A Clear Mystery: Recognizing Lipomatosis of the Nerve

: Lipomatosis of the nerve (LN) commonly presents with neurologic dysfunction due to massive fibro-fatty enlargement of the peripheral nerves. It is uniquely associated with adipose proliferation in the subcutaneous tissue and muscle in the innervated territory, along with osseous abnormalities. Herein, we present the case of a 56-year-old woman who presented with severe right ulnar distribution pain involving the medial forearm and hand (9/10 on a numerical rating scale), declining right-hand strength, movement-dependent hypoesthesias, paresthesias, and a pronounced claw deformity of the right hand with intrinsic atrophy. Electrodiagnostic studies demonstrated pronounced fibrillations, decreased voluntary activation, and minimal collateral reinnervation in the abductor digiti minimi and abductor pollicis brevis, consistent with dysfunction of the lower trunk of the right brachial plexus. Magnetic resonance imaging (MRI) and computed tomography (CT) of the brachial plexus were interpreted as a tumor on the right supra- and infraclavicular brachial plexus. At surgery, the brachial plexus was embedded in relatively tight connective tissue with a typical lipoma posteriorly. The lipoma was resected, and the plexus was explored extensively. This case is the 10th report of LN involving the brachial plexus and demonstrated the cardinal features of LN. It provides insight into the pattern of lesions associated with innervation by LN.