Clinical Review - Hydrocephalus

Contributed by Mr Keyoumars Ashkan, consultant neurosurgeon and senior lecturer, King's College Hospital, south east London.

CNS tumours are a major cause of obstructive hydrocephalus
CNS tumours are a major cause of obstructive hydrocephalus

Section 1: Epidemiology and aetiology
The term hydrocephalus arises from the Greek words 'hydro' meaning water and 'cephalus' meaning the head and is commonly known as 'water on the brain'.

It remains one of the oldest recognised neurosurgical conditions and descriptions appear in medical texts of the ancient Egyptians, Greeks and Persians. Today, the incidence of congenital hydrocephalus is reported as 0.5-3.5/1,000 live births.

Communication hydrocephalus with enlargement
of the ventricles

Cerebrospinal fluid dynamics
Hydrocephalus is essentially a disease of cerebrospinal fluid (CSF) dynamics.

Around 500ml of CSF is produced daily, primarily by the choroid plexus within the walls of the ventricular system. The bulk of the CSF is formed within the lateral ventricles, circulating into the third ventricle via the foramina of Monro and then into the fourth ventricle via the aqueduct of Sylvius.

From there, the CSF leaves the ventricular system to enter the cisterna magna and the subarachnoid space through the foramina of Luschka and Magendie. Once circulated, the CSF is predominantly absorbed from the cortical subarachnoid spaces into the venous sinuses via the arachnoid granulations. Therefore, hydrocephalus may be caused by overproduction or under-absorption of the CSF.

In practice, diseases leading to over-production of CSF, such as tumours of the choroid plexus, are extremely rare and the vast majority of hydrocephalus is due to CSF under-absorption.

There are many classification systems for hydrocephalus, such as infantile versus adult onset, congenital versus acquired or acute versus chronic.

From a practical point of view, hydrocephalus is best divided into obstructive and communicating subtypes (see images).

The former is characterised by a physical obstruction within the CSF pathway while the latter is thought to be due to functional impairment of the arachnoid granulations to absorb CSF.

The major causes of obstructive hydrocephalus include CNS tumours, particularly those within the ventricular system, or those within the cerebellum, intraventricular bleeds and congenital conditions, such as aqueduct stenosis or Chiari malformations with or without myelomeningocele.

Communicating hydrocephalus may be a sequel of intracranial haemorrhage, with head injury or meningitis. Normal pressure hydrocephalus (NPH), a chronic disease usually affecting patients in the seventh decade of life and beyond, is also often classified under communicating hydrocephalus.

Given its unique clinical features, prevalence (up to 1.5 per cent) and clinical importance, as a potentially reversible cause of dementia, NPH will be given particular attention in the sections to follow.

Section 2: Making the diagnosis
The presenting features of hydrocephalus largely depend on its rate of development and the underlying aetiology.

The age of the patient also influences the presentation. In the paediatric population, the patient may present with disproportionate head enlargement, tense or full fontanelles, engorgement of the scalp veins, sixth nerve palsy, failure to thrive, irritability or cardiopulmonary instability.

The 'setting sun' sign due to failure of up gaze as a result of pressure on the midbrain quadrigeminal plate is a striking sign in this age group, but is now rare.

In older patients, symptoms of raised intracranial pressure, such as headaches, visual failure, nausea and vomiting, are common. Rapidly progressive hydrocephalus is a neurosurgical emergency.

Patients with NPH present in a more chronic fashion with few symptoms of raised pressure. The classical clinical triad of NPH is progressive deterioration in memory or cognition, gait disturbance and incontinence.

In terms of investigation, serial measurement of the occipito-frontal head circumference (OFHC) is a useful method of assessment and follow up for suspected infantile hydrocephalus.

Disproportionate OFHC, rapidly growing OFHC or that approaching the second standard deviation all need further attention. In such patients and all others with an acute presentation, neuroimaging of the brain is the investigation of choice. Ultrasound scan is a useful non-ionising means of imaging the brain if fontanelles are still open. In older age groups, CT and MRI will provide invaluable information.

The hallmark of hydrocephalus on imaging is enlarged ventricles. In obstructive hydrocephalus the enlargement is confined to the part of the ventricular system which is proximal to the level of the obstruction. Periventricular low density on the scans represents transventricular spread of CSF into the brain parenchyma, indicating acutely raised pressure.

