Age / Sex : 28 / M

Age/Sex: 28/M

Chief Complaint: Limited IP flexion of Lt thumb (6 MA)

anterior interosseous nerve syndrome

Discussion

Findings: 

FDP, FPL, and pronator quadratus muscle show subtle hyperintense signal on T2WI and T1WI with mild enhancement.

Diagnosis:

EMG/NCS : Lt anterior interossous neuropathy, partial axonotmesis state (Degree of involvement: FPL>FDP>PQ)

Operation: releasing compressed AIN between two heads of FDS by cutting the humeral head of FDS.

Discussion:

Anterior interosseous syndrome or Kiloh-Nevin syndrome is a medical condition in which damage to the anterior interosseous nerve, a motor branch of the median nerve, causes pain in the forearm and a characteristic weakness of the pincer movement of the thumb and index finger.

 Anterior interosseous nerve, the largest branch of the median nerve, supplies the flexor pollicis longus, the pronator quadratus, and the radial part of the flexor digitorum profundus muscle (for second finger). The anterior interosseous branch runs along the interosseous membrane after branching off of the median nerve at the inferior part of the cubital fossa. The anterior interosseous nerve lies between flexor digitorum profundus and flexor pollicis longus. Flexor pollicis longus originates on the distal aspect of the radius and the lateral half of the interosseous membrane. It inserts into the distal phalanx of the thumb. The anterior interosseous nerve supplies the lateral half of flexor digitorum profundus, flexor pollicis longus and pronator quadratus. The pronator quadratus arises on the anterior, distal ¼ of the ulna and inserts on the anterior distal ¼ of the radius. The anterior interosseous nerve ends by supplying the carpals.

Most cases are due to compression of the nerve as a result of humeral or forearm fracture, fibrous band, anomaly (muscles, vessels), tumors, and sports activity (e.g., weightlifting, throwing).

MR imaging is potentially useful and safe in the investigation of anterior interosseous nerve syndrome, excluding muscle and tendon trauma from the differential diagnosis and providing a good alternative to EMG in the confirmation that the abnormality lies in the distribution of the anterior interosseous nerve. STIR images shows increased signal intensity in the involved muscles, which in itself is a nonspecific response with a variety of possible causes, including myositis, delayed-onset muscle soreness, and muscle strain. It is the characteristic isolated involvement of muscles supplied by the anterior interosseous nerve, however, that indicates antenor interosseous nerve syndrome as theunderlying cause. In chronic denervation phase, fatty atrophy may be depicted. A potential advantage of MR imaging over EMG is in the evidence that increased signal intensity on STIR images is seen before changes in denervation become detectable at EMG. West et al found that the first changes on STIR images were seen as early as 4 days after complete nerve injury, in contrast to changes seen at EMG, which are not detectable before 2-3 weeks. There is no certainty, however, that all neuropathies will give rise to muscle changes at similar times after the initial insult, and early imaging in patients with anterior interosseous nerve syndrome may show that changes at MR imaging have different times of onset, depending on the underlying cause.

References

Grainger AJ, Campbell RSD, Stothard J. Anterior interosseous nerve syndrome:appearance at MR imaging in three cases. AJR 1998: 208:381-384