1330 - Not Your Usual Foot Drop: Rare Sacral Plasmacytoma Case Report

A 66-year-old male presented with severe low back pain and difficulty ambulating. Physical exam revealed a right foot drop and limited ankle range of motion. MRI showed a large, expansile, lobulated mass replacing the sacrum with extension into the L5-S1 disc space and bilateral lumbosacral plexus. Biopsy confirmed plasmacytoma (IgG Kappa, M spike 1 g/dL); bone marrow biopsy was negative for systemic involvement. Conservative management, including pharmacologic treatment, physical and occupational therapy, was exhausted. Given sacral destruction, foraminal stenosis, and persistent weakness and neuropathic pain, the patient underwent L4-L5 and L5-S1 laminectomies and neuroforaminotomies, with decompression and sacral lesion resection. He was admitted to acute inpatient rehabilitation, reporting full resolution of preoperative neuropathic pain, improved mobility, from moderate assistance for 10 feet to independent ambulation for 150 feet, despite persistent right foot drop.

Discussions:

This case highlights both a rare cause of foot drop, as well as the unusual neurologic manifestations that a sacral plasmacytoma can present with. Right foot drop has a wide range of etiologies, including trauma, surgery, herniated disc, stroke, and nerve injury from compression at the fibular head. Solitary plasmacytoma of the sacrum is an especially rare neoplasm as plasmacytomas typically occur in the region of the head, neck, and spine. A destructive sacral plasmacytoma invading presacral soft tissue and plexus led to the constellation of symptoms our patient presented with, including lower back pain, lower extremity weakness, and inability to stand on the right heel.

Conclusions:

Physiatrists play a critical role in the initial evaluation, diagnosis, and post-surgical functional optimization. The surgical decompression and resection performed in this case provided significant pain relief; however, acute inpatient rehabilitation and long-term support remain essential for improving function, addressing residual deficits, and maximizing independence.