Nerve Injury: MCCQE1 Preparation Guide

CanMEDS Focus: Medical Expert

For the MCCQE1, candidates must demonstrate the ability to diagnose, classify, and manage peripheral nerve injuries. Understanding the anatomy, pathophysiology (Wallerian degeneration), and the timeline for recovery is crucial for the Medical Expert role.

Introduction

Peripheral nerve injuries (PNI) are a common presentation in Canadian emergency departments and primary care clinics. They range from transient neurapraxia to complete transection. Mastery of this topic is essential for MCCQE1 preparation, as it integrates anatomy, neurology, and physical medicine and rehabilitation.

Definition: A peripheral nerve injury involves damage to the nerve structure (axon, myelin sheath, or connective tissue), resulting in a deficit of motor, sensory, or autonomic function in the distribution of that nerve.

Classification of Nerve Injuries

Understanding the classification is high-yield for the MCCQE1. The two primary systems used are Seddon’s Classification and Sunderland’s Classification.

Comparison of Classification Systems

Seddon Classification	Sunderland Grade	Pathophysiology	Prognosis	Recovery Rate
Neurapraxia	Grade I	Focal demyelination; axon continuity preserved. No Wallerian degeneration.	Excellent	Days to weeks (complete recovery)
Axonotmesis	Grade II	Axonal disruption; endoneurium intact. Wallerian degeneration occurs.	Good	~1 mm/day (guided by endoneurial tube)
Neurotmesis	Grade III	Axon + Endoneurium disrupted; Perineurium intact.	Variable	Incomplete without surgery
Neurotmesis	Grade IV	Axon + Endoneurium + Perineurium disrupted; Epineurium intact.	Poor	Scarring blocks regeneration; Surgery usually required
Neurotmesis	Grade V	Complete transection of the entire nerve trunk.	None	Requires surgical repair

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MCCQE1 Tip: Remember the “Rule of 1”: Axonal regeneration occurs at approximately 1 mm per day (or roughly 1 inch per month). This helps in estimating recovery time for Axonotmesis.

Pathophysiology: Wallerian Degeneration

Process of Wallerian Degeneration

When an axon is severed, the segment distal to the injury undergoes degeneration. This is a critical concept for understanding electrodiagnostic timing.

0–24 Hours: Axonal swelling and fragmentation begin.
24–96 Hours: Myelin sheath disintegration.
1–2 Weeks: Macrophages clear debris (critical for regeneration space).
Schwann Cells: Proliferate and form the “Bands of Bungner” to guide regenerating axons.

Clinical Presentation and Specific Nerve Injuries

Symptoms depend on the nerve type (sensory, motor, mixed) and the level of injury.

High-Yield Nerve Injuries for MCCQE1

Upper Limb

Upper Limb Nerve Injuries

1. Median Nerve

Etiology: Carpal Tunnel Syndrome (compression at wrist), Supracondylar fracture (proximal).
Motor: Loss of thumb opposition (“Ape Hand”), weak wrist flexion.
Sensory: Loss over palmar aspect of thumb, index, middle, and radial half of ring finger.
Sign: Phalen’s test, Tinel’s sign at wrist.

2. Ulnar Nerve

Etiology: Cubital Tunnel Syndrome (elbow), Guyon’s canal compression (wrist/cyclists), Medial epicondyle fracture.
Motor: Weakness of interossei (abduction/adduction), loss of thumb adduction.
Sensory: Loss over little finger and ulnar half of ring finger.
Sign: Froment’s sign (compensatory thumb flexion due to adductor pollicis paralysis), Claw hand (if distal lesion).

3. Radial Nerve

Etiology: Humeral shaft fracture, “Saturday Night Palsy” (compression at spiral groove), Crutch palsy (axilla).
Motor: Wrist drop (loss of extensors), weak supination.
Sensory: Loss over dorsal web space (between thumb and index).
Sign: Unable to extend wrist and fingers.

4. Axillary Nerve

Etiology: Anterior shoulder dislocation, surgical neck fracture of humerus.
Motor: Deltoid weakness (abduction >15 degrees).
Sensory: “Regimental badge” area over lateral deltoid.

Diagnostic Approach

Step 1: Detailed History

Identify the mechanism of injury (sharp transection vs. crush vs. stretch). Inquire about the time course (acute vs. chronic).

Canadian Context: Ask about occupation (WSIB claims) and functional impact on Activities of Daily Living (ADLs).

Step 2: Physical Examination

Perform a focused neurological exam:

Motor: Grade power (MRC scale 0–5).
Sensory: Test light touch, pinprick, and two-point discrimination.
Provocative Tests: Tinel’s sign (percussion over nerve) to track regeneration (advances 1mm/day).

Step 3: Electrodiagnostic Studies (NCS/EMG)

This is the gold standard for localization and prognostication.

Timing is key: Generally performed 3–4 weeks post-injury.
Why? Wallerian degeneration takes time. Performing EMG too early (< 2 weeks) may yield false negatives for denervation (fibrillation potentials).

