Hydrocephalus

Hydrocephalus literally means water on the brain (Greek). As a definition, this is non-specific because atrophic dementia results in a passive increase in the volume of CSF in the head and such patients also have large ventricles but do not have hydrocephalus. Rather, hydrocephalus should be defined as an increase in the volume of the CSF within the skull due to an abnormality in its production, circulation, or absorption. This definition embraces those conditions in which there is at some stage an increased volume, and usually pressure, of CSF within the cranial cavity.

Aetiology

Hydrocephalus may be due to:

  • increased formation of CSF;
  • obstruction to the flow of fluid at some point between the choroid plexuses of the lateral ventricles from which it is secreted and the arachnoidal villi in the sagittal sinus through which it is reabsorbed;
  • impaired absorption of the fluid due to inflammation of the arachnoid (as in meningitis) or to thrombosis of the sagittal sinus;
  • normal-pressure hydrocephalus is a clinical syndrome characterized by dementia, gait apraxia, and urinary incontinence.

Increased formation of CSF

Increased formation of CSF occurs with a choroid plexus papilloma (Guthkelch and Riley 1969). In such cases, removal of the tumour is usually curative, but occasionally hydrocephalus persists despite successful removal (McDonald 1969). Another rare syndrome of overproduction of CSF with deficient absorption is due to squamous metaplasia of the arachnoid villi (Davson et al. 1986) but whether it occurs in humans remains undecided.

Obstruction

Obstruction to the CSF circulation may occur at any point of its course. Within the ventricles the most common cause is a neoplasm compressing one or both interventricular foramina or filling the third ventricle. The cerebral aqueduct may be obstructed by a tumour arising in the third ventricle, midbrain, or pineal body, or may be congenitally narrowed or even absent. Owing to its small calibre, slight swelling of its ependymal lining may lead to aqueduct obstruction, and cases have been reported in which hydrocephalus has been due to gliosis caused by ependymitis in this region. Aqueduct stenosis is a cause of infantile hydrocephalus but may give rise to increased intracranial pressure for the first time in adult life (Harrison et al. 1974).

Posterior fossa tumours

Posterior fossa tumours may obstruct the fourth ventricle. Its foramina may be blocked by a congenital septum (the Dandy–Walker syndrome), by adhesions following meningitis, or by displacement of the medulla into the foramen magnum by the pressure of a tumour. The Dandy–Walker syndrome may be due to atresia of the foramina of Magendie and Luschka or to dysplasia of the cerebellum developing early in fetal life, as the cerebellar vermis is often absent or vestigial in such cases (Hart et al. 1972). The malformation may be accompanied by extra-axial leptomeningeal cysts in the posterior fossa (Haller et al. 1971), while such cysts alone may give a similar clinical and radiological picture. Within the subarachnoid space, obstruction may again be due to tumour, to traumatic adhesions, parasitic cysts (Kuper et al. 1958; Allcutt and Coulthard 1991), inflammation, haemorrhage, or to congenital abnormalities such as basilar impression or the Arnold–Chiari malformation.

The Arnold–Chiari malformation

The Arnold–Chiari malformation consists of congenital displacement of the cerebellar tonsils and of an elongated medulla oblongata downwards into the cervical canal (Fig. 17.5; see also Fig. 4.2). It prevents the egress of CSF from the fourth ventricle into the subarachnoid space. It is sometimes associated with lumbosacral spina bifida and with meningocele or meningomyelocele. When the caudal displacement of the cerebellar tonsils, fourth ventricle, and medulla into the cervical canal is associated with myelodysplasia, they may extend down to the mid-cervical region, and this anomaly is known as a Chiari type II. The Chiari type I anomaly is a simple ectasia of the cerebellar tonsils (down to C1) without any other primary malformation of the neuraxis. Congenital narrowing of the cerebral aqueduct sufficient to cause hydrocephalus was found in 10 of 20 such cases. MacFarlane and Maloney (1957) suggested that a Chiari malformation or, less often, the Dandy–Walker syndrome may result in dilatation of the central canal of the spinal cord early in life (hydromyelia) and that this is probably the most common mechanism causing syringomyelia (Gardner 1965).

