Teaching Case

The 54-year female was referred for an MRI brain for the persistent headache for three months. Physical examination was unremarkable.

T2 axial image shows a signal void at the expected location of the occipital sinus. T2 gradient axial image show loss of signal void at the expected location of the occipital sinus. Axial, coronal and sagittal MR venography show patent bilateral occipital sinuses around the foramen magnum extending towards torcula Heropheli with absent bilateral transverse sinuses.

The persistent occipital sinus, along with absent transverse sinus or sigmoid sinus, is a rare entity observed in <10% of the human population. It connects the marginal sinus to the torcula and is the smallest in calibre among all cerebral dural venous sinuses [1]. This entity is commonly observed in neonates but gradually disappears during adulthood. This condition has a critical surgical significance in selecting various surgical approaches for a lesion in the posterior fossa, including retrosigmoid procedures [2]. The occipital sinus originates as a small venous channel from primitive torch-la and medial portions of transverse sinuses. It is seen most prominently in the 5th month intrauterine and, after that, it reduces in calibre and is only seen after birth in a minority of the population. It may show variable courses. It courses inferiority from the torcula to marginal veins close to the foramen magnum and then drains into the suboccipital veins, paravertebral venous plexus or jugular veins [3]. Since it communicates with the posterior internal vertebral plexus, the occipital sinus receives a small amount of blood from the spine and the spinal cord and hence be considered an emissary vein. The occipital sinus function as an alternative venous drainage route from the vertebral column when the venous return through the internal jugular vein is compromised. In the case of an absent transverse sinus, the occipital sinus becomes the main pathway of venous drainage from the brain [4].

Patients are asymptomatic, with no gender predilection noted. Conventional CT has no role in diagnosis. CT venography may identify the anomaly with a clear anatomical depiction. Conventional T1 and T2 weighted images may show a signal void at the expected location of the occipital sinus, and gradient imaging shows a loss of signal void. MR venography is the investigation of choice which depicts detailed anatomy and drainage route along with other associated venous anomalies.

As the entity is incidental, no management is required except treatment for other pathological conditions, e.g., venous thrombosis and occlusion. However, a patent occipital sinus with an absent transverse sinus represents a significant clinical entity. It obviates the need for an efficient neurosurgical team if the posterior fossa approach is planned surgically [5].

Take home message

A radiologist should be aware of persistent occipital sinus and its imaging features and should report patent sinuses in cases of posterior fossa abnormalities. This guides crucial modification of surgical planning during the posterior fossa approach of surgical procedure and avoids catastrophic bleeding, brain swelling and neurosurgical morbidity due to sinus injury.