USMLE Neurology: Movement Disorders (Parkinson, Huntington, Hemiballismus)

What This Is and Why It Matters for USMLE

Movement Disorders: Parkinson, Huntington, Hemiballismus is a high-yield topic for Step 1 and Step 2 CK. It appears in both basic science and clinical contexts, with a focus on classic presentations, diagnostic approaches, and management strategies.

High-Yield Facts (What You Must Memorize)

• Pathophysiology:
  • Parkinson's disease: loss of dopaminergic neurons, Lewy bodies, and alpha-synuclein aggregation
  • Huntington's disease: expansion of CAG repeats in HTT gene, leading to neuronal degeneration
  • Hemiballismus: lesion to subthalamic nucleus, causing involuntary movements
• Classic presentation and physical exam findings:
  • Parkinson's disease: tremors, bradykinesia, rigidity, postural instability
  • Huntington's disease: chorea, cognitive decline, psychiatric symptoms
  • Hemiballismus: violent, involuntary movements of the arm or leg
• Diagnostic approach:
  • Imaging: MRI to rule out other causes of movement disorders
  • Labs: blood glucose to rule out hypoglycemia, thyroid function tests to rule out hypothyroidism
• First-line treatment and management:
  • Parkinson's disease: levodopa, dopamine agonists, MAO-B inhibitors
  • Huntington's disease: tetrabenazine, antipsychotics for chorea
  • Hemiballismus: botulinum toxin injections or deep brain stimulation
• Red flags, complications, and follow-up:
  • Parkinson's disease: dysphagia, falls, and dementia
  • Huntington's disease: suicide risk, cognitive decline, and emotional instability
  • Hemiballismus: seizures, stroke, and infection

Clinical Pearls & Buzzwords

• "Dopa-responsive dystonia" (Parkinson's disease)
• "Chorea" (Huntington's disease)
• "Subthalamic nucleus lesion" (Hemiballismus)
• "Lewy bodies" (Parkinson's disease)
• "CAG repeat expansion" (Huntington's disease)

Step-by-Step Clinical Reasoning

1. Identify the syndrome or presentation (e.g., tremors, chorea, or violent movements).
2. Generate a differential (most likely and must-not-miss):
   • Parkinson's disease
   • Huntington's disease
   • Hemiballismus
   • Other movement disorders (e.g., dystonia, tics)
3. Order appropriate initial tests:
   • Imaging (MRI)
   • Labs (blood glucose, thyroid function tests)
4. Interpret results:
   • Imaging: rule out other causes of movement disorders
   • Labs: confirm or rule out underlying conditions
5. Initiate treatment and monitoring:
   • Parkinson's disease: levodopa, dopamine agonists, MAO-B inhibitors
   • Huntington's disease: tetrabenazine, antipsychotics for chorea
   • Hemiballismus: botulinum toxin injections or deep brain stimulation

Common Mistakes & Exam Traps

• The mistake: Failing to consider other causes of movement disorders (e.g., hypoglycemia, hypothyroidism).
• Why it happens: Misunderstanding the classic presentations and diagnostic approaches.
• How to avoid it: Review the high-yield facts and consider alternative diagnoses.
• Exam board insight: The examiners will penalize you for not considering other causes of movement disorders.
• The mistake: Failing to recognize the complications of Parkinson's disease (e.g., dysphagia, falls, dementia).
• Why it happens: Rushing through the question and not considering the long-term consequences.
• How to avoid it: Take your time and consider the potential complications of each condition.
• Exam board insight: The examiners will penalize you for not considering the long-term consequences of each condition.

How It’s Tested on USMLE

• Step 1: Basic science vignette (e.g., molecular mechanism, pathology slide, pharmacology).
  • Example: "A 45-year-old patient presents with tremors and rigidity. What is the underlying cause of this condition?"
• Step 2 CK: Clinical vignette (e.g., "A 45-year-old with chest pain...").
  • Example: "A 45-year-old patient presents with chorea and cognitive decline. What is the next step in diagnosis and management?"
• Step 3: Similar to Step 2 CK, plus prognosis, risk factors, and occasionally CCS management.

CCS (Step 3) Relevance (If Applicable)

If this topic appears in Step 3 Computer-based Case Simulations, provide a short strategy:

• Initial orders: Order imaging (MRI) and labs (blood glucose, thyroid function tests) to rule out other causes of movement disorders.
• Monitoring and follow-up: Monitor the patient's response to treatment and adjust as needed.
• Common mistakes: Failing to consider other causes of movement disorders, not ordering indicated tests, and delaying treatment.

Practice Questions (3-5 single-best-answer)

Question 1: A 45-year-old patient presents with tremors and rigidity. What is the underlying cause of this condition? A) Hypoglycemia B) Hypothyroidism C) Parkinson's disease D) Dystonia

Answer: C) Parkinson's disease

Explanation: The patient's symptoms (tremors and rigidity) are classic for Parkinson's disease. The underlying cause is the loss of dopaminergic neurons and Lewy bodies.

Question 2: A 45-year-old patient presents with chorea and cognitive decline. What is the next step in diagnosis and management? A) Order imaging (MRI) B) Order labs (blood glucose, thyroid function tests) C) Start tetrabenazine and antipsychotics D) Refer to a neurologist

Answer: C) Start tetrabenazine and antipsychotics

Explanation: The patient's symptoms (chorea and cognitive decline) are classic for Huntington's disease. The next step in diagnosis and management is to start tetrabenazine and antipsychotics.

Quick Reference Card (60-Second Summary)

• Parkinson's disease: levodopa, dopamine agonists, MAO-B inhibitors
• Huntington's disease: tetrabenazine, antipsychotics for chorea
• Hemiballismus: botulinum toxin injections or deep brain stimulation
• Lewy bodies: Parkinson's disease
• CAG repeat expansion: Huntington's disease

If You Get Stuck on Test Day

• How to eliminate obviously wrong answers: Look for contradictions in the patient's history and physical exam.
• How to use the "next best step" hierarchy (least invasive, most specific): Start with the least invasive test (e.g., labs) and work your way up to more specific tests (e.g., imaging).
• For Step 3 CCS: What to order when unsure (basic labs, vitals, IV access): Order basic labs (e.g., CBC, electrolytes) and vitals to assess the patient's overall condition.

Related USMLE Topics

• Dystonia: Connects to Parkinson's disease and tetrabenazine treatment.
• Tics: Connects to Huntington's disease and antipsychotics treatment.
• Chorea: Connects to Huntington's disease and tetrabenazine treatment.