Parkinson’s Disease¶
Chapter 446 | Part 13: Neurologic Disorders
KEY CLINICAL POINTS¶
- Cardinal motor features include bradykinesia, rest tremor, rigidity, and postural instability.
- α -Synuclein aggregation and Lewy body formation are central to pathogenesis.
- Levodopa remains the gold standard for symptomatic treatment, but motor complications are common.
- Differential diagnosis includes atypical parkinsonism (MSA, PSP, CBS) and secondary parkinsonism.
- Nonmotor features (e.g., depression, sleep disturbances, autonomic dysfunction) significantly impact quality of life.
1. DEFINITION & OVERVIEW¶
Parkinson’s disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron loss in the substantia nigra pars compacta (SNc), leading to motor and nonmotor symptoms. The classical features include bradykinesia, rest tremor, rigidity, and postural instability. Nonmotor features such as cognitive impairment, sleep disturbances, and autonomic dysfunction are increasingly recognized as critical aspects of the disease. The Braak hypothesis proposes that α -synuclein pathology spreads from the olfactory bulb and enteric nervous system to the brainstem and cortex.
Table 446-1: Clinical Features of Parkinson’s Disease¶
| CARDINAL MOTOR FEATURES | OTHER MOTOR FEATURES | NONMOTOR FEATURES |
|---|---|---|
| Bradykinesia | Micrographia | Anosmia |
| Rest tremor | Masked facies | Sensory disturbances |
| Rigidity (hypomimia) | Reduced eye blinking | Mood disorders (depression, anxiety, apathy) |
| Postural instability | Drooling | Sleep disturbances (fragmented sleep, RBD) |
| Soft voice (hypophonia) | Autonomic disturbances | |
| Dysphagia | Orthostatic hypotension | |
| Freezing | Gastrointestinal disturbances | |
| Falling | Genitourinal disturbances | |
| Sexual dysfunction | ||
| Cognitive impairment/dementia |
1.1 Cardinal Motor Features¶
Bradykinesia (slowness of movement), rest tremor, rigidity (stiffness), and postural instability are the core motor features. These are termed the 'cardinal' features of PD.
1.2 Nonmotor Features¶
Nonmotor symptoms include anosmia (loss of smell), constipation, rapid eye movement (REM) sleep behavior disorder (RBD), depression, anxiety, and autonomic dysfunction. These often precede motor symptoms and contribute significantly to disability.
1.3 Pathophysiology¶
Dopaminergic neuron loss in the SNc leads to reduced striatal dopamine, impaired motor control, and abnormal synaptic plasticity. α -Synuclein aggregation into Lewy bodies is a hallmark of the disease.
2. EPIDEMIOLOGY¶
Global prevalence is ~10.8 million, with incidence increasing with age (mean age of onset ~60 years). Genetic factors account for 5–15% of cases, with GBA1 mutations being the strongest risk factor. Environmental exposures (e.g., pesticides, manganese) may contribute, though evidence remains inconclusive. Nonmotor symptoms (e.g., constipation, RBD) often precede motor features by years.
2.1 Risk Factors¶
Age, family history, GBA1 mutations, and environmental toxins (e.g., pesticides, manganese) are key risk factors. Early-onset PD (<40 years) is often genetic (e.g., LRRK2, PRKN mutations).
2.2 Demographics¶
Men are more commonly affected than women. Ethnicity influences risk (e.g., GBA1 variants in Ashkenazi Jews).
3. ETIOLOGY & PATHOPHYSIOLOGY¶
PD is caused by a combination of genetic and environmental factors. α -Synuclein misfolding and aggregation into Lewy bodies are central. Genetic mutations (e.g., GBA1, LRRK2, PRKN) and environmental toxins (e.g., MPTP) contribute to neuronal degeneration. Oxidative stress, mitochondrial dysfunction, and inflammation are implicated in pathogenesis.
Table 446-4: Confirmed Genetic Causes of Parkinson’s Disease¶
| DESIGNATION & REFERENCE | CLINICAL CLUES | COMMENTS |
|---|---|---|
| PARK-SNCA | Early-onset, severe parkinsonism with cognitive dysfunction | a-Synuclein main component of Lewy bodies |
| PARK-LRRK2 | Typical PD with slightly slower progression | Most common genetic form of PD |
| PARK-GBA1 | Faster progression, greater cognitive impairment | Strongest known genetic risk factor |
| PARK-PRKN | Early-onset dystonia, often in a leg | Most common early-onset genetic PD |
| PARK-PINK1 | Prominent psychiatric features | Clinically similar to PARK-PRKN |
3.1 Genetic Factors¶
GBA1 mutations are the strongest genetic risk factor. LRRK2 mutations account for ~3% of cases. PRKN, PINK1, and PARK7 mutations cause autosomal recessive early-onset PD.
3.2 Environmental Factors¶
Exposure to pesticides, solvents, and heavy metals (e.g., manganese) may increase risk. MPTP, a byproduct of illicit drug synthesis, causes parkinsonism in humans.
