IH and Related Disorders
IH and Chiari Malformations
A Chiari (kee-ar-ee) malformation is a neurological condition in which an area at the back of the brain, known as the cerebellar tonsils, is pushed down beyond the bottom of the skull into the spinal canal. As a result, the tonsils can physically block the circulation of cerebrospinal fluid in the sub-arachnoid space surrounding the brain and spine, and may cause secondary intracranial hypertension and other serious problems.
A Chiari Malformation
A Chiari malformation is often congenital. However, there is evidence that, over time, sustained increased intracranial pressure may cause an acquired Chiari malformation. While an acquired Chiari malformation can also be the result of overdrainage from an LP shunt or multiple spinal taps, some research suggests that an acquired Chiari malformation is eight times more common in chronic IH patients who have not had shunting procedures.
Additionally, for unknown reasons, certain people with congenital Chiari malformations develop chronic intracranial hypertension, even after neurosurgery (surgical decompression) to remedy any obstruction to CSF circulation.
There are many questions that need to be answered about the relationship between chronic intracranial hypertension and Chiari malformations. Does chronic intracranial hypertension play a role in the development of acquired Chiari malformations? What biological mechanism leads to increased intracranial pressure? With research, the answers to these questions can be found.
IH and Hydrocephalus
There is considerable confusion regarding intracranial hypertension and hydrocephalus. Both are disorders of abnormal cerebrospinal fluid dynamics. In other words, something goes wrong with the normal cycle of CSF production, absorption and drainage in both IH and hydrocephalus.
Intracranial pressure is determined by the three main components within the skull —brain tissue, blood and CSF— working together. Under normal circumstances, these three components maintain a dynamic equilibrium. In order for this balance to be maintained, it is believed that CSF, which is produced at approximately .3 cc per minute, must also exit the skull at the same rate.
However, since the skull is a fixed container, made of bone that cannot expand, changes in the volume of any one of these three components leads to abnormal intracranial pressure. Intracranial hypertension in adults is defined as CSF pressure at 250mmH2O or above. In chronic IH, the exiting (or egress) of CSF is thought to be impaired while simultaneously the CSF productions continues, which leads to elevated intracranial pressure.
In hydrocephalus, the ventricles (four small cisterns located within the brain) become enlarged and contain an excess of CSF. As part of the normal cycle of CSF production, CSF circulates through the ventricles before passing into the sub-arachnoid space surrounding the brain. If obstruction to CSF flow occurs within the ventricular system, then the ventricles up to the point of obstruction will become pressurized, fill with cerebrospinal fluid and in time, become enlarged. This is known as obstructive hydrocephalus.
Non-obstructive hydrocephalus occurs after CSF has left the ventricular system uneventfully. But, in this case, the CSF is either restricted in its flow over the brain surface (the sub-arachnoid space) or at the site of exiting or absorption (similar to chronic IH.) One question remains particularly puzzling: Why is there no ventricular enlargement in chronic IH (unlike non-obstructive hydrocephalus), if both disorders have the same site of absorption or CSF egress that is not adequately functioning?