Hydrocephalus*

The following article has been written by a neurosurgeon for the General Practice/Family Physician community, and for nursing and paramedical staff and medical students.

Article outline:

Introduction

What is hydrocephalus and how common is it?

• Hydrocephalus or "water on the brain", refers to a relative increase in the amount of cerebrospinal fluid (CSF) in the brain. This can cause a rise in pressure in the brain referred to as raised intracranial pressure (ICP). For the purposes of this article, the definition of hydrocephalus is increased cerebrospinal fluid and pressure in the brain.
• In Western populations, the prevalence or frequency of hydrocephalus at birth is approximately 1 in 1200 live births (< 0.1%). The prevalence of hydrocephalus in the community is higher than 0.1% because many cases of hydrocephalus are acquired some time after birth (see below).

What is cerebrospinal fluid and how does it circulate in the brain?

• Cerebrospinal fluid (CSF) is a thin and normally clear tear-like liquid containing ions (especially sodium, potassium and chloride) and glucose, a few white blood cells (i.e., a few lymphocytes but normally no polymorphs or blood cells), and some other molecules such as beta-2 transferrin.
• CSF is formed by the choroid plexus (see Image 1, below), a fibrovascular structure located in certain portions of the fluid-filled cavities of the brain known as ventricles. CSF circluates throughout the ventricles, and exits the brain at its base, and then circulates over the surfaces of the brain hemispheres and also into and around the spinal cord. Around the brain and spinal cord, the circulating CSF is located in the normally water-tight "subarachnoid space". CSF is absorbed by "arachnoid granulations" (see diagram, below) found near the midline and top region of the brain (i.e., near the superior sagittal sinus). There is a gentle pressure gradient here that drives CSF from the subarachnoid space one way across the arachnoid villi of the arachoid granulations, the fluid then draining into the superior sagittal sinus as part of the normal venous drainage of the brain.

Image 1 (above): CSF Circulation. SSS = superior sagittal sinus. AG = arachnoid granulation. LV = lateral ventricle. CP = choroid plexus. IIIrd = 3rd ventricle. CAqS = Cerebral aqueduct of Sylvius. IVth = 4th ventricle. SC = Spinal cord. See above text for details.

• CSF is produced and absorbed at a rate of approximately 20 mL per hour (about one-half of a liter per day) in an adult. The volume of CSF in the central nervous system is approximately 150 mL, with only a small amount contained within normal-sized ventricles (total of 25-30 mL). The pressure of brain CSF (i.e., normal intracranial pressure) is approximately 10-15 cm of water (or 10 mm of mercury) in adults.
• Note: Cerebrospinal fluid circulation can be impaired anywhere between its points of production and absorption. Microscopic debris and adhesions/scars (often due to protein and cellular breakdown products following brain infection, inflammation or hemorrhage, sometimes tumour cells) can clog up/choke the arachnoid granulations and lead to hydrocephalus. A brain plumbing problem.

How is hydrocephalus classified and what causes it?

Hydrocephalus is classified as:

• "Congenital" (i.e., present at the time of birth) versus "acquired" (i.e., diagnosed some time after birth, i.e., maybe years or decades later).
• "Communicating" (i.e., where CSF can circulate freely in and around the brain; see Image 2) versus "non-communicating" (i.e., where CSF cannot circulate freely in and around the brain, due to some physical "obstruction" - like a blocked pipe; see Image 3). Note that non-communicating hydrocephalus is also referred to as obstructive hydrocephalus.

Image 2 (above): Communicating hydrocephalus. Left image (CT scan, axial w contrast): Moderate communicating hydrocephalus. LVFH = lateral ventricle's frontal horn. LVOH = lateral ventricle's occipital horn. Right image (MRI scan, axial T2): Mild communicating hydrocephalus. LV = lateral ventricle. FOM = foramen of Monro. III = 3rd ventricle. See text for details.

Image 3 (above): Non-communicating (obstructive) hydrocephalus. In this case, the obstruction to CSF circulation causing hydrocephalus is due to the presence of a large posterior fossa (hindbrain) tumor. Left image (MRI scan, sagittal T1 w contrast): Tu = tumor. LV = lateral ventricle. Right image (CT scan, axial wo contrast): LV = lateral ventricle. III = 3rd ventricle. Tumor (a choroid plexus papilloma) in red circle. See text for details.

The congenital causes of hydrocephalus include:

• Intraventricular hemorrhage (IVH) during development of the fetus - hemorrhage from the foetal brain's "germinal matrix" layer which gives rise to the brain's neurons and support cells (glia). Intraventricular hemorrhage is more prevlaent in premature babies.
• Congenital acqueductal stenosis (CAS) from abnormal development and further narrowing (or blockage) of the already normally narrow "cerebral aqueduct of Sylvius" (see CSF circulation, Image 1, above). Developmental aberrations here include bifurcation (forking), gliosis (scarring), or septation (internal walls). There may be a genetic basis to this particular condition, namely, x-linked recessive.
• Dandy-Walker malformation (DWM) which is a developmental abnormality of the fourth ventricle (see CSF circulation, Image 1, above) where its fluid outlets are poorly developed or missing (atretic). Medically, the specific abnormalities here include: atresia of the foramina of Magendie and Luschka, agenesis of the cerebellar vermis, and a large posterior fossa cyst that communicates with the fourth ventricle. Hydrocephalus is present in 90% of Dandy-Walkfer malformation patients. In Dandy-Walker syndrome (DWS), the child has the Dandy-Walker malformation in addition to other brain developmental abnormalities (e.g., agenesis of the corpus callosum, occipital encephalocoele) and abnormalities of other body regions such as the facial skeleton, eyes, and heart.
• Arnold Chiari Type 2 (AC 2) malformation which involves a number of hindbrain (cerebellar tonsils and vermis; brainstem medulla and fourth ventricle) and lower spinal (spina bifida / myelomeningocoele) structural abnormalities, plus hydrocephalus.