3.3 Pathophysiology¶
Dopaminergic neuron loss in the SNc leads to striatal dopamine deficiency. α -Synuclein aggregates disrupt synaptic function and trigger neuroinflammation. The Braak hypothesis suggests peripheral α -synuclein pathology spreads to the brain.
4. CLINICAL FEATURES¶
PD presents with a mix of motor and nonmotor symptoms. Motor features include bradykinesia, tremor, rigidity, and postural instability. Nonmotor features encompass cognitive impairment, depression, sleep disorders, and autonomic dysfunction. Cognitive decline may precede motor symptoms in dementia with Lewy bodies (DLB).
4.1 Motor Symptoms¶
Bradykinesia, rest tremor, rigidity, and postural instability are the cardinal features. Freezing of gait and dyskinesias are common complications.
4.2 Nonmotor Symptoms¶
Nonmotor features include depression, anxiety, REM sleep behavior disorder (RBD), constipation, orthostatic hypotension, and cognitive impairment. These significantly impact quality of life.
4.3 Cognitive and Behavioral Features¶
Cognitive impairment (e.g., executive dysfunction) and dementia occur in ~80% of advanced PD. Psychosis (hallucinations, delusions) is common, often linked to dopaminergic medications.
5. DIFFERENTIAL DIAGNOSIS¶
PD must be differentiated from atypical parkinsonism (MSA, PSP, CBS), secondary parkinsonism (drugs, toxins), and other neurodegenerative disorders (e.g., Huntington’s disease, Wilson’s disease). Key features include response to levodopa, motor asymmetry, and presence of nonmotor symptoms.
Table 446-2: Differential Diagnosis of Parkinsonism¶
| Parkinson’s Disease | Atypical Parkinsonism | Secondary Parkinsonism | Other Neurodegenerative Disorders |
|---|---|---|---|
| Sporadic | Multiple-system atrophy (MSA) | Drug-induced | Wilson’s disease |
| Genetic | Cerebellar type (MSA-c) | Tumor | Huntington’s disease |
| PD with dementia/dementia with Lewy bodies | Parkinson type (MSA-p) | Infection | Neurodegeneration with brain iron accumulation |
| Progressive supranuclear palsy (PSP) | Vascular | SCA 3 (spinocerebellar ataxia) |
| Parkinson’s Disease | Atypical Parkinsonism | Secondary Parkinsonism | Other Neurodegenerative Disorders |
|---|---|---|---|
| Parkinsonian variant | Normal-pressure hydrocephalus | Fragile X–associated ataxi a-tremor-parkinsonism | |
| Richardson variant | Trauma | Prion diseases | |
| Corticobasal syndrome | Liver failure | X-linked dystonia-parkinsonism | |
| Toxins (e.g., carbon monoxide, manganese) | Alzheimer’s disease with parkinsonism | ||
| Dopa-responsive dystonia | Dopa-responsive dystonia |
5.1 Atypical Parkinsonism¶
Multiple-system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal syndrome (CBS) share features with PD but have distinct clinical and pathological profiles. MSA often presents with autonomic dysfunction and cerebellar signs.
5.2 Secondary Parkinsonism¶
Drug-induced parkinsonism (e.g., neuroleptics), toxins (e.g., carbon monoxide), and metabolic disorders (e.g., Wilson’s disease) can mimic PD. These often respond to levodopa or address the underlying cause.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis relies on clinical features, imaging, and biomarkers. MRI can exclude vascular parkinsonism, while dopamine transporter (DaT) scans and α -synuclein biomarkers aid in differentiation. CSF analysis and genetic testing are used for specific subtypes.
Table 446-3: Features Suggesting Atypical or Secondary Parkinsonism¶
| SYMPTOMS/SIGNS | ALTERNATIVE DIAGNOSIS |
|---|---|
| Early speech and gait impairment (lack of tremor, motor asymmetry, early falls) | Atypical parkinsonism |
| Poor/no response to levodopa | Atypical or secondary parkinsonism |
| Dementia as first or early feature | Dementia with Lewy bodies |
| Prominent orthostatic hypotension | MSA |
| Prominent cerebellar signs | MSA-c |
| Slow saccades with impaired downgaze | PSP |
| High-frequency (6–10 Hz) symmetric postural tremor | Essential tremor |
6.1 Imaging¶
MRI excludes vascular causes. DaT scans show reduced striatal dopamine uptake. PET with [18F]FDOPA or [123I]FP-CIT is used to assess dopaminergic function.
6.2 Biomarkers¶
CSF α -synuclein levels, skin biopsy for α -synuclein deposits, and plasma biomarkers (e.g., phospho-tau) are emerging diagnostic tools. SAA (seed amplification assay) detects α -synuclein seeds in CSF.
6.3 Diagnostic Criteria¶
UK Brain Bank Criteria and MDS Clinical Diagnostic Criteria for PD emphasize motor features, response to levodopa, and exclusion of other causes. DLB is diagnosed using the McKeith criteria.
7. MANAGEMENT & TREATMENT¶
Levodopa remains the cornerstone of treatment, but motor complications (wearing-off, dyskinesias) are common. Dopamine agonists, COMT inhibitors, and MAO-B inhibitors are used adjunctively. Surgical options (DBS) and novel therapies (gene therapy, stem cells) are being explored.
