Hydrocephalus

Hydrocephalus

Hydrocephalus is defined as a disproportionate increase in the amount of CSF within the cranuum, 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% of it being derived from the choroid plexus and the rest from the parenchyma. The rate of production is between 0.2- 0.4 ml  min with a daily production

    rate of approximately 480 ml  CSF volume is replaced 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 

Basic science of CSF

• Volume is around 140 ml

• It is replaced about three limes per day

• CSF production and reabsorption is regulated by ICP

• 80% is produced by the choroid plexus

• It is absorbed over the cortex by the arachnoid villi

• Overproduction, failure of resorption or obstruction to its flow will cause hydrocephalus

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, passing laterally and inferiorly out of the foramina of Luschka and Magendie, respectively, to pass over the surface of the cortex for reabsorption at the arachnoid villi 

When diagnosing hydrocephalus, the first condition to be excluded from a pathophysiological point of view is hydro­ cephalus exvacuo, a negative mass effect. 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

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, an obstruction, or impaired of resoption.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 in which CSF resorption is impaired, there is a rare congenital condition where in 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

There is a further concept of obstructive or communicating hydrocephalus. Obstructive hydrocephalus is seen if 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,  obvious obstruction to CSF flow can be observed and all of the ventricles appear to be communicating freely and uniformly enlarged with respect to the basal cisterns and sub­arachnoid spaces. In fact, these so-called cases of communicating hydrocephalus can have some obstructive element underlying them, the level usually being in the basal cisterns, the subarach­noid space or at the level of the arachnoid villi Hydrocephalus may be congenital and occur in conjunction with other abnormalities of the central nervous system (CNS) , such as spina bifida, as a result of congenital aqueduct stenosis or as a result of intrauterine infections. Hydrocephalus acquired post-natally 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

In older children and adults, hydrocephalus may be manifest
principally by gradual development of symptoms of raised lCP 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 tumours 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. Occasionally, hydrocephalus can pre­sent chronically with a subtle, progressive decline with respect to cognition. gait and urinary continence. This occurs more com­ monly following meningitis, subarachnoid haemorrhage or head injuries. Recognition of this condition is important as itis one of the few potentially reversible causes of dementia

Presentation of hydrocephalus

• In neonotes with increasing head circumference, tense fontanelle and failure to thrive

• In children and adults with headeches, nausea, vomiting ond decreasing consciousness

• In the elderly, it is one of the treatable causes of dementia

• In infants it may cause insidious visual failure and developmental delay

Investigation

Records of the head circumference and its comparison with body weight and length are an integral part of the post-natal follow-up of any child. Although this is an essentially somewhat crude method of determining the onset of hydrocephalus, it is nonethe­less an easy and non-invasive sequential ivestigation 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', whereas in severe cases it may be possi­ ble to transilluminate the head due to a thinned cortical mantle.

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 

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, when 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 MRl to follow patients with hydrocephalus is attractive, as no ionising radiation is involved, and has become a routine investigation for hydrocephalus. 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. A lumbar puncture can be performed in cases of communicating hydrocephalus. even in the presence of raised ICP, as both a diag­nostic and therapeutic manoeuvre

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 mayreduce CSF production by as much as 60%. 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, by passing a blockage to normal CSF flow, drainage of CSF externally or finally drainage of CSF in another absorptive viscus  Although 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% of the CSF is produced by the non-choroidal 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 bypass technique was that described by Torkildsen, who overcame aqueduct stenosis by passing a catheter from the lateral ventricle into the cisternal 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 . The proximal site of CSF. diversion is usually the lateral ventricle and, although many distal sites have been attempted, the current favoured distal site of CSF absorption is the peritoneum. Four main complications can occur following shunt insertion, namely (1) infection, (2) haemorrhage, (3) shunt malfunction and overdrainage resulting in low pressure symptoms and (4) possibly chronic subdural haematomas. Infection occurs in about 5% of adult cases and approximately10% 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 of 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, slit-like ventricles

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