Clinical history: A 29 year-old female presents with increasing swelling of the volar 1st web space for several years. She has numbness in the thumb and pain with gripping. Proton density-weighted (1a) and STIR (1b) axial images and T1-weighted sagittal (1c) and coronal (1d) images are provided. What are the findings? What is your diagnosis?
Accessory flexor digitorum superficialis indicis muscle, most likely of the digastric variety.
Many accessory muscles have been described in the hand. Most are asymptomatic, but they can be a cause of compressive neuropathy or a palpable soft tissue mass1,4. Detection can be difficult because the muscles have the same signal intensity and overall appearance as the adjacent normal muscles and therefore do not attract the eye in the fashion of a typical mass3. The key to diagnosis is knowledge of normal muscle anatomy, in particular being aware of spaces where muscles should not exist normally.
Ulnar-sided Accessory Muscles
The accessory abductor digiti minimi is the most common of the accessory muscles, present in as many as 24% of wrists, and bilateral in 50%1,2,3,4,5. It is seen radial and volar to the pisiform and inserts with the abductor digiti minimi (ADM) at the ulnar base of the 5th proximal phalanx, projecting into Guyon’s canal1,2,3,4. Normally there should be no muscle tissue in Guyon’s canal at the level of the pisiform3.
A normal muscle that can be mistaken for a variant is the palmaris brevis, which lies in the subcutaneous tissues volar to the neurovascular structures of Guyon’s canal1. The palmaris brevis is distal to the pisiform and inserts into the skin2.
Midline Accessory Muscles
Palmaris longus variants are midline and superficial to the flexor retinaculum. The normal palmaris longus has the muscular component in the proximal to mid forearm and is tendinous in the wrist. The variants that cause muscle tissue to be present at the wrist include an inverted configuration with the tendon proximal and the muscle distal, a digastric configuration with muscle at each end separated by tendon and a non-tendinous variant with muscle along the entire width. The last variant can be solid or bifid in the wrist1.
In most people, the lumbrical muscles begin distal to the carpal tunnel. Proximal origin of the lumbricals in the carpal tunnel can occur in as many as 22% of people and can cause carpal tunnel syndrome1,4.
Radial-sided Accessory Muscles
The accessory flexor digitorum superficialis indicis is a replacement of the FDS tendon by muscle at the carpals or second metacarpal. Like the palmaris longus, there are several variants including complete replacement, a digastric muscle belly and distal extension of the muscle into the carpal tunnel1,2.
The flexor carpi radialis brevis vel profundus extends from the distal radius to the capitate and base of the third and fourth metacarpals. It passes between the pronator quadratus and FCR muscles2.
Dorsal Accessory Muscles
The extensor digitorum brevis manus is found ulnar to the extensor tendon of the index finger. Diagnosis is easier to make if you keep in mind that the muscle bellies of the extensor tendons should not extend to the carpal bones1,3.
Accessory muscles of the hand and wrist are usually incidental findings but can be a cause of compressive neuropathy or present as a palpable mass. Diagnosis depends on familiarity with the normal muscular anatomy of the wrist and being aware of places in the wrist where muscles should not exist.
1 Timins M. Muscular anatomic variants of the wrist and hand: Findings on MR imaging. AJR. 1999; 172: 1397-1401.
2 Sookur PA, Naraghi AM, Bleakney RR, Jalan R, Chan O, White LM. Accessory Muscles: Anatomy, symptoms and radiologic evaluation. Radiographics. 2008;28(2).
3 Chung C, Steinbach L. MRI of the Upper Extremity: Shoulder, Elbow, Wrist and Hand. 2009. Lippincott Williams & Wilkins.
4 Pfirrmann CWA, Zanetti M. Variants, pitfalls and asymptomatic findings in wrist and hand imaging. European Journal of Radiology. 2005; 56: 286-295.
5 Zeiss J, Jakab E, Khimji T, Imbriglia J. The ulnar tunnel at the wrist (Guyon’s canal): Normal MR anatomy and variants. AJR. May 1992; 158: 1081-1085.