Imaging description

Two normal muscle variants that are commonly described in the literature are the accessory abductor digiti minimi muscle and the extensor digitorum brevis manus muscle (Figure 43.1). On MRI, normal variant muscle tissue signal characteristics will follow that of normal muscle. The accessory abductor digiti minimi can originate at the palmar carpal ligament, the tendon of the palmaris longus muscle, or the antebrachial fascia of the forearm. It inserts at the volar, ulnar aspect of the base of the proximal phalanx of the small finger. On transverse MR images, it is a fusiform mass with the signal characteristics of muscle located volar, lateral to the pisiform bone. The extensor digitorum brevis manus originates from the distal radious and dorsal radiocarpal ligament with an insertion on the index or long finger. On MRI, it is identified as an accessory muscle just ulnar to the extensor tendon of the index or long finger at or distal to the wrist.

Importance

These muscles can be misdiagnosed clinically as a mass or inflammatory process. This usually occurs when these variant muscles are prominent or hypertrophied. The accessory abductor digiti minimi muscle is somewhat common, occurring in as many as 25% of the population. In addition, when an accessory abductor digiti minimi muscle is prominent it can cause a compressive neuropathy of the ulnar nerve.