Lumboperitoneal Shunt Malfunction Due to Misplacement of the Lumbar Catheter Into the Spinal Subdural Extra-arachnoid Space: A Case Report.

Lumboperitoneal (LP) shunting is an established treatment for idiopathic normal pressure hydrocephalus (iNPH). Although generally considered less invasive, LP shunts carry the risk of lumbar catheter malposition, including rare misplacement into the spinal subdural extra-arachnoid space (SSES), a potential cause of shunt malfunction. A 72-year-old woman presented with gait disturbance, cognitive decline, and urinary incontinence. Magnetic resonance imaging (MRI) revealed ventriculomegaly and disproportionately enlarged subarachnoid space (DESH). Her symptoms improved after a tap test, and an LP shunt was placed under fluoroscopy using a CERTAS™ Plus programmable valve. Initial postoperative improvement was observed. However, her symptoms worsened six months later despite normal imaging and valve adjustments. Shuntography revealed localized contrast pooling along the thoracolumbar nerve roots without spinal canal spread. Computed tomography (CT) confirmed catheter placement in the SSES. Revision surgery using the one-piece method with fluoroscopic guidance achieved accurate catheter repositioning. The patient's symptoms subsequently improved. SSES is a potential but often unrecognized anatomical space created between the dura and arachnoid mater during puncture. Although the cerebrospinal fluid (CSF) may drain initially, the lack of communication with the subarachnoid space can result in delayed shunt failure. Diagnosis requires imaging, and initial clinical improvement may be misleading. The one-piece method enables safe, precise catheter reinsertion and avoids reconnection errors. Clinicians should consider catheter misplacement into SSES as a cause of LP shunt failure, even when early postoperative improvement is observed. Fluoroscopy-guided placement and vigilant postoperative follow-up are essential for optimal outcomes.