A rare symptomatic L5-S1 anterior retroperitoneal lumbar disc herniation causing foot drop and mimicking a peripheral nerve sheath tumor: illustrative case.

backgroundSymptomatic anterior retroperitoneal lumbar disc herniations (ARLDHs) are exceedingly uncommon and pose a significant diagnostic challenge. The challenge arises when the disc herniation directly compresses the peripheral nerve and mimics a peripheral nerve sheath tumor (PNST). This case report highlights the difficulty in accurate radiological differentiation between a PNST and ARLDH, despite sophisticated neuroimaging techniques. OBSERVATIONS: The authors present the case of a 70-year-old male with symptomatic right-sided L5 radiculopathy and foot drop. Initial contrast MRI suggested a PSNT anterior to the sacral ala with mild uptake on fluorodeoxyglucose positron emission topography. A right-sided anterior retroperitoneal approach was performed for an excisional biopsy, which revealed a migrating disc herniation originating from the anterior L5-S1 disc space. Postoperatively, the patient reported significant improvement of radiculopathy symptoms; however, the foot drop remained. LESSONS: This case highlights the difficulty in differentiating enhancing presacral retroperitoneal lesions using neuroimaging alone, and that tissue biopsy and histopathological evaluation remain essential for accurate diagnosis. The anterior retroperitoneal corridor proved a safe and effective approach for obtaining tissue for accurate diagnosis and guiding the optimal ongoing management. https://thejns.org/doi/10.3171/CASE25977.