Parkinson's disease Study Guide

📖 Core Concepts Parkinson’s disease (PD) – a neurodegenerative synucleinopathy: abnormal α‑synuclein builds up in neurons, especially loss of dopaminergic cells in the substantia nigra → movement disorder. Parkinsonism – the quartet of resting tremor, bradykinesia, rigidity, postural instability. Prodrome – early non‑motor signs (anosmia, constipation, REM‑sleep behavior disorder) that can appear years before motor symptoms. Dopamine replacement – the mainstay of symptomatic treatment; levodopa is converted to dopamine in the brain. Disease‑modifying vs. symptomatic – currently no therapy proven to halt PD; research focuses on α‑synuclein aggregation, neuroprotection, gene/cell therapy. --- 📌 Must Remember Diagnostic criterion: bradykinesia plus resting tremor or rigidity. Resting tremor: “pill‑rolling” 4–6 Hz, improves with action. Orthostatic hypotension: ≥20 mm Hg systolic or ≥10 mm Hg diastolic drop on standing. Key genetic risk genes: Dominant: SNCA, LRRK2, VPS35 Recessive: PRKN, PINK1, DJ1 GBA1 → higher dementia risk. Environmental hazards: pesticides (paraquat, rotenone), metals, solvents, TCE, traumatic brain injury, type 2 diabetes. Levodopa combo: levodopa + peripheral decarboxylase inhibitor (carbidopa/benserazide/entacapone). Deep Brain Stimulation (DBS) – indicated for medication‑refractory rigidity/tremor; contraindicated with significant cognitive impairment. Prevalence: 1 % > 60 yr; rises to >4 % > 85 yr; projected >12 M worldwide by 2040. --- 🔄 Key Processes α‑Synuclein aggregation Misfolded monomers → oligomers → protofibrils → Lewy bodies (fibrillar shell + granular core). Prion‑like spread: aggregates seed neighboring neurons → progressive pathology. Braak staging (propagation) Stage 0‑1: olfactory bulb / enteric nervous system → vagus → brainstem. Stage 2‑3: substantia nigra degeneration → motor signs. Stage 4‑6: limbic & neocortical involvement → dementia, autonomic failure. Mitochondrial dysfunction Complex I inhibition → ↑ ROS, ↓ ATP, impaired Ca²⁺ buffering. PINK1/parkin mutations → defective mitophagy → accumulation of damaged mitochondria. --- 🔍 Key Comparisons Parkinson’s disease vs. Dementia with Lewy bodies PD: motor signs precede dementia; Lewy bodies start in cortex → early cognitive decline. Levodopa vs. Dopamine agonist Levodopa: most potent motor relief, risk of dyskinesia & motor fluctuations. Agonist: lower dyskinesia risk, higher impulse‑control disorder risk, less potent. Typical vs. Atypical parkinsonism Typical: good levodopa response, slower progression. Atypical (MSA, PSP, CBD): poor levodopa response, early autonomic/eye‑movement signs, rapid progression. --- ⚠️ Common Misunderstandings “All tremors are Parkinsonian.” → Essential tremor is action‑type, higher frequency, no rigidity/bradykinesia. “Smoking causes PD.” → Epidemiologically linked to lower risk; causality unclear. “Levodopa cures PD.” → It only replaces dopamine temporarily; does not stop neurodegeneration. “Dementia automatically means DLB, not PD‑dementia.” → In PD, dementia occurs after ≥5 yr of motor disease (PD‑dementia). --- 🧠 Mental Models / Intuition “Neuro‑traffic jam” – Think of α‑synuclein aggregates as traffic accidents that block neuronal “roads”; they spread downstream, causing a cascade of “road closures” (neuron loss). “Battery depletion” – Dopaminergic neurons are the brain’s battery; loss of “cells” = reduced charge → motor slowing (bradykinesia) and tremor. --- 🚩 Exceptions & Edge Cases Early‑onset PD (<50 yr) – higher likelihood of recessive gene mutations (PRKN, PINK1). Antipsychotic‑induced parkinsonism – drug‑blocked dopamine receptors → reversible symptoms after discontinuation. Vascular parkinsonism – gait‑dominant, minimal tremor; imaging shows extensive white‑matter disease. --- 📍 When to Use Which Levodopa → severe motor disability, especially in older patients where dyskinesia risk is lower. MAO‑B inhibitor → early disease, to delay levodopa initiation, or as adjunct to smooth fluctuations. Dopamine agonist → younger patients needing motor control without immediate levodopa exposure (to postpone dyskinesia). DBS → motor complications (fluctuations, dyskinesia) refractory to optimized meds and cognitive status acceptable. Low‑protein meals → advanced PD when levodopa absorption is compromised. --- 👀 Patterns to Recognize Prodromal triad: constipation + anosmia + REM‑sleep behavior disorder → high suspicion for upcoming PD. Motor “shuffling” + festination → classic gait pattern indicating disease progression. Motor fluctuations: “wearing‑off” → levodopa dose‑interval too long; “on‑off” dyskinesia → peak levodopa levels. --- 🗂️ Exam Traps Distractor: “Resting tremor improves with movement → not Parkinson’s.” Wrong – tremor does improve with voluntary movement, which is a hallmark. Distractor: “Orthostatic hypotension is a primary diagnostic criterion.” Wrong – supportive, not required. Distractor: “All patients with PD develop dementia.” Wrong – 30 % develop PD‑dementia; many remain cognitively intact. Distractor: “Levodopa should be taken with protein‑rich meals for better absorption.” Wrong – protein competes with levodopa transport; take on empty stomach. ---