Psoas Muscle Schwannoma Mimicking Abdominal Pathology with Lumbar Plexus Involvement

Abstract

Background: Schwannomas are benign nerve sheath tumours that rarely occur in the retroperitoneum, with involvement of the psoas muscle being particularly uncommon. Due to their deep location and nonspecific presentation, they often mimic more common pathologies, leading to diagnostic uncertainty. Case Presentation: A 47-year-old male presented with dull aching pain in the left lower abdomen and flank for four months, radiating to the lateral thigh and associated with mild numbness. Clinical examination revealed a vague, non-tender mass in the left lumbar region with features suggestive of psoas irritation. Imaging studies demonstrated a well-defined cystic lesion within the left psoas muscle measuring around 9 × 7 cm, provisionally diagnosed as a hydatid cyst. The patient underwent open surgical excision via a lateral approach. Intraoperatively, a well-encapsulated intramuscular cystic lesion was identified and completely excised. Histopathological examination revealed spindle cells with strong S-100 positivity, confirming the diagnosis of schwannoma. Postoperatively, the patient developed transient thigh weakness and numbness, which resolved with physiotherapy. At three-month follow-up, the patient remained asymptomatic with no recurrence. Conclusion: Psoas schwannoma is a rare differential diagnosis for chronic abdominal pain and retroperitoneal cystic lesions. Imaging may be inconclusive, and definitive diagnosis requires histopathological confirmation. Complete surgical excision is curative, and the lateral approach provides effective access with minimal morbidity in selected cases. Early recognition and appropriate surgical management result in excellent outcomes. Clinicians should maintain a high index of suspicion for nerve sheath tumors in atypical cystic retroperitoneal lesions to avoid misdiagnosis and guide appropriate management.