Hydrocephalus is a term coming from prefix hydro- (water) and cephalos (head). It is also often mentioned as “water on the brain”. The “water” is in fact the so-called cerebrospinal fluid. The fluid is constantly formed by cells lining the cavities inside the brain (brain ventricles). The fluid flow through the ventricles and through narrow channels it even gets into space around the brain. Out of this area it is absorbed into cerebral blood veins into the bloodstream. Cerebrospinal fluid protects the brain against mechanic damage and allows the brain to remove its waste products. If the outflow of cerebrospinal fluid is anyhow blocked, it starts to accumulate and causes elevation of intracranial pressure (intracranial hypertension). In an adult this is especially dangerous as his skull bones are firmly fused ad there is no way to compensate the pressure. In little children, however, the skull bones are not fused and growing pressure leads to pushing away the bones resulting in enlargement of the diameter of the head. This slows the onset of symptoms and gives the physicians more time to solve the condition.
The outflow of the cerebrospinal fluid may be disrupted by an obstacle within the brain ventricles or blockage of absorption into venous blood. The causes include inherited defects of the brain ventricles and connecting canals, obstruction by a blood clot (following a brain bleeding), damage after a head injury, damage due to infections of brain meninges (meningitis) or brain tissue (encephalitis) and obstruction by brain tumors and their metastases.
The major symptoms in children are enlargement of the head as the fluid pushes the unfused skull bones away. Compression of brain by intracranial hypertension can lead to epileptic seizures, vomiting and irreversible mental retardation. Children with hydrocephalus also often have special look, which is referred to as the "sign of the setting sun" - the pupils are located by the lower eyelids and above them we can see the white of the eye as if the child permanently looked down.
The hydrocephalus in adults is much more dangerous as the skull bones do not allow compensatory head enlargement. Therefore, hydrocephalus occurrence is quickly followed by more severe signs of intracranial hypertension such as vomiting, headache, abnormal gait, impaired consciousness and eventually death.
Note for medical students: There is also a subtype of hydrocephalus called “hydrocephalus e vacuo”. This is basically a pseudohydrocephalus, when there is normal intracranial pressure and brain atrophy. Atrophic brain tissue looses volume and this leads to compensatory enlargement of brain ventricles. It typically occurs in the elderly and in chronic alcoholics.
The treatment approach is mainly neurosurgical. An obstruction is usually treatable with a shunt implantation. The shunt is a hollow tune, which dilates any narrow or obstructed part of the brain ventricular system. If there is disrupted absorption of cerebrospinal fluid into the bloodstream, it is possible to use a special long shunt to connect the ventricular system to another body cavity (e.g. abdominal cavity). This ensures easy drainage of the cerebrospinal fluid but on the other hand, it is related to increased risk of brain infections.