Table 446-5: Drugs Commonly Used for Treatment of Parkinson’s Disease¶
| AGENT | AVAILABLE DOSAGES | TYPICAL DOSING |
|---|---|---|
| Levodopa | Carbidopa/levodopa 10/100, 25/100, 25/250 mg | 200–1000 mg/day |
| Benserazide/levodopa | 25/100, 50/200 mg | |
| Carbidopa/levodopa CR | 25/100, 50/200 mg | |
| Benserazide/levodopa MDS | 25/200, 25/250 mg | |
| Parcopa | 10/100, 25/100, 25/250 mg | |
| Rytary (carbidopa/levodopa) | 23.75/95, 36.25/145, 48.75/195, 61.25/245 mg | See conversion tables |
| Carbidopa/levodopa/entacapone | 12.5/50/200, 18.75/75/200, 25/100/200, 31.25/125/200, 37.5/150/200, 50/200/200 mg | |
| Pramipexole | 0.125, 0.25, 0.5, 1.0, 1.5 mg | 0.25–1.0 mg tid |
| Pramipexole ER | 0.375, 0.75, 1.5, 3.0, 4.5 mg | 1–3 mg/d |
| Ropinirole | 0.25, 0.5, 1.0, 3.0 mg | 6–24 mg/d |
| Ropinirole XL | 2, 4, 6, 8 mg | 6–24 mg/d |
| Rotigotine patch | 2-, 4-, 6-, 8-mg patches | 4–24 mg/d |
| Apomorphine SC | 2–8 mg | 2–8 mg |
| Entacapone | 200 mg | 200 mg with each levodopa dose |
| Tolcapone | 100, 200 mg | 100–200 mg tid |
| Opicapone | 50 mg | 50 mg HS |
| Selegiline | 5 mg | 5 mg bid |
| Rasagiline | 0.5, 1.0 mg | 1 mg QAM |
| Safinamide | 100 mg | 100 mg QAM |
| Inhaled levodopa | 5–40 mg | Up to 5 doses per day |
| Apomorphine sublingual strip | Up to 5 doses per day | |
| Istradefylline | 20, 40 mg | 20 or 40 mg/d |
| Amantadine | 100–400 mg |
7.1 Levodopa Therapy¶
Levodopa is the most effective symptomatic treatment. It is combined with carbidopa to reduce peripheral metabolism. Continuous delivery via intestinal infusion (Duodopa) or subcutaneous apomorphine reduces motor complications.
7.2 Dopamine Agonists¶
Pramipexole, ropinirole, and rotigotine are used to reduce levodopa requirements. They are associated with dyskinesias and impulse control disorders (e.g., hypersexuality).
7.3 Other Pharmacologic Agents¶
COMT inhibitors (entacapone, tolcapone) and MAO-B inhibitors (selegiline, rasagiline) prolong levodopa efficacy. Amantadine is used for dyskinesias and early PD.
7.4 Surgical Treatment¶
Deep brain stimulation (DBS) of the STN or GPi is effective for severe motor complications. MRI-guided ultrasound and novel targets (e.g., pedunculopontine nucleus) are being explored.
8. PROGNOSIS & COMPLICATIONS¶
PD is a progressive disorder with variable disease course. Nonmotor symptoms (e.g., dementia, depression) are major sources of disability. Motor complications (wearing-off, dyskinesias) develop over time, reducing quality of life. Mortality is increased due to complications like falls and infections.
8.1 Disease Progression¶
Disease progression is slow in early stages but accelerates with age. Nonmotor symptoms often precede motor features and contribute to disability.
8.2 Complications¶
Complications include falls, freezing of gait, dyskinesias, cognitive decline, and autonomic dysfunction. Psychosis and depression are common and worsen prognosis.
9. SPECIAL CONSIDERATIONS¶
Management of nonmotor features (e.g., depression, sleep disorders) is critical. Caregiver burden is significant, requiring multidisciplinary support. Special considerations include pregnancy, pediatric PD, and elderly patients with comorbidities.
9.1 Nonmotor Symptom Management¶
Antidepressants, melatonin for RBD, and cholinesterase inhibitors may help manage nonmotor symptoms. Amantadine is used for dyskinesias.
9.2 Caregiver Support¶
Caregivers face high burden due to behavioral symptoms and caregiving demands. Education, social services, and respite care are essential.
9.3 Special Populations¶
Pregnancy and lactation require careful medication selection. Pediatric PD is rare but often genetic. Elderly patients face increased risks of falls and drug interactions.
10. KEY POINTS & CLINICAL PEARLS¶
- Levodopa is the mainstay of treatment but must be balanced against motor complications. 2. Nonmotor symptoms (e.g., depression, sleep disorders) are underdiagnosed and undermanaged. 3. Early diagnosis with biomarkers and imaging improves treatment outcomes. 4. DBS is effective for severe motor complications but not for nonmotor features. 5. Genetic testing is crucial for familial PD and may guide disease-modifying therapies.