Communicating hydrocephalus is, by definition, a hydrocephalus in an individual whose cerebrospinal fluid (CSF) circulation pathways are competent from the ventricles inside of the brain to the fluid spaces just below the the third ventricle, the so called basilar cisterns. Classically it was thought to arise when the arachnoid villi became obstructed and were no longer able to transmit the CSF from the subarachnoid space investing the brain into the blood vessels draining the brain. Recent work by several pediatric neurosurgeons have suggested that there may be several causes for communicating hydrocephalus with some being due to altered or compromised blood circulation within the brain, within the skull or within the chest. As these special causes become better understood, alternative treatments may become available. For now though the treatment for this condition is some form of a shunt.
Obstructive hydrocephalus typically refers to hydrocephalus which develops as a result of a blockage in the normal circulation of cerebrospinal fluid (CSF) within the brain. In most cases it refers to a blockage between the third and fourth ventricles at the level of the Aqueduct of Sylvius, so called aqueductal obstruction. This can be due to scarring of this passage (aqueductal stenosis), or a tumor (in most cases a tectal glioma). When the obstruction is due to a tectal glioma it is important to realize that the tumor has only a 10% chance for growth and the only treatment required is for the hydrocephalus. Additional treatments for the tumor are only delivered if growth is documented.
Obstruction in the CSF pathways can occur at other levels. The lateral ventricles can become isolated when the pathways into the third ventricle (the foramen of Monroe) become obstructed. Obstruction can occur within the outlets of the fourth ventricle with dilation of all four ventricles resulting. One example of this would be hydrocephalus associated with a Chiari Malformation.
With the introduction of endoscopic management of hydrocephalus there has been a broadening of the definition of obstructive hydrocephalus. Increasingly it is thought to be represented by any obstruction of the CSF pathways below the level of the suprasellar cisterns, the fluid spaces outside the brain directly underneath the floor of the third ventricle. Consequently many would include obstruction in the pathways around the outside of the brainstem in the definition of obstructive hydrocephalus.
The management of obstructive hydrocephalus has undergone a major shift during the past two decades. Most physicians experienced with endoscopy would seriously consider managing this form of hydrocephalus with a third ventriculostomy. For many, a third ventriculostomy is the treatment of choice for obstructive hydrocephalus. This is because of a lower long term complication rate. It is import to realize that this procedure does not cure the hydrocephalus. Rather, it is simply a treatment just as a shunt is a treatment and not a cure.
Normal Pressure Hydrocephalus or NPH is a type of hydrocephalus which typically arises in adults in their late middle life and beyond. The condition gives rise to gait instability owing to a sense of imbalance, particularly when turning, urinary incontinence and progressive dementia. Unlike other forms of hydrocephalus headaches are rare with this condition.
NPH has classically been felt to be a communicating hydrocephalus. With the advent of endoscopic management of some forms of hydrocephalus it has been found that some with NPH can be successfully treated with a third ventriculostomy. This would suggest that at least some individuals with NPH have an obstructive hydrocephalus.
For most management of this form of hydrocephalus is done with a shunt. It can be difficult to determine. This can lead to some difficulties. Shunts are designed to drain to a certain pressure differential between the pressure within the head and with the distal cavity the system is draining into. Over drainage in an elderly adult carries a significant risk of the brain collapsing into the space created by the removal of the fluid. When this happens bleeding can occur over the surface of the brain and this in turn can compromise the brain's functioning. To avoid this problem shunts with higher resistance to drainage can be used but this can lead to inadequate treatment of the hydrocephalus (remember this is "normal pressure" hydrocephalus and treatment must decrease pressure in order to work). Recently a work around for this has recently become available. Several programmable valves are now available which allow adjustment of their drainage resistance noninvasively during an office visit. These shunts can be set at a high resistance at the time of implant so as to avoid brain collapse and then the resistance to drainage can be gradually lowered overtime until the desired symptomatic has been obtained.
It is not uncommon for there to be a period of benefit from the implantation of the shunt followed by return in deterioration in gait, mentation and urinary continence. A recent study found that in this situation it is important to check the shunt's functioning as a high percentage of patients experiencing this type of progression are doing so because of a shunt malfunction.