• Levodopa-induced dyskinesia: choreiform peak-dose movements from pulsatile stimulation and postsynaptic receptor plasticity; risk rises with disease duration and cumulative dose.
• Impulse control disorders: pathologic gambling, hypersexuality, compulsive shopping or eating — driven by D3-rich mesolimbic stimulation and strongly associated with dopamine agonists. Ask about it directly; patients rarely volunteer it.
• Dopamine dysregulation syndrome and dopamine agonist withdrawal syndrome (dysphoria, panic, autonomic symptoms on taper).
• Sudden-onset sleep attacks with dopamine agonists — counsel about driving.
• Drug-induced psychosis, more common with anticholinergics, amantadine, and agonists.
• Parkinsonism-hyperpyrexia (neuroleptic malignant–like) syndrome: fever, rigidity, altered mentation, and rhabdomyolysis after abrupt dopaminergic withdrawal or a D2 blocker — a true emergency requiring dopaminergic reinstitution and supportive care.

• Bradykinesia is mandatory: Under the MDS criteria you cannot diagnose parkinsonism without bradykinesia plus rest tremor or rigidity. A pure tremor without slowness is essential tremor until proven otherwise — and essential tremor improves with alcohol and propranolol, which does nothing for PD.
• Asymmetry is the tell: Unilateral onset with persistent side-to-side asymmetry favors idiopathic PD. A symmetric parkinsonian patient on metoclopramide or a typical antipsychotic has drug-induced parkinsonism — the single best next step is to stop the offending drug, not to start levodopa.
• The most tested association: MPTP (a contaminated meperidine analog) is converted by MAO-B to MPP+, which inhibits mitochondrial complex I in nigral neurons and produces abrupt, irreversible parkinsonism in young drug users.
• REM sleep behavior disorder is the highest-yield prodrome: dream enactment with loss of REM atonia predicts an eventual synucleinopathy (PD, DLB, or MSA) with striking specificity; hyposmia and constipation are the other prodromal pair.
• DaTscan answers one question only: it separates degenerative parkinsonism from essential tremor, drug-induced parkinsonism, and functional tremor. It is abnormal in PSP, MSA, and CBD as well, so it cannot confirm Parkinson disease.
• Red flags for atypical parkinsonism: early falls (PSP), vertical gaze palsy (PSP), prominent early dysautonomia or cerebellar signs (MSA), alien limb and asymmetric apraxia (CBD), and — across all of them — a poor levodopa response.
• Distractor to avoid: cogwheel rigidity is velocity-*independent*; if the stem describes velocity-dependent, clasp-knife tone with hyperreflexia and Babinski, that is upper motor neuron spasticity, not PD.
• Never abruptly stop dopaminergic therapy — including before surgery — because parkinsonism-hyperpyrexia syndrome can follow. For hallucinations, choose quetiapine, clozapine, or pimavanserin; haloperidol and risperidone will make the patient rigid.

• Prevalence: Most common movement disorder; increases with age (peak 60-80 years)
• Pathology: Loss of dopaminergic neurons in substantia nigra pars compacta; Lewy bodies (α-synuclein aggregates) are hallmark
• Clinical diagnosis: No biomarkers; based on motor features + response to levodopa
• Mortality: Increased mortality from aspiration, falls, infections—not the disease itself
• Genetic forms: 5-10% familial; SNCA, PARKIN, PINK1, DJ-1, LRRK2 mutations

1. 1.Confusing essential tremor with Parkinson tremor: Essential tremor is postural/kinetic (worse with action), symmetric, no rigidity/bradykinesia. Parkinson tremor is resting, often unilateral, with motor signs.
2. 2.Assuming all parkinsonism responds to levodopa: Atypical parkinsonian syndromes (PSP, MSA, CBD) respond poorly and progress faster; lack "pill-rolling" tremor.
3. 3.Missing non-motor features: Many patients present with anosmia, depression, or REM sleep behavior disorder years before motor symptoms; these are highly specific for PD.