The arachnoid villi

The arachnoid villi may be obstructed by inflammatory, neoplastic, or leukaemic cells in infective or neoplastic meningitis. Obstruction of the subarachnoid space and the arachnoid villi by blood accounts for the hydrocephalus seen with subarachnoid haemorrhage and head injury. Absorption of fluid from the arachnoid villi may also be restricted by a rise in the intracranial venous pressure, by compression of venous sinuses by an intracranial tumour, or by impairment of venous drainage from the head due to raised intrathoracic pressure in cases of intrathoracic neoplasm or pulmonary hypertension. In general, venous obstruction results in brain swelling with a reduction in the size of the ventricles. Thrombosis of the superior sagittal sinus (Section 27.11), caused by extension of inflammation from the transverse sinus, is one cause of the condition ‘otitic hydrocephalus’, in which symptoms of hydrocephalus complicate otitis media or mastoiditis (Symonds 1937).

Classification

Hypertensive hydrocephalus can be further subdivided into:

  1. Obstructive hydrocephalus (once called internal hydrocephalus, and also known as non-communicating hydrocephalus), in which there is an obstruction to the circulation of the CSF, either within the ventricles or aqueduct, or at the outlet from the fourth ventricle. It prevents free communication between the ventricles and the subarachnoid space.

  2. Communicating hydrocephalus (once called external hydrocephalus, and also known as non-obstructive hydrocephalus), in which hydrocephalus is due either to disturbance in the formation and absorption of CSF, or to an obstruction to its circulation in the subarachnoid space itself.

Congenital abnormalities are the most common cause of obstructive hydrocephalus in autopsy series, especially in neonates and perinatally (Pinar et al. 1998). In 100 consecutive post-mortem examinations, malformation was the sole cause in only 14 per cent of cases, but in association with infection or trauma it accounted for 46 per cent (Laurence 1969). Inflammatory reaction due to infection or haemorrhage but without malformation accounted for another 50 per cent, the remaining 4 per cent being due to tumours.

In the past, a distinction was often made between ‘congenital’ and ‘acquired’ hydrocephalus, but this distinction is artificial. A congenital abnormality alone is the most likely cause of hydrocephalus developing before birth, but congenital and acquired factors often both contribute to hydrocephalus in infancy. Nor do congenital factors cease to operate later, since hydrocephalus developing in adult life may be the late result of aqueduct stenosis or Chiari malformation. The more common causes of hydrocephalus developing in the absence of congenital abnormality are meningeal adhesions following meningitis or haemorrhage, arachnoiditis of obscure origin, thrombosis of intracranial venous sinuses, and intracranial tumour. Chronic inflammatory meningitis or arachnoiditis following tuberculosis are rare causes. Obstruction within the third or fourth ventricle or in the subarachnoid space, may be due occasionally to parasitic cysts.

Incidence

The incidence of all neural malformations, including hydrocephalus, varies considerably between different countries, being much higher, for instance, in Scotland and Ireland than in Japan. In the United States, between 1958 and 1995, 12.4 per cent of all congenital malformations of the central nervous system were due to hydrocephalus (Pinar et al. 1998). The incidence is higher in the east, and especially the north-east, than elsewhere (Kurtzke et al. 1973). In China the incidence was 0.89 per 1000 births (Hu et al. 1996). In Sweden the rate increased from the 1970s to the 1980s, partly due to an increase in ventricular haemorrhage among preterm infants, but declined again in the 1990s (6.99, 25.37, 13.69 per 1000, respectively) (Fernell and Hagberg 1998). The incidence is lower in Eastern Europe (0.44/1000) (Sipek et al. 1998) and higher in the Middle East (Rajab et al. 1998).

Pathophysiology

The rate of at which hydrocephalus develops determines the ventricular size. Acute obstructive hydrocephalus produces high intracranial pressure with only slight ventricular enlargement. Chronic obstruction may produce massive ventricles with only slightly elevated ICP. When obstruction occurs in the aqueduct, only the lateral and third ventricles are distended. When the obstruction is more caudal, the aqueduct and fourth ventricle may also be enlarged. Ventricular distension causes thinning of the cerebral hemispheres which, in severe cases, may be extreme, and is associated with marked atrophy of the white matter and loss of cortical ganglion cells. The ventricular ependyma is normal, except in inflammatory cases, when a localized or diffuse ependymitis may be present. Meningeal adhesions indicate previous meningitis. Distension of the ventricles leads to pressure upon the calvarium, which becomes thin, especially over the cerebral gyri. Separation of the sutures occurs when hydrocephalus develops in early life, but is not seen, as a rule, after adolescence. Compression of the base of the skull causes erosion of the clinoid processes and excavation of the sella turcica. The olfactory tracts and optic nerves are often atrophic.

