In chronic cases, additional hyperintense signal changes can occur on T1w imaging secondary to lipomatous atrophy. In AINS, the flexor pollicis longus (FPL), the radial part (2nd and 3rd digit) of the flexor digitorum profundus (FDP) and pronator quadratus (PQ) will be accordingly affected. Look for the pattern of muscle signal changes on fluid-sensitive (STIR, PD or T2w fat sat) sequences to uncover the affected nerve. Gantzer muscle), recognizing pathological muscle signal patterns on MRI can inversely point to the affected nerve. primary nerve or sheath tumors, ganglion cysts, osseous spurs, anatomical variants (e.g. Next to directly visualizing direct causes [e.g. Ultrasound and MRI are the two imaging modalities that best lend themselves to investigating entrapment syndromes. In complete anterior interosseous nerve syndrome, the flexor pollicis longus (FPL), the radial part (2nd and 3rd digit) of the flexor digitorum profundus (FDP) and pronator quadratus (PQ) are affected. Two common causes of AINS are compression neuropathy and brachial plexus neuritis 4.ĪINS can be caused by compression between the heads of the pronator teres muscle and the proximal edge of the FDS arch. The anterior interosseous nerve is furthermore prone to entrapment by anatomical variants, Gantzer’s muscle, an anomalous head of the FPL (flexor pollicis longus muscle) 1, being the most noteworthy, as it is found in up to 52% of the population 2. Weakness of the pronator quadratus muscle manifests itself in pronation weakness with a flexed elbow 2.ĪINS 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. Typically, patients fail to make an “O.K.”-sign, as flexion of the interphalangeal joint of the thumb and the distal interphalangeal joint of the index finger is impaired.Īnother sensitive test is the pinch test: a patient with AINS will also not be able to pinch a sheet of paper between his thumb and index finger, instead of clamping the sheet between his extended thumb and index fingers, akin to a tong rather than a clamp. Clinical presentationĪINS is a pure motor neuropathy, as the anterior interosseous nerve contains no sensory fibers dull forearm pain is however sometimes mentioned by patients.

It is said to account for less than 1% of neuropathies of the upper limb 1.

Anterior interosseous nerve syndrome is a rare entrapment syndrome, with comparatively little robust epidemiological data.

Pronator syndrome