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Anterior Interosseous Nerve Syndrome

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Anterior Interosseous Nerve Syndrome ANTERIOR INTEROSSEOUS NERVE SYNDROME BY DOUGLAS H.C.L. CHIN, MD, AND ROY A. MEALS, MD Anterior interosseous nerve syndrome (Kiloh-Nevin Syndrome) is the triad of weakness of the flexor pollicis longus, the flexor digitorum profundus of the index finger, and the pronator quadratus. It is a manifestation of neuropathy affecting either the anterior interosseous nerve itself (anterior interosseous neuropathy) or its fascicles more proximally within the median nerve or brachial plexus (pseudo– anterior interosseous neuropathy). Anterior interosseous neuropathy in the presence of normal anatomic variation of the anterior interosseous nerve must be distin- guished clinically from pseudo–anterior interosseous neuropathy, which can present with telltale signs in addition to the signature weaknesses of anterior interosseous nerve syndrome. A history of penetrating injury mitigates toward early explora- tion and nerve repair. A history of sudden onset and rapid progression, partic- ularly when accompanied by a prodrome of pain and fatigue, suggests the presence of a focal neuritis, which typically resolves completely within 6 to 12 months without surgical intervention. If no improvement is noted within 6 to 12 months or if the neurologic condition worsens, surgical exploration may be warranted. In the presence of untreatable injury to the anterior interosseous nerve, with perma- nent muscular atrophy, functional tendon transfers of the flexor digitorum superficialis of the ring or middle finger or of the brachioradialis may be helpful. Copyright © 2001 by the American Society for Surgery of the Hand inel1 probably offered the original description peculiar triad of findings, identifying an isolated neu- of a characteristic pattern of weakness of me- ritis of the anterior interosseous nerve (AIN) as one Tdian innervated muscles, referring to it as a potential cause. Their anatomic correlation thus “dissociated paralysis of the median nerve.” Kiloh and named the syndrome, which now bears their eponym. Nevin2 offered a clear anatomic explanation for the Several other reports commonly are cited as early descriptions of AIN syndrome, such as the classic description of an acute brachial neuritis by Parsonage From the Division of Hand and Upper Exptremity Surgery, Depart- and Turner3 in 1948. Their description included a ment of Orthopedic Surgery, UCLA Center for Health Sciences, Los weakness of the shoulder girdle, making it impossible Angeles, CA. for this to have represented a neuropathy of the AIN. Address reprint requests to Roy A. Meals, MD, 100 UCLA Medical Plaza, Suite 305, Los Angeles, CA 90024. E-mail: [email protected] Others have reported observing the same constellation of findings associated with supracondylar humeral Copyright © 2001 by the American Society for Surgery of the Hand 4,5 6-8 1531-0914/01/0104-0004$35.00/0 fractures, forearm fractures, an anomalous median doi:10.1053/jssh.2001.28806 artery,9 the use of a lateral epicondylitis forearm JOURNAL OF THE AMERICAN SOCIETY FOR SURGERY OF THE HAND ⅐ VOL. 1, NO. 4, NOVEMBER 2001 249 250 ANTERIOR INTEROSSEOUS NERVE SYNDROME ⅐ CHIN & MEALS band,10 thrombophlebitis,11 and antecubital vein example of the pseudo–anterior interosseous neurop- catheterization or phlebotomy.12 athies.3 From the numerous reports supposing a broad range of causes—inflammatory, compressive, post- ANATOMY traumatic—it becomes clear that AIN syndrome refers not to a single pathologic entity, but to a ust after coursing between the two heads of the common clinical manifestation of several different Jpronator teres muscle, the median nerve gives rise pathologies. These may or may not affect the AIN to the AIN from its radial aspect. This take-off of the itself. AIN from the median nerve occurs 5 to 8 cm distal to Much of the controversy surrounding the manage- the lateral epicondyle14 and 22.4 to 23.4 cm proximal ment of anterior interosseous nerve syndrome arises to the radial styloid.15,16 Coursing beneath the fibrous from subtle but important semantic differences be- arch of the flexor digitorum superficialis muscle, the tween the terms anterior interosseous nerve syndrome and AIN then enters the flexor digitorum profundus mus- anterior interosseous neuropathy, which, unfortunately, cle belly an average of 30% the forearm length distal 17 have tended even among experienced hand surgeons to to the medial epicondyle. The nerve then courses be used rather interchangeably. Many types of pathol- distally on the volar surface of the interosseous mem- ogies may result in the signature triad of palsies brane. Approximately 4 cm distal to its takeoff from representing the syndrome. the median nerve, the AIN gives rise to motor Strictly speaking, anterior