M28 kayaker with exertional dorsolateral forearm pain when paddling.
Functional ‘entrapment’ of the posterior interosseous nerve
Long-axis ultrasound images of the posterior interosseous nerve (arrowheads) are shown at elbow level in both forearm supination and forearm pronation [distal is to the left of image]. No focal compression or segmental swelling of the nerve is evident, but an angular kink in the course of the nerve is demonstrated within proximal supinator tunnel due to hypertrophy of the humeral head of supinator muscle (*) relative to the ulnar head. The kink accentuated in maximal forearm supination, a likely cause for dynamic nerve stretch & irritation when paddling due to the normal physiological phenomenon of ~ 20% increase in muscle volume with exercise. The clinical impression was also that of posterior interosseous nerve entrapment. Note prominent recurrent radial artery (leash of Henry) that does not appear to compress the nerve. From an imaging perspective, this case illustrates the importance of considering tissue dynamics (i.e. anatomical function), a particular strength of ultrasound over MRI.
NOTE: PIN = posterior interosseous nerve