Posterior Interosseous Nerve Schwannoma Presenting as a Painless Forearm Mass: A Case Report.

Schwannomas are benign peripheral nerve sheath tumors that typically exhibit slow growth and may present diagnostic challenges when arising from uncommon sites. We report the case of a 65-year-old female who presented with a six-year history of a gradually enlarging, painless swelling over the extensor aspect of the left forearm. The lesion initially appeared as a small nodule and progressively enlarged to approximately 4 × 3 cm, with intermittent tingling and occasional numbness but no significant functional impairment. Clinical examination revealed a firm, well-defined mass with a positive Tinel's sign. Ultrasonography and contrast-enhanced MRI demonstrated a well-circumscribed, encapsulated lesion along the course of the posterior interosseous nerve (PIN) in the intermuscular plane, suggestive of a benign peripheral nerve sheath tumor. The patient underwent planned microsurgical excision under general anesthesia. Intraoperatively, the tumor was found to arise eccentrically from the PIN and was meticulously dissected from the surrounding nerve fascicles, allowing complete excision while preserving nerve continuity. Histopathological examination confirmed the diagnosis of schwannoma, demonstrating characteristic Antoni A and Antoni B areas, Verocay bodies, and thick-walled hyalinized vessels. The postoperative course was uneventful, with preservation of motor and sensory function and no evidence of recurrence on follow-up. This report underscores the importance of thorough clinical evaluation and appropriate imaging in the diagnosis of peripheral nerve tumors of the forearm and highlights the role of meticulous microsurgical technique in achieving complete tumor excision with excellent functional outcomes.