Step 4: Imaging

Ultrasound: Increasingly used in Canada for cost-effective, dynamic visualization of nerves.
MRI: Useful for plexus injuries or evaluating mass lesions/tumors.

Medical Abbreviations

Common abbreviations used in neurology notes:


CTS  : Carpal Tunnel Syndrome
NCS  : Nerve Conduction Studies
EMG  : Electromyography
SNAP : Sensory Nerve Action Potential
CMAP : Compound Muscle Action Potential
WSIB : Workplace Safety and Insurance Board (Ontario specific, varies by province)

Management

Management depends on the type of injury (Open vs. Closed).

1. Closed Injuries (e.g., Crush, Traction)

Usually managed conservatively initially.

Observation: Monitor for 3–6 months for signs of re-innervation.
Physical Therapy: Maintain Range of Motion (ROM) to prevent contractures.
Pain Management: Neuropathic agents (Gabapentin, Pregabalin, TCAs).

2. Open Injuries (e.g., Lacerations)

Clean sharp laceration: Immediate primary surgical repair (neurorrhaphy).
Dirty/ragged laceration: Delayed repair (2–3 weeks) once the wound is clean and the extent of nerve damage is demarcated.

Canadian Guidelines & Choosing Wisely

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Canadian Practice Point: According to Choosing Wisely Canada (Neurology/Physical Medicine), do not order EMG/NCS for simple entrapment neuropathies (like typical CTS) if the diagnosis is clear clinically, unless surgery is being considered or the diagnosis is atypical.

Red Flags Requiring Urgent Referral

Rapidly progressive motor weakness.
Acute trauma with complete functional loss (concern for neurotmesis).
Suspected compartment syndrome (pain out of proportion).

Key Points to Remember for MCCQE1

Neurapraxia has the best prognosis and recovers within weeks; Neurotmesis requires surgery.
Wallerian Degeneration dictates that EMG/NCS should generally be delayed 3–4 weeks post-injury for accurate assessment of denervation.
Radial Nerve Palsy (Wrist drop) is often associated with humeral shaft fractures.
Common Peroneal Nerve injury causes foot drop and is the most common lower limb neuropathy.
Tinel’s Sign tracks the progress of axonal regeneration distally.
Compartment Syndrome is a surgical emergency; do not wait for nerve signs (paresthesia/paralysis are late signs).

Study Checklist

Memorize the brachial plexus anatomy (C5–T1).
Differentiate between Ulnar (Claw hand) and Median (Ape hand) signs.
Review the sensory dermatomes vs. peripheral nerve distributions.
Understand the timeline for Wallerian degeneration.

Sample Question

Clinical Scenario

A 42-year-old male construction worker presents to your clinic complaining of weakness in his right hand. He reports that 4 weeks ago, he sustained a fracture of the hook of the hamate while operating a jackhammer. The fracture was managed conservatively. On physical examination, you note atrophy of the hypothenar eminence and weakness of finger abduction and adduction. He has difficulty holding a piece of paper between his thumb and index finger. Sensation is decreased over the palmar aspect of the fifth digit.

Question

Which of the following nerves is most likely injured in this patient?

Explanation

The correct answer is:

C. Ulnar nerve

Detailed Explanation: This patient presents with signs of distal Ulnar nerve compression, likely at Guyon’s canal (wrist), which is anatomically related to the hook of the hamate.

Motor Findings: Atrophy of the hypothenar eminence and weakness of the interossei muscles (responsible for finger abduction/adduction) are classic for ulnar neuropathy.
Froment’s Sign: The difficulty holding paper between the thumb and index finger indicates weakness of the adductor pollicis (ulnar innervated). The patient compensates by flexing the interphalangeal joint of the thumb (Flexor Pollicis Longus - Median nerve), which is a positive Froment’s sign.
Sensory Findings: Sensory loss in the fifth digit is consistent with ulnar distribution.

Why other options are incorrect:

A. Median nerve: Would present with thenar atrophy (“Ape hand”) and sensory loss in the thumb/index/middle fingers.
B. Radial nerve: Would present with wrist drop and sensory loss on the dorsum of the hand.
D. Anterior interosseous nerve: A branch of the median nerve; injury causes pure motor deficits (weakness of FPL and FDP to index finger - “OK sign” deficit) with no sensory loss.
E. Musculocutaneous nerve: Innervates the biceps/brachialis; injury causes weak elbow flexion and sensory loss on the lateral forearm.

References

Medical Council of Canada. MCCQE Part I Clinical Decision-Making and Multiple-Choice Question Guidelines. Available at mcc.ca .
Choosing Wisely Canada. Neurology and Physical Medicine & Rehabilitation Guidelines. Available at choosingwiselycanada.org .
Preston, D. C., & Shapiro, B. E. (2020). Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic Correlations (4th ed.). Elsevier.
Public Health Agency of Canada. Injury in Canada. Data and epidemiology.
Campbell, W. W. (2013). DeJong’s The Neurologic Examination. Lippincott Williams & Wilkins.