Saturday, January 24, 2009

Darwin and an Anomalous Muscle

The theory of evolution posits that natural selection acts on the phenotypic expression of genetic variation, driving population changes over time. The classic example of genetic variation, observed by Charles Darwin, was the different appearance of the beaks of finches in the Galapagos islands:

The different beak shapes are adapted to different food sources, and affect the relative survival of various finch species. From these seminal observations, Darwin developed his theory of evolution.

Humans, like finches, also exhibit variations in their bodies. This variation is often plain to see, with different body shapes and sizes, skin color, and the like. Other variations are more subtle, and require cross-sectional imaging to detect.

In this case, a 34 year-old woman presented to a hand surgeon with a lump on the dorsum (back) of her wrist. The lump had been present for a few years, but had become more bothersome recently, exacerbated by certain activities. She was sent for an MRI:

Axial T1 and T2 fatsat images reveal a mass-like area (red arrows) corresponding to the clinical finding. The mass is close to muscle in signal intensity. A coronal image depicts the sharp contours of the mass-like area (red arrow), situated between the extensor tendons (green arrows):

Sagittal images help confirm the diagnosis of an accessory muscle, the extensor digitorum brevis manus (red arrow, figure A). Compare to an image from a normal individual (image B):

The extensor digitorum brevis manus (EDBM) muscle occurs in 1-3% of the population and may be mistaken for a mass or ganglion. The EDBM origin is variable, but the most common origin is the dorsal wrist capsule deep to the extensor retinaculum. The muscle may also arise from the distal radius or the deep carpal fascia. It typically inserts onto the extensor hood of the index finger or middle finger but may also insert into the extensors of the fourth and fifth fingers by way of either a tendon or a slip.

On MRI, extensor tendons should not have a muscular component at and distal to level of carpus. The presence of muscle tissue in this region indicates the presence of an accessory muscle.

The EDBM is usually asymptomatic, but increased use of the hand may lead to pain. Conservative treatment is pursued in most cases, but surgical removal may be necessary. Note that resection of EDBM should be avoided in cases where the EDBM is compensating for the absence of the extensor indicis proprius.

In this particular case, the patient elected to modify her activity level, rather than undergo surgery.

Nature gives finches different beaks, and humans different muscles.

Vic David MD

1 comment:

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