Symptoms and signs

These are so dependent upon age and the deformability of the skull that it is useful to divide symptoms and signs into infantile and post-infantile. Nevertheless, infants with hydrocephalus may grow up with or without neurological and cognitive impairment, depending on the effectiveness and timeliness of treatment. The features of infantile hydrocephalus may therefore manifest in later life; leading to a degree of overlap when it comes to long-term disability.

Infantile hydrocephalus

Enlargement of the head is the most conspicuous sign in infantile hydrocephalus. It is being diagnosed with increased frequency before birth. The disorder becomes evident during the first few weeks of life owing to the large head, prominent scalp veins, and down-turning of the eyes, ‘sun-setting sign’. It is slowly progressive. If untreated, the head may attain a huge size with a circumference of 75 cm or even more. The cranial sutures are widely separated and the anterior fontanelle is much enlarged. There is marked congestion of scalp veins. Enlargement of the head occurs in all diameters and in extreme cases it is translucent. The frontal region bulges forwards, and downward pressure upon the orbital plates causes the eyes to be protruded forwards and downwards. Fortunately such cases are rare today.

Owing to expansibility of the skull in infancy, symptoms of increased intracranial pressure are slight or absent. Hydrocephalic children seem little troubled by headache and rarely vomit. Convulsions are common. In neglected cases bilateral anosmia may occur.

Optic atrophy due to pressure upon the nerves is usually present, but in rare cases there is papilloedema. Visual acuity may be progressively reduced until the child becomes blind. Paralysis of other cranial nerves may occur, and squint is not uncommon. Nystagmus may be present.

In the limbs there are usually weakness and incoordination, generally more marked in the lower than in the upper limbs. Spasticity with exaggeration of tendon reflexes is common in the lower limbs, although sometimes tendon reflexes are lost. The plantar reflexes are usually extensor. There may be little or no disturbance of sensibility.

The mental state varies. In severe cases there is usually reduced cognitive function and poor memory, but in milder cases this is slight or absent. Intelligence in later life may be surprisingly unimpaired in some cases, even when ventricular dilatation has progressed such that only 1 cm thickness of cerebral substance remains between the ventricles and the inner skull table.

In milder cases there may be obesity and/or diabetes insipidus, due to compression of the hypothalamus and pituitary, and, in more severe cases, wasting. Cerebrospinal fluid rhinorrhoea is a rare complication. A unique hydrocephalic ‘bobble-head doll syndrome’ is characterized by two to four oscillations of the head per minute with psychomotor retardation and results from obstructive lesions in or near the third ventricle or aqueduct (Tomasovic et al. 1975; Menkes 1980).

Hydrocephalus after infancy

The clinical picture of hydrocephalus developing after infancy varies according to its cause. In obstructive hydrocephalus, symptoms of increased intracranial pressure are conspicuous. Headache and vomiting are early symptoms and are often followed by the development of papilloedema. The headache is initially paroxysmal, but later becomes constant; there are sometimes intense exacerbations characterized by severe headache radiating down the neck associated with head retraction and even with opisthotonos, vomiting, and impairment of consciousness. Giddiness is a common symptom. Some mental deterioration usually occurs after a time, especially in later life.

Hallucinations, delusions, and mood changes may occur. Convulsions are less common than in the infantile variety, and enlargement of the head does not occur after the age of 3 years, although slight separation of the sutures may occur until teenage years. In older children this slight separation of the cranial sutures yields a ‘cracked-pot sound’ on percussion and may be associated with venous congestion of the scalp. The skull remains of normal size following onset in adulthood, for instance in delayed presentation of cerebral aqueduct stenosis. Cranial-nerve palsies may occur, especially paresis of the sixth and seventh nerves, and symptomatic trigeminal neuralgia or facial sensory loss have been reported (Maurice Williams and Pilling 1977).

Slight exophthalmos is not uncommon. Gross weakness of the limbs is absent, though clumsiness and slight incoordination are common. The tendon reflexes may be exaggerated or diminished. The plantar reflexes are often extensor. There is usually no sensory loss. Symptoms of hypopituitarism, obesity, and genital atrophy are common in children and adolescents.

Normal-pressure hydrocephalus

A form of late-onset communicating hydrocephalus is called ‘low-pressure hydrocephalus’ since, although the ventricles are dilated, the pressure within them at the time of measurement was either normal or only slightly raised (Hakim and Adams 1965).

