Hydrocephalus refers to the abnormal enlargement of cerebral ventricles and/or subarachnoid space as a result of excess cerebrospinal fluid (CSF) accumulation. There are two types of true hydrocephalus: communicating hydrocephalus, which occurs due to decreased CSF absorption or increased CSF production in absence of any CSF-flow obstruction, and noncommunicating hydrocephalus, which occurs due to the obstructed passage of CSF from the ventricles to the subarachnoidal space. Both forms cause elevated intracranial pressure (ICP), which leads to headache, nausea, and/or vomiting. Specific clinical manifestations include changes in vital signs resulting from brainstem compression and, in congenital hydrocephalus, macrocephaly. Normal pressure hydrocephalus (NPH) is a chronic form of communicating hydrocephalus that occurs in older individuals (> 60 years of age). NPH occurs due to decreased CSF absorption and manifests with normal ICP because of effective compensation for the slow CSF accumulation through ventricular dilation. This ventricular distention leads to the classic presentation of urinary incontinence, dementia, and ataxic gait. Hydrocephalus ex vacuo is the enlargement of the ventricles and subarachnoid space due to loss of brain tissue (e.g., cerebral atrophy) and the subsequent filling of the void with CSF. It is not considered a true hydrocephalus because ventricular enlargement does not result from CSF accumulation and, accordingly, does not affect intracranial pressure or flow of cerebrospinal fluid. CT or MRI (and ultrasound for infants) are important diagnostic procedures for all types of hydrocephalus. A CSF tap test confirms the diagnosis of NPH. Treatment involves surgical insertion of a shunt, which drains excess CSF into another area of the body (usually the peritoneum).
- Hydrocephalus is a condition in which CSF accumulates within the cerebral ventricles, causing their enlargement.
- In hydrocephalus, ICP can be elevated or normal.
|Overview of hydrocephalus|
|NPH) (|| |
|Hydrocephalus ex vacuo|| || |
- Prevalence: communicating hydrocephalus is more common than noncommunicating hydrocephalus
- Age 
- Sex: ♂ = ♀
Epidemiological data refers to the US, unless otherwise specified.
Etiology and pathophysiology
- ↓ CSF absorption
- ↑ CSF production
Noncommunicating hydrocephalus (obstructive hydrocephalus)
Obstruction of the obstructed passage of CSF from the ventricles to the subarachnoidal space., the , or the results in
Congenital: Noncommunicating hydrocephalus is the most common form of congenital hydrocephalus.
- Dandy-Walker malformation: congenital malformation caused by failure of the fourth ventricle to close, which leads to persistence of Blake's pouch (cyst in the 4th ventricle) and cerebellar vermis hypoplasia
- Intrauterine infections: e.g., congenital toxoplasmosis (see “”)
- Colloid cyst obstructing the interventricular foramen
- Congenital stenosis of the cerebral aqueduct of Sylvius
- Acquired: medulloblastomas, pinealoma, ependymomas, and astrocytomas) (especially
- Features of increased ICP
- Additional features in infants
- Other findings
Because the fontanelles of infants are still open, the accumulation of CSF can lead to macrocephaly; this accommodation offsets the elevation in ICP, meaning that neurological symptoms often develop later than in older patients, whose fontanelles are closed.
Subtypes and variants
Normal pressure hydrocephalus
- Definition: a form of chronic communicating hydrocephalus that primarily affects elderly individuals (> 60 years) and is characterized by a distinct clinical triad (urinary incontinence, dementia, ataxic gait) and normal or episodic increase in ICP
- Pathophysiology: ↓ CSF absorption → CSF accumulation → enlargement of the ventricle
Clinical features: a classic triad
- Gait abnormalities: frequent falls, broad-based gait with short steps (ataxic gait, also known as magnetic gait)
Urinary incontinence 
- Initially, only increased urgency and frequency of micturition; later also urge incontinence
- Dilation of the lateral ventricles results in a distortion of the fibers of the corona radiata, which causes a loss of inhibition of detrusor activity.
- Incontinence worsens as cognitive impairment progresses.
- Gait disturbances can make reaching the toilet more difficult, leading to further incontinence-related issues.
- Dementia: short attention span, loss of motivation, and disturbances of spatial orientation. 
- MRI (initial test), CT
- CSF tap test: confirmatory test
- Rule out other causes of symptoms
MRI or CT
- Features of acute hydrocephalus
- Enlarged ventricles
- Temporal horn dilation > 2 mm
- Mickey Mouse ventricles (enlarged lateral and third ventricle that take on the appearance of Mickey Mouse)
- Sulcal enlargement
- Evans ratio (largest width of frontal horns vs largest biparietal diameter ratio) > 30%
- Upward bowing of the corpus callosum
Hydrocephalus ex vacuo
- Definition: Hydrocephalus ex vacuo is often classified as a form of hydrocephalus, however, this is a misnomer as it is not a true hydrocephalus. The ventricles and subarachnoid space appear enlarged secondary to loss of brain tissue; however, intracranial pressure and flow of cerebrospinal fluid are normal.
- Clinical features
The differential diagnoses listed here are not exhaustive.
Most types of hydrocephalus are progressive and carry a risk of neurological damage. Definitive treatment of hydrocephalus involves the drainage of excess CSF via a cerebral shunt, usually into the peritoneum (e.g., ventriculoperitoneal or VP shunt).
- Definition: a long-term treatment for hydrocephalus intended to return excess cerebrospinal fluid from the ventricle to systemic circulation, most commonly through the peritoneum.
Important components 
- Inflow catheter
- Adjustable one-way pressure valves
- Outflow catheter
- Shunt infection
- Shunt malfunction (partial or complete blockage of the shunt)
- Underdrainage (CSF not removed quickly enough) → ↑ ICP
- Overdrainage (drainage of too much CSF): See ” .”
Slit ventricle syndrome: a condition due to chronic overdrainage of CSF
- The exact mechanisms underlying this syndrome are still debated.
- One hypothesis states that the overdrainage of one ventricle results in the collapse of its walls and occlusion of the shunt catheter. This, in turn, leads to underdrainage of the contralateral ventricle. 
- Clinical features