Parkinson's Disease for the American Board of Psychiatry & Neurology (ABPN) Exam
Pathophysiology
- Neurodegeneration:
- Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta, part of the basal ganglia.
- This dopaminergic depletion leads to an imbalance between the direct and indirect pathways in the basal ganglia, resulting in the hallmark motor symptoms of the disease.
- Lewy Bodies:
- Intracellular aggregates of ?-synuclein, known as Lewy bodies, are a pathological hallmark of PD. These inclusions are found in the remaining neurons and are believed to play a role in neurodegeneration.
- Neurochemical Changes:
- There is a significant reduction in dopamine within the striatum, leading to the motor symptoms of PD. Other neurotransmitters, such as norepinephrine, serotonin, and acetylcholine, are also affected, contributing to non-motor symptoms.
Clinical Presentation
- Cardinal Motor Features:
- Bradykinesia: Slowness of movement, a required feature for diagnosis. Often manifests as a reduction in the amplitude and speed of movements.
- Rigidity: Increased resistance to passive movement, often described as "cogwheel rigidity" due to its ratchet-like quality.
- Resting Tremor: A "pill-rolling" tremor that typically starts unilaterally and is most apparent at rest.
- Postural Instability: Impaired balance and coordination, often leading to falls. This feature typically appears in the later stages of the disease.
- Other Motor Symptoms:
- Gait Disturbances: Shuffling gait with short steps, reduced arm swing, and festination (a tendency to speed up involuntarily).
- Hypomimia: Reduced facial expression, also known as "masked facies."
- Micrographia: Small, cramped handwriting that becomes progressively smaller.
Non-Motor Symptoms
- Neuropsychiatric:
- Depression and Anxiety: Common in PD, often preceding the onset of motor symptoms.
- Cognitive Decline: Can progress to Parkinson's disease dementia, characterized by executive dysfunction, visuospatial impairment, and fluctuating attention.
- Psychosis: Hallucinations and delusions, often exacerbated by dopaminergic medications.
- Autonomic Dysfunction:
- Orthostatic Hypotension: Due to autonomic failure, leading to dizziness or fainting on standing.
- Constipation: Often one of the earliest non-motor symptoms.
- Urinary Dysfunction: Urgency, frequency, and incontinence are common.
- Sleep Disorders:
- REM Sleep Behavior Disorder (RBD): Acting out dreams, often preceding the diagnosis of PD.
- Excessive Daytime Sleepiness: Common in advanced stages.
- Olfactory Dysfunction:
- Hyposmia/Anosmia: Reduced or lost sense of smell, often an early sign of the disease.
Diagnosis
- Clinical Diagnosis:
- The diagnosis of PD is primarily clinical, based on the presence of the cardinal motor features. Bradykinesia, along with either rigidity, resting tremor, or postural instability, is required.
- Response to Dopaminergic Therapy:
- A positive response to dopaminergic medications, such as levodopa, supports the diagnosis.
- Differential Diagnosis:
- Important to differentiate PD from other parkinsonian syndromes such as multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD), which have overlapping features but distinct clinical courses and treatment responses.
Treatment
- Pharmacologic Management:
- Levodopa/Carbidopa: The most effective treatment for motor symptoms. Levodopa is converted to dopamine in the brain, and carbidopa inhibits peripheral decarboxylation of levodopa, reducing side effects.
- Dopamine Agonists: (e.g., pramipexole, ropinirole) Can be used as monotherapy in early disease or as adjuncts to levodopa in advanced stages.
- MAO-B Inhibitors: (e.g., selegiline, rasagiline) Help to increase dopamine levels by inhibiting its breakdown.
- COMT Inhibitors: (e.g., entacapone, tolcapone) Extend the effect of levodopa by inhibiting its metabolism.
- Anticholinergics: (e.g., benztropine, trihexyphenidyl) Useful primarily for tremor, especially in younger patients.
- Amantadine: May help with dyskinesias and mild motor symptoms.
- Surgical Treatment:
- Deep Brain Stimulation (DBS): Considered for patients with advanced PD who have motor fluctuations or dyskinesias not well-controlled with medication. The subthalamic nucleus and globus pallidus interna are common targets.
- Non-Pharmacologic Management:
- Physical Therapy: Focused on improving mobility, balance, and strength.
- Occupational Therapy: Assists with daily living activities and maintaining independence.
- Speech Therapy: Helps with speech and swallowing difficulties.
- Nutritional Support: Addressing issues like constipation and weight loss, common in advanced stages.
Complications
- Motor Complications:
- Motor Fluctuations: "On-off" phenomena where patients experience fluctuations in motor performance due to varying plasma levels of levodopa.
- Dyskinesias: Involuntary movements, often a side effect of chronic levodopa therapy.
- Neuropsychiatric Complications:
- Dementia: Affects a significant proportion of PD patients in the later stages.
- Psychosis: Can be a side effect of dopaminergic therapy or a feature of disease progression.
- Autonomic Complications:
- Severe Orthostatic Hypotension: Can significantly impact quality of life and may require pharmacologic intervention.
Prognosis
- PD is a progressive disease with a variable course. With appropriate management, many patients maintain a good quality of life for several years. However, the disease often leads to significant disability over time, particularly due to motor complications and cognitive decline.
Commonly Tested Items on the ABPN Exam
- Cardinal Features: Recognition of the motor symptoms of PD, especially bradykinesia, rigidity, and tremor.
- Differential Diagnosis: Distinguishing PD from other parkinsonian syndromes.
- Pharmacologic Management: Understanding the use of levodopa and dopamine agonists, along with their side effects.
- Complications: Managing motor fluctuations, dyskinesias, and neuropsychiatric symptoms.