Spinal cord-spinal canal disproportion following treatment for syringomyelia associated with Chiari malformation type I: illustrative case.

backgroundChiari malformation type I (CM-I) is a common pediatric neurosurgical condition that is often associated with syringomyelia (SM) and spine deformity. The association of these three conditions is well recognized, but the pathophysiology linking them has yet to be fully elucidated. OBSERVATIONS: This case report describes the unusual course of a 13-year-old male with CM-I, a large holocord syrinx, and progressive scoliosis who developed an angiography-negative subarachnoid hemorrhage (SAH) 4 months after successful posterior fossa decompression with duraplasty (PFDD). Notably, his cervical spinal canal diameter was increased relative to normative data. After presenting with SAH, he was treated with a course of lumbar cerebrospinal fluid drainage, which relieved his symptoms, and he had no further incidents. His syrinx responded to PFDD with a dramatic decrease in spinal cord diameter, in contradistinction to his dilated bony spinal canal. He is presently 4 years out from his SAH and doing well clinically. LESSONS: This report describes the first case of nonaneurysmal SAH following PFDD for CM-I and SM and explores a possible link between SM and bony spinal canal diameter. https://thejns.org/doi/10.3171/CASE24272.