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Symptomatic sciatic nerve schwannoma: the challenges of management

A 27-year-old female presented with an extraperitoneal pelvic mass found during investigation of 4 months of pelvic pain radiating to her left knee, associated with 28 kg of unintentional weight loss. On examination, initial motor and sensory function was normal, the mass could not be palpated externally and a positive Tinel’s sign could be elicited immediately lateral to sacrum at the level of S2/S3. Gastroscopy, colonoscopy and routine blood tests were normal. This occurred on the background of endometriosis, the initial differential diagnosis. Magnetic resonance imaging (MRI) revealed a 3 2 3 cm well-circumscribed cystic lesion deep to piriformis immediately adjacent to the left sciatic nerve displacing the sacral plexus superomedially into the pelvis (Figs 1,2). Differential diagnoses included a schwannoma or malignant peripheral nerve sheath tumour (PNST). A subsequent positron emission tomography scan showed no local avidity or distant disease. Pain worsened, particularly with hip extension. The patient was able to touch weight bear only, and a sensory deficit developed involving the left sural and tibial nerves. A computed tomography-guided fine-needle aspirate was arranged by the state sarcoma service, revealing benign features, making schwannoma as the most likely diagnosis. Due to daily intractable pain causing loss of function and the development of sensory changes, the lesion was excised. A Kocher–Langenbeck approach was made with longitudinal splitting of gluteus maximus to access the left sciatic nerve as it emerged inferior to piriformis (Fig. 3). Intraoperatively, the lesion was a well-circumscribed, avascular cystic mass, adherent to the sciatic nerve at the sciatic foramen. The team was able to dissect proximal to the lesion into the pelvis via this approach and undertook an intrafascicular neurolysis to remove it with the sacrifice of a single sensory fascicle. Neurogen wrap (Integra Lifescience, New Jersey, USA) was then placed on the sciatic nerve to help prevent neuroma. Histopathology subsequently confirmed schwannoma. Two months post-operatively, the patient’s sensory defect showed improvement with an advancing Tinel’s sign over the sural nerve and no motor or gait deficit. The pelvic pain continued and endometriosis was confirmed. However, the pain radiating to the left knee improved significantly with the cessation of oral analgesia and the ability to sit without pain. PNSTs encompass both benign and malignant lesions, but the most common are benign tumours including schwannomas and neurofibromas. PNSTs of the sciatic nerve are rare, with limited cases described in the literature. MRI sensitivity and specificity for differentiating malignant and benign PNST are estimated to be 68% and 93%, respectively. This case presented several challenging clinical questions.