PAUL H. CHAPMAN
PATHOPHYSIOLOGY AND DEFINITION OF TERMS
Hydrocephalus is the term used to describe an excessive volume of cerebrospinal fluid within the head. This condition is typically the result of abnormal circulation or resorption of cerebrospinal fluid. Although brain atrophy also increases the volume of intracranial spinal fluid, this phenomenon, which is known as hydrocephalus ex vacuo, does not reflect an abnormality of spinal fluid circulation. To understand the dynamics of hydrocephalus and its treatment, it is useful to consider its pathophysiology.
Spinal fluid is produced at a rate of 20 ml/h. At least 80 per cent of this volume is generated by the choroid plexus of the lateral ventricles. Once produced, the fluid passes through the foramina of Monro into the third ventricle and from there through the aqueduct into the fourth ventricle. It reaches the subarachnoid space through the fourth ventricular foramina of Luschka and Magendie. Within the subarachnoid space the fluid circulates into the spinal canal and ultimately over the surfaces of the brain. It is reabsorbed into the venous circulation, principally through the arachnoid villi of the sagittal sinus. A problem at any point in the sequence between production and resorption will cause an imbalance leading to progressive spinal fluid accumulation.
Overproduction of fluid due to a tumour of the choroid plexus (choroid plexus papilloma) is a rare cause of hydrocephalus. More often there is obstruction to flow at some point along the circulatory pathways. The term non-communicating hydrocephalus refers to obstruction within the ventricular system, while the term communicating hydrocephalus applies to a blockage in the subarachnoid space. Accumulation of cerebrospinal fluid proximal to the point of obstruction causes enlargement of the ventricles (ventriculomegaly). Distension of the ventricular wall is associated with transudation of fluid into the periventricular white matter. There is disruption of the ependymal lining of the ventricles, degeneration of axons, gliosis, and drop-out of neurones. If hydrocephalus occurs abruptly and is severe, life-threatening compression of the brain-stem results.
Hydrocephalus is common in infancy and childhood; in this age group the predominant causes are congenital and reflect a structural brain abnormality. At least 80 per cent of infants with myelomeningocele develop hydrocephalus due to a congenital deformity of the hindbrain (Chiari malformation), which prevents cerebrospinal fluid flowing from the fourth ventricle into the subarachnoid space. Congenital narrowing of the aqueduct of Sylvius (aqueductal stenosis) is responsible for many cases of childhood hydrocephalus. This can occur in association with myelomeningocele or as an isolated phenomenon. Congenital posterior fossa cyst (Dandy-Walker malformation) is also associated with infantile hydrocephalus.
Any pathological process that produces inflammation and scarring of the areas which act as pathways for cerebrospinal fluid passage may result in hydrocephalus. This is the mechanism by which intrauterine infection with toxoplasma or cytomegalovirus causes acquired congenital hydrocephalus. Bacterial meningitis, contracted at any age, can cause obstruction of the subarachnoid spaces. Blood products in the spinal fluid due to haemorrhage have a similar effect. The most frequent cause of post-haemorrhagic hydrocephalus in infants is intraventricular haemorrhage associated with prematurity and respiratory distress. In adults posthaemorrhagic hydrocephalus is most commonly due to rupture of a cerebral aneurysm or vascular malformation. Hydrocephalus occasionally follows severe head trauma, reflecting bleeding in the subarachnoid space.
The most common cause of non-communicating hydrocephalus is a tumour within or adjacent to the ventricular system. Depending on the type of tumour involved, hydrocephalus may be entirely responsible for the presenting symptoms and signs. A colloid cyst of the anterior third ventricle will obstruct the foramina of Monro. Large tumours at the base of the brain, such as craniopharyngioma and pituitary adenoma, may impede flow of cerebrospinal fluid at the same site. Intraventricular ependymomas and astrocytomas arising near the walls of the ventricles obstruct the third and fourth ventricles. Pineal region tumours and gliomas of the midbrain compress the aqueduct of Sylvius. Cerebellar tumours often block the fourth ventricle. Half of all brain tumours affecting children are medulloblastomas or benign cerebellar astrocytomas. These tumours are typically associated with obstructive hydrocephalus.
The extent to which hydrocephalus is responsible for a patient's symptoms depends on the underlying disease process. Hydrocephalus may be the only manifestation of an intraventricular tumour. On the other hand, symptoms attributable to hydrocephalus may be overshadowed by the effects of severe bacterial meningitis, intracranial haemorrhage, or an invasive tumour. Ventricular enlargement may be discovered in utero during routine prenatal ultrasound examination. Many such fetuses have associated lethal anomalies.
A patient's age influences the mode of clinical presentation. While the cranial sutures and anterior fontanelle are open, accumulation of intracranial fluid causes enlargement of the head, splitting of the sutures, and bulging of the anterior fontanelle. Serial head circumference measures are routinely plotted on standard growth curves during infancy for the early detection of hydrocephalus. Associated symptoms during infancy include irritability, poor feeding, and a high-pitched cry. There may be tonic downward deviation of the eyes (sunsetting) due to brain-stem compression. Once the skull has become rigid and the fontanelle is closed, the signs and symptoms of hydrocephalus are typically those associated with increased intracranial pressure. These include headache, nausea and vomiting, lethargy, papilloedema, and sixth nerve paralysis.