More subtle signs of hydrocephalus on imaging include ballooning of the third ventricle and increased width of the usually slit-shaped temporal horns of the lateral ventricles. Evan's ratio represents an attempt to quantify the imaging definition of hydrocephalus as the ratio of the maximum width of the frontal horns of the lateral ventricles to the biparietal diameter of greater than 30 per cent.

In the absence of significantly increased intracranial pressure, imaging features of NPH are more subtle and may be difficult to distinguish from enlarged ventricles seen in patients with brain atrophy and dementia.

Enlarged ventricles in the presence of preserved cortical gyri may be a useful sign. Nonetheless, before starting treatment, patients with suspected NPH will require specialist investigations including psychometry, gait analysis, uro-dynamic and CSF dynamic analyses.

Section 3: Managing the condition
CSF diversion is one of the most well established techniques in neurosurgery to treat hydro-cephalus. The decision as to which diversion technique to use depends on the type of hydrocephalus, the clinical presentation and the age of the patient.

In many patients with post-haemorrhagic hydrocephalus, the condition may be transient. In such patients, ventricular drainage via an external drain may buy sufficient time until the condition is resolved. In cases of communicating hydrocephalus, serial lumbar punctures may achieve the same results.

In patients with obstructive hydrocephalus secondary to a tumour, excision of the tumour, where possible, may relieve the hydrocephalus and eliminate the need for other means of CSF diversion.

Ventricular shunts
In neonates weighing less than 2kg and requiring permanent CSF diversion, a ventricular catheter connected to a subcutaneous reservoir to allow serial ventricular taps is preferable to a shunt due to the low body weight and the risks of skin erosion and infection.

In older age groups a shunt, a piece of tubing with one end in the ventricular system and the other in a body cavity with sufficient surface area to allow CSF absorption, remains the most widely used treatment modality for permanent hydrocephalus.

Ventriculo-peritoneal, ventriculo-atrial and ventriculo-pleural shunts are the three most common types. Recent advances in shunt technology include antibiotic or silver impregnated shunts to reduce the risk of infection.

Another important development is the programmable shunt system which allows the clinician to adjust the shunt, using a device held externally over the skin on top of the shunt, to determine the amount of the CSF being drained.

This reduces problems associated with over or under-drainage, which may be associated with shunts, and has proved particularly useful in the treatment of patients with NPH.

Endoscopic third ventriculostomy
Despite advances in technology, shunt complications such as infection, disconnection, migration or blockage remain a problem, leading to a search for alternative surgical techniques.

Endoscopic third ventriculostomy (ETV) is one such technique that has gained increasing interest in the recent times.

ETV is a minimally invasive, hardware independent technique, in which a hole is placed endoscopically in the floor of the third ventricle to allow flow of CSF out of the ventricular system and into the basal cisterns and the subarachnoid space.

The technique is particularly useful for treating obstructive hydrocephalus although it has also been used in other forms of hydrocephalus.

In well-selected patients, ETV can avoid the need for a shunt in up to 70 per cent of cases. However, the success rate is lower in the neonatal age group, at around 20 per cent. Complications include haemorrhage and vascular injuries.

Section 4: Prognosis
The prognosis of patients with hydrocephalus depends on the underlying diagnosis and delivery of prompt treatment.

In patients with neonatal post-haemorrhagic hydrocephalus, although survival has improved in the recent decades, epilepsy, motor and cognitive impairments remain problems.

In older patients, rapid diagnosis and treatment have resulted in favourable outcomes with many patients leading a normal life and in active employment.

Similarly, in carefully selected patients with NPH, shunt insertion may lead to significant improvement, particularly in gait and continence, in more than two thirds of patients.

Because of the often chronic nature of the disease and the potential complications, including those of the shunt itself, most neurosurgical units in the UK continue to keep the patients under follow up or operate an open door policy. This is especially important in the paediatric population as the shunt may require elective revision to cope with the child's growth.

The role of the GP remains vital both in the timely diagnosis and referral of patients with acute hydrocephalus or blocked shunts and in supporting the patient and family while they learn about and live with this disease.

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