Hydrocephalus is a condition in which cerebrospinal fluid (CSF) builds up within and/or around the brain, which can cause pressure to increase in the skull. Symptoms may vary according to age. Headaches and double vision are common. Elderly adults with normal pressure hydrocephalus (NPH) may have poor balance, difficulty controlling urination or mental impairment. In babies, there may be a rapid increase in head size. Other symptoms may include vomiting, sleepiness, seizures, and downward pointing of the eyes.

Hydrocephalus can occur due to birth defects (primary) or can develop later in life (secondary). Hydrocephalus can be classified via mechanism into communicating, noncommunicating, ex vacuo, and normal pressure hydrocephalus. Diagnosis is made by physical examination and medical imaging, such as a CT scan.

Hydrocephalus is typically treated through surgery. One option is the placement of a shunt system. A procedure called an endoscopic third ventriculostomy has gained popularity in recent decades, and is an option in certain populations. Outcomes are variable, but many people with shunts live normal lives. However, there are many potential complications, including infection or breakage. There is a high risk of shunt failure in children especially. However, without treatment, permanent disability or death may occur.

Hydrocephalus affects about 0.1–0.6% of newborns. Rates in the developing world may be higher. Normal pressure hydrocephalus affects about 6% of patients over 80. Description of hydrocephalus by Hippocrates dates back more than 2,000 years. The word hydrocephalus is from the Greek ὕδωρ, hydōr, meaning 'water' and κεφαλή, kephalē, meaning 'head'.

The clinical presentation of hydrocephalus varies with age as well as chronicity.

Hydrocephalus is difficult to detect clinically before delivery, although enlarged ventricles can be spotted on ultrasonography as early as 18–20 weeks gestation. Since infants' skulls are not fully fused together at the cranial sutures yet, they have soft spots on their skulls known as open fontanelles. This anatomic characteristic means that infants' skulls can visibly grow in size when cerebrospinal fluid accumulates. Therefore, infants with hydrocephalus may present with an enlarged skull (or rapid growth in skull size), bulging fontanelles, or separated cranial sutures. Parents or physicians may also note that the infant is more irritable or tired than normal. Other symptoms include seizures, inability to look upwards ("sunset eyes" or "setting sun" sign), and pauses in breathing. Infants may also present with lack of weight gain or failure to meet motor and developmental milestones. Imaging can be done to confirm the suspected diagnosis of hydrocephalus. In infants, the open fontanelles allow for use of head ultrasonography. This allows pediatricians to minimize radiation exposure and come up with a diagnosis quickly. If further information is needed, an MRI can be done.

Of note, hydrocephalus in infants can occur as part of a syndrome, and therefore patients may present with other characteristic symptoms. An example of one such syndrome is the Walker-Warburg syndrome, in which patients may also have holoprosencephaly and several other cranial defects. The VACTERL disorders (vertebral anomalies, anal atresia, cardiac defects, tracheoesophageal fistula, renal and radial anomalies, and limb defects) may also include hydrocephalus, and can then be called VACTERL-H. Hydrocephalus can also occur as part of neurocutaneous disorders such as neurofibromatosis type I (NF I) and tuberous sclerosis. Lastly, patients with trisomy disorders (trisomy 9,19p, 13, 18, and 21) as well as triploidy have been noted to have hydrocephalus.

In older children, the fontanelles are closed, so there is no visible change in head size. Since there is limited expansion of the skull, symptoms are more representative of the effects of increased intracranial pressure on a child's developing brain. The most common presenting features in this age group are memory and concentration issues as well as motor and gait abnormalities. Nausea, vomiting, and a tremor of the arms and legs are also common features in older children. Patients may also have papilledema (swelling of the optic disc), worsening vision, and difficulty looking upwards on examination. A key feature in this age group includes headaches, due to the intracranial hypertension caused by the increased CSF in the closed space of the skull. These headaches tend to occur early in the morning as patients have been in a horizontal position throughout the night, which increases ICP.