Hydrocephalus

Hydrocephalus is defined as a disproportionate increase in the amount of CSF within the cranium, usually in association with a rise in ICP

Physiology and circulation of cerebrospinal fluid

The normal volume of circulating CSF is in the region of 140 ml. The fluid both protects and supports the brain and spinal cord, as well as maintaining homeostasis by acting as a transport medium for transmitters and as a method of removing the end products of metabolism. CSF is produced by an active process, 80 per cent of it being derived from the choroid plexus and the rest from the parenchyma. The rate of production is between 0.2 and 0.4 ml/minute, with a daily production rate of approximately 480 ml. This means that the turnover of CSF is approximately three times daily. Production of CSF is regulated, not only by the homeostatic environment, but also neurogenically and in response to alterations in CSF pressure. Resorption of CSF is almost entirely pressure dependent as a result of a hydrostatic gradient existing between the CSF in the subarachnoid space and the arachnoid villi, the point of reabsorption of the CSF into the venous system. There is also some absorption of CSF via the sleeves of the nerve roots.

Most fluid is produced in the lateral ventricles. Normal flow is then down through the foramina of Monro into the third ventricle and subsequently the aqueduct of Sylvius into the fourth ventricle, to pass laterally and inferiorly out of the fourth ventricle via the foramina of Luschka and Magendie, to circulate over the surface of the cortex for reabsorption at the arachnoid villi (Fig. 35.19).

Hydrocephalus means an imbalance between the ratio of the CSF to cerebral tissue within the cranium. The first condition to be excluded immediately from a physiological point of view is hydrocephalus exvacuo. This occurs when the ratio is altered as a result of atrophy of the cerebrum with an increase in CSF, purely as a compensatory mechanism. This condition does not have treatment implications, other than being part of the differential diagnosis in a patient with suspected hydrocephalus (Fig. 35.20).

Aetiology

In those patients with the pathophysiological condition of hydrocephalus, an imbalance has occurred between the nor­mal physiological production of CSF and its absorption. This imbalance can be as a result of overproduction of CSF or impaired absorption. Conditions in which CSF is overproduc­ed are uncommon. Typically the choroid plexus papilloma is cited as the most common cause of overproduction and, in most cases, there is no doubt that this does occur. However, in these cases there are compounding problems, such as obstruction of CSF flow, haemorrhage and change in the protein level of the CSF, which may exacerbate the hydrocephalus. As regards conditions where CSF resorption is impaired, there is a rare congenital condition wherein there is a congenital absence of the arachnoid villi. Failure of absorp­tion is usually as a result of alterations in the hydrostatic gradient responsible for CSF absorption or failure of the CSF to circulate adequately to allow absorption to take place.

There is a further concept of obstructive or communicating hydrocephalus. Obstructive hydrocephalus is seen where the normal pathways of CSF flow are for some reason occluded. This may be as a result of conditions such as aqueduct stenosis or as a result of local compression from a tumour. In communicating hydrocephalus no obvious obstruction to CSF flow can be observed and all of the ventricles appear to be communicating freely. In fact, these so-called cases of communicating hydrocephalus usually do have some obstructive element underlying them, the level usually being in the basal system, the subarachnoid space or at the arachnoid villi.

Hydrocephalus may be congential and occur in conjunction with other abnormalities of the central nervous system, such as spina bifida, as a result of congenital aqueduct stenosis or as a result of intrauterine infections. Hydro­cephalus acquired postnataly is commonly secondary either to intraventricular and intraparenchymal haemorrhage or to meningitis.

Clinical features

The presenting signs and symptoms related to hydrocephalus are very much dependent upon the age of the patient at presentation. In the neonatal period an increasing head circumference, tense fontanelle and failure to thrive may be the only initial signs, although feeding problems and ‘sunsetting’ (early down-turning of the eyes) associated with bradycardias may become apparent in the extreme cases (Fig. 35.21).

In older children and adults, hydrocephalus may be manifest principally by gradual development of symptoms of raised ICP, so that headache, nausea and vomiting occur, ultimately followed by a deterioration in the level of consciousness. There may also be associated ataxia and visual disturbance. With increasing age, hydrocephalus secondary to turnouts becomes increasingly common and therefore in the older age group the symptoms of hydrocephalus may be combined with those symptoms attributable to the neoplasm itself.