The acquired causes of hydrocephalus include:

Clinical Presentation and Evaluation of Hydrocephalus

What are the symptoms and signs of hydrocephalus?

• In infants, there may be a disproportionate increase in the size of the head (specifically, the occipito-frontal circumference/OFC increases out of proportion to the area of the facial skeleton) and a firm or bulging anterior fontanelle. Additionally, an infant with evolving hydrocephalus may be quite irritable, feed poorly, and exhibit vomiting.
• In older children and adults, there is headache, nausea, vomiting, and impairment of vision [double vision/diplopia from sixth cranial nerve (abducens) palsy, impaired upward gaze, and/or papilledema with blurred optic disc margins during retinal examination/fundoscopy], and dysfunction of gait and balance (ataxia).
• In patients who have known hydrocephalus that has already been treated by placement of a ventriculoperitoneal shunt (see below), there may be symptoms and signs due to recurrent hydrocephalus or meningitis from shunt dysfunction (blockage/breakage/infection). These can include one or more of the above symptoms and signs, plus fever, neck stiffness, redness along the shunt tracts, and/or abdominal pain.

Differential Diagnosis of Communicating and Non-communicating hydrocephalus

i.e., what conditions can mimic communicating and non-communicating hydrocephalus? (i.e., but are not "true" hydrocephalus as defined in this article; see top of page)

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Treatment of Hydrocephalus

Shunts (e.g., ventriculoperitoneal/VP shunt)

• A shunt is a piece of soft tubing that runs entirely under the skin surface from the ventricle or from the lumbar cistern to some other part of the body, most commonly the abdomen, but sometimes the lung (ventriculopleural shunt) or the neck (ventriculoatrial shunt).
• Shunts allow CSF to be continually diverted in patients who cannot absorb enough CSF for whatever reason. Shunts may be placed in patients who: (1) Are born with hydrocephalus; (2) develop hydrocephalus later in life following, say, brain aneurysm rupture, growth of an obstructive brain tumor, or some form of inflammation of the brain tissues such as in meningitis or ventriculitis; (3) cannot be successfully "weaned" from an external ventricular drain (EVD) and/or lumbar drain; (4) have certain diseases such as NPH or BIHT (see below).
• Shunt placement is carried out in the operating room with the patient asleep. If neuroavigation ("GPS for the brain") is used to assist the accuracy of shunt placement, the procedure is referred to as stereotactic shunting.
• For the most common of shunt configurations referred to as the ventriculoperitoneal (VP) shunt, the shunt tubing runs from the ventricle to the abdomen’s peritoneal space or peritoneum. The tubing runs in a fatty tunnel between the two locations. There will be a small, curved skin incision somewhere in the scalp overlying a small burr hole drilled in the adjacent skull bone, a second tiny straight incision somewhere behind the ear, and a slightly longer third straight incision somewhere in the abdomen. The tubing is tunneled by the surgeon between these three incisions (see Image 5, below). There is typically a bump felt near the scalp incision, which arises from placement of a shunt valve, a highly specialised, small and delicate piece of technology that regulates the flow of CSF. This should be expected to be felt in this location. The tubing can also be felt, but is otherwise invisible to the naked eye. The tubing and the valve are left in permanently unless a need for their removal arises, say, if the system becomes infected or blocked.

Image 5 (above): Surgical diversion of CSF in patients with hydrocephalus. Left image: Ventriculoperitoneal (VP) shunt. Right image: Endoscopic third ventriculostomy (ETV). See text for details.

Endoscopic Third Ventriculostomy (ETV)

This is an alternative to ventricular shunt placement, and involves no permanent tubing being left behind in the patient. A small telescopic instrument (neuro-endoscope) is advanced by the neurosurgeon via a small coin-sized surgical opening in the skull (burr hole) into the frontal horn of the lateral ventricle (see Image 5, above). Through a natural opening in this region called the foramen of Monro, the telescope is advanced towards and then a small way through the floor of the third ventricle. The hole made here allows CSF to circulate via an alternate pathway if there is blockage downstream (e.g. at the aqueduct of Sylvius or in the fourth ventricle). In some patients, this hole can seal itself over again, thereby requiring shunt placement.

Special Conditions

This is a "diagnosis of exclusion" - i.e., when all other things are ruled out as the cause of the progressive headaches.
The key thing to look for is thrombosis of the intracranial venous sinuses.
Benign intracranial hypertension (BIH) is also known as pseudotumor cerebri because of its typical association with increased intracranial (brain) pressure.
The classic presentation of this condition is an overweight female in her forties, presenting with (i) progressive, often daily headaches with eye region/orbital pressure-discomfort, and (ii) impaired visual field from papilledema (the latter of which can lead to permanent blindness if the condition is unrecognised and untreated).
Lumbar puncture reveals an elevated opening pressure (often > 25 cm of water) and examination of the retina/fundoscopy reveals papilledema. Brain CT scanning might be normal or it may show narrowed ventricle size (NOT HYDROCEPHALUS), and brain MRI scanning may pick up a thrombosed venous sinus.
Treatment may be by a combination of one or more of the following: weight loss, fluid and salt restriction, diuretic (Diamox / acetazolamide which reduces CSF production by inhibiting the enzyme carbonic anhydrase); repeated lumbar punctures; stereotactic ventriculoperitoneal shunting (see above); optic sheath fenestration; worst case scenario: the above, plus bi-cranial decompression. If there is sinus thrombosis diagnosed during the work-up, investigation and treatment of that also needs to occur (beyond the scope of this article).

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