The Venous Drainage central nervous system consists of the cerebrum, cerebellum, brain-stem and spinal cord. Their venous drainage is complex, and rather uniquely, does not follow the arterial supply. The cerebrum, cerebellum, and brain stem are drained by numerous veins, which empty into the dural venous sinuses. The spinal cord is supplied by anterior and posterior spinal veins, which drain into the internal and external vertebral plexuses.
Dural Venous Sinuses of Venous Drainage
The dural venous sinuses lie between the periosteal and meningeal layers of the dura mater. They are best thought of as collecting pools of blood, which drain the central nervous system, the face, and the scalp. All the dural venous sinuses ultimately drain into the internal jugular vein. Unlike most veins of the body, the dural venous sinuses do not have valves.
There are eleven venous sinuses in total. The straight, superior, and inferior sagittal sinuses are found in the falx cerebri of the dura mater. They converge at the confluence of sinuses (overlying the internal occipital protuberance). The straight sinus is a continuation of the great cerebral vein and the inferior sagittal sinus.
From the confluence, the transverse sinus continues bi-laterally and curves into the sigmoid sinus to meet the opening of the internal jugular vein.
The cavernous sinus drains the ophthalmic veins and can be found on either side of the sella turcica. From here, the blood returns to the internal jugular vein via the superior or inferior petrosal sinuses.
Veins of the Cerebrum
The veins of cerebrum are responsible for carrying blood from the brain tissue, and depositing it in the dural venous sinuses.
They can be divided into superficial and deep groups, which are flamboyantly arranged around the gyri and sulci of the brain. Upon exiting the cerebral parenchyma, the veins run in the subarachnoid space and pierce the meninges to drain into the dural venous sinuses.
The superficial system of veins is largely responsible for draining the cerebral cortex:
- Superior cerebral veins: Drain the superior surface, carrying blood to the superior sagittal sinus.
- Superficial middle cerebral vein: Drains the lateral surface of each hemisphere, carrying blood to the cavernous or sphenopalatine sinuses.
- Inferior cerebral veins: Drain the inferior aspect of each cerebral hemisphere, depositing blood into cavernous and transverse sinuses.
- Superior anastamotic vein (Trolard): Connects the superficial middle cerebral vein to the superior sagittal sinus.
- Inferior anastamotic vein (Labbé): Connects the superficial middle cerebral vein to the transverse sinus.
- Subependymal veins – There are numerous subependymal veins, which will not be described here in detail. These receive blood from the medullary veins and carry it to the dural venous sinuses. The great cerebral vein (vein of Galen) is worthy of a mention; it is formed by the union of two of the deep veins, and drains into the straight sinus.
- Medullary veins: Originate 1-2cm below the cortical grey matter, and drain into subependymal veins. These drain the deep areas of the brain.
Other Structures in the Central Nervous System that helps in Venous Drainage
There are two main veins responsible for the venous drainage of the cerebellum – the superior and inferior cerebellar veins. They empty into the superior petrosal, transverse and straight dural venous sinuses.
Venous Drainage Brain stem
Venous drainage of the brain stem is carried out by numerous vessels – many of which are beyond the scope of this article. Examples of veins that drain the brain stem include the transverse pontine vein, anteromedian medullary vein, and the anterior and posterior spinal veins.
The spinal cord is supplied by three anterior and three posterior spinal veins. These veins are valveless, and form an anastamotic network along the surface of the spinal cord. They also receive venous blood from the radicular veins.
The spinal veins drain into the internal and external vertebral plexuses, which in turn empty into the systemic segmental veins. The internal vertebral plexus also empties into the dural venous sinuses superiorly.
Cerebral Venous Sinus Thrombosis
Cerebral venous sinus thrombosis (CVST) describes the presence of a thrombus within one of the dural venous sinuses.
The thrombus occludes venous return through the sinuses, and causes an accumulation of deoxygenated blood within the brain parenchyma. This in turn can lead to venous infarction. The situation is complicated by an accumulation of cerebrospinal fluid, which can no longer drain through the thrombosed venous sinus.
Common clinical features are headache, nausea and vomiting, and neurological defects.
The definitive diagnosis is usually made by CT or MRI scan with contrast, which demonstrates obstruction of the venous sinuses. Treatment is with anticoagulation.