Spinal intradural arachnoid cyst in an infant with motor developmental delay showing intraoperative MEP improvement: illustrative case.

backgroundSpinal intradural arachnoid cysts (SIACs) are relatively rare, but they can cause neurological symptoms. The authors report a case of an infant with an SIAC presenting with motor developmental delay, highlighting the intraoperative improvement of motor evoked potentials (MEPs). OBSERVATIONS: A 14-month-old girl presented with a lower limb motor developmental delay. Spinal MRI revealed an SIAC at the C7-T4 level, compressing the spinal cord from the dorsal side. She underwent surgical fenestration of the cyst. Intraoperatively, transcranial MEP amplitudes in the lower extremities were initially lower than those in the upper extremities. Following cyst fenestration and decompression, the MEP amplitudes increased more than 10-fold compared to the baseline and were maintained. Postoperatively, her motor functions gradually improved. By 5 months after surgery, her developmental delay had resolved, and she achieved age-appropriate motor milestones. LESSONS: In this pediatric case of an SIAC, surgical decompression led to a significant and immediate increase in MEP amplitudes. This physiological change was followed by an improvement in the patient's motor development, suggesting that intraoperative MEP monitoring can be a useful tool for evaluating the immediate effect of decompression in infants with an SIAC. https://thejns.org/doi/10.3171/CASE26285.