Large Retroperitoneal Schwannoma Abutting the Psoas Muscle: A Case Report

Introduction

Schwannomas are benign nerve sheath tumors. They are infrequently retroperitoneal, composing only approximately 3% of all schwannomas (Chiang et al., 2008; Bocchini et al., 1987). Due to their retroperitoneal location, these tumors often go undetected and can grow quite large (Kuo et al., 1990). This case report describes a large lumbar schwannoma extending to the psoas muscle, and causing significant pain.

Case Report

A 72-year-old woman with a history of breast cancer and germline ATM mutation presented with severe, burning right anteromedial thigh pain. The pain began approximately 4 years ago, but recently worsened. A prior PET scan revealed a retroperitoneal nerve sheath tumor near the right psoas, and a subsequent biopsy identified a benign schwannoma.

There was significant muscle guarding on exam with co-contraction of lumbo-pelvic muscles when ranging her right hip. The motor and sensory exam of the lower extremities was otherwise normal. 

Lumbar MRI re-demonstrated a punctate annular tear of the posterior L4-5 disc, bilateral L5-S1 foraminal narrowing abutting the L5 nerve roots and the schwannoma lateral to the right psoas. Repeat PET scan demonstrated interval increase in FDG avidity of the right retroperitoneal nerve sheath tumor with enlargement measuring 4.5 x 3.6 cm in greatest dimensions (Image 1), previously 3.8 x 2.8 cm. MRI neurogram showed the 5.3 cm schwannoma lateral to the right psoas muscle arising from the right L3 and L4 nerves (Image 2). 

NCV/EMG of the lower extremities was normal. Medial branch blocks and L2/L3 epidural steroid injections were attempted without relief. The neurosurgical team advised against resection due to  difficulty pinpointing the precise cause of pain and the potential for functional decline if the right psoas schwannoma were resected. Pharmacologic optimization with gabapentin 800mg TID was advised. 

Discussion

In this case, a patient with a large schwannoma arising from the L3 and L4 nerve sheaths had grown laterally to the right psoas muscle resulting in thigh pain. Cases of nerve sheath tumors are frequently hard to manage and often unresponsive to central interventions, especially when abutting peripheral structures. There may be a role for perineural steroid injections as suggested in a case by Jevotovsky et al. After a multidisciplinary approach, it was ultimately decided to proceed with pharmacologic management of pain and symptoms. This case demonstrates the value of pain medicine physicians as part of the multidisciplinary teams when treating complex pain cases resulting from tumors.