Hydrocephalus

Hydrocephalus is 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 is caused by decreased CSF resorption or increased CSF production in the absence of CSF flow obstruction, and noncommunicating hydrocephalus, which is caused by the obstructed passage of CSF from the ventricles to the subarachnoid space. Both forms typically manifest with signs of elevated intracranial pressure (ICP). Other clinical manifestations may include changes in vital signs resulting from brainstem compression and, in congenital hydrocephalus, macrocephaly. Normal pressure hydrocephalus (NPH) is a gradual-onset chronic form of communicating hydrocephalus that primarily occurs in adults > 60 years of age. ICP is normal or mildly elevated; manifestations include a classic triad of gait apraxia, dementia, and urinary incontinence. All types of hydrocephalus are diagnosed using MRI brain or CT head; ultrasound is used in children < 18 months of age with an open anterior fontanelle. A lumbar puncture may be required (once intracranial lesions with mass effect have been ruled out) to measure opening pressure, obtain CSF samples for analysis, and, if NPH is suspected, perform a CSF tap test. Temporizing measures to reduce CSF volume, such as external ventricular drain placement or serial lumbar punctures, may be used in the acute setting. Definitive treatment often involves surgical insertion of a shunt that drains excess CSF into another area of the body. Hydrocephalus ex vacuo is the enlargement of the ventricles and subarachnoid space due to loss of brain tissue (e.g., cerebral atrophy). It is not considered a true hydrocephalus because ventricular enlargement does not result from CSF accumulation and it does not affect ICP or CSF flow.

• Hydrocephalus is a condition in which CSF accumulates within the cerebral ventricles, causing their enlargement.
• In hydrocephalus, ICP can be elevated or normal.

• Prevalence: communicating hydrocephalus is more common than noncommunicating hydrocephalus
• Age ^[1]
  • 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.

Communicating hydrocephalus

Dysfunction of subarachnoid cisterns or arachnoid villi resulting in decreased CSF absorption or increased CSF production.

• ↓ CSF absorption
  • Inflammatory diseases of the central nervous system (e.g., meningitis) → inflamed arachnoid villi → scarring and obliteration of arachnoid villi
  • Subarachnoidal or intraventricular hemorrhage → inflammatory response → fibrosis → obliteration of arachnoid villi
  • Congenital absence of arachnoid villi
• ↑ CSF production
  • Choroid plexus papilloma
    • Rare benign tumor of the choroid plexus
    • Typically manifests with headache (and other symptoms of intracranial hypertension) due to hydrocephalus secondary to cerebrospinal fluid overproduction
  • Choroid plexus carcinoma
  • Inflammation of the choroid plexus ^[2]

Noncommunicating hydrocephalus (obstructive hydrocephalus)

Obstruction of the cerebral aqueduct of Sylvius, the lateral foramen of Luschka, or the median foramen of Magendie results in obstructed passage of CSF from the ventricles to the subarachnoidal space.

• Congenital: Noncommunicating hydrocephalus is the most common form of congenital hydrocephalus.
  • Arnold-Chiari malformation
  • 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 4^th ventricle) and cerebellar vermis hypoplasia
    • Causes a variety of neurologic abnormalities (e.g., ataxic gait) and noncommunicating hydrocephalus
    • Associated with a variety of extracranial abnormalities (e.g., craniofacial abnormalities, cardiac defects, spina bifida)
  • Intrauterine infections: e.g., congenital toxoplasmosis (see “Congenital TORCH infections”)
  • Colloid cyst obstructing the interventricular foramen
  • Congenital stenosis of the cerebral aqueduct of Sylvius
• Acquired
  • Brain tumor (especially medulloblastomas, pinealoma, ependymomas, and astrocytomas)
  • Other space-occupying lesions, e.g., brain abscesses ^[3]
  • Clots due to hemorrhage

• Features of increased ICP
  • Headache, nausea, and vomiting
  • Papilledema
  • Abnormal gait
  • Impaired consciousness
  • Cushing triad (irregular breathing, widening pulse pressure, bradycardia)
  • Abducens nerve palsy
• Clinical features in infants
  • Macrocephaly: an enlarged head with a circumference greater than the average for age and sex by two standard deviations (as indicated by a percentile growth chart)
  • Tense fontanelle
  • Setting sun sign: persistent downward deviation of the eyes due to increased ICP in infants and young children
  • Developmental delays (e.g., psychomotor delays)
  • Behavioral changes (e.g., irritability)
  • MacEwen sign (cracked pot sign): percussion of the skull near the junction of the frontal, temporal, and parietal bones evokes an unusually resonant sound
    • Caused by separation of cranial sutures due to increased intracranial pressure
    • Found in late hydrocephalus
• Other findings
  • Changes in vital signs resulting from brainstem compression due to herniation
  • Lower extremity spasticity, hyperreflexia

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.

