Hydrocephalus

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.

• 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)

• Features of increased ICP
  • Headache, nausea, and vomiting
  • Papilledema
  • Abnormal gait
  • Impaired consciousness
  • Cushing triad (irregular breathing, widening pulse pressure, bradycardia)
  • Abducens nerve palsy
• Additional 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.

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, apraxic gait) and normal or episodic increase in ICP
• Etiology
  • Idiopathic (most common in adults > 60 years)
  • 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: ↓ CSF absorption → CSF accumulation → enlargement of the ventricle
• Clinical features: a classic triad
  • Gait abnormalities: frequent falls, broad-based gait with short steps (gait apraxia, also known as magnetic gait)
  • Urinary incontinence ^[3]
    • 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. ^[4]
• Diagnostics
  • MRI (initial test), CT
    • Ventriculomegaly without sulcal enlargement
    • Periventricular hypointensity (on CT) or hyperintensity (on T2 MRI sequence) due to periventricular edema
  • CSF tap test: confirmatory test
    • Note severity of symptoms before the test.
    • Opening pressure is normal or slightly elevated.
    • Remove a small amount of CSF fluid (30–50 mL).
    • Improvement of symptoms after CSF removal via lumbar puncture or shunt confirms NPH.
    • Lumbar puncture is both diagnostic and therapeutic.
  • Rule out other causes of symptoms
    • Cognitive assessment
    • Vitamin B12 level and thyroid function tests

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).

• Ultrasonography
  • Indication: clinical suspicion during antenatal period or in infants < 6 months of age (through the anterior fontanelle when it is still open)
  • Findings: enlarged lateral ventricles
• MRI or CT
  • Indication
    • Older infants: when fontanelle is already closed, typically patients > 6 months of age (MRI is preferred to CT)
    • Adults
  • Features of acute hydrocephalus
    • Enlarged ventricles
      • Communicating hydrocephalus: all ventricles are dilated
      • Noncommunicating hydrocephalus: ventricles upstream of the obstruction are dilated
    • 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.
• Etiology
  • Primary cerebral atrophy
  • Cerebral destructive lesions or degeneration
    • Alzheimer disease
    • Huntington disease
    • Pick disease
    • 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. ^[5]
• 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.

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).

Cerebral 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 ^[6]
  • Inflow catheter
  • Adjustable one-way pressure valves
  • Outflow catheter
  • Reservoir
• Complications ^[6]
  • Shunt infection
    • Bacterial contamination (e.g., Staphylococcus epidermidis), often associated with biofilm formation
    • Therapy: shunt explantation surgery
  • Shunt malfunction (partial or complete blockage of the shunt)
  • Underdrainage (CSF not removed quickly enough) → ↑ ICP
  • Overdrainage (drainage of too much CSF): See ”Intracranial hypotension syndrome.”
  • Slit ventricle syndrome: a condition due to chronic overdrainage of CSF
    • 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. ^[7]
    • Clinical features
      • Chronic intermittent headache
      • Nausea/vomiting
      • Altered mental status
      • Cranial neuropathies

Possible interim therapy

• Diuretics
• Fibrinolysis
• Serial lumbar punctures
• Acetazolamide

Alternative surgical procedures

• Endoscopic third ventriculostomy
• Choroid plexectomy (or coagulation)
• 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.