Posterior interosseous nerve entrapment due to bilobed parosteal lipoma

Abstract

Entrapment of posterior interosseous nerve (PIN) can be due to fracture dislocation of elbow, fibrous arcade of Frohse, neoplasms (lipoma, schwannoma), ganglion cysts and rheumatoid synovitis. Parosteal lipomas are extremely rare. These tumors grow slowly and as they grow, they can compress a nearby nerve producing sensory and motor disturbances. Till date less than 50 cases of PIN entrapment due to parosteal lipoma have been reported in literature. However, to the best of our knowledge, none was bilobed. A 54-year-old female patient presented with progressive weakness of the right-hand extensors including thumb for the last 5 months with no sensory loss. Clinico-radiological findings and electophysiological studies revealed parosteal lipoma causing entrapment of PIN. Surgical excision of the lesion was done through posterior approach. The excised mass was sent for histopathological examination which confirmed the diagnosis of lipoma. Appreciable recovery was first noticed at 3 months and complete recovery was seen at 7 months. No recurrence was seen until 2 years of follow up. Urgent surgical excision is necessary to prevent entrapment of this nerve and facilitate early functional and neurological recovery.

1. Introduction

Radial nerve divides into two branches at the level of lateral epicondyle: superficial radial nerve (sensory branch) and deep branch of radial nerve (motor branch). The deep branch winds around the neck of radius, enters the arcade of Frohse and pierces the supinator muscle where it is known as posterior interosseous nerve (PIN). Entrapment of PIN can be caused by a number of conditions like fracture-dislocations around elbow, fibrous arcade of Frohse, neoplasms (lipoma, schwannoma), aneurysms, ganglion cyst and enlarged bursae. The common potential sites for entrapment of PIN are: a) origin of the extensor carpi radialis brevis, b) adhesions around radial head and neck, c) the recurrent radial arterial fan d) arcade of Frohse (where the nerve enters the supinator) e) distal edge of supinator where the nerve exits.

Lipomas are benign soft tissue tumors which generally occur in subcutaneous location. Rarely they can occur deep to the fascia.¹ Depending upon their location they can be further subdivided into intramuscular, intermuscular, interosseous and parosteal lipomas.² Parosteal lipomas are extremely rare and account for 0.3% of all lipomas.³ They are usually indolent in nature and symptoms of nerve compression caused by them are unusual.⁴ Till date less than 50 cases of PIN entrapment due to parosteal lipoma have been reported in literature, however to the best of our knowledge, none was bilobed. We hereby present a case of PIN entrapment by bilobed parosteal lipoma in a 54-year-old woman.

2. Case report

A 54-year-old female patient presented in the OPD with progressive weakness of the right-hand extensors including thumb for the last 5 months. There was no history of trauma to her right upper limb or neck. Her past medical history did not reveal any metabolic disease including diabetes mellitus, vitamin deficiency, alcohol abuse or chemical exposure. Neck and shoulder examination were found to be normal. Clinical examination around elbow revealed a soft, painless swelling in the posterolateral aspect of the proximal forearm just distal to the elbow. The size of the swelling was about 4 cm × 3 cm. There was decrease in strength of the extensors of the metacarpophalangeal joints of all fingers and thumb (Power 2/5). However, there was no weakness of wrist extensors. Elbow movements were found to be normal and there was no instability or laxity. Sensation to light touch and pinprick was intact throughout the upper extremity. Vascular examination was also found to be normal.

Plain radiographs of elbow and proximal forearm were found to be normal. MRI of the elbow and forearm revealed a bilobed encapsulated homogenously T1 hyperintense lesion located in juxtacortical region of proximal shaft and neck of radius suggestive of lipoma (Fig. 1). NCV studies showed posterior interosseous neuropathy. Electromyography (EMG) revealed denervation of the posterior interosseous innervated muscles with sparing of radial innervated muscles and a preserved superficial sensory radial nerve. Based on clinico-radiological findings and electophysiological studies, a diagnosis of parosteal lipoma causing entrapment of PIN was made.

