Posterior interosseous nerve entrapment by intramuscular lipoma arising from the supinator muscle - a case report

Background: Lipomas are benign, slow-growing tumors frequently subcutaneous and asymptomatic, intramuscular lipoma constituting a rare subtype. However, a lipoma occurring nearby the proximal radius may cause posterior interosseous nerve (PIN) entrapment.

Case presentation: We described an uncommon case of a 45-year-old-man with a history of progressive, painless proximal right (dominant) forearm swelling for 4 months associated with PIN entrapment syndrome, presenting as fingers extension weakness. Intramuscular lipoma was observed in the supinator muscle in the magnetic resonance imaging (MRI). Lipoma surgical excision and release of the PIN through proximal forearm direct anterior approach was performed.

Results: The histopathological examination confirmed the diagnosis of benign intramuscular lipoma. The follow-up of the patient showed full recovery within three months postoperatively.

Discussion: Intramuscular lipoma is rare; however, it could originate from supinator muscle in the forearm, presenting with vague pain and could lead to compression of nearby neurovascular structures such as the PIN. Clinical evaluation and imaging studies, especially MRI, are crucial for diagnosis. If neural compression is evident, surgical resection is mandatory.

Conclusion: Intramuscular lipoma entrapping PIN is rare and requires early clinical diagnosis confirmed by imaging and neurophysiological studies, surgical excision being the method of choice for optimum functional outcomes.