Uncovering Anconeus Epitrochlearis in Persistent Ulnar Neuropathy: A Case Report

Case Diagnosis: Accessory Anconeus Epitrochlearis and Persistent Ulnar Neuropathy at the Elbow (UNE)

Case Description or Program Description: A 24-year-old right-hand-dominant male presented to outpatient musculoskeletal (MSK) clinic with a two-month history of persistent numbness and tingling in the left fourth and fifth digits and medial forearm, exacerbated by elbow flexion and exercise. Examination revealed decreased sensation in the ulnar nerve distribution without significant motor deficits.

Setting: Outpatient MSK Clinic

Assessment/Results: Initial electrodiagnostic testing demonstrated moderate-to-severe left UNE, characterized by focal demyelination and conduction block with partial axonal loss. Despite conservative management, including nighttime splinting, flexion block bracing, and a home exercise program, symptoms persisted, prompting surgical evaluation.
The patient underwent cubital tunnel release with initial symptom improvement. However, two months postoperatively, he endorsed persistent sensory deficits. Musculoskeletal ultrasound revealed an enlarged ulnar nerve with abnormal morphology within the cubital tunnel, along with a suspected accessory anconeus epitrochlearis muscle. Repeat nerve testing confirmed persistent ulnar neuropathy, with predominantly focal demyelination.

Discussion (relevance): Ulnar neuropathy at the elbow (UNE) is the second most common compressive neuropathy of the upper extremity, often leading to functional impairments due to sensory disturbances and intrinsic hand weakness(1). In this case, the presence of an anconeus epitochlearis muscle – an anatomical variation found in only 5.4% of patients with UNE – was a likely but rare contributor to the persistent neuropathy(2). Interestingly, this variation has also been associated with protection against UNE, adding complexity(2).
The use of musculoskeletal ultrasound, despite prior surgical decompression and repeat electrodiagnostic studies, was critical in identifying the accessory muscle as a contributor to the persistent dysfunction(3).

Conclusions: This case highlights the importance of considering rarer causes of UNE, specifically the accessory anconeus. Further, it illustrates the benefits of musculoskeletal ultrasound in evaluating rarer etiologies, including the accessory anconeus, to help guide future management.