Parkinson Disease

Parkinson disease (PD) is a chronic, progressive neurodegenerative disease caused by loss of dopaminergic neurons in the substantia nigra pars compacta, producing a striatal dopamine deficiency that underlies its cardinal motor features — resting tremor, bradykinesia, and rigidity, with postural instability emerging later in the disease course.^[1] Nonmotor features — cognitive impairment, depression, psychosis, and autonomic dysfunction such as orthostatic hypotension and bladder dysfunction — often predate the motor diagnosis by years and become the more disabling problem as disease progresses. No treatment has been definitively shown to modify the underlying neurodegenerative process; every currently available therapy is symptomatic, replenishing or mimicking dopamine within the brain, though treatment meaningfully reduces morbidity and improves function.

Cardinal motor features

Classic PD presents with an asymmetric or unilateral-onset resting tremor (a pill-rolling tremor of the hand is typical), bradykinesia (slowness and a progressive decrement in the amplitude of repetitive movements, e.g., on finger tapping), and rigidity (cogwheel or lead-pipe resistance to passive movement). Postural instability and gait impairment (shuffling steps, reduced arm swing, freezing of gait) typically emerge only later in the disease course — early, prominent postural instability is itself a red flag for an alternate diagnosis rather than an expected early finding (see Red Flags & Refer).

Prodromal and early nonmotor features

Nonmotor symptoms often precede the motor diagnosis, sometimes by more than a decade, and should raise suspicion when clustered together: hyposmia (reduced sense of smell), constipation, REM sleep behaviour disorder (dream-enactment behaviour during sleep), fatigue, depression, and subtler changes such as micrographia (progressively smaller handwriting) or a tremulous sensation in one arm without an obviously visible tremor.^[1]

• Increasing age — the strongest known risk factor.
• Male sex.
• Family history of Parkinson disease, or a known pathogenic mutation (e.g., LRRK2, GBA, SNCA, and PARK2/PINK1 in particular for younger-onset disease).
• Environmental pesticide or herbicide exposure.
• Prior significant head trauma.
• Rural living and well-water use have been proposed as contributing exposures.
• Cigarette smoking and caffeine intake have both shown an inverse association with PD risk in observational data, though neither is a recommended preventive intervention.

Differentiating idiopathic Parkinson disease from its mimics

Diagnostic criteria

PD remains a clinical diagnosis. Parkinsonism is defined as bradykinesia plus rest tremor and/or rigidity. Once parkinsonism is confirmed, the Movement Disorder Society (MDS) clinical diagnostic criteria are used to determine whether idiopathic PD, rather than an alternate cause, is the most likely diagnosis — weighing supportive criteria (e.g., a clear and dramatic response to dopaminergic therapy, levodopa-induced dyskinesia, rest tremor) against absolute exclusion criteria and red flags that must be counterbalanced by supportive features before a PD diagnosis is made. The presence of dementia, even early in the disease course, is no longer an exclusion criterion.^[3]

History

• Ask specifically about early, easily-missed complaints: fatigue, hyposmia, constipation, REM sleep behaviour disorder, general slowness, deteriorating handwriting, or a tremulous sensation in one arm without a visible tremor.
• Take a complete medication history targeting dopamine-blocking drugs — first- and second-generation antipsychotics and central dopamine-blocking antiemetics (metoclopramide, prochlorperazine) are the most common drug causes of parkinsonism, and screening for these is a medication-review task pharmacists are well positioned to lead.
• Ask about family history and age at onset — younger-onset disease raises the relevance of a genetic cause and of excluding Wilson disease.

Examination

Look specifically for asymmetric or unilateral onset of rest tremor, bradykinesia (decrement in amplitude on repetitive finger tapping), and cogwheel rigidity. Early, prominent postural instability, impaired eye movements, or a symmetric presentation from onset are not expected in early PD and should prompt reconsideration of the diagnosis.

Investigations

• Routine neuroimaging is not indicated for a classic, straightforward presentation — reserve MRI for young-onset disease or when atypical features are present, primarily to exclude a structural or vascular cause.
• In young patients, exclude Wilson disease with a serum ceruloplasmin, a slit-lamp examination for Kayser-Fleischer rings, and a 24-hour urine copper.
• A diagnostic trial of at least 1 month of properly dosed dopaminergic therapy can help confirm the diagnosis in an uncertain case — remember that levodopa may take several weeks to reach its full effect once fully titrated, so an inadequate trial duration or dose should not be misread as a poor response.^[1]

No scoring tool is used to diagnose PD or to guide routine drug therapy decisions in practice — the Unified Parkinson Disease Rating Scale (UPDRS/MDS-UPDRS) is a comprehensive, clinician-administered, multi-domain rating instrument used mainly in research and specialist follow-up, not a simple point-sum a pharmacist would calculate at the counter, so no interactive calculator is provided for it here.