The clinical syndrome is characterized by the triad of progressive dementia, gait apraxia, and urinary incontinence. The CT scan has proved helpful but the findings are not invariably conclusive (Jacobs and Kinkel 1976).

Large ventricles and small sulci shown on the CT or MRI, however, are suggestive of this condition and continuous intracranial monitoring has shown significant B waves for at least 2 hours a day (Symon et al. 1972; Crockard et al. 1977). This has been helpful in identifying patients who may benefit from surgery. Thus, while CSF pressure is usually normal or low, transient episodes of raised pressure occur in many cases. With MRI scanning it is possible to measure the volume of the ventricular CSF and to compare it with the volume of CSF in the subarachnoid spaces and fissures (Grant et al. 1987). In a large prospective study, the most reliable predictive feature of a good response to shunting was the absence of white matter changes on MRI scan (Pickard et al. 1992).

The most widely accepted test by neurologists and neurosurgeons is the CSF tap test, where there is clinical improvement following a lumbar puncture (Wikkelso et al. 1982). Of all the tests available, the response to lumbar puncture is thought to be the most reliable (Vanneste and van Acker 1990). Serial psychometric tests are useful in assessing the response to treatment. These, together with a simple walking test, should be documented carefully before and after lumbar puncture (Singh and Crockard 1999).

The condition usually presents in middle or late life, sometimes with dementia alone (Crowell et al. 1973) or with the clinical picture of the parkinsonism–dementia complex (Sypert et al. 1973) and ‘drop’ attacks may occur (Botez et al. 1977). The cause of the communicating hydrocephalus is unexplained in most such cases, although it may follow subarachnoid haemorrhage or may develop many years after recovery from meningitis or head injury. Differential diagnosis from cerebral atrophy in dementia is clearly important, as shunting operations are of no value in the latter condition; however, no single method provides an absolutely reliable distinction.

Radiological diagnosis

Radiographs of the skull in hydrocephalus may show enlargement of the calvarium, with suture diastasis, thinning, and exaggeration of convolutional markings. However, this latter finding alone may be normal and is an unreliable guide to raised intracranial pressure. Separation of the sutures may be present in children. The clinoid processes are often eroded and the sella turcica is deepened and expanded anteroposteriorly. CT and MR imaging give a clear picture of the ventricular size, and often the underlying cause is identified (Fig. 17.7 and Fig. 17.8). MRI is superior to CT in identifying posterior fossa causes (Teasdale et al. 1989). The use of three-dimensional fast asymmetric spin-echo (FASE) MR imaging sequences strongly predicts a good response to shunting (Yoshihara et al. 1998). Other MRI techniques to evaluate CSF flow have yielded conflicting results (Bradley et al. 1996; Hakim and Black 1998).

Monitoring / compliance

The most frequent form of monitoring is measurement of the head circumference in infants. This is routine in most hospitals and in primary care. Deviation from the percentile for a particular child should trigger referral for further assessment. Also, regular measurement of head circumference allows the efficacy (or otherwise) of treatment to be monitored over time. Intracranial pressure monitoring may be useful when there is uncertainty about the need for CSF diversion. This may take place via the chamber of a shunt or reservoir inserted specifically for this purpose. In normal-pressure hydrocephalus, intracranial pressure monitoring may be helpful, particularly if pressure waveform analysis is undertaken (Williams et al. 1998).

Non-invasive intracranial pressure monitoring is possible via the fontanelle in infants or with the tympanic membrane displacement test in adults (Madan et al. 1998).

Compliance (the reciprocal of elastance) may reflect the reserve capacity to withstand small changes in volume. It can now be measured directly with a continuous compliance monitor (Piper et al. 2000).

Prognosis

It has been recognized for many years that untreated infantile hydrocephalus is fatal during the first few years of life, on average a hydrocephalic alive at 3 months has a 26 per cent chance of reaching adult life without surgery, and survival to between 1 and 2 years is associated with a 50 per cent chance of reaching adulthood (Macnab 1966). Some who survive have mental retardation, epilepsy, or blindness. Even with underlying aqueduct stenosis, the hydrocephalic process often arrests spontaneously (Laurence 1969). In 41 per cent of 70 cases without spina bifida the IQ after 6 years was over 85, in 29 per cent below 50. The severity of hydrocephalus bears a close relationship to IQ and physical disability such as spasticity and ataxia. In the past many children with the Arnold–Chiari malformation and myelomeningocele died from infection of the sac or hydrocephalus.

