Weakness Not Caused By Cerebrovascular Accident

Introduction

Welcome to this comprehensive MCCQE1 study guide on weakness not caused by cerebrovascular accident (CVA). This resource is tailored for Canadian medical students preparing for the Medical Council of Canada Qualifying Examination Part I (MCCQE1). Understanding the various causes of weakness is crucial for success in your MCCQE1 and future medical practice in Canada.

Differential Diagnosis of Weakness

When approaching a patient with weakness in a Canadian healthcare setting, consider the following differential diagnosis:

Key Neurological Causes of Weakness

Guillain-Barré Syndrome (GBS)

GBS is an important cause of acute flaccid paralysis in Canada. Understanding its presentation and management is crucial for MCCQE1 preparation.

Multiple Sclerosis (MS)

MS has a higher prevalence in Canada compared to many other countries, making it a high-yield topic for the MCCQE1.

Key points for MCCQE1:

• Relapsing-remitting course
• Optic neuritis as a common initial presentation
• MRI showing demyelinating lesions
• McDonald criteria for diagnosis
• Disease-modifying therapies approved in Canada

Myasthenia Gravis (MG)

MG is an autoimmune disorder affecting neuromuscular junctions. Canadian guidelines emphasize the following:

1. Fatigable weakness, often affecting ocular and bulbar muscles
2. Ice pack test for diagnosing ocular MG
3. Anti-acetylcholine receptor antibodies in most patients
4. Thymectomy consideration in selected patients

Amyotrophic Lateral Sclerosis (ALS)

ALS is a progressive neurodegenerative disorder affecting both upper and lower motor neurons.

Muscular Causes of Weakness

Muscular Dystrophies

Muscular dystrophies are genetic disorders causing progressive muscle weakness. Key types to remember for MCCQE1 include:

1. Duchenne Muscular Dystrophy
2. Becker Muscular Dystrophy
3. Myotonic Dystrophy

Polymyositis and Dermatomyositis

These inflammatory myopathies are important considerations in the differential diagnosis of weakness.

Metabolic Causes of Weakness

Electrolyte imbalances can lead to significant muscle weakness. Key points for MCCQE1:

Canadian Guidelines for Weakness Evaluation

The Canadian Neurological Sciences Federation provides guidelines for the evaluation of patients presenting with weakness:

1. Detailed history, including onset, progression, and associated symptoms
2. Thorough neurological examination
3. Appropriate use of diagnostic tests (EMG, NCS, MRI) based on clinical suspicion
4. Consideration of referral to a neurologist for complex cases

Key Points to Remember for MCCQE1

• 🍁 Recognize the higher prevalence of MS in Canada and its impact on healthcare resources
• 🍁 Understand the Canadian approach to GBS management, including early recognition and treatment
• 🍁 Be familiar with the Canadian guidelines for ALS care, including multidisciplinary management
• 🍁 Consider metabolic causes of weakness, especially in the context of Canadian dietary habits and medication use
• 🍁 Know the indications for specialist referral in the Canadian healthcare system

MCCQE1 Practice Question

Sample Question

A 28-year-old woman presents to a Canadian emergency department with progressive weakness in her legs over the past week. She reports a recent upper respiratory tract infection. On examination, she has symmetric weakness in both legs, absent deep tendon reflexes, and intact sensation. Which one of the following is the most appropriate next step in management?

• A. Immediate brain and spine MRI
• B. Start high-dose corticosteroids
• C. Perform lumbar puncture for CSF analysis
• D. Begin plasmapheresis
• E. Administer pyridostigmine

Explanation

The correct answer is:

• C. Perform lumbar puncture for CSF analysis

This patient's presentation is highly suggestive of Guillain-Barré Syndrome (GBS), given the progressive symmetric weakness, areflexia, and recent history of infection. In the Canadian healthcare context, when GBS is suspected, the most appropriate next step is to perform a lumbar puncture for CSF analysis.

CSF analysis in GBS typically shows albuminocytologic dissociation (elevated protein with normal cell count), which is a key diagnostic feature. This finding, along with the clinical presentation, helps confirm the diagnosis and guides further management.

The other options are less appropriate:

• A: While MRI can be helpful in some cases, it's not the first-line diagnostic test for GBS.
• B: Corticosteroids are not recommended for GBS treatment in Canada.
• D: Plasmapheresis is a treatment option, but it's not initiated before confirming the diagnosis.
• E: Pyridostigmine is used for myasthenia gravis, not GBS.

Remember, in the Canadian healthcare system, prompt diagnosis and management of GBS are crucial to prevent complications and improve outcomes.

References

1. Pringle CE, et al. Canadian Guidelines on the pharmacological treatment of neuromuscular disorders. Can J Neurol Sci. 2019;46(2):172-185.

2. Public Health Agency of Canada. (2018). Multiple Sclerosis in Canada. Retrieved from https://www.canada.ca/en/public-health/services/publications/diseases-conditions/multiple-sclerosis-infographic.html (opens in a new tab)

3. Guidon AC, et al. Consensus disease definitions and treatment guidelines for myasthenia gravis. Muscle Nerve. 2020;62(3):282-295.

4. Shoesmith C, et al. Canadian best practice recommendations for the management of amyotrophic lateral sclerosis. CMAJ. 2020;192(46):E1453-E1468.

5. Fokke C, et al. Diagnosis of Guillain-Barré syndrome and validation of Brighton criteria. Brain. 2014;137(Pt 1):33-43.