• Improve and maintain functional independence and quality of life throughout the disease course.
• Minimize acute and long-term medication adverse effects, including delaying the onset of motor fluctuations and dyskinesia where achievable.
• Achieve consistent, predictable symptom control across the day, minimizing off-time.
• Identify and treat nonmotor symptoms proactively, since these often become more disabling than the motor features as disease progresses.
• Prevent falls and injury from postural instability and orthostatic hypotension.
• Avoid any abrupt interruption of dopaminergic therapy at every care transition — surgery, hospitalization, NPO status, and medication reconciliation are all high-risk moments for an inadvertent missed or stopped dose.
• Support a realistic, positive outlook for the patient and caregiver, given the chronic and progressive nature of PD.

• Provide patient and caregiver education through Parkinson Canada and other reputable local/national resources from the time of diagnosis onward.
• Recommend a regular, structured exercise program combining aerobic, strength, balance, and gait training (e.g., dance, tai chi, yoga) — this reduces fall risk and improves both motor and nonmotor symptoms, with benefits most pronounced when started early.^[1]
• Refer early and proactively to allied health professionals — physiotherapy, occupational therapy, and speech-language pathology — rather than waiting for advanced disease; involve home care services as disability increases.
• Counsel every patient starting a dopamine agonist or levodopa on the possibility of impulse control disorders and sudden sleep attacks before the first dose, not only if problems later emerge.
• Counsel every patient and caregiver, from the first prescription, that dopaminergic medication must never be stopped or reduced abruptly.

Surgical / device-aided therapy

Deep brain stimulation (DBS) is the surgical treatment of choice for motor fluctuations and dyskinesia that remain disabling despite optimized medical therapy. The subthalamic nucleus and globus pallidus interna are the most common targets (improving bradykinesia, rigidity, and dyskinesia); a thalamic target is reserved for tremor-predominant disease. DBS is typically offered to patients younger than 70 years of age with more advanced disease whose motor symptoms can no longer be adequately controlled with medication alone.^[1]

DBS is not a cure and does not treat symptoms that do not respond to dopaminergic therapy — functioning improves only up to the level of the patient's best on-state, and postural instability typically does not improve. Significant cognitive impairment is a contraindication to DBS, so formal neuropsychological assessment and confirmation of a robust levodopa response are standard parts of the pre-surgical workup.

Breastfeeding: dopamine agonists suppress lactation and are generally avoided during breastfeeding; data on levodopa and other agents in lactation are limited, so specialist input is advised when treatment is genuinely needed during this period.

Pediatrics

PD is a disease of middle-aged and older adults; it does not occur in children. Parkinsonism presenting in a young patient needs a broader differential — genetic and metabolic causes, including Wilson disease — rather than a default assumption of idiopathic PD.

Renal and hepatic impairment

Pramipexole and amantadine are renally cleared and require dose reduction with longer dosing intervals as renal function declines; amantadine in particular can accumulate to toxic levels in significant renal impairment. Rasagiline and safinamide require dose capping or avoidance in hepatic impairment (rasagiline: maximum 0.5 mg/day in mild impairment, avoid in moderate-severe impairment; safinamide: maximum 50 mg/day in moderate impairment, avoid in severe impairment).

Older adults

Avoid starting a dopamine agonist as initial or add-on therapy in patients over 70 years of age, given the disproportionate risk of confusion, hallucinations, and falls in this group. Avoid anticholinergic antiparkinsonian drugs (benztropine, trihexyphenidyl, ethopropazine) in older patients given their anticholinergic burden. Start every antiparkinsonian medication at a low dose and titrate slowly in older patients to minimize acute adverse effects.^[1]

At every visit

• Ask directly about impulse control symptoms (gambling, shopping, hypersexuality, binge eating) in any patient on a dopamine agonist — patients rarely volunteer these unprompted.^[2]
• Screen for psychosis (hallucinations, delusions) and assess whether it is tolerated by the patient/caregiver or requires intervention.
• Check orthostatic blood pressure (lying and standing) in any patient on levodopa or a dopamine agonist, especially after a dose increase.
• Ask about falls, freezing episodes, and off-time — a simple symptom/motor-fluctuation diary can help quantify this between visits.
• Review the full medication list for any new dopamine-blocking drug (antipsychotic, antiemetic) that could worsen parkinsonism.
• Screen nonmotor symptoms specifically — mood, sleep quality and REM sleep behaviour disorder symptoms, constipation, and urinary symptoms — since these are easy to miss if not asked about directly.^[7]

Periodic

• Cognitive screening at intervals appropriate to disease stage and clinical suspicion, particularly before considering a dopamine agonist, amantadine, or anticholinergic (all can worsen cognition) and before DBS candidacy assessment.
• Fall-risk assessment and home safety review as disease progresses.
• Reassess whether domperidone pretreatment is still needed once GI/orthostatic tolerance to dopaminergic therapy is established.

