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 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).

	Pathophysiology	Clinical features	Diagnosis
Communicating hydrocephalus	↑ CSF production ↓ CSF absorption	Typical findings of raised ICP Headache, nausea, and vomiting Papilledema 6th nerve palsy Abnormal gait Impaired consciousness Cushing triad (irregular breathing, widening pulse pressure, bradycardia)	Ultrasonography (in infants < 6 months) MRI or CT (older infants or adults)
Noncommunicating hydrocephalus	Obstructed passage of CSF from the ventricles to the subarachnoidal space
Normal pressure hydrocephalus (NPH)	↓ CSF absorption	Classic triad Wet: urinary incontinence Wacky: dementia Wobbly: frequent falls, broad-based gait with short steps (ataxic gait)	MRI (initial test) CSF tap test (confirmatory test)

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 (CNS) → inflamed arachnoid villi
  • Subarachnoidal or intraventricular hemorrhage → inflammatory response → fibrosis
  • Congenital absence of arachnoid villi
• ↑ CSF production
  • Choroid plexus papilloma: rare benign tumor of the choroid plexus. Typically presents 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

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

• Congenital
  • Arnold-Chiari malformation
  • 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 4^th 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 congenital infections)
• Acquired
  • Brain tumor (especially medulloblastomas, pinealoma, ependymomas, and astrocytomas)
  • Subarachnoid hemorrhage or intraventricular hemorrhage → inflammatory response → fibrosis
  • Inflammation (e.g., following recovery from bacterial meningitis)

• Features of increased ICP
  • Headache, nausea, and vomiting
  • Papilledema
  • Abnormal gait
  • Impaired consciousness
  • Cushing triad (irregular breathing, widening pulse pressure, bradycardia)
  • 6^th 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
• 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!

Normal pressure hydrocephalus

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

Impaired CSF absorption results in CSF accumulation

• 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
• ↓ CSF absorption
• Secondary: impaired CSF absorption caused by hemorrhage and/or inflammation and subsequent fibrosis of the subarachnoid villi

Clinical features

• Classic triad
  • Urinary incontinence
    • Initially, only increased urgency and frequency of micturition , later also urge incontinence.
    • Incontinence worsens as dementia progresses.
    • Gait disturbances can make the way to the toilet more difficult, leading to further incontinence related issues
  • Dementia
  • Frequent falls, broad-based gait with short steps (ataxic gait)

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

Patients present with the classic triad of wet, wacky, and wobbly.

Diagnostics

• Rule out other causes of symptoms
  • Cognitive assessment
  • Vitamin B₁₂ level and thyroid function tests
• MRI (initial test), CT
  • Ventriculomegaly without sulcal enlargement
  • Periventricular hypodensity 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 confirms NPH
  • Lumbar puncture is both diagnostic and therapeutic

• 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 (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:
    • Enlarged ventricles
      • Communicating hydrocephalus: all ventricles are dilated
      • Non-communicating hydrocephalus: ventricles upstream of the obstruction are dilated

Hydrocephalus ex vacuo

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.,
  • Alzheimer disease
  • Huntington disease
  • Pick disease
  • AIDS

Clinical features

• Usually asymptomatic
• Symptoms of the underlying condition (see “Etiology and pathogenesis” above)

Diagnostics

• Resembles hydrocephalus on imaging (i.e., enlarged CSF spaces, especially lateral ventricles)
• Cortical atrophy may be prominent
• ICP is normal

References: ^[3]

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).

Cerebral shunt

• Important components
  • Inflow catheter
  • Adjustable one-way pressure valves
  • Outflow catheter
  • Reservoir
• Complications
  • Underdrainage → ↑ ICP
  • Overdrainage
    • Symptoms: see intracranial hypotension syndrome
  • Shunt infection (∼ 5% of cases)
    • Bacterial contamination (e.g., Staphylococcus epidermidis), often associated with biofilm formation
    • Therapy: shunt explantation surgery
  • Slit ventricle syndrome
    • Symptoms:
      • Chronic intermittent headache
      • Nausea/vomiting
      • 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

Diuretics, fibrinolysis, serial lumbar punctures, (acetazolamide)

References:^[4][5]