Anterior interosseous nerve syndrome (AINS), also known as Kiloh-Nevin syndrome, is one of three common median nerve entrapment syndromes, the other two being pronator teres syndrome and the far more common carpal tunnel syndrome.
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Epidemiology
Anterior interosseous nerve syndrome is a rare entrapment syndrome, with comparatively little robust epidemiological data. It is said to account for less than 1% of neuropathies of the upper limb 1.
Clinical presentation
Anterior interosseous nerve syndrome is a pure motor neuropathy, as the anterior interosseous nerve contains no sensory fibers; dull forearm pain is however sometimes mentioned by patients.
Typically, patients fail to make an “OK sign", as flexion of the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger is impaired. Another sensitive yet similar test is the "pinch test" whereby a patient with anterior interosseous nerve syndrome will not be able to pinch a sheet of paper between their thumb and index finger, but will instead of clamp the sheet between their extended thumb and index fingers. Weakness of the pronator quadratus muscle manifests itself in pronation weakness with a flexed elbow 2.
Anterior interosseous nerve syndrome can be confounded by the Martin-Gruber anastomosis, present in up to 25% of the population: in these cases, the anterior interosseous nerve gives off branches to the ulnar nerve, creating atypical motor innervation patterns of the forearm and hand and thus effacing the typical clinical symptoms.
Pathology
The etiology is highly debated. Two common causes of anterior interosseous nerve syndrome are compression neuropathy and brachial plexus neuritis (Parsonage-Turner syndrome) 4.
In compressive etiologies, the anterior interosseous nerve can be compressed between the heads of the pronator teres muscle and the proximal edge of the flexor digitorum superficialis (FDS) muscle arch. The anterior interosseous nerve is furthermore prone to entrapment by anatomical variants, Gantzer muscle, an anomalous head of the flexor pollicis longus muscle 1, being the most noteworthy, as it is found in up to 52% of the population 2.
Distribution
In complete anterior interosseous nerve syndrome, the flexor pollicis longus (FPL) muscle, the radial part (2nd and 3rd digit) of the flexor digitorum profundus (FDP) muscle and pronator quadratus muscle are affected.
Radiographic features
Ultrasound and MRI are the two imaging modalities that best lend themselves to investigating entrapment syndromes. Next to directly visualizing direct causes [e.g. primary nerve or sheath tumors, ganglion cysts, osseous spurs, anatomical variants (e.g. Gantzer muscle), recognizing pathological muscle signal patterns on MRI can inversely point to the affected nerve.
MRI
Look for the pattern of muscle signal changes on fluid-sensitive (STIR, PD or T2 fat sat) sequences to uncover the affected nerve.
In anterior interosseous nerve syndrome, the the flexor pollicis longus (FPL) muscle, the radial part (2nd and 3rd digit) of the flexor digitorum profundus (FDP) muscle and pronator quadratus muscle will be accordingly affected. In chronic cases, additional hyperintense signal changes can occur on T1 imaging secondary to lipomatous atrophy. Care must be taken in assessing the pronator quadratus muscle, however, as hyperintense signal here has been shown to be a frequent normal finding of unclear etiology 2.
Although not strictly necessary in secondary entrapment syndromes (i.e. those not caused by space-occupying lesions etc.), intravenous application of contrast medium will show enhancement of denervated muscles.
Treatment and prognosis
Management of anterior interosseous nerve syndrome depends on the cause. In cases due to brachial neuritis, corticosteroids have been used. In cases due to entrapment, surgical decompression had a success rate of approximately 70% in one series 3.
History and etymology
Anterior interosseous nerve syndrome (Kiloh-Nevin syndrome) was first described in 1948 by Parsonage and Turner and further defined in 1952 by Kiloh and Nevin 4.
Differential diagnosis
Anterior interosseous nerve syndrome must be differentially diagnosed from:
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other median nerve entrapment syndromes