Refer to a movement disorder specialist

For diagnostic uncertainty, poor or waning levodopa response, disabling motor fluctuations or dyskinesia despite oral/transdermal optimization, or a possible candidate for device-aided therapy.

1. Confirm true nonadherence or timing problems — missed doses, or levodopa taken with a high-protein meal — before assuming the medication itself has failed.
2. Reconsider the diagnosis if response to an adequately dosed levodopa trial is genuinely poor — this pattern favours atypical parkinsonism or a non-parkinsonian cause over idiopathic PD.
3. Optimize the current levodopa dose and dosing frequency before adding a new drug class.
4. Add adjunct therapy targeted to the specific problem — a COMT inhibitor or MAO-B inhibitor for wearing-off, or amantadine for dyskinesia — rather than simply increasing the levodopa dose further.
5. Reassess nonmotor symptom burden separately; depression, sleep disruption, and autonomic symptoms often need their own targeted management rather than more dopaminergic therapy.
6. Refer to a movement disorder specialist for device-aided therapy (deep brain stimulation or infusion therapy) candidacy once oral/transdermal optimization has been genuinely exhausted.

• Stopping or substantially reducing dopaminergic medication abruptly, or allowing an inpatient NPO order or a medication-reconciliation gap to interrupt dosing — risks parkinsonism-hyperpyrexia syndrome.
• Missing drug-induced parkinsonism — continuing an antipsychotic or metoclopramide while a patient is being worked up or treated for presumed idiopathic PD, rather than reviewing the medication list first.
• Using a conventional (typical) antipsychotic, or any atypical antipsychotic other than quetiapine or clozapine, for PD-related psychosis — these worsen motor symptoms.
• Starting a dopamine agonist as initial therapy in a patient over 70 years of age.
• Withholding or delaying levodopa in a functionally impaired patient out of an outdated concern about preserving it for later or triggering earlier dyskinesia.
• Not asking directly about impulse control symptoms in a patient on a dopamine agonist — patients and families often do not recognize or volunteer these as medication-related.
• Giving levodopa with a high-protein meal in a patient who already has motor fluctuations, without recognizing why the response has become erratic.
• Missing the sudden-sleep-attack driving-safety conversation when starting or increasing any dopaminergic medication.

How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. Parkinson disease does not occur in children, so no pediatric dosing line is given for any drug below. Select any drug name to open its complete Pharmasist Drug Database page.

Levodopa preparations

The most effective symptomatic therapy at every disease stage; keep the total daily dose as low as consistently effective.

Dopamine agonists

Non-ergot agents (pramipexole, ropinirole, rotigotine) are preferred over ergot-derived bromocriptine given its pulmonary/cardiac fibrosis risk. Avoid initiating any agent in this class in patients over 70.

Monoamine oxidase B (MAO-B) inhibitors

Modest symptomatic effect; rasagiline and safinamide are also useful adjuncts for wearing-off. No convincing evidence that any agent in this class is disease-modifying.

Catechol-O-methyltransferase (COMT) inhibitors

Adjunct for wearing-off only — has no effect if not given together with levodopa; not indicated in non-fluctuating early disease.

Opicapone, a longer-acting, once-daily COMT inhibitor, is approved in some other jurisdictions but is not available in Canada.

N-methyl-D-aspartate (NMDA) receptor antagonist

Used specifically to reduce established levodopa-induced dyskinesia, not for early motor symptoms.

Anticholinergic agents

Should not be used as first-line PD therapy given limited efficacy (mainly tremor, little effect on bradykinesia) and a substantial side-effect burden; avoid in older patients and in anyone with dyskinesia or motor fluctuations.

Nonmotor symptom management

PD does not have a separate Clinical Guidelines chapter of its own for these problems, so real dosing for the most pharmacist-relevant nonmotor issues is included directly below. Domperidone carries a specific dose-related cardiac risk in this population.^[7][8]

Evidence note: Drug inventory, dosing, adverse effects, and comments reflect standard Canadian practice, cross-checked against the current Canadian Parkinson disease guideline.^[1]