Sacral CSF-venous fistula: a rare entity. Illustrative case.

backgroundCSF-venous fistula (CVF) is an increasingly recognized cause of CSF leakage resulting in spontaneous intracranial hypotension (SIH). CVFs are most commonly identified in the thoracolumbar spine and are rarely reported in the sacral region. OBSERVATIONS: A man in his 60s presented with symptoms consistent with SIH. CT of the brain demonstrated bilateral subdural hematomas and a decreased pontomesencephalic angle. Spinal MRI showed degenerative changes without evidence of a CSF leak. Lateral decubitus CT myelography (CTM) with delayed imaging identified a sacral CVF at the L5-S1 level, which was confirmed with an adjusted technique within digital subtraction myelography (DSM). The fistula was determined to be the etiology of the patient's symptoms. Transvenous embolization via femoral venous access using Onyx-18 resulted in successful obliteration of the fistula. LESSONS: This case represents only the third reported sacral CVF. It highlights the importance of advanced diagnostic imaging when routine studies are negative but clinical suspicion for SIH remains high. Key technical considerations for CTM and DSM of the lumbosacral junction and sacrum are discussed. https://thejns.org/doi/10.3171/CASE2632.