Anterior interosseous nerve syndrome is a clinical entity
characterized by pain in the proximal part of the forearm and paresis of the
pronator quadratus, the flexor pollicis longus, and the radial half of the
flexor digitorum profundus. It was first described in association with
brachial neuritis by Parsonage and Turner in
19481 and was later
identified as a distinct entity by Kiloh and Nevin in
19522. Although
anterior interosseous nerve syndrome often arises spontaneously, it has been
associated with trauma and extrinsic compression. We report the cases of three
patients in whom anterior interosseous nerve syndrome developed immediately
after ipsilateral shoulder arthroscopy. The three arthroscopic procedures were
performed by different surgeons. Two patients underwent subsequent exploration
of the anterior interosseous nerve. All three recovered motor function nine to
sixteen months after the arthroscopy.
To our knowledge, anterior interosseous nerve syndrome as a complication of
shoulder arthroscopy has not been previously described. In this report, we
describe the complication and discuss three possible mechanisms. Our patients
were informed that data concerning the cases would be submitted for
publication.
Case 1. A forty-four-year-old healthy man presented with
a six-month history of right shoulder pain. The physical examination suggested
a rotator cuff tear, the presence of which was confirmed by magnetic resonance
imaging. The symptoms failed to decrease despite six weeks of physical therapy
and one corticosteroid injection. Shoulder arthroscopy was performed with use
of regional anesthesia with an interscalene block and with the patient in the
left lateral decubitus position with the arm extended. Ten pounds (4.5 kg) of
traction was applied with use of a boom attached to an adhesive foam sling on
the distal part of the forearm. A partial supraspinatus tear was diagnosed,
and débridement, subacromial decompression, and release of the
coracoacromial ligament were performed.
One week postoperatively, mild pain in the forearm and weakness in the
thumb and index finger developed. At six weeks after the operation, the pain
had worsened and there was total paralysis of the flexor pollicis longus and
flexor digitorum profundus to the index finger. Electrodiagnostic studies
confirmed a lack of motor unit potentials to these muscles. The patient was
managed conservatively with splinting and physical therapy. Ten weeks
postoperatively, he began to regain motor function. The anterior interosseous
nerve syndrome resolved completely by nine months after the arthroscopy.
Case 2. A fifty-two-year-old man presented with right shoulder
pain that was especially bothersome when he worked overhead. Magnetic
resonance imaging demonstrated tears of the supraspinatus and infraspinatus
portions of the rotator cuff. Shoulder arthroscopy was performed with use of
regional anesthesia with an interscalene block and with the patient in the
left lateral decubitus position. Ten pounds (4.5 kg) of traction was applied
with use of a boom attached to an adhesive foam sling on the distal part of
the forearm. Arthroscopy revealed chondromalacia of the humeral head and
fraying of the superior aspect of the labrum. The humeral head and the labrum
were débrided, and subacromial decompression was performed. A mini-open
approach was used to visualize and repair the supraspinatus and infraspinatus
portions of the rotator cuff.
The patient awoke from surgery with weakness of the left thumb. During the
first postoperative month, pain in the proximal part of the left forearm,
paresis of the flexor pollicis longus, and paralysis of the flexor digitorum
profundus to the index finger developed. Electrodiagnostic studies confirmed
the presence of anterior interosseous nerve palsy. Seven months after the
arthroscopy, the patient had continued pain and total paralysis of the flexor
pollicis longus and the flexor digitorum profundus to the index finger. The
anterior interosseous nerve was explored. A compressive fibrous band on the
proximal aspect of the flexor digitorum superficialis was identified and
divided. Function of the flexor digitorum profundus returned two weeks after
the exploration, and motor units to the flexor pollicis longus began to fire
at two months. Five months after the exploration, the patient had recovered
completely.
Case 3. A thirty-five-year-old man was diagnosed with an
undersurface tear of the posterior portion of the rotator cuff. The shoulder
demonstrated posterior capsular tightness and symptomatic acromioclavicular
joint arthritis. Shoulder arthroscopy was performed with the patient in the
beach-chair position and under regional anesthesia with an interscalene block.
An assistant held the arm during the procedure. Labral and rotator cuff
débridement, subacromial decompression, posterior capsular release, and
distal clavicle excision were performed. Several days postoperatively, forearm
pain and paresis of the flexor pollicis longus and flexor digitorum profundus
to the index finger developed. Nine months after the operation, flexion of the
index finger showed minimal improvement; the thumb still had no active
interphalangeal joint flexion. The patient underwent exploration of the
anterior interosseous nerve. A fibrous band along the dorsum of the
superficial head of the pronator teres was found immediately over the anterior
interosseous nerve and was divided. Improvement of motor function was noted
four months later. Seven months after the exploration, the patient had
recovered completely.