Fig. 1.

MRI showing a bilobed encapsulated homogenously T1 hyperintense lesion in juxtacortical region of proximal shaft and neck of radius suggestive of lipoma.

An excisional biopsy was performed using posterior approach of forearm under tourniquet control and general anaesthesia. Intermuscular interval was developed between external digitorum communis and external carpi radialis brevis. PIN was identified (Fig. 2). The encapsulated mass was excised meticulously making every effort not to damage the nerve or its branches (Fig. 3). After excision of the mass, the nerve was examined. Maximum compression of the nerve was seen at the arcade of Frohse. It was found to be edematous with no fibrosis within or around it. No neurolysis was performed. The excised mass was sent for histopathological examination which confirmed the diagnosis of lipoma (Fig. 4).

Fig. 2.

Isolation of Posterior interosseous Nerve.

Fig. 3.

Excision of encapsulated mass meticulously making every effort not to damage the nerve or its branches.

Fig. 4.

Excised bilobed soft tissue mass sent for HPE which was histologically confirmed to be lipoma.

Post-operatively patient was advised dynamic cock-up splint. Physiotherapy was started after suture removal. Post-operative course was uneventful. Regular follow-up was done every 6 weeks to assess the progress of nerve recovery. Appreciable recovery was first noticed at 3 months and complete recovery was seen at 7 months. No recurrence was seen until 2 years of follow up.

Full informed consent was taken from the patient for participation in the case report and her privacy rights were observed.

3. Discussion

Parosteal lipomas are rare, occur deep to the fascia and account for 0.3% of all lipomas.³ Common sites are proximal radius, humerus, femur, tibia, clavicle, ribs and pelvis.³ Compression of PIN due to parosteal lipoma around neck and proximal shaft of radius is not common.⁵ Entrapment of PIN due to bilobed parosteal lipoma has not been reported till date.

Classically, a patient with PIN entrapment due to parosteal lipoma presents with insidious onset weakness of the digital extension without any forearm pain and sensory deficit. In addition to entrapment of PIN, these lipomas can also entrap the superficial radial nerve thereby causing sensory loss.⁵ In our case there was no involvement of the superficial radial nerve and sensations were intact. The patient never complained of any forearm swelling or pain.

Even though MRI is the imaging modality of choice for these conditions, skiagrams may reveal a soft tissue radiolucent area consistent with a lipoma.^6,7 In our case plain radiographs were found to be normal.

Surgical excision is the mainstay treatment of these cases. This is done either to prevent entrapment of the PIN or to increase the chances of functional recovery.⁵ The chances of complete recovery depend upon the duration of symptoms. Early surgical intervention is necessary for achieving optimal functional outcome. We performed surgical excision after 5 months of onset of symptoms and gained complete recovery at 7 months after surgery. This signifies the importance of relieving the compression and highlights the fact that the earlier the compression is relieved, the better are the chances of recovery. In literature, the longest disease duration prior to surgery, which gained full recovery following surgical excision is 2 years.⁵ However, cases have also been reported where excision of the mass failed to result in functional recovery and required tendon transfer.⁷ If surgery is delayed for about 18 months, it is more likely that fibrosis of the affected muscles will occur. In such cases tendon transfers become the only viable option for achieving functional restoration.

We used the posterior approach for gaining access to the bilobed lesion and for proper visualization of PIN. This approach provides excellent visualization of PIN and its branches and prevents iatrogenic damage to the nerve while dissecting out and excising the lesion. Some studies have advocated use of anterior approach for excision of these lipomas.^7,8 However, we didn’t find any difficulty using posterior approach. Regardless of the incision and approach, the aim of the surgery should be meticulous en bloc excision of the mass without causing iatrogenic damage to PIN.

4. Conclusion

Entrapment of PIN can occur due to bilobed parosteal lipoma of proximal radius. Urgent surgical excision is necessary to prevent entrapment of this nerve and facilitate early functional and neurological recovery.

Declaration of competing interest

Authors declare no competing interests and no funding was taken from any source.