An Uncommon Case of Atraumatic Palsy of a Branch of the Anterior Interosseous Nerve with a Late Spontaneous Recovery.

Atraumatic palsy of the anterior interosseous nerve (AIN) is rarely encountered, presenting an uncertain etiology which provokes a weakness of the flexor pollicis longus (FPL), flexor digitorum profundus (FDP), and pronator quadratus, while a lesion of one of the AIN branches is even rarer. In many cases, the diagnosis is based in motor deviations due to nerve's palsy. A palsy of the AIN can be "complete" or "incomplete." In an incomplete palsy, only the FPL or the FDP of the index finger is paretic or paralyzed. There is a scientific debate concerning the effectiveness between surgical and conservative treatment approaches. Moreover, a patient may have the opportunity to decide whether to be submitted in an interventional procedure or not. The purpose of this paper is to report a case of an AIN's branch palsy and to suggest a possible delay of the surgical exploration, since a late self-recovery may occur.

Introduction

Atraumatic palsy (AP) of the anterior interosseous nerve (AIN) is considered as a rather rare phenomenon with an uncertain etiology, while an isolated total or subtotal loss of function of a branch of the AIN is even rarer. De novo AIN palsy is most likely neuritis, caused by a nonmechanical compression, with a low incidence, as it accounts for < 1% of all compression syndromes in the upper limb. AP of the AIN usually provokes a weakness of the flexor pollicis longus (FPL), flexor digitorum profundus (FDP), and pronator quadratus (PQ).[12] The AIN is a purely motor nerve. It arises 5–8 cm distal to the lateral epicondyle, then passes through or under the pronator teres, and travels along the anterior aspect of the interosseous membrane, terminating as articular branches to joints of the distal foramen and wrist. AIN innervates PQ, FDP to the index and long fingers, as well as the FPL.[3] A communication between the proximal AIN or median nerve and the ulnar nerve may exist as part of an anatomical variation known as the Martin–Gruber anastomosis, which may be found in up to 23% of individuals. Lesions of the AIN can result in a variety of clinical manifestations, depending on the location and the degree of axonal damage. An incomplete AIN syndrome may also occur, featuring various components of the fully developed syndrome. There is a conflict concerning the effectiveness between surgical and conservative treatment. The purpose of this paper is to report a case of an AIN's branch palsy and to suggest a possible delay of the surgical exploration, since a late self-recovery may occur as Goulding and Schady noted in 1993.[4]

Case Report

A 32-year-old female dentist of Caucasian (Hellenic) origin realized while eating her lunch that she was unable of holding a knife properly. She could not flex the metacarpophalangeal joint of the right thumb. Moreover, she presented subtle pain on the radical surface, as well as on the base of the thumb of the same hand. Fifteen days later, as there was no improvement, she consulted a physician, who proposed magnetic resonance (MR) scan on the cervical part of the spine to rule out a possible disc herniation. The MR scan showed only mild degenerative lesions which were not consistent to disc herniation and irrelevant to the symptoms of our patient [Figure 1]. She was then admitted to our medical practice where the muscular strength of the thumb was examined and she was graded zero out of five on the Medical Research Council Scale (MRC) for testing of the muscle strength. The patient was incompetent to make the “OK gesture” presenting the “straight thump sign” [Figure 2], a fact which depicts a typical sign of palsy of the AIN. A tendon rupture was excluded as the patient demonstrated an intact tenodesis effect. The performed electrophysiologic studies showed normal electromuscular activity and no sign of pathology which could produce external pressure. Since there was only a loss of movement present with a preserved sensitivity, carpal tunnel syndrome was ruled out. Therefore, AP of the selective branch of the AIN, which nerves the FPL muscle, was finally diagnosed.

Figure 1

T2-weighted sagittal magnetic resonance imaging showing mild degenerative lesions with a very small bulging of the disc at C5–C6 level, which was irrelevant to the symptoms of our patient

Figure 2

Patient performing with both his hands the “OK” sign. Diagnosis, note the difference of posturing the fingers normally in the left hand and the depiction of a pinch disturbance, a straight thump sign in the right hand, revealing an anterior interosseous nerve palsy (left side). The same patient after a spontaneous recovery 1 year later. Note the lines demonstrating complete recovery (right side)

The patient chose to follow a nonoperative treatment with intake of Vitamin B complex supplements, electrostimulation, and physiotherapy. Since no improvement was noted after a 3-month time period, a series of new electromyographic studies was conducted, depicting the same results. At this point, an operative approach for release the AIN was suggested according to literature. However, the patient declined this option due to personal reasons, continuing the noninvasive therapeutic protocol. A year later, a spontaneous recovery occurred and the MRC demonstrated a four out of five grade.

