Dominant Occipital Sinus: A Rare Anatomical Variant With Potentially Catastrophic Consequences if Unrecognized Preoperatively

The occipital sinus is often thought of as a redundant vestigial structure in adults. However, in rare cases, it can form the dominant route of intracerebral venous drainage, with a risk of significant surgical morbidity if unrecognised. We present an illustrative case describing this anatomical variant and tailoring of a midline suboccipital craniotomy to allow resection of a fourth ventricular epidermoid tumour with preservation of a dominant occipital sinus, and a review of the published literature. A 48-year-old female patient was diagnosed with a large fourth ventricular tumour with marked diffusion restriction, consistent with an epidermoid tumour. Imaging demonstrated bilateral hypoplastic transverse sinuses and a widely patent occipital sinus draining the straight and superior sagittal sinuses into the marginal sinus. A midline posterior fossa craniotomy, C1 laminectomy, and paramedian durotomy with the division of the left marginal sinus allowed for gross total resection of the epidermoid tumour with preservation of the occipital and right marginal sinuses. Given the significant potential surgical morbidity resulting from injury or ligation of a dominant occipital sinus, as seen in the literature review, we highlight the importance of recognising anatomical variants of the dural venous sinuses preoperatively to modify surgical approaches and minimise potential complications.