Hydrocephalus and shunts: what the neurologist should know.

The second most common reason for being sued for negligence in neurosurgery is a problem related to hydrocephalus management (the first being spinal surgery!). However, the good news is that the overall standard of care for patients with hydrocephalus appears to have greatly improved over the last 10 years with the advent of better facilities for investigation, new approaches to treatment, and a greater awareness of the need for adequate follow up. In the possible absence of a local neurosurgeon with an interest in hydrocephalus, a neurologist who is faced with the ongoing care of a patient with hydrocephalus should ideally have a clear idea of what exactly constitutes appropriate follow up and which clinical and radiological warning signals of shunt problems to look out for.

DEFINITIONS

Hydrocephalus is an excessive accumulation of cerebrospinal fluid (CSF) within the head caused by a disturbance of formation, flow or absorption.

"Hydrocephalus ex vaccuo" is a misnomer. It refers to asymptomatic ventricular enlargement caused by generalised loss of cerebral tissue, from severe head injury, infarction or cerebral hypoxia.

"Normal pressure hydrocephalus" is also a misnomer. It describes a condition in older adults of low grade hydrocephalus with intermittently raised intracranial pressure (ICP) (usually at night) causing the classic Adam's triad of symptoms--gait apraxia, incontinence, dementia.

BASIC HYDROCEPHALUS PATHOPHYSIOLOGY IN ADULTS

The normal CSF production rate in an adult is 0.35 ml/mm (20 ml/hour or 500 ml/24 hours). The capacity of normal lateral and third ventricles is approximately 20 ml, whereas the total CSF volume in an adult is 120-150 ml. Hence, in normal circumstances CSF is recycled over three times each day.

ICP rises if production of CSF exceeds absorption, but CSF production will fall as ICP rises to high levels, and compensation (stabilisation of hydrocephalus at a new steady state) may occur through transventricular absorption of CSF. Dural absorption may also be important through nerve root sleeves and unrepaired meningocoeles. Of vital significance is the fact that compensated hydrocephalus (for example, in patients with a longstanding non-functional shunt) is not necessarily permanent because of the sometimes precarious nature of the balance between production and absorption of CSF. Clinicians involved in the care of patients with so called stable hydrocephalus should always be alert to the possibility of insidious subclinical progression or late decompensation of this condition, that may occur spontaneously or after a minor head injury.

As hydrocephalus develops the temporal and frontal horns dilate first, often asymmetrically. There is then elevation of the corpus callosum and stretching of the white matter tracts followed by thinning of the convexity grey matter of the brain (fig 1).

CAUSES OF HYDROCEPHALUS

The causes of hydrocephalus are listed in table 1.

CLINICAL FEATURES

The clinical features of hydrocephalus are notoriously variable, depending on the rapidity of onset of the condition (see box). The most rapid deteriorations are seen in young adults with colloid cysts of the third ventricle where the acute rise in ICP caused by the ball valve plugging of the third ventricle can lead to sudden death. The least rapid presentations occur in older patients with soft compliant brains where the only clue to the presence of progressive hydrocephalus may be a subtle slowing of gait or mentation. This slow presentation also characteristically occurs after severe head injury or subarachnoid haemorrhage.

INVESTIGATIONS

Computerised tomography

A computerised tomographic (CT) scan should be undertaken to assess the overall size of the ventricles, and to determine if periventricular oedema or "lucency" is present. A CT scan is also useful to assess the size of the fourth ventricle--if large, this suggests a communicating hydrocephalus, whereas a relatively small fourth ventricle implies obstructive hydrocephalus that …