interosseous syndrome branches to the flexor pollicis longus, the flexor digi- torum profundus to the index finger, and, variably, refers to that constellation of signs and symptoms the flexor digitorum profundus to the middle finger.14 referable to weakness of the pronator quadratus, the It then supplies a motor branch to the pronator quad- flexor pollicis longus, and the flexor digitorum pro- ratus before terminating as sensory branches to the fundus to the index finger. Although the AIN sup- radiocarpal, midcarpal, and carpometacarpal joints. plies sensory fibers to the radiocarpal, midcarpal, and At the level of the elbow, motor fibers of the carpometacarpal joints, AIN syndrome by definition median nerve that are destined to become the constit- refers to a purely motor constellation of signs and uents of the AIN distally lie posteriorly relative to the symptoms. main nerve trunk.18 Sunderland reported that fibers Although AIN syndrome is strictly motor, it may ultimately becoming the AIN form a distinct bundle be associated with additional extrasyndromic signs within the median nerve 2.5 cm proximal to its and symptoms. Additional findings may suggest ei- take-off as the AIN.19 Spinner20 has shown that this ther that the underlying pathology resides outside of anatomic feature makes it possible for this fascicular the AIN itself (median nerve or brachial plexus) or component of the median nerve to sustain an isolated that aberrant anatomic features exist distal to the injury, giving rise to a clinical pattern mimicking an pathologic lesion in the AIN. anterior interosseous neuropathy. Wertsch and col- Among the many potential underlying pathologies leagues13 reported cases of AIN syndrome associated manifesting as AIN syndrome are abnormalities either with sensibility deficits in a median nerve distribu- within or proximal to the AIN. Thus, causes of AIN tion. On exploration, the median nerve, including syndrome are appropriately divided into 2 broad cat- fascicles comprising the AIN distally, were com- egories. Anterior interosseous neuropathies include pressed at the antecubital fossa, proximal to the take- those compression neuropathies, neuritides, congeni- off of the AIN. tal anomalies, and anatomic lesions and discontinui- Spinner and Schrieber 4,20 have identified at least 8 ties of the AIN itself. Pseudo–anterior interosseous anatomic features that seem to predispose an individ- neuropathies,13 on the other hand, represent patholo- ual toward an anterior interosseous palsy (Table 1). gies affecting more proximal anatomic sites, but in- Among a series of patients with AIN syndrome, Hill volve nerve fascicles contributing to the anterior in- and associates21 reported fibrous bands spanning from terosseous nerve more distally. Parsonage-Turner the deep head of the pronator teres to the brachialis syndrome, in which AIN syndrome is associated with fascia to be the most common source of anatomic weakness of the parascapular muscles, is a classic compression. Less common causes of compression were ANTERIOR INTEROSSEOUS NERVE SYNDROME ⅐ CHIN & MEALS 251 With anterior interosseous or pseudo–anterior in- TABLE 1 terosseous neuritides, onset of neurologic symptoms is Anatomic Features Predisponsing Toward Anterior Interosseous Compression Neuropathy typically sudden and rapidly progressive. Patients of- ten relate an antecedent prodrome of proximal volar Tendinous origin of the deep head of the pronator forearm or shoulder pain, and these patients are gen- teres Tendinous origin of the flexor superficialis to the long erally suspected of exhibiting an inflammatory neu- finger ropathy. Pain will often be of sudden onset and may Tendinous origin of variant muscles (palmaris pro- be related to minor trauma. The prodrome may con- fundus, flexor carpi radialis brevis) Thrombosis of crossing ulnar collateral vessels sist of systemic complaints, including generalized fa- Accessory muscle and tendon from the flexor digitorum tigue and fever. Hepatitis has been reported to be superficialis to the flexor pollicis longus associated with acute brachial neuritis.23 Gantzer’s muscle (accessory head of the flexor pollicis longus) Aberrant radial artery EXAMINATION Enlarged bicipital bursa near the origin of the AIN Data from Spinner.24 IN syndrome is suggested by the resting repose Aof the hand, which will exhibit an unnatural extension of the distal interphalangeal (DIP) joint of the index finger and interphalangeal (IP) joint of the fibrous bands arising from the superficial head of the thumb, compared with the gentle flexion arcade of the pronator teres, a nerve running deep to both heads of remaining fingers. The metacarpophalangeal joint of the pronator teres, and a double lacertus fibrosus. the thumb may compensate by assuming hyperflexion. The signature finding on physical examination is weakness of the flexor
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