Anterior interosseous nerve palsy as a result of prolonged shoulder immobilisation

Abstract

This is a rare case of an anterior interosseous nerve (AIN) palsy in a patient as a result of a prolonged period of shoulder immobilisation. The patient had an open reduction internal fixation of a midshaft clavicle fracture. They subsequently underwent removal of metal due to symptomatic prominence of the metal work. The patient was in a shoulder immobiliser for a period of 5 months in total. They developed progressive AIN palsy as a result of a positional compression due to prolonged wearing of a shoulder immobiliser. This resolved with conservative management and careful observation.

Background

Anterior interosseous nerve (AIN) palsy is described as a motor palsy of the muscles, which derive supply from the nerve. Three muscles are involved, the flexor pollicis longus (FPL), pronator quadratus and the radial half of the flexor digitorum profundus (FDP). It is not a common entity.¹ The quoted rate in the literature is <1% of all upper limb compressive neuropathies. Clinically this manifests as difficulty in flexing the interphalangeal joint of the thumb and/or index finger. A similar entity, which it can be confused with, is pronator syndrome (PS).²

This is described as a compressive neuropathy of the median nerve. However, more recently a neuritis component to the pathology has been described.³ AIN palsy usually is as a result of a neuritis rather than compression.⁴ Characteristically the compression occurs as the median nerve enters the forearm between the heads of the pronator teres muscle. The symptoms of PS and AIN palsy are similar but distinct. PS is associated with aching and pain in the forearm volar surface. They may have pain along the radial three and a half digits. Motor function is usually spared in PS. This unusual presentation has not been described before in the literature.

Case presentation

This case describes the unusual presentation of an AIN palsy in the setting of prolonged shoulder immobilisation. The patient was a 23-year-old right hand dominant man. He injured his right clavicle and underwent open reduction internal fixation (ORIF). This was after being in a shoulder immobiliser for 4 weeks. He had a midshaft comminuted and shortened fracture. He remained in a shoulder immobiliser for a further 4 weeks. Then he was encouraged to wean out of his sling with physiotherapy. He began to complain of metal prominence and partial dehiscence of the distal edge of the wound upon commencing vigorous physiotherapy. He returned to use a shoulder immobiliser and had scheduled removal of his metal. He remained in the sling for a further 4 weeks. In total, the calculated time in sling was 5 months. He had reported aching in his forearm in the interval between his ORIF and metal removal. It was noted at his 6-week postoperative check that he had inability to flex his thumb interphalangeal joint (IPJ). His power grading was 2/5 for his thumb IPJ and 3/5 for his index finger long flexor. Prompt nerve conduction studies were obtained.

This demonstrated mild attenuation in the amplitude of the median motor response from the FPL and FDP supporting an anterior interosseous neuropathy.

An MRI of the forearm was conducted to identify for any compressive lesions or obvious pathology. The MRI demonstrated patchy oedema in the T2 in the muscles supplied by the AIN, which was reported as favouring a PS distribution (figure 1).

Figure 1.

Oedema in muscles of forearm.

A decision was made to clinically observe, as per the literature.

The patient was referred to hand therapy and physiotherapy to strengthen the forearm muscles and work on flexor strength.

The patient began to clinically recover; however, the recovery was prolonged. Serial nerve conduction studies were obtained to guide the treatment. The second report demonstrated improvement in amplitude of the median motor response.

He had regained full FPL function after 10 months of observation.

Investigations

Radiographs were performed and full blood count including inflammatory markers. This patient had electrophysiological testing and an MRI to exclude a compressive forearm lesion. The MRI delineated oedema in the forearm muscles while the electromyography (EMG) confirmed the clinical suspicion of AIN palsy.

Differential diagnosis

AIN palsy.

Pronator quadratus syndrome.

Compressive muscular lesion.

Treatment

Observation and discontinuation of the sling. Hand therapy was obtained for the patient as well.

Outcome and follow-up

Indirect compression of the AIN nerve can occur following a period of prolonged forearm immobilisation. It was proposed that the compression happened because of the bulk of the proximal forearm muscles causing compression due to positioning. The diagnosis is achieved through a careful history, augmented by electrophysiology and advanced imaging with MRI. The clinical course can be tracked by repeat nerve conduction studies and careful liaison with a neurophysiologist. This patient underwent complete resolution of symptoms after a period of 6 months and was discharged from the clinic.

Discussion

AIN is classically described as a pure motor neuropathy. However, other studies have described the association of a dull pain in the forearm with this syndrome.⁵ Many cases have been documented in the literature of either unknown, spontaneous or traumatic aetiology.⁶ The more common traumatic associations include direct injuries, fractures, fracture fixation with injury to the nerve.⁷ Spontaneous pathology is related to compression or neuritis.⁸ The authors postulate that this neuropathy was as a result of indirect compression in the form of prolonged forearm immobilisation at 90°.

An important adjunct in the diagnosis is the use of electrophysiology. This affirms the clinical suspicion and guides treatment as well.⁹ It can help differentiate compression from a neuritis.

MRI was used to good effect in this case. The MRI demonstrated increased signal uptake on the short tau inversion recovery (STIR) sequences in all of the muscles supplied by the AIN.

The literature supports the use of MRI to evaluate for the extent of involved muscles. The most commonly involved muscle is the pronator quadratus which demonstrates increased signal in STIR sequences or T2 stir sequences.¹⁰

Generally conservative treatment is advocated as a first-line treatment for AIN palsy.¹¹ In this patient, it was important to recognise the cause of the indirect compression. The patient was prescribed flexor muscles physiotherapy and the sling was discontinued immediately. Non-operative treatment can be supplement with NSAIDS and physiotherapy as described by Miller-Breslow et al.¹² Ulrich et al ¹³ advocated a wait time of 3 months then surgical intervention in a cohort.

This patient underwent conservative treatment which was reliably directed by improving nerve conduction studies. These directed the treatment course.

Learning points.

• Unusual presentation of a compressive palsy, prolonged shoulder immobilisation is a risk.

• Serial clinical examinations and investigations with electrophysiology confirmed the treatment course.

• There is a benefit in observing clinically as most cases will resolve if the cause is identified and rectified.

• Adjunctive investigations with radiology are useful to exclude compressive lesions in the forearm.