Isolated Thrombosis of Occipital Sinus: A Rare Entity

Sir, Cerebral venous thrombosis (CVT) is seen commonly in the southern part of India. The most frequent sites are superior sagittal and transverse sinuses.[1] Posterior cranial fossa due to its collateral supply is rarely involved in venous infarction.[1] The occipital sinus is an unpaired venous sinus that lies on the inner surface of the occipital bone, enclosed between leaflets of falx cerebelli and communicates cranially with the torcula herophili and caudally with the vertebral venous plexus or the marginal sinus.[2] Thrombosis of the occipital sinus is an extremely rare condition, to our knowledge, only one case report has been published so far.[3] A 53-year-old female patient presented with a complaint of chronic headache in the occipital region which had worsened in the last 2 months. On examination, her Glasgow coma scale (GCS) was E4V5M6. There were no cranial nerve palsies, focal neurological deficits or cerebellar signs. Signs of meningeal irritation were absent. Baseline ophthalmological evaluation was normal and no signs of raised intracranial tension (ICP) and intraocular pressure (IOP) were present. Cerebrospinal fluid (CSF) examination revealed initial blood-tinged fluid but later clear fluid. Eight cells were seen, one was a degenerated cell and others were lymphocytes. CSF glucose and protein were within normal limits. India ink preparation and cryptococcal antigen tests were negative. CT brain was done which showed no abnormality. MRI revealed T2/FLAIR hyperintensity in the paramedian aspect of the left cerebellar hemisphere [Figure 1a and b]. There was no blooming on SWI [Figure 1c] or diffusion restriction [Figure 1d]. On contrast study, there was filling defect within the occipital sinus [Figure 2a-c] which showed blooming on SWI (not shown) s/o thrombosis.Figure 1: (a) Axial FLAIR image showing hyperintensity in the posteromedial aspect of the left cerebellar hemisphere (arrow); (b) Axial T2 image showing hyperintensity in the posteromedial aspect of the left cerebellar hemisphere (arrow) and hypointensity within the thrombosed occipital sinus (dotted arrow); (c) Axial SWI image showing blooming within the thrombosed occipital sinus (dotted arrow) with maintained signal intensity in the B/L transverse-sigmoid sinus junctions (black arrows); (d) Axial DWI shows no evidence of diffusion restriction within the cerebellar hemispheresFigure 2: Post-contrast T1 weighted images in three orthogonal planes (a - axial; b - sagittal and c - coronal) showing the prominent filling defect (arrow) in the occipital sinus suggestive of intraluminal thrombus. Dotted arrows (a and c) show normal opacification of the sinuses at the B/L transverse-sigmoid sinus junctionsCVT has varied causes ranging from dehydration, pregnancy, puerperium, contraceptives, and protein C deficiency, protein S deficiency, mastoiditis, tumor invasion of a venous sinus, vasculitis, head trauma, intracranial, and systemic infections. The most common sinuses involved are superior sagittal and transverse sinuses. Thrombosis of these sinuses usually causes hemorrhagic infarction of the supratentorial parenchyma. Isolated cerebellar infarct secondary to venous sinus thrombosis is very rare due to the abundance of the collateral venous drainage; it usually occurs when there is deficiency in this collateral venous drainage.[1] Occipital sinus is a dural venous sinus enclosed in the falx cerebelli and passes along the crista occipitalis interna.[4] Cranially, it communicates with the confluence of sinuses – torcula herophili and caudally with the vertebral venous plexus; however, it can communicate with transverse sinus, sigmoid sinus, or the marginal sinus. Tributaries of occipital sinus are diploic veins, vein of hypoglossal canal, basilar plexus, and occipital emissary veins. Its course is usually in midline but may be deviated to one side and its size increases as it approaches the torcula. The size of sinus varies with age; usually, it regresses with age. It becomes prominent if the transverse sinus is occluded or rudimentary. It is important to know the course of occipital sinus before surgery for a posterior fossa lesion as anatomical variation in its course and size often produces difficulty. Widjaja et al. have done a study based on time of flight (TOF) MR venography in children and they found that in 41 (82%) of 50 patients, occipital sinus was not visualized. A possible explanation for it could be atretic occipital sinuses or alternatively slow flow within the sinus, being undetectable by 2D TOF techniques.[5] Given that the presence of the sinus itself is rare, its thrombosis is expectedly rarer. A review of the literature showed only one case report published. The imaging in our case revealed thrombosis in the occipital sinus with T2/FLAIR signal hyperintensity in the left paramedian cerebellar hemisphere representing venous ischemia. The cerebellar signal change was not reported in the prior case. The unenhanced CT in our patient did not reveal any abnormality which could be attributed to the chronic nature of the thrombus as was noted on T1 and T2 weighted imaging which showed isointense signal intensity within. Post-contrast MRI showed nonenhancing filling defect within the occipital sinus extending till the confluence of sinuses. We conclude that occipital sinus is a rare anatomical variation and its thrombosis producing venous ischemia is still rarer. Chronic thrombus of dural venous sinuses can be overlooked on unenhanced CT. MRI with contrast can detect these thrombi with a higher degree of sensitivity. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.