The abductor pollicis brevis is the most superficial thenar muscle, forming the rounded prominence of the lateral thenar eminence and the primary muscle for palmar abduction of the thumb. Innervated by the recurrent branch of the median nerve, it is the thenar muscle most sensitive to carpal tunnel syndrome and its wasting is a late sign of advanced median nerve compression. Testing APB strength by resisting palmar abduction is a standard part of hand neurological assessment.
| Origin | Flexor retinaculum, scaphoid tubercle, and trapezium |
|---|---|
| Insertion | Radial side of the base of the proximal phalanx of the thumb |
| Nerve Supply | Recurrent branch of the median nerve (C8, T1) |
| Blood Supply | Superficial palmar arch |
| Actions | Palmar abduction of the thumb (perpendicular to the palm); Assists in opposition of the thumb |
|---|
Palmar abduction positions the thumb perpendicular to the palm ready for opposition, the movement that distinguishes human hand function. It is the initiating movement of the opposition sequence and its failure significantly impairs the ability to grasp objects.
APB weakness and wasting are the most important motor signs of carpal tunnel syndrome and indicate advanced compression requiring urgent surgical decompression. The thenar wasting visible in severe carpal tunnel syndrome affects the APB most prominently, producing the characteristic flat thenar eminence. Electrodiagnostic studies that show prolonged or absent median motor latency to the APB confirm the diagnosis.
The APB is the prominent rounded muscle forming the radial thenar eminence, easily palpated during resisted palmar thumb abduction. Atrophy produces visible flattening of this eminence compared to the normal side.
Progressive APB atrophy from advanced carpal tunnel syndrome producing a flat radial thenar eminence, weak palmar abduction, and impaired opposition requiring urgent carpal tunnel decompression.
