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,
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).
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.