The anterior interosseous nerve is principally a motor branch of the
median nerve. It arises 5 to 8 cm distal to the lateral epicondyle, although
fibers destined to form the anterior interosseous nerve may be isolated as far
proximally as the level of the brachial
plexus3. The nerve
passes through or under the pronator teres and travels along the anterior
aspect of the interosseous membrane to the wrist. It innervates the pronator
quadratus, the flexor digitorum profundus to the index and long fingers, and
the flexor pollicis longus. The terminal anterior interosseous nerve branches
are sensory in nature, supplying the volar aspect of the wrist and the
intercarpal joints. Paresis or paralysis of the anterior interosseous nerve
manifests clinically as weakness in flexion of the interphalangeal joint of
the thumb or of the distal interphalangeal joints of the index and long
fingers. Clinically, a patient with this condition is unable to make an
"O" with the thumb and index finger.
Anatomically, a variety of local compression points are known to predispose
patients to the development of spontaneous anterior interosseous nerve
syndrome4. Trauma is
another common
etiology5-7.
Iatrogenic injury to the nerve may result from
surgery8,9,
dressings10,11,
slings12, and
venipuncture13,14.
Anterior and posterior interosseous nerve dysfunctions have also been reported
after elbow
arthroscopy15,16.
There are several possible explanations for the postoperative anterior
interosseous nerve syndrome that we observed in our three patients. Peripheral
neuropathy is a recognized complication of regional anesthesia. The chief
mechanisms are thought to be mechanical trauma, compressive hematoma, and
direct anesthetic
neurotoxicity17. A
study performed at our institution involved the follow-up of 693 patients who
had undergone a single-injection interscalene block with levobupivacaine and
epinephrine18. In
the patients in that study, the majority of neuropathies stemmed from lesions
proximate to the injection site. That series included the patient described in
case 1; he was the only subject in that report to present with flexor pollicis
longus and flexor digitorum weakness. In two other studies, neuropathy was
found in twelve of 512
patients19 and in
seventy-four of 520
patients20, but
motor nerves were not involved in any of those patients.
Extravasation of irrigation fluid is a well-established complication of
arthroscopy21.
Temporary local compartment syndrome and femoral neuropathy have been reported
in association with knee
arthroscopy22-25.
In one case report, transient hypoesthesia and a wristdrop were described in a
patient in whom extravasation occurred during shoulder
arthroscopy26. In
the three patients in our report, extravasated fluid may have caused an
intrafascicular compartment syndrome leading to nerve
compression27. The
occurrence of this mechanism would have required the extravasated fluid to
track distally into the forearm. However, none of the surgeons had used
high-pressure irrigation and none of the patients had clinical evidence of
distal extravasation, such as forearm swelling.
Postoperative neurapraxic injuries related to patient positioning and limb
traction have been
described28.
Indeed, the beach-chair position was introduced to address concerns related to
traction caused by lateral decubitus
positioning29.
Pitman et al. reported a four-in-ten prevalence of abnormal somatosensory
evoked potentials in the median and ulnar nerves during shoulder
arthroscopy26. In
one cadaveric study, traction on the proximal portion of the median nerve was
shown to be the cause of specific tension on the anterior interosseous nerve,
which is less freely mobile than the distal portion of the median
nerve30. In our
case series, it is possible that traction was placed on the median nerve, and
thus also on the anterior interosseous nerve, even with use of the beach-chair
position.
There is no clearly defined etiology among these three patients with
anterior interosseous nerve syndrome. The distance of the anterior
interosseous nerve from the injection site and the anatomic specificity of the
findings that we observed suggest that the anesthetic block was not
responsible. Both patients in whom the nerve was explored had fascial bands
overlying the anterior interosseous nerve, which may have predisposed that
nerve to compression. Perhaps the most plausible explanation for the
occurrence of this syndrome in the three patients in our case series is a
traction-type neurapraxia in anatomically at-risk nerves.
There is a lack of consensus regarding the optimal management of patients
with anterior interosseous nerve syndrome. This is due to several issues: many
etiologic factors appear to play roles in the development of the syndrome;
recovery patterns vary greatly; and no prospective studies, to our knowledge,
have compared operative management with conservative management. Recovery can
take more than one
year31. Several
authors have suggested operative intervention if there is complete paralysis
and if symptoms do not resolve within three to six
months32-34.
In keeping with these recommendations, we surgically explored the anterior
interosseous nerve of the two patients who failed to show appreciable
improvement by six months. The rapid return of function to the flexor
digitorum profundus in one of our patients (Case 2) suggests that
decompression hastened recovery in that patient. On the basis of our
experience, we continue to offer exploration to patients who do not show
improvement at six months. The provision of informed consent, after the lack
of efficacy data and the risks of the procedure are described, is of paramount
importance.
Anterior interosseous nerve syndrome is likely to remain a rare but
underdiagnosed phenomenon. As such, it is important for surgeons to be
familiar with the findings related to this condition and to be aware that the
syndrome is a potential complication of shoulder arthroscopy.
Note: The authors acknowledge Dr. Jason Koh, Dr. Gordon Nuber,
and Dr. Stephen Gryzlo for contributing cases to this report.