Hydrocephalus is a condition caused by accumulation of cerebrospinal fluid in the brain.
Anatomy and physiology
The brain is located in the cranial cavity which is eclosed space with constant volume. It is surrounded by cerebrospinal fluid (CSF). CSF is continuously produced in the brain by choroid plexus and passes through ventricular system to reach the outer surface. There are four interconnected ventricles in the brain connected to subarachnoid space.
CSF passes from ventricles to subarachnoid space and then it is reabsorbed into blood circulation by the system of arachnoid granulations or villi. Cerebro-spinal fluid is constantly produced and reabsorbed. Daily CSF production is about 600 ml and total volume of cerebro-spinal fluid in the central nervous system is approximately 150 ml. Therefore, CSF is renewed 4-5 times every day. CSF production is pressure independent - meaning that no matter what is the pressure inside the brain the amount of newly formed CSF is constant.
On the other hand, absorbtion of the CSF is is pressure dependent. The higher the pressure in the subarachnoid space the higher the reabsorbion rate. This peculiar mechanism maintains normal pressure inside the cranial cavity and optimises brain functions.
Hydrocephalus is the disturbance of this balance when the fluid accumulates in the brain. Usually, the cause is either a blockage of the flow or insufficient absorption. Fluid builds up in the brain raising the pressure inside the head.
Since the cranim is a closed space even smal amount of extra fluid causes a significant increase in pressure. Elevated pressure in the brain is an extremely dangerous condition and should be treated promptly. There is one exceptional form of hydrocephalus when the pressure does not raise. It is called normal pressure hydrocephalus (NPH) and it is seen only in elderly patients.
Hydrocephalus is caused either by increased production of CSF, blockage of flow or decreased absorption. Here are few conditions associated with hydrocephalus
Obstruction or narrowing in the ventricular system. Aqueductal stenosis, foramen Monro stenosis, colloid cyst, intraventricular arachnoid cysts, tumors, bleeding into ventricles are examples of this type. In these cases, CSF cannot flow freely causing hydrocephalus. The mechanism is quite similar to gallbladder or kidney outflow obstruction by stones.
Obstruction within subarachnoid space. Meningitis, subarachnoid hemorrage, cancer spreading into subarachnoid space are well known examples. This form of hydrocephalus is caused by both obstruction of CSF flow in the subarachnoid space and blockage of absorption by arachnoid granulations.
Increased CSF secretion by choroid plexus hyperplasia or tumor.
Normal pressure hydrocephalus. It is a specific form of disease seen only in elderly patients. In this condition the pressure is not high rather brain cannot tolerate normal pressure and hydrocephalus develops.
Symptoms of hydrocephalus drastically different in babies and adults. In babies, the skull is capable of growing and therefore fluid accumulation does not significantly increase the pressure. Rather it leads to the increased size of the head. In older children and adults hydrocephalus cannot distort the head. Rather it inreases intracranial pressure compromising brain functions.
Headache is the most common symptom of increased intracranial pressure. This pressure stretches the brain covers - especially the dura mater. Dura mater has many pain receptors and is very sensitive to pressure. The brain itself does not feel the pain. Other symptoms include nausea and vomiting, altered consciousness. Vision disturbances are very common in hydrocephalus. Blurred vision due to optic nerve compormise is a very common finding. Eye movements are also frequently compromised. Parinaud symptom or upward gaze palsy is very common in hydrocephalus. Also VI cranial nerve can be compromised by increased intracranial pressure and cause inability to look to the side.
The severity of symptoms depends on the pressure and onset time. Acute hydrocephalus is poorly tolerated because brain cannot switch on compensatory mechanisms. It is usually a life-threatening condition and should be treated promptly. Chronic hydrocephalus may not be symptomatic for a long time.
Normal-pressure hydrocephalus (NPH) is different from other forms. It is always chronic and characteristically has three main symptoms - dementia, gait disturbance and urinary incontinence. Any elderly patient with dementia should have a brain scan to rule out the possibility of NPH since it is the only treatable form of dementia.
Clinical examination with careful history of the disease is the first step for establishing the diagnosis. In babies increased head size is easily detected and diagnosis can be made rigth away. Adult patients can be unconsious and therefore history might not be obtained.
Surgery is the only treatment option for hydrocephalus. There are two types of surgery – shunting and ventriculostomy. CSF diversion or shunting is a procedure when the surgeon places a device called shunt to redirect the fluid from the brain to another part of the body for absorption. Most commonly the CSF is shunted to belly – so-called ventriculoperitoneal shunting. Ventriculostomy or endoscopic third ventriculostomy (ETV) is a procedure when an artificial window is made to allow the CSF to by-pass the blockage. Both shunting and ETV are effective means if used properly. The main handicap of shunting is the implantation of the foreign device which has to stay for the rest of the patient’s life. The shu"nt may get infected or have a mechanical failure. ETV, on the other hand, is an elegant procedure yet it is most effective when hydrocephalus is due to a blockage of fluid flow.