In both Anterior Interosseous Nerve Syndrome (AINS) and ulnar nerve palsy, there is a difficulty in holding an object, usually a flat object such as a piece of paper, between the thumb and index finger (pinch grip).
Froment’s sign is a special test of the wrist that tests for palsy of the ulnar nerve, specifically, the action of adductor pollicis. The patient experiences difficulty maintaining a hold and will compensate by flexing the flexor pollicis longus (FPL) of the thumb, which is innervated by the anterior interosseous branch of the median nerve, to maintain grip pressure causing a pinching effect. Clinically, this compensation manifests as flexion of the IP joint of the thumb (rather than adduction, as would occur with correct use of the adductor pollicis).
AINS, on the other hand, is a pure motor neuropathy, as the AIN contains no sensory fibers. Typically, patients are unable to make the ‘O.K.’ sign, as flexion of the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger are impaired. A patient with AINS will also not be able to pinch a sheet of paper between his thumb and index finger, instead will clamp the sheet between his extended thumb and index fingers, rather like a tong.
In summary, for Froment’s sign, the patient pinches the paper, while in AINS, the patient cannot pinch the paper.
Tip: The AIN syndrome is also known as the Kiloh-Nevin syndrome.
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