Adults with chronic hydrocephalus may present with insidious onset of nonspecific headaches and visual disturbances without other features of elevated ICP. ^[4]

Normal pressure hydrocephalus

Definition ^[5]

• A form of chronic communicating hydrocephalus that primarily affects adults > 60 years of age
• Characterized by a distinct clinical triad (gait apraxia, dementia, and urinary incontinence) and normal or only mildly elevated ICP

Etiology ^[5][6]

• Idiopathic (iNPH, most common in adults > 60 years of age)
• Possible secondary causes that result in obstruction and/or fibrosis of subarachnoid villi
  • Inflammatory diseases of the CNS (e.g., meningitis)
  • Intraventricular hemorrhage
  • Subarachnoidal hemorrhage

Pathophysiology ^[5][6]

• ↓ CSF absorption → CSF accumulation → enlargement of the ventricle
• ICP: normal or only mildly elevated because ventricular dilation compensates for the slow accumulation of CSF

Clinical features ^[5][6]

A classic triad of:

• Gait abnormalities: frequent falls, broad-based gait with short shuffling steps (gait apraxia, also known as magnetic gait)
• Dementia: short attention span, loss of motivation, disturbances of spatial orientation ^[7]
• 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.

Normal pressure hydrocephalus does not manifest with signs of increased ICP (e.g., headache, papilledema).

Patients present with the classic triad of the 3 Ws: Wet (urinary incontinence), Wacky (dementia), and Wobbly (gait apraxia).

Diagnostics ^[5][8][9]

For additional diagnostics in patients with dementia, see also “Initial studies in dementia.”

• Imaging: MRI brain (preferred) ; , CT head
  • Ventriculomegaly without or out of proportion to sulcal enlargement
  • No evidence of CSF flow obstruction
  • Periventricular white matter changes are often seen.
• CSF tap test: lumbar puncture with measurement of opening pressure, followed by removal of 30–50 mL of CSF
  • Opening pressure is typically normal or mildly elevated: 5–18 mm Hg
  • Gait is assessed immediately before and after the test.
  • Gait improvement following CSF removal supports the diagnosis and can help predict response to cerebral shunt placement.
• CSF analysis: to assess for causes of secondary NPH (e.g., chronic meningitis)

NPH is diagnosed based on typical clinical features, evidence of communicating hydrocephalus on imaging, and normal or only mildly elevated opening pressure. ^[9]

Gait improvement following high-volume CSF removal supports the diagnosis of NPH, but lack of improvement does not rule it out. ^[6]

Differential diagnosis

For differential diagnoses based on the initial clinical presentation, see:

• “Differential diagnoses of ataxia”
• “Causes of dementia”
• “Causes of urinary incontinence”

Patients with NPH may initially present with urinary frequency and urgency, which may lead to an incorrect diagnosis of urinary tract infection. ^[10]

Treatment ^[5][11]

• Refer to neurosurgery for cerebral shunt placement.
• Manage treatable causes of secondary NPH, if identified.
• Initiate supportive care for dementia.

The diagnosis of hydrocephalus is made on imaging. For the initial approach to patients with signs of elevated ICP, see “Diagnostics for elevated ICP.”