The introduction of ventricular diversion procedures reduced the mortality of hydrocephalus with myelomeningocele to 30 per cent at the end of 2 years, and that of uncomplicated hydrocephalus to less than 20 per cent. Two-thirds of such cases treated surgically had an IQ of 75 or more (Milhorat 1972). More recent figures have claimed IQs of more than 100 in 31 per cent and greater than 70 in 85 per cent. The prognosis of hydrocephalus after infancy depends upon its cause and how far this is amenable to treatment. In non-tumorous hydrocephalus, IQs were greater than 90 in 32 per cent and greater than 70 in 60 per cent (Hoppe-Hirsch et al. 1998). Also, adults who were treated for hydrocephalus due to spina bifida had poorer verbal and visuospatial memory performance than those with aqueduct stenosis (Hommet et al. 1999).

Treatment by drainage

In an emergency, obstructive hydrocephalus can be treated by ventricular drainage through a burrhole or twistdrill. Drainage is usually maintained for up to 5 days via a closed system with a fixed-height drip-chamber. By contrast, communicating hydrocephalus can be treated by lumbar puncture or via a lumbar drain, provided that imaging has excluded any intracranial mass or tonsillar herniation. This is often the case with subarachnoid haemorrhage or meningitis. Because they can be set-up under local anaesthesia, such drainage methods may be used in emergency situations and in patients unfit for general anaesthesia. Permanent CSF diversion requires a general anaesthetic.

CSF diversion and shunts

In the past many operations, including excision of the choroid plexus, Torkildsen's ventriculo-cisternostomy, ventriculosubdural, ventriculo-ureteric, ventriculo-atrial and ventriculo-pleural drainage, were used, with varying degrees of success. Now there is general agreement that the treatment of choice is use of various shunts inserted into the lateral ventricle, with catheter drainage into the peritoneum. These shunts have differing hydrodynamic properties, which have been carefully evaluated (Czosnyka et al. 1998). Most of them have low hydrodynamic resistance so that flow increases due to a siphoning effect when connected to a long distal catheter. Some shunts are programmable but they are all susceptible to siphoning. Colonization of the valves with Staphylococcus albus or diphtheroids may be asymptomatic for some time. The UK shunt registry has documented varying infection and revision rates in the 44 UK units. Revision rates ranged from 18 to 70 per cent and infection rates from 0 to 21 per cent (Pickard 2000).

The appropriate treatment of hydrocephalus after infancy depends upon its cause. A causative intracranial tumour must receive appropriate surgical treatment whenever possible. When there is a tumour in the third ventricle or midbrain or in some other area that is causing obstruction but cannot be removed or treated effectively by radiotherapy, temporary improvement may result from a shunting operation with insertion of a ventriculo-peritoneal (VP) shunt.

While early reports of ventriculo-peritoneal shunting in cases of low-pressure hydrocephalus were encouraging, with about two-thirds of all patients showing early intellectual as well as physical improvement, after 3 years less than half demonstrated continuing benefit (Greenberg et al. 1977; Gustafson and Hagberg 1978). In yet another series of patients, only a third were improved, and 50 per cent of a similar group of patients not operated upon failed to deteriorate over a 3-year period; the surgical group also showed a high incidence of complications (Hughes et al. 1978). Thus early optimism has not been borne out by good long-term results in all cases. In some longstanding cases, there are irreversible neuropathological changes somewhat similar to those of Alzheimer's disease (Ball 1976), although this is an inevitable occurrence in any large group of patients presenting with mild dementia. With good clinical diagnostic criteria and a positive CSF tap test, shunting undoubtedly leads to improvement in many patients (Vanneste and van Acker 1990).

Other treatments

Third ventriculostomy has been reintroduced with advances in endoscopic techniques. It is particularly useful when hydrocephalus is due to aqueduct stenosis Lumbo-peritoneal shunting is as successful as VP shunting in communicating hydrocephalus, normal-pressure hydrocephalus, and benign intracranial hypertension, and has the advantage of not requiring ventricular cannulation through brain tissue. Rare underlying causes need to be considered in all cases of hydrocephalus and may need prolonged treatment in their own right. Examples include antituberculous treatment for tuberculous meningitis or praziquantel for parasitic cysts.