L5 mononeuritis, an uncommon cause of foot drop: illustrative case

BACKGROUND: New-onset adult foot drop is commonly encountered in neurosurgical practice and has a broad differential, including radiculopathy, peroneal nerve palsy, demyelinating diseases, and central causes. Etiology is commonly identified with comprehensive history, examination, imaging, and investigations. Despite familiarity with the management of lumbar spondylosis and peroneal nerve compression causes, rare or uncommon presentations of nonsurgical causes are important to consider in order to avoid nonbeneficial surgery. OBSERVATIONS: The authors report a very uncommon cause of foot drop: new-onset isolated L5 mononeuritis in a 61-year-old nondiabetic male. They provide a review of the etiology and diagnosis of foot drop in neurosurgical practice and detail pitfalls during workup and the strategy for its nonsurgical management. LESSONS: Uncommon, nonsurgical causes for foot drop, even in the setting of degenerative lumbar spondylosis, should be considered during workup to reduce the likelihood of unnecessary surgical intervention. The authors review strategies for investigation of new-onset adult foot drop and relate these to an uncommon cause, an isolated L5 mononeuritis, and detail its clinical course and response to treatment.