We report a very unusual case of a patient with a previously undiagnosed congenital absence of the left dural venous drainage system and a prominent system on the right. Due to its prominence it was at risk during a cortical mastoidectomy performed as part of an endolymphatic sac decompression procedure. Manipulation of the lateral venous sinus resulted in partial occlusion of the already compromised venous drainage. The result was a rise in intra-cranial pressure and papilloedema, from which the patient recovered. The abnormality was only detected on post-operative imaging.
