The venous hypothesis of hydrocephalus.

Pressure in the central nervous system (CNS) depends upon the volume of tissue that it contains. This includes blood, cerebrospinal fluid (CSF), nerves and any space occupying lesions. The dependency of pressure on volume arises because the CNS is confined by bone. Venous and CSF pressure is linked to overall pressure. Arterial pressure can increase in response to overall pressure to maintain arterial supply. Continuous arterial supply can be maintained because venous blood flows out of the CNS. Reduced volumes of arterial blood will enter the system if venous outflow is interrupted. Increase in CNS volume, as occurs with space occupying lesions, causes compression of veins. This may result in increased venous pressure and reduction in flow of blood out of the CNS. Cerebrospinal fluid (CSF) is extracellular fluid; its absorption back into the circulation is influenced by venous pressure. Any increased in CNS tissue volumes can therefore lead to CSF accumulation. This may then exacerbate the hydrocephalus by further increasing overall CNS volume. Free flow of CSF around the CNS facilitates venous drainage. Blockages to CSF flow can act like space occupying lesions. Chiari malformations, where the cerebellar tonsils obstruct the foramen magnum lead to reductions in CSF flow that can occur intermittently. This leads to impairment of venous drainage which may result in accumulation of CSF. The head or the spine can be affected together or separately. The manifestation of excess fluid accumulation is hydrocephalus and syringomyelia. The speed and origin of venous insufficiency influences the morphology of individual cases particularly with regard to lateral ventricle size. When pressure increases rapidly there may be little time for CSF accumulation. Oedema, compression of intracranial CSF spaces and cerebral ischaemia follows. When venous pressure is only slightly elevated CSF will accumulate and the manifestations of ischaemia may be less apparent, although ischaemia will be a feature of all instances of pathologically raised CNS pressure.