Hydrocephalus & Shunts
Hydrocephalus & Shunts
Hydrocephalus & Shunts
Vaseem Zamair)
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.
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 ventricleif large, this suggests a communicating hydrocephalus, whereas a relatively small fourth ventricle implies obstructive hydrocephalus that might be best treated by endoscopic third ventriculostomy rather than a ventriculo-peritoneal shunt.
Young adults: o Symptomsheadache, vomiting, failing vision, drowsiness, muzziness of the head, fatigue
Signspapilloedema, enlarged blind spots on visual field analysis or reduced visual acuity, failure of upward gaze, general clumsiness, dyspraxic gait, large head Older adults/elderly: o Symptomsslowing of mental capacity, unsteady on feet/frequent falls, incontinence, drowsiness, headaches less frequently o Signsgait dyspraxia (slow, hesitant shuffling gait), dementia (reduced mini-mental score), rarely papilloedema
o
Lumbar CSF infusion tests measure CSF outflow resistance, which in simple terms represents the overall compliance of the intracranial and spinal CSF compartment. During this test saline or artificial CSF is constantly infused via a lumbar puncture needle or catheter, and the subsequent gradient of rise in the ICP with time is recorded. A low outflow resistance corresponds to high cerebral compliance and vice versa. Normal values are 510 mm Hg/ml/minute and a value > 18 mm Hg/ml/minute appears to be the approximate cut off point for diagnosing active hydrocephalus in the elderly. Other compliance monitors have recently been developed that are placed as bolts through small twist drill holes in the skull. These tests can be used to guide treatment of patients with newly diagnosed ventricular enlargement; they can also be useful in patients with possible blockage of their shunts or delayed occlusion of their third ventriculostomy site.
Transcranial Doppler
Transcranial Doppler involves the non-invasive measurement of the middle cerebral artery flow velocities and pulsatility index. The latter index appears to correlate fairly