Long-standing ventricular dilatation can cause gait ataxia and spasticity of the lower extremities because of mechanical stretching and disruption of frontal cortical axons. Individuals beyond the fifth decade of life are at risk for so-called normal pressure hydrocephalus. This form of communicating hydrocephalus is characterized by the clinical triad of gait ataxia, urinary incontinence, and a variable degree of dementia. It may be difficult to distinguish such cases from ex vacuo hydrocephalus due to an underlying atrophic process of the brain, such as multi-infarct dementia or Alzheimer's disease.
To diagnosis hydrocephalus one must demonstrate some degree of ventricular enlargement. The only exception to this dictum is benign external hydrocephalus of infancy, in which the sub-arachnoid spaces over the cerebral convexities are enlarged. Modest ventricular dilatation may also be present. The condition is self-limited and its only manifestation is a large head: it does not usually require treatment.
During infancy, while the fontanelle is open, cranial ultrasound is a useful tool for the detection of ventricular enlargement. In older patients, hydrocephalus is usually detected by CT or MRI. Once ventriculomegaly has been demonstrated its cause must be determined. This may be evident from historical information or from the presence of a problem such as subarachnoid haemorrhage or bacterial meningitis. Further imaging studies may be necessary to identify a tumour or to characterize a congenital anomaly of the brain.
MANAGEMENT CONSIDERATIONS AND TREATMENT
The first step in formulating a plan of management is to decide whether the patient requires treatment: one occasionally encounters a patient with large ventricles who has no symptoms that seem referable to this finding. In this circumstance it is necessary to decide whether the hydrocephalus is progressive or stable. Further enlargement of the ventricles is likely to occur if the hydrocephalus is progressive (active hydrocephalus). Arrested hydrocephaly is characterized by asymptomatic and non-progressive ventriculomegaly. In the latter case no treatment is necessary, although careful follow-up with clinical examination and serial imaging is appropriate.
Patients with progressive hydrocephalus require some form of treatment. This may be removal of a tumour which is obstructing flow of cerebrospinal fluid. Temporary means of controlling hydrocephalus are appropriate if one expect the problem to be self-limited. Acetazolamide and frusemide decrease spinal fluid production and are occasionally employed in the treatment of mild infantile hydrocephalus. The osmotic diuretic isosorbide has also been used with some success. Hydrocephalus associated with intracranial haemorrhage is transient in up to 80 per cent of patients, and a temporary measure such as external ventricular drainage is often appropriate.
Most patients with active hydrocephalus require surgical treatment. The goal of any operation is to divert spinal fluid around or away from the site of obstruction. This typically involves draining fluid from the enlarged lateral ventricles to a site elsewhere in the body, where it can be reabsorbed. The device typically used for this purpose, known as a shunt, consists of a small diameter catheter, the proximal end of which is placed in the dilated lateral ventricle through a small drill hole in the frontal or occipital skull. The tubing is then tunnelled subcutaneously into the area of the body where the fluid is to be directed. Fluid is most commonly diverted into the peritoneal cavity, which is entered through a small incision over the anterior abdomen (ventriculoperitoneal shunt). Miniature valve assemblies are incorporated in shunts to regulate pressure and ensure unidirectional flow. Most shunts also have a small plastic reservoir that can be tapped percutaneously to allow sampling of fluid and irrigation of the system. The valve and reservoir are positioned beneath the scalp for easy access (Fig. 1) 2302.
Other sites in the body are less commonly used for shunting. Fluid may be diverted from the ventricle into the right atrium of the heart via the internal jugular vein (ventriculoatrial shunt) or into the pleural space (ventriculopleural shunt). In communicating hydrocephalus, fluid may be shunted from the lumbar subarachnoid space to the peritoneum (lumboperitoneal shunt). In non-communicating hydrocephalus it is theoretically possible to bypass the site of obstruction with a valveless shunt tube from the lateral ventricle to the cervical subarachnoid space. This operation, known as a Torkildsen shunt, is rarely used because of the relative ease with which the other types of shunts can be performed.
One must assume that a shunt inserted for treatment of hydrocephalus will be needed indefinitely. Any problem with shunt function will cause recurrent hydrocephalus, the usual symptoms being headache, nausea, vomiting, and lethargy. If the malfunction is left uncorrected, the problem may become life-threatening. The most common site of obstruction is the ventricular catheter, which becomes clogged with choroid plexus or tissue debris. If there is uncertainty regarding function of the shunt, the reservoir may be tapped through the skin to test flow in the system and to measure pressure.
Infection develops in 2 to 10 per cent of shunts. Bacterial contamination of the shunt may simply cause it to malfunction; it may also lead to meningitis, peritonitis, and generalized sepsis. Non-pathogenic bacteria such as Staphylococcus epidermidis and diphtheroids are a particular problem; they may be difficult to detect and eradicate. Indolent shunt infections are a common cause of obstruction of the peritoneal end of the shunt.
Although shunts effectively relieve hydrocephalus and associated intracranial hypertension, they do not restore normal pressure dynamics within the head. The commonly used shunt valves are activated by a pressure difference across the device. This can cause intracranial pressure to drop to unusually low levels when the patient assumes the upright position. Markedly negative intracranial pressure can cause troublesome postural headache, similar to that which occurs after a spinal tap. The same mechanism is responsible for subdural haematoma, which complicates up to 10 per cent of shunt operations performed in elderly patients with normal pressure hydrocephalus.
Chronic over-drainage of fluid in children treated with a shunt during infancy may result in slit-like ventricles. Five per cent of such children will suffer recurrent headaches, with or without obstruction of the ventricular catheter (slit-ventricle syndrome). Shunt over-drainage, also known as siphoning, may be treated by including specialized equipment such as the antisiphon device in the shunt system.
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