Imaging ^[10][12]

• Modalities
  • Antenatal period and children with an open anterior fontanelle (< 18 months of age): ultrasonography
  • Adults and children with a closed anterior fontanelle (≥ 18 months of age): MRI brain; (preferred) , CT head ^[3][12]
• Findings
  • Ventriculomegaly without or out of proportion to sulcal enlargement
    • Communicating hydrocephalus: All ventricles are dilated.
    • Noncommunicating hydrocephalus: Ventricles upstream of the obstruction are dilated.
  • Evans ratio (ratio of largest width of frontal horns to largest biparietal diameter): > 30%
  • Findings of underlying cause, e.g., congenital malformations or tumors

Lumbar puncture ^[3]

Consider the following with specialist guidance if no contraindications to lumbar puncture are present:

• Opening pressure measurement: to estimate ICP
• CSF tap test: if NPH is suspected
• CSF analysis: e.g., to assess for evidence of meningitis or neoplastic seeding

To avoid precipitating brain herniation, consider lumbar puncture only after intracranial lesions with mass effect have been ruled out on imaging. ^[3]

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.
• Etiology
  • Primary cerebral atrophy
  • Cerebral destructive lesions or degeneration
    • Alzheimer disease
    • Huntington disease
    • Frontotemporal dementia
    • AIDS
• Pathophysiology
  • 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. ^[13]
• Clinical features
  • Usually asymptomatic (signs of elevated ICP and NPH triad are absent)
  • Symptoms of the underlying condition (see “Etiology” and “Pathophysiology” above)
• Diagnostics
  • Resembles hydrocephalus on imaging (i.e., enlarged CSF spaces, especially lateral ventricles)
  • Cortical atrophy may be prominent.
  • ICP is normal.

Other differential diagnoses

For differential diagnoses based on the initial clinical presentation, see:

• “Differential diagnoses of headache”
• “Causes of elevated ICP”
• “Causes of altered mental status and coma”

The differential diagnoses listed here are not exhaustive.

General principles ^[10]

• Management must be guided by neurosurgery.
• Temporizing CSF drainage and/or medications may be used in the acute setting for management of elevated ICP.
• Definitive treatment involves surgical intervention for permanent CSF drainage (e.g., through a ventriculoperitoneal shunt).

Initiate treatment promptly, as most types of hydrocephalus are progressive and can result in neurological damage if left untreated.

Temporizing measures ^[10][14]

• Interim drainage procedures: typically used for acute posthemorrhagic hydrocephalus or if infection precludes definitive treatment
  • Serial lumbar punctures or lumbar drain placement: for communicating hydrocephalus
  • Ventricular tap: for preterm infants with intraventricular hemorrhage (performed through the open anterior fontanelle)
  • External ventricular drain: for noncommunicating hydrocephalus (e.g., after subarachnoid hemorrhage)
• Pharmacotherapy: may be considered, although the benefit is likely limited ^[14]
  • Diuretics; (e.g., furosemide, acetazolamide): to reduce CSF production
  • Intraventricular fibrinolytic agents: for acute hydrocephalus following subarachnoid or intraventricular hemorrhage

Definitive treatment ^[10]

Cerebral shunt

• Definition: a definitive treatment for hydrocephalus that diverts excess CSF from the ventricles to a body cavity where it is absorbed into the systemic circulation
• Types
  • Ventriculoperitoneal shunt: most common
  • Others: e.g., ventriculoatrial shunt, ventriculopleural shunt
• Components
  • Inflow catheter
  • Reservoir
  • Shunt valve
  • Outflow catheter
• Complications ^[15]
  • Shunt infection
    • Most commonly with gram-positive bacteria (e.g., Staphylococcus epidermidis)
    • Clinical features include fever, headache, and signs of elevated ICP.
    • Initial diagnostics: CSF analysis with cultures , further septic workup as indicated
    • Management: immediate neurosurgical consult for shunt removal and antibiotic therapy
    • See also “Management of neurosurgical device-related infection.”
  • Shunt obstruction
    • A neurosurgical emergency that manifests with signs of elevated ICP
    • Initial diagnostics: shunt series to confirm shunt placement and integrity, CT head
    • Management: immediate neurosurgical consult for shunt revision
  • Shunt overdrainage or underdrainage
    • Underdrainage: can lead to elevated ICP
    • Overdrainage: can lead to intracranial hypotension syndrome
    • Slit ventricle syndrome: ventricular collapse and shunt obstruction due to chronic CSF overdrainage ^[16]

Shunt infection and obstruction are neurosurgical emergencies.

Other surgical procedures ^[10]

• Endoscopic third ventriculostomy: for hydrocephalus due to obstruction at or distal to the cerebral aqueduct
• Cerebral aqueductoplasty: a procedure in which a stenotic or closed cerebral aqueduct is opened to allow adequate passage of CSF between the third and fourth ventricles
• Excision of obstructive lesion: e.g., posterior fossa tumor