A rare variation of intrinsic and extrinsic hand muscles represented by a bi-ventered first lumbrical extending into the carpal tunnel combined with bilateral fifth superficial flexor digitorum tendon regression

Atavistic muscles in human anatomy: Evolutionary origins and clinical implications

The evolution and variations of human anatomy are of great interest to physicians and anatomists. Variations can be categorized as vestigial, accessory or atavistic structures. Vestigial muscles are frequently encountered structures that are normally present but have become rudimentary through evolution. Muscles that disappeared during evolution sometimes arise again, although rarely; such muscles are referred to as atavistic. They arise from failure of suppression of genetic loci. Some common atavistic muscles seen clinically are the extensor digitorum brevis manus, chondroepitrochlearis and plantaris. Atavistic muscles appear more frequently in the upper limb than in any other region of the human body. One explanation for the appearance of these muscles, mainly within the upper limbs, is based on the evolution of the complex upper extremities formed in humans today. Often, the presence of atavistic muscles is asymptomatic, but they can compromise the function of normal anatomical structures and complicate clinical situations if their presence is unknown. They can cause complications if they are confused with soft tissue pathology, if they compress or displace surrounding structures, or if they require an additional blood supply during times of exercise and stress. The purpose of this paper was to describe the common atavistic muscles, their hypothesized evolutionary origins, their potential complications and possible treatments for the diagnosing clinician.