Discussion

A palsy of the AIN (known also as Kiloh-Nevin syndrome) can be “complete” or “incomplete.” In an incomplete palsy, only the FLP or the FDP of the index finger is paretic or paralyzed. The causes for AIN palsy can be divided in traumatic and nontraumatic/spontaneous. Entrapment neuropathy, neuralgic amyotrophy, or an isolated neuritis may result to AIN AP. The nerve is an entrapment vulnerable entity, unsheltered to be snared by soft tissue and by vascular and bony structures. To distinguish between the two types of palsy is of paramount importance as a series of pathology should be accounted in the differentiated diagnosis. A flexor tendon rupture, a flexor tendon adherence or adhesion, a stenosing tenosynovitis, a FPL rupture due to rheumatoid arthritis,[56] a viral neuritis-like the Parsonage-Turner syndrome,[7] and a brachial neuritis mimicking the clinical manifestations of an AIN neuropathy,[8] all fall into the account.

The clinical presentation with an isolated palsy of the FPL muscle as an incomplete AIN syndrome is difficult to distinguish from other local pathology, especially after trauma. Due to the relative rarity of this syndrome, few controlled studies exist to determine the most effective treatment techniques.[9] Diagnosis of the AIN syndrome may be confirmed by electrophysiological studies showing abnormalities confined to the three muscles innervated by it (usually electromyography showing denervation). They not only confirm the diagnosis but objectively evaluate the severity of the neuropathy as well. MR imaging is not commonly used in the diagnosis of the AIN syndrome lesions. However, some studies consider it useful as it may unveil both neuropathy and other local pathology.[51011] Concerning AIN palsy treatment, it is strongly related to the specific disease etiology and in some cases controversial. The indications of an operative treatment for the AIN AP remain debatable. Once the diagnosis of an AIN syndrome is made, observation combined with an avoidance of handwork, immobilization, limb rest, and anti-inflammatory medication for several months is suggested before interventional decompression. The majority of the patients with an AIN syndrome enjoy, in later stages, an improvement without being subjected to any surgical procedure. The time interval proposed by the literature for the improvement to appear, in case it appears, varies from several weeks to 1 year.[59]

All studies agree that if no improvement appears, surgical neurolysis is eventually to be applied. On the other hand, the proposed onset time of such an approach differs widely. Thus, Spinner advocated exploration if no signs of clinical and electromyographic improvement occur within a time period from 6 up to 8 weeks,[12] with Nigst and Dick to idealize it to exactly 8 weeks,[13] and Ulrich et al. to prolong it up to 12 weeks.[9] Nevertheless, some researchers recommend a conservative approach for up to 1 year as a spontaneous partial to complete (rare) recovery may show up in most patients between 9 and 24 months. Seki et al. recommended intervention only after 1 year of continuous symptomatology if not at all especial among younger patients.[14] If motor function does not recover after 12 months, tendon transfers will restore the function satisfactorily.[4]

In patients who present complete paralysis of either muscle-tendon unit and who have shown no improvement as determined by physical examination and/or repeat electromyography after 1–2 years of clinical observation (depending on the case), we recommend exploration and neurolysis of the AIN, an approach which offers a rapid and often a complete recovery. Patients with paresis should be monitored, most probably for a longer time period. However, as virtually complete spontaneous recovery may occur in all patients, regardless severity, it is concluded that atraumatic AIN lesions are most likely to reflect a circumscribed form of brachial neuritis and that surgical decompression should be deferred for at least 12 up to 24 months or even indefinitely if recovery is proceeding.

Thus, as a very late recovery of the AIN AP is possible, it is of great importance to insist in nonoperative treatment modalities for longer time periods before we aggressively intervene. Moreover, we need to keep follow-up a patient who refused or who does not urgently need a surgical neurolysis, to urge and ensure his/her constant participation in physiotherapy-kinesiotherapy programs. This will surely preserve both joints and muscles healthier and more functional until the moment when the AIN will start to function again.

Conclusion

Although the AP of the AIN is infrequent, clinical suspicion should arise in the presence of weakness in the innervated musculature, concerning the nerve centrally, or one of its branches (incomplete syndrome). Therapeutic approach should be personalized concerning the usage of interventional procedures or not.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understand that name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.