Trauma-induced Neuropathy of the Ulnar Branch to the Abductor Digiti Minimi

Background Innervated by the ulnar nerve, the abductor digiti minimi (ADM) is an intrinsic muscle of the hand that serves as an abductor of the fifth digit. Denervation of the ADM can result in fifth digit weakness and atrophy. Classically, ulnar nerve entrapment at the cubital tunnel or Guyon's canal may present with these findings, and this would be evident on electrodiagnostic evaluation. There is no current literature documenting a case of fifth digit weakness and atrophy with electrodiagnostic studies showing an isolated neuropathy to the ulnar branch of the ADM. Case Presentation A 45-year-old male presented to an orthopedic clinic with weakness of his right fifth digit. As the patient arose from bed two months prior, he heard something pop and felt a sharp pain in his right fifth digit and hypothenar aspect. Physical exam showed diminished abduction of the right fifth digit and hypothenar atrophy. The patient was referred for an electromyogram (EMG) and nerve conduction study (NCS). EMG/NCS of the ulnar nerve, tested distally using the first dorsal interosseous muscle, was normal. Afterwards, he was sent to follow up with the orthopedic surgeon, who requested another EMG to specifically test the ADM. The repeated EMG study showed isolated neuropathy of the ADM. Discussion This patient's clinical case highlights the significance of using the history and physical exam findings to probe deeper despite unremarkable initial electrodiagnostic studies. This case also exemplifies the importance of exploring less common pathologies, as this is a unique and uncommon diagnosis.