Congenital hydrocephalus results from a complex interaction of genetic and environmental factors and is present at birth. It is important to remember that the term genetic does not imply that it is hereditary. Often the exact cause of congenital hydrocephalus cannot be determined. Though it might not be recognised and diagnosed immediately, congenital hydrocephalus is often diagnosed before birth through routine ultrasound. Hydrocephalus diagnosed in adulthood may have existed since birth and can still be considered congenital and may be referred to as compensated hydrocephalus.
Acquired hydrocephalus develops after birth as a result of neurological conditions. This type of hydrocephalus can affect individuals of all ages and may be caused by head trauma, brain tumour, cyst, intraventricular haemorrhage or infection of the central nervous system.
Within both of these areas, congenial and acquired, hydrocephalus can be described as communicating or non-communicating. Communicating hydrocephalus occurs when the flow of CSF is blocked after it exits the ventricles. This form is called communicating because the CSF can still flow between the ventricles, which remain open. Non-communicating hydrocephalus - also called "obstructive" hydrocephalus occurs when the flow of CSF is blocked along one or more of the narrow passages connecting the ventricles.
There are two other forms of hydrocephalus which do not fit exactly into the categories mentioned above and primarily affect adults: benign external hydrocephalus and normal pressure hydrocephalus.
Benign External Hydrocephalus (Communicating)
Benign external hydrocephalus (also referred to as external hydrocephalus) occurs when an accumulation of CSF is found outside the brain, which usually presents itself at birth or soon thereafter. The infants head size will increase, but scans show no international difficulties in the ventricles or pathways. This condition usually corrects itself within 18 months of age.
Normal Pressure Hydrocephalus (Non-Communicating)
Normal pressure hydrocephalus can happen to people at any age, but it is most common among the elderly. It may result from a subarachnoid haemorrhage, head trauma, infection, tumour, or complications of surgery. However, many people develop normal pressure hydrocephalus even when none of these factors are present for reasons that are unknown. (For further information see 'What is Normal Pressure Hydrocephalus').