Intradural contractions: spinal lipoma surgery with equivocal electromyography activity. Illustrative case.

BACKGROUND

Intradural spinal lipomas are a form of closed dysraphism and a frequent cause of tethered cord. Although usually adipocyte predominant, rare lesions may contain additional histological elements that can yield unexpected intraoperative behavior.

OBSERVATIONS: A 13-year-old boy presented with progressive right foot deformity, distal weakness, gait limitation, and a low conus at S1-2 with a large terminal lipoma on spine MRI. He underwent detethering with debulking and L5 osteoplastic laminoplasty under intraoperative neuromonitoring (IONM). During microsurgical dissection, a pink band along the right aspect of the lipoma appeared unlike a nerve root. Targeted stimulation produced contraction within the thecal sac. The segment was isolated and divided under IONM guidance. Histopathological analysis confirmed skeletal muscle fibers intertwined with lobulated adipose tissue and fibrovascular elements of the filum terminale, consistent with a myolipoma. The patient had no postoperative sensory changes and progressed satisfactorily.

LESSONS: Electrically evoked lipoma contraction is a physiological signature suggestive of myolipoma and can refine the extent of safe untethering. Although IONM adds value beyond evoked potential mapping by eliciting myogenic contraction and helping discriminate resectable elements from neural structures, the resulting EMG tracing becomes difficult to interpret in the presence of muscle and detethering proceeds based on visual assessment. https://thejns.org/doi/10.3171/CASE25811.