Age / Sex : 68 / M

Chief complaint: Right lateral hand weakness without sensory deficit

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Courtesy: Sang Hee Choi, Samsung medical center Seoul hospital

Entrapment of the deep motor branch of the ulnar nerve.(Cyclist’s palsy)

Discussion

Findings:

 On axial T2 weighted and proton density weighted image, the signal intensity of right adductor pollicis muscle and flexor pollicis brevis muscle, palmar and dorsal intraosseus muscle increase comparing with other hand muscle. These muscles are innervated by deep motor branch of ulnar nerve. But there is no space occupying lesion along ulnar nerve tract and guyon’s canal.

Differential Diagnosis:

Diagnosis: Entrapment of the deep motor branch of the ulnar nerve.(Cyclist’s palsy)

Discussion:

<Ulnar nerve entrapment at wrist>

1)     Pathophysiology : Nerve is stretched around the pisohamate ligament with hand extension, making it vulnerable to external compression or laceration

2)     Causes : Ganglia, lipomas ; anomalies of ligaments or muscles; ulnar artery aneurysms; fractures of the radius, pisiform bone, hook of the hamate ; chronic repetitive trauma, called cyclist’s palsy

3)     Clinical manifestation

–      Type 1 : Proximal to the Guyon canal

               Sensory loss combined with weakness of all ulnar intrinsic hand muscles

–      Type 2 : Deep motor branch in a location immediately distal to bifurcation

               All ulnar intrinsic hand muscles but produces no sensory loss

–      Type 3 : Deep motor branch in a location distal to the hypothenar branches

               Interosseous and lumbrical muscles but spares the hypothenar muscles

–      Type 4 : Superficial branch

               Sensory loss without weakness

4)     MR Findings

The normal ulnar nerve : best identified on T1WI, appearing as a round or oval low SI structure in the Guyon canal that is surrounded by fat

Diffuse swelling (intermediate signal) or enlargement of nerve.

MRI can help identifing mass (usually with hyperintense signal) such as ganglion cyst, aneurysm, fracture, anomalous muscle.

Indirect evidences include muscular denervation edema or atrophy depending on the site of the lesion.

References:

RadioGraphics 2010; 30:1373–1400

RadioGraphics 2006; 26:1267–1287

ARCH NEUROL/VOL 62, MAY 2005