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Atypical Ulnar Neuropathy Presenting With an Atypical Wartenberg's Sign and Atypical Ultrasound.

A 62-year-old man with a history of wellcontrolled myasthenia gravis presented with a 3-month history of left hand weakness. He noticed difficulty opening jars and was unable to fully adduct his left fourth and fifth digits (Fig. 1). There was no associated paresthesia. His neurologic examination was notable for Medical Research Council grade 4 weakness of the left first dorsal interosseous, 5 weakness of the abductor digiti minimi, and normal strength of the fourth and fifth flexor digitorum profundus. Sensation was preserved in the hand. On electrodiagnostic testing, his left ulnar compound muscle action potential amplitude recorded from first dorsal interosseous was low (0.9 mV, compared with 9.4 mV on the right). The left ulnar abductor digiti minimi compound muscle action potential and antidromic ulnar sensory nerve action potential recorded from the fifth digit were normal. Needle examination demonstrated fibrillation potentials and reduced recruitment in the left first dorsal interosseous and a few fibrillations in the abductor digiti minimi. Other proximal ulnar and C8T1 innervated muscles were normal. Neuromuscular ultrasonography (Fig. 2A) and magnetic resonance imaging (Fig. 2B) showed a multiseptated cystic mass with an associated mass effect on the ulnar nerve in Guyon’s canal, consistent with a ganglion cyst. A lesion of the ulnar nerve at or distal to Guyon’s canal, as was identified in this case, primarily affects the deep palmar motor branch and spares the superficial branch. This results in weakness of the ulnar intrinsic hand muscles while relatively sparing the abductor digiti minimi, and patients may demonstrate Wartenberg’s sign. Classically described Wartenberg’s sign is an involuntary abduction of the fifth digit and can be a clinical marker of ulnar neuropathy. Our patient presented with an unusual variant of Wartenberg’s sign in which both the fourth and fifth digits were abducted. This variation was previously described by Wartenberg1 in his original manuscript describing his eponymous sign.bib1 The mechanism of