Investigation

Records of the head circumference and its comparison with body weight and length are an integral part of the postnatal follow-up of any child. While this is an essentially somewhat crude method of determining the onset of hydrocephalus, it is nonetheless an easy and noninvasive sequential investi­gation with an excellent rate of diagnosis. On clinical examination, disproportion of the head to the rest of the body may immediately be evident and palpation of the head will reveal a tense fontanelle and separation of the sutures. Percussion of the head may produce the so-called ‘crack-pot sign’, while in severe cases it may be possible to transilluminate the head.

In older children and adults, the effects of chronic raised intracranial pressure may be evident on a skull radiograph with separation of the sutures and ‘copper beating’ of the skull, as well as erosion of the pituitary fossa (Fig. 35.22).

When the anterior fontanelle is patent it is possible to carry out ultrasonography to visualise the ventricular system, and this is also the way in which hydrocephalus is picked up antenatally. In older or younger children where further infor­mation is required, CT may be performed to aid in diagnosis. If a tumour is suspected then both enhanced and unenhanced tomography should be performed. The use of MRI to follow patients with hydrocephalus is attractive as no radiation dose is involved. However, accessibility, cost and time preclude this from being a routine investigation. MRI may be required when a tumour is seen to determine the surgical strategy, and when aqueduct stenosis occurs, to rule out a tectal plate tumour.

Management

Medical

Treatment of hydrocephalus is primarily directed towards methods of reducing CSF production. This can be achieved

by using acetazolamide, which is a carbonic anhydrase inhibitor and may reduce CSF production by as much as 60 per cent. Frusemide also has an effect on CSF production and both drugs, therefore, may be used in the short term. In the long term the effect of these medications appears to be relatively limited and therefore surgical intervention is required.

Surgical

Where an obstruction to the flow of CSF is present, removal of that obstruction, particularly if it is neoplastic in origin, should be the primary goal of surgery. In most patients with obstructive hydrocephalus secondary to tumours, removal of the tumour will result in resolution of the hydrocephalus.

In others with long-standing hydrocephalus and the chronic changes as a result of this, long-term CSF diversion will be required. Surgical management of the hydrocephalus may be directed towards reducing CSF production, bypassing a blockage to normal CSF flow drainage of CSF externally or finally drainage of CSF in another absorptive viscus. While obliteration of the choroid plexus was first described by Dandy it has failed for several reasons to act as a cure for hydrocephalus. First, the surgical procedure is directed towards the choroid plexus at the lateral ventricles and, as described above, 20 per cent of the CSF is produced by the nonchoroidal surface of the ventricles. Ablation of the intra­ventricular choroid plexus possibly has a role in the reduction in incidence of shunt obstructions. Bypassing obstruction to CSF flow may be achieved by a variety of means, such as cannulation of the aqueduct of Sylvius or third ventriculostomy, which may be performed endoscopically or at open operation. The first described bypass technique was that described by Torkildsen, where aqueduct stenosis was overcome by passing a catheter from the lateral ventricle into the cisterna magna, thereby reconstituting the normal circulatory pathway. External drainage of CSF may help in the temporary management of acute hydrocephalus but the risk of infection precludes this as a means of long term management.

The final and most common form of surgical management of hydrocephalus is by an internal diversion. Methods of CSF diversion were first tried in the nineteenth century when attempts were made to divert the CSF by the use of silver wires into the lumbar vertebral bodies. The modern era of internal CSF diversion began in the 1950s when implantable devices with regulatory valves were developed (Fig. 35.23). the proximal site of CSF diversion is usually the lateral ventricle and, while many distal sites have been attempted, he current favoured distal site of CSF absorption is the peritoneum. Two main complications can occur following shunt insertion. The first is infection and the second is shunt malfunction. Infection occurs in about 5 per cent of adult cases and approximately 10 per cent of paediatric cases depending on age mix, as the rate of infection in neonates is much higher than that in older children.

Manifestations of infection usually become apparent within the first few weeks or months following implantation, and treatment of choice is to remove the infected system and treat the patient with intrathecal and intravenous antibiotics. Because infection is often with low-grade organisms, infec­tion may take some time to diagnose. It is for this reason that the previously favoured distal shunting site at the right atrium was abandoned, as chronic bacteraemia was asso­ciated with the development of immune complexes leading to renal and pulmonary damage.

     Shunt obstruction may occur at any time following shunt insertion and may be due to either ventricular catheter obstruction, valve malfunction or a distal obstruction at the peritoneal catheter. Regrettably, no ideal valve mechanism for CSF diversion has been developed and together with occlusion of the system, the system may also malfunction by overdraining. This is a result of the siphoning effect of the catheter into the peritoneal cavity and can lead to symptoms of headache as a result of the low intracranial pressure and the appearances on tomography of small, slitlike ventricles