Accessory abductor digiti minimi muscle, less muscular than thought: an update on prevalence, morphology, and review of the literature

Objective: To update the prevalence and morphology of the accessory abductor digiti minimi muscle using 3T MRI. Materials and methods: A retrospective study of 3T wrist MRI was performed. Presence and morphology of the accessory abductor digiti minimi were evaluated in addition to its relationship to the ulnar nerve. Charts were reviewed for demographics, EMG, neuropathic symptoms, and initial MRI interpretation. Results: Analysis of 396 wrist MRI studies yielded 25% prevalence of the accessory abductor digiti minimi, of which the majority (69%) demonstrated fascial-type morphology proximal to Guyon’s canal. A minority (31%) demonstrated a contiguous muscle belly throughout its course. Overall, prevalence of a muscular type was only 8%. All patients were asymptomatic with normal EMG, unremarkable ulnar nerve, and no significant nerve compression. Anterior-posterior muscle dimension was larger in males. Only 1 of 98 accessory abductor digiti minimi muscles was prospectively identified. Conclusion: The accessory abductor digiti minimi is not uncommon; however, patients are usually asymptomatic without neuropathic symptoms. The most common anatomic variant is a small muscle belly at the level of the pisiform with fascial morphology proximal to Guyon’s canal and origin from the distal antebrachial fascia. A contiguous muscle belly extending from the distal forearm through the wrist occurs much less frequently. Clinicians should be familiar with these anatomic variants to enhance its recognition, understanding that a muscular variant is rarer than previously reported, but historically can serve as a potential source of clinical symptomatology and could be encountered during physical examination or wrist surgery.