Untitled Quiz

Ulnar Neuropathy at the Wrist

• UNW (ulnar neuropathy at the wrist) is a rare condition, sometimes confused with ulnar neuropathy at the elbow (UNE) or early motor neuron disease.
• Knowledge of the ulnar nerve's wrist anatomy is crucial for understanding its unique clinical and electrophysiological presentations.
• Diagnosis might involve extra- or intra-wrist compression from structural lesions.
• Ultrasound plays a valuable role in aiding electrodiagnostic studies for localizing ulnar neuropathy.

Anatomy

• At the wrist, the ulnar nerve enters Guyon's canal near the distal wrist crease.
• Guyon's canal's boundaries include the pisiform, hook of the hamate, transverse carpal ligament and adjacent hamate/triquetrum bones.
• The ulnar nerve divides into superficial and deep branches within the canal.
• The deep branch (deep palmar motor branch) supplies motor innervation to the hypothenar muscles (ABductor digiti minimi [ADM], flexor digiti minimi, and opponens digiti minimi).
• The superficial branch provides sensory function to the volar fifth and medial fourth digits and innervates the palmaris brevis.
• The deep palmar motor branch also innervates the third and fourth lumbricals, dorsal and palmar interossei, the adductor pollicis, and the deep head of the flexor pollicis brevis.

Clinical Presentation

• Several UNW subtypes exist, depending on the lesion's location and affected fibers.
• Distal Deep Palmar Motor Lesion: affects all deep palmar motor branch muscles but spares the hypothenar muscles and sensory fibers.
• Proximal Deep Palmar Motor Lesion: impacts all ulnar-innervated hand muscles, excluding the palmaris brevis.
• Proximal Canal Lesion: involves all ulnar nerve branches (deep and superficial), including sensory areas.
• Superficial Branch Lesion: primarily affects the superficial branch, leading to sensory loss in the volar fifth and medial fourth digits, with palmaris brevis potentially spared.
• Symptoms often include painless weakness and atrophy of ulnar intrinsic hand muscles, leading to characteristic hand postures like Benediction hand. The "palmaris brevis sign" may be present in severe cases.

Etiology

• Ulnar nerve entrapment at the wrist is less frequent than at the elbow.
• It can be associated with trauma or wrist fractures.
• A ganglion cyst in Guyon's canal is a more common cause of compression.
• Other possible causes include anomalous muscles, nerve tumours (e.g., lipomas, aneurysms), and occupational or repetitive use of hand tools that apply pressure to the hypothenar eminence.

Differential Diagnosis

• Early motor neuron disease may be confused with distal UNW lesions due to similar symptoms of painless weakness.
• In UNW, the abductor pollicis brevis (median nerve-supplied) muscle remains unaffected.
• Atypical motor neuron disorders (like multifocal motor neuropathy) may also present similar symptoms.
• Accurate identification of the specific lesion location (wrist vs. elbow) is crucial.
• Features like intact median nerve innervation and patterns of weakness/atrophy aid in differential diagnosis.
• Features of proximal lesions can indicate a need to exclude disorders such as ulnar neuropathy at the elbow (UNE).