Ataxia Gait: MCCQE1 Preparation Guide

Introduction

Ataxia is a non-specific clinical manifestation implying dysfunction of the parts of the nervous system that coordinate movement, such as the cerebellum, sensory pathways (posterior columns), or vestibular system. For MCCQE1 preparation, understanding the differential diagnosis based on the onset (acute vs. chronic) and the localization (cerebellar vs. sensory vs. vestibular) is crucial.

This guide is structured to help you master the Medical Council of Canada (MCC) objectives regarding gait disturbances and coordination, integrating the CanMEDS framework, particularly the Medical Expert and Health Advocate roles.

Anatomy and Pathophysiology

To effectively diagnose ataxia, one must understand the three main systems governing coordination:

1. Cerebellum: The center for coordination. Lesions here cause ipsilateral deficits.
2. Sensory System (Proprioception): Dorsal columns of the spinal cord and peripheral nerves.
3. Vestibular System: Inner ear and brainstem connections.

The “DANISH” Mnemonic

A classic mnemonic for cerebellar signs often tested on the MCCQE1:

D - Dysdiadochokinesia (impaired rapid alternating movements)
A - Ataxia (limb/gait)
N - Nystagmus
I - Intention tremor
S - Slurred speech (dysarthria) / Scanning speech
H - Hypotonia

Clinical Assessment

History Taking

In the Canadian context, history taking should be patient-centered. Determine the tempo of onset, which is the most powerful discriminator.

Physical Examination

Use the following systematic approach to examine a patient with suspected ataxia.

Differential Diagnosis

The differential diagnosis is best categorized by the time course of the illness.

Comparison: Cerebellar vs. Sensory Ataxia

Investigations

For MCCQE1, order investigations based on the clinical suspicion (Choosing Wisely).

• Bedside:
  • Capillary glucose (rule out hypoglycemia mimicking stroke).
• Laboratory:
  • CBC, Electrolytes, Urea, Creatinine.
  • Toxicology screen: Alcohol level.
  • Vitamin levels: B12, Thiamine, Vitamin E.
  • Thyroid function: Hypothyroidism can cause reversible ataxia.
• Imaging:
  • CT Head: First line in acute setting to rule out hemorrhage. Note: CT is poor at visualizing the posterior fossa.
  • MRI Brain: Gold standard for posterior fossa (cerebellum/brainstem), demyelination (MS), and tumors.
• Specialized:
  • Lumbar puncture (if infectious/MS suspected).
  • Genetic testing (referral to genetics).

Management and Canadian Guidelines

Management is etiology-specific. The CanMEDS framework emphasizes a multidisciplinary approach.

1. Acute Stroke (Ischemic or Hemorrhagic)

Follow Canadian Stroke Best Practice Recommendations:

• Time is Brain: If ischemic and within window (<4.5 hours), consider thrombolysis (tPA) or endovascular thrombectomy (EVT).
• Admit to a dedicated Stroke Unit.
• Secondary prevention (antiplatelets, statins, BP control).

2. Wernicke’s Encephalopathy

• Medical Emergency: Administer IV Thiamine before any glucose loading to prevent precipitation of Korsakoff’s psychosis.
• Dosing: Typically 500mg IV q8h x 2-3 days in acute phase.

3. Multiple Sclerosis

• Referral to Neurology.
• Acute relapse: High-dose IV methylprednisolone.

4. Degenerative/Genetic Conditions

• Supportive Care: There is often no cure.
• Rehabilitation: PT/OT for gait aids (walkers) and fall prevention.
• Genetic Counselling: Essential for family planning (Health Advocate role).

Key Points to Remember for MCCQE1

• Alcohol: The most common cause of acute and chronic ataxia in Canada.
• HINTS Exam: More sensitive than MRI in the first 48 hours for distinguishing posterior circulation stroke from vestibular neuritis.
• Friedreich’s Ataxia: Most common hereditary ataxia. Look for pes cavus (high arches), kyphoscoliosis, and hypertrophic cardiomyopathy (cause of death).
• Vitamin B12 Deficiency: Causes Subacute Combined Degeneration (Dorsal columns + Corticospinal tracts). Result: Ataxia + UMN signs (Babinski) + LMN signs (absent ankle jerks).
• Phenytoin Toxicity: Causes nystagmus and ataxia. Check levels.

Sample Question

Case Presentation

A 68-year-old male presents to the Emergency Department with the sudden onset of severe vertigo, nausea, vomiting, and inability to walk without assistance. He has a history of hypertension, hyperlipidemia, and type 2 diabetes. On physical examination, he has a normal mental status. He is unable to sit upright without support. Eye examination reveals direction-changing nystagmus. The Head Impulse Test is normal (no saccade). He has significant ataxia on the left side during finger-to-nose testing.

Question

Which one of the following is the most appropriate next step in the management of this patient?

• A. Discharge with a prescription for betahistine and outpatient follow-up
• B. Perform the Epley maneuver
• C. Administer IV tissue plasminogen activator (tPA) immediately
• D. Urgent MRI of the brain
• E. Urgent CT angiogram of the chest

Explanation

The correct answer is:

• D. Urgent MRI of the brain

Detailed Explanation

Why D is correct: This patient presents with Acute Vestibular Syndrome (AVS). The differentiation between a peripheral cause (e.g., vestibular neuritis) and a central cause (e.g., cerebellar/brainstem stroke) is critical. The patient has vascular risk factors (age, HTN, Diabetes). The physical exam findings are highly suggestive of a central lesion (Stroke):

1. Normal Head Impulse Test: In AVS, a normal HIT is worrisome for stroke (indicating the vestibulo-ocular reflex is intact, so the problem is central).
2. Direction-changing nystagmus: Central sign (Peripheral nystagmus is unidirectional).
3. Severe ataxia: Inability to sit upright is a “red flag” for cerebellar stroke (Peripheral vertigo patients can usually sit still).

Given these findings (HINTS exam pointing to central cause), an urgent MRI is the gold standard to visualize a posterior circulation stroke, as CT scans often miss small infarcts in the posterior fossa due to bone artifact.

Why other options are incorrect:

• A: Discharging a patient with a likely posterior circulation stroke is dangerous.
• B: The Epley maneuver is for Benign Paroxysmal Positional Vertigo (BPPV), which presents with episodic vertigo triggered by head movement, not continuous AVS.
• C: While this might be a stroke, you generally need imaging (CT to rule out bleed, MRI to confirm ischemia) before giving tPA, and you must confirm the diagnosis and contraindications first. “Immediately” without imaging is incorrect.
• E: CT chest is irrelevant to the presenting complaint.

References

1. Medical Council of Canada. (n.d.). MCCQE Part I Clinical Decision-Making and Multiple-Choice Question Objectives.
2. Canadian Stroke Best Practices. (2022). Acute Stroke Management. Heart and Stroke Foundation of Canada.
3. Kattah, J. C., et al. (2009). “HINTS to diagnose stroke in the acute vestibular syndrome: three-step bedside oculomotor examination more sensitive than early MRI diffusion-weighted imaging.” Stroke, 40(11), 3504-3510.
4. Choosing Wisely Canada. (n.d.). Neurology: Five Things Physicians and Patients Should Question.
5. Toronto Notes. (2023). Neurology Chapter: Ataxia and Gait Disorders.