Weakness Not Caused By Cerebrovascular Accident

Introduction

For the MCCQE1, approaching a patient with weakness requires a structured clinical reasoning process. While Cerebrovascular Accidents (CVA) are a leading cause of acute weakness, a broad differential diagnosis is essential for the Medical Expert role in the CanMEDS framework.

This guide focuses on the evaluation and management of weakness arising from etiologies other than stroke, including spinal cord pathology, peripheral neuropathies, neuromuscular junction disorders, and myopathies. Mastering the anatomical localization of the lesion is the single most important skill for this topic in MCCQE1 preparation.

Anatomical Localization

The first step in evaluating weakness is determining where the lesion is. This guides your investigations and management.

Comparison of UMN and LMN Signs

Differential Diagnosis by Etiology

1. Spinal Cord Pathologies (UMN/LMN Mix)

• Compressive Myelopathy: Tumor, epidural abscess, disc herniation.
  • Red Flag: Cauda Equina Syndrome (Saddle anesthesia, bowel/bladder retention/incontinence). This is a surgical emergency.
• Transverse Myelitis: Inflammation of the cord (often post-viral or MS-related).
• Amyotrophic Lateral Sclerosis (ALS): Unique presentation of mixed UMN and LMN signs in multiple body segments.

2. Peripheral Neuropathies (LMN)

• Guillain-Barré Syndrome (GBS): Acute inflammatory demyelinating polyneuropathy.
  • Classic presentation: Ascending weakness, areflexia, often post-GI (Campylobacter) or respiratory infection.
• Mononeuritis Multiplex: Vasculitis, Diabetes.
• Tick Paralysis: Rare but relevant in Canadian rural settings (BC, Alberta). Ascending paralysis similar to GBS but resolves upon tick removal.

3. Neuromuscular Junction Disorders

• Myasthenia Gravis (MG): Autoantibodies against ACh receptors.
  • Clinical: Fluctuating weakness, ptosis, diplopia.
• Lambert-Eaton Myasthenic Syndrome (LEMS): Paraneoplastic (Small cell lung cancer).
  • Clinical: Proximal weakness, improves briefly with exercise.
• Botulism: Descending paralysis.

4. Myopathies (Muscle)

• Inflammatory: Polymyositis, Dermatomyositis (Gottron’s papules, Heliotrope rash).
• Drug-induced: Statins (common in Canada), Corticosteroids.
• Endocrine: Hypothyroidism, Cushing’s.
• Electrolytes: Severe Hypokalemia (Periodic Paralysis).

Clinical Approach to Weakness

Follow these steps to systematically evaluate a patient for the MCCQE1.

Canadian Guidelines & Management

1. Guillain-Barré Syndrome (GBS)

• Canadian Management: Requires hospitalization, often ICU monitoring.
• Treatment: IVIG (Intravenous Immunoglobulin) or Plasmapheresis. Corticosteroids are not indicated.
• Monitoring: Frequent vital capacity checks; intubate if FVC < 20 mL/kg.

2. Multiple Sclerosis (MS)

• Diagnosis: McDonald Criteria (Dissemination in time and space).
• Acute Relapse: High-dose IV Methylprednisolone.
• Disease Modifying Therapies (DMT): Canada has access to a wide range of DMTs (e.g., Ocrelizumab, Natalizumab) funded provincially.

3. Spinal Cord Compression

• Guideline: Immediate MRI of the spine.
• Treatment: High-dose Dexamethasone (usually 10mg IV bolus followed by 4mg q6h) immediately if neoplastic compression is suspected, followed by urgent neurosurgical or oncological consultation.

4. Choosing Wisely Canada

• Low Back Pain: Do not image for low back pain within the first 6 weeks unless red flags (weakness, cauda equina, cancer history, fever) are present.

Key Points to Remember for MCCQE1

• Hypokalemia can cause acute generalized weakness and is a reversible metabolic cause.
• Myasthenia Gravis symptoms worsen at the end of the day; Lambert-Eaton improves with repetition.
• ALS is a diagnosis of exclusion characterized by the coexistence of UMN and LMN signs in the same or multiple limbs, with no sensory loss.
• Cauda Equina Syndrome is the most urgent “cannot miss” spinal diagnosis.
• Polymyositis presents with painless proximal muscle weakness and elevated CK; Polymyalgia Rheumatica presents with pain/stiffness but normal strength and normal CK.

Sample Question

Scenario

A 34-year-old female presents to the emergency department with a 2-day history of progressive weakness in her legs. She reports tripping frequently and difficulty climbing stairs. Two weeks ago, she had a self-limiting episode of diarrhea. On physical examination, she is alert and oriented. Vital signs are stable. Cranial nerves are intact. Motor examination reveals 3/5 strength in hip flexors and 4/5 in ankle dorsiflexors bilaterally. Deep tendon reflexes are absent at the patella and Achilles. Sensation is intact to pinprick and vibration.

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

Options

• A. Initiate high-dose intravenous methylprednisolone
• B. Order an urgent MRI of the lumbar spine
• C. Measure Forced Vital Capacity (FVC)
• D. Prescribe oral pyridostigmine
• E. Refer for outpatient electromyography (EMG)

Explanation

The correct answer is:

• C. Measure Forced Vital Capacity (FVC)

Detailed Explanation: The clinical presentation is highly suggestive of Guillain-Barré Syndrome (GBS). Key features include:

1. Ascending weakness: Legs affected first.
2. Areflexia: Absent reflexes are a hallmark of GBS (LMN lesion).
3. Antecedent infection: History of diarrhea (likely Campylobacter jejuni).
4. Intact sensation: While paresthesias are common, objective sensory loss is often mild compared to motor deficits.

In GBS, the most life-threatening complication is respiratory failure due to neuromuscular weakness of the diaphragm and intercostal muscles. Therefore, the immediate priority is to assess respiratory function using Forced Vital Capacity (FVC) or Negative Inspiratory Force (NIF).

• Option A: Steroids are not effective for GBS and are the treatment for acute MS relapses or spinal cord compression.
• Option B: MRI Spine is used to rule out cord compression (UMN signs, sensory level, bladder issues), which is less likely given the areflexia and absence of sensory level.
• Option C: Correct. Monitoring for respiratory failure is the ABC (Airway/Breathing) priority.
• Option D: Pyridostigmine is the treatment for Myasthenia Gravis (fluctuating weakness, ocular signs), not GBS.
• Option E: EMG confirms the diagnosis but is not the immediate management priority over assessing airway safety.

References

1. Medical Council of Canada. MCCQE Part I Clinical Decision-Making and Multiple-Choice Question Objectives. Available at mcc.ca.
2. Toronto Notes 2024. Neurology Chapter: Weakness and Neuromuscular Disorders. Toronto: Toronto Notes for Medical Students, Inc.
3. Wijdicks EF, et al. Practice parameter: immunotherapy for Guillain-Barré syndrome. Neurology.
4. Choosing Wisely Canada. Neurology and Family Medicine Recommendations. Available at choosingwiselycanada.org.
5. Public Health Agency of Canada. Lyme Disease: For Health Professionals. Canada.ca.