Types of hydrocephalus

Congenital hydrocephalus

"Congenital" means present at birth. Congenital hydrocephalus affects roughly 1 in 1,000 newborns. It can be caused by something that happened during the baby's development in the womb, by a genetic condition, or by a problem like a brain bleed before or shortly after birth. Sometimes hydrocephalus is diagnosed before birth on prenatal ultrasound or fetal MRI.

Acquired hydrocephalus

"Acquired" means it developed at some point after birth. People of any age can develop acquired hydrocephalus. It can follow a head injury, a brain hemorrhage, a brain infection like meningitis, or a tumor that gets in the way of CSF flow. In premature babies, bleeding inside the brain shortly after birth is a common cause.

Non-communicating (obstructive) hydrocephalus

In non-communicating hydrocephalus, also called obstructive hydrocephalus, something is physically blocking the normal path of CSF. The fluid can't move out of the ventricles because the doorway it would normally use is narrowed or closed.

A common example is aqueductal stenosis — a narrowing of the thin channel (the cerebral aqueduct) that connects two of the ventricles deep inside the brain. Tumors, cysts, and scarring from old bleeding or infection can also cause obstruction.

Communicating hydrocephalus

In communicating hydrocephalus, the ventricles are still connected to one another — CSF can move between them and out into the space around the brain. The problem is further downstream: the fluid is not being absorbed back into the bloodstream the way it should, or, as recent research suggests, large molecules in the fluid are pulling water in faster than it can move out.

Many cases of post-hemorrhagic hydrocephalus and most cases of normal pressure hydrocephalus are communicating.

Aqueductal stenosis

A narrowing of the cerebral aqueduct, the small channel between the third and fourth ventricles. It is one of the most common causes of obstructive hydrocephalus in children. It can be present at birth or develop later.

Hydrocephalus with spina bifida

Many babies born with the open form of spina bifida (myelomeningocele) also develop hydrocephalus. This is related to a brain malformation called Chiari II, which affects how CSF flows.

Post-hemorrhagic hydrocephalus of prematurity

Babies born very early are at higher risk for bleeding inside the ventricles in the first days of life. When that blood doesn't clear properly, hydrocephalus can develop. This is a major cause of acquired hydrocephalus in infants.

Post-infectious hydrocephalus

Infections of the brain or its lining — like meningitis — can leave scarring that interferes with CSF absorption. This is more common in some parts of the world than others and is a leading cause of childhood hydrocephalus globally.

Post-traumatic hydrocephalus

A head injury can cause hydrocephalus, either soon after the injury or weeks to months later. Bleeding inside the head and the inflammation that follows can both contribute.

Tumor-associated hydrocephalus

Some brain tumors block CSF pathways directly, and others produce extra fluid. Treating the underlying tumor sometimes resolves the hydrocephalus; other times a separate procedure is needed.

Normal pressure hydrocephalus (NPH)

NPH mainly affects adults over 60. It is called "normal pressure" because the pressure measured during a spinal tap is often within the normal range, even though the ventricles are enlarged. NPH is most often communicating and is widely under-recognized — its three main signs (trouble walking, urinary changes, and memory or thinking problems) are often mistaken for general aging or for Alzheimer's disease.

Hydrocephalus ex vacuo

This describes ventricles that look enlarged on a scan because the surrounding brain has shrunk — for example, after a stroke or with some forms of dementia. The amount of CSF is not actually too high. This is not true hydrocephalus and is generally not treated with shunting or ETV.

Arrested hydrocephalus

This term is sometimes used for older cases where the ventricles are enlarged but the condition appears to have stabilized without active progression. Whether and how to treat it is a decision made with a neurosurgeon based on the full picture.