- Clinical science
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 to the slow CSF accumulation by ventricular dilation. This ventricular distension 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 since 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).
- Prevalence: communicating hydrocephalus > noncommunicating hydrocephalus
- Congenital hydrocephalus (∼ 60% of cases): 3/1.000 live births in the US 
- Acquired hydrocephalus (∼ 40% of cases): affects all ages
- NPH, a common form of acquired hydrocephalus, primarily affects individuals > 60 years.
- Sex: ♂ = ♀
Epidemiological data refers to the US, unless otherwise specified.
↓ CSF absorption
- Inflammatory diseases of the central nervous system (CNS) → inflamed arachnoid villi
- Subarachnoidal or intraventricular hemorrhage → inflammatory response → fibrosis
- Congenital absence of arachnoid villi
↑ CSF production
- Choroid plexus papilloma or carcinoma
- Inflammation of the choroid plexus 
Obstruction of the obstructed passage of CSF from the ventricles to the subarachnoidal space., the , or the results in
Dandy-Walker malformation: A 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.
- Causes a variety of neurologic abnormalities (e.g., ataxic gait) and hydrocephalus
- Associated with a variety of extracranial abnormalities (e.g., craniofacial abnormalities, cardiac defects)
- Intrauterine infections: e.g., congenital toxoplasmosis (see )
- Features of increased ICP
- Additional features in infants
- Lower extremity spasticity, hyperreflexia
- Changes in vital signs resulting from brainstem compression due to herniation
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!
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.
Etiology and pathogenesis
- Idiopathic (most common in adults > 60 years)
- Possible secondary causes that result in obstruction and/or fibrosis of subarachnoid villi
- ↓ CSF absorption
- Secondary: impaired CSF absorption caused by hemorrhage and/or inflammation and subsequent fibrosis of the subarachnoid villi
- Urinary incontinence
- Frequent falls, broad-based gait with short steps (ataxic gait)
Patients present with the classic triad of wet, wacky, and wobbly.
- Rule out other causes of symptoms
MRI (initial test), CT
- Ventriculomegaly without sulcal enlargement
- Periventricular hypodensity due to periventricular edema
- CSF tap test: confirmatory test
MRI (preferred for children) or CT
- Indication: older infants (when fontanelle is already closed, typically patients > 6 months of age) or adults
- Features of acute hydrocephalus:
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.
Etiology and pathogenesis
Loss of brain tissue results in the expansion of the subarachnoid space filled with CSF. The ventricles appear dilated as well, with an apparent increase in CSF because of reduced brain tissue. However, there is no increase in CSF production, decreased CSF absorption, or obstruction.
- Primary cerebral atrophy
- Cerebral destructive lesions or degeneration e.g.,
- Usually asymptomatic
- Symptoms of the underlying condition (see “Etiology and pathogenesis” above)
- Resembles hydrocephalus on imaging (i.e., enlarged CSF spaces, especially lateral ventricles)
- Cortical atrophy may be prominent
- ICP is normal
The differential diagnoses listed here are not exhaustive.
Most types of hydrocephalus are progressive and present 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).
- Inflow catheter
- Adjustable one-way pressure valves
- Outflow catheter
- Underdrainage → ↑ ICP
- Symptoms: see
- Shunt infection (∼ 5% of cases)
- Bacterial contamination (e.g., Staphylococcus epidermidis), often associated with biofilm formation
- Therapy: shunt explantation surgery
Slit ventricle syndrome
- Chronic intermittent headache
- Altered mental status
- Cranial neuropathies
- Pathophysiology: 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.
Possible interim therapy