By Michael Rubin, MD
Professor of Clinical Neurology, Weill Cornell Medical College
Dr. Rubin reports he is a consultant for Merck Sharp & Dohme Corp.
SYNOPSIS: Anterior interosseous nerve syndrome is part of the median neuropathy spectrum and often has been attributed to compression near the elbow. Modern magnetic resonance neurography and ultrasound studies have ruled-out compression in most cases.
SOURCE: Sneag DB, Arányi Z, Zusstone EM, et al. Fascicular constrictions above elbow typify anterior interosseous nerve syndrome. Muscle Nerve 2019; Nov. 26. doi: 10.1002/mus.26768.
Originating from cervical roots C7-T1, the anterior interosseous nerve (AIN) is the largest branch of the median nerve. Purely motor in nature with no cutaneous innervation, it is crucial for pinching with the thumb and index fingers, and supplies the flexor pollicis longus (FPL), flexor digitorum profundus to digits 2 and 3 (FDP), and pronator quadratus muscles. Also known as Kiloh-Nevin syndrome, spontaneous AIN syndrome (AINS) is rare and of uncertain etiology, often attributed to compression in the forearm or idiopathic brachial neuritis (Parsonage-Turner syndrome). Using magnetic resonance neurography (MRN) or ultrasound (US), intrinsic fascicular constrictions (FCs) of anterior interosseous fascicles of the median nerve proper previously have been reported in AINS, and evidence now suggests that AINS is essentially a non-compressive neuropathy.
Sneag et al retrospectively analyzed imaging and electrodiagnostic (EDX) data of 45 patients with a clinical diagnosis of AINS by reviewing databases, spanning January 2005 to December 2018, from Hospital for Special Surgery in New York, and the Department of Neurology at Semmelweis University in Budapest. AINS diagnosis required spontaneous AIN neuropathy, documented by complete paralysis of either or both the FPL and FDP, with electromyography (EMG) evidence of axonal loss, documented by minimal to no motor unit recruitment in AIN-innervated muscles. For both MRN and US studies, pathology was characterized uniformly, with an FC defined as an abrupt, focal decrease of fascicle diameter, and fascicular enlargement defined as focal enlargement of the fascicle, with gradual tapering to a normal diameter proximal and distal to the enlargement. MRN and US studies were interpreted retrospectively by radiologists or a neurologist with at least five years of dedicated nerve imaging experience. EDX studies were performed by examiners with extensive clinical experience in idiopathic brachial neuritis.
Among the 45 patients, 74 FCs were found, with bilateral involvement in two patients. Ipsilateral pain for an average of 5.9 days prior to onset of weakness was reported in 71%. MRN, performed an average of eight months following symptom onset in 22 New York patients, revealed a total of 63 FCs, with FCs found in all patients, at a mean distance of 5.4 cm proximal to the elbow and in the posterior/posteromedial topographical region of the median nerve in 91% (n = 20). Within the forearm, MRN demonstrated normal size, signal intensity, and morphology of the median nerve, without FCs. Using US, only one of eight patients was found to have an FC, despite having FCs on MRN, but in the remaining seven, fascicular enlargement was found just proximal to the elbow joint line. Surgical internal neurolysis of constrictions was performed in eight patients with, as yet, unknown outcomes. Among 14 patients treated conservatively, two were lost to follow-up. Based on neurological examination, three had complete recovery, six had partial recovery, and three had no recovery of AIN function.
Among 23 Budapest patients studied with US, 11 FCs were seen in nine of 25 limbs, 91% (n = 10) located at or proximal to the elbow, three posteromedial, and seven in a medial fascicle that could not be further differentiated, with the single remaining FC 1 cm distal to the joint line and posterolateral in position. Fascicular enlargement of the median nerve was identified in 13 limbs that did not exhibit FCs, with no abnormalities detected in the three remaining limbs, and US of the forearm revealed no median nerve abnormality distal to the site of all fascicular lesions.
AINS appears to be a non-compressive neuropathy characterized by median nerve FCs, overwhelmingly in the upper arm.
COMMENTARY
AINS appears to be a form of idiopathic brachial neuritis (Parsonage-Turner syndrome), and may be autoimmune in origin or result from an ischemic event leading to fibrosis and thickening of epineurial and/or perineurial bands. Surgery or tendon transfer is the final option for patients who have demonstrated no recovery. At the Hospital for Special Surgery, the preference is to wait a minimum of nine months prior to surgery, but it would be equally reasonable to wait up to two years, as recovery has been shown to occur late. Surgery, if performed, involves internal microsurgical neurolysis of involved fascicles. Thus, prior imaging is crucial if surgery is considered. When the median nerve is exposed, nothing is apparent to the naked eye. Only when internal neurolysis is performed and the fascicles identified are the constrictions identified.
