One of the first questions after a hydrocephalus diagnosis is simply "why?" Hydrocephalus has many possible causes, and they differ by age. This article pulls them together into one overview. In every case, the common thread is a disruption in the balance of CSF — too little flow, too little absorption, or (rarely) too much production.
The three basic mechanisms Whatever the specific cause, hydrocephalus generally arises through one of these:
- Blocked flow — something obstructs CSF within the ventricular system.
- Impaired absorption — CSF can't be reabsorbed properly into the bloodstream.
- Overproduction — rarely, too much CSF is made (for example, by certain choroid plexus tumors).
Causes present at or before birth (congenital) In babies, common causes include:
- Aqueductal stenosis — narrowing of the cerebral aqueduct.
- Spina bifida (myelomeningocele) — frequently associated with hydrocephalus via the Chiari II malformation.
- Dandy-Walker malformation — affecting the fourth ventricle and cerebellum.
- Infections during pregnancy and genetic factors.
Causes that develop later (acquired) At any age after birth, hydrocephalus can result from:
- Bleeding (hemorrhage). Intraventricular hemorrhage in premature babies is a major cause; in adults, subarachnoid hemorrhage (often from a ruptured aneurysm) is important.
- Infection. Meningitis and other infections can scar CSF pathways and impair absorption.
- Brain tumors. Tumors can block the ventricular system or disrupt CSF dynamics.
- Head injury (trauma).
- Complications of brain surgery.
The special case of NPH In older adults, normal pressure hydrocephalus is most often idiopathic — meaning no clear cause is found — though it sometimes follows bleeding, infection, or trauma. It's thought to involve altered CSF dynamics and absorption rather than a single blockage.
When no cause is found In a meaningful number of cases — across ages — no specific cause is identified despite investigation. This can be frustrating, but it doesn't change the fact that the hydrocephalus can still be effectively treated. The treatment addresses the fluid buildup itself, whether or not the original trigger is known.
Why identifying the cause matters Finding the cause, when possible, helps in two ways: it may allow the underlying problem to be treated (for example, removing a tumor or treating an infection), and it helps the team choose the best procedure (a discrete blockage may suit an ETV, while an absorption problem suits a shunt). Even so, the priority is always relieving the dangerous fluid buildup.
If you're searching for "why," know that the cause sometimes shapes treatment and sometimes remains unknown — but in either case, hydrocephalus is treatable, and your team will focus on protecting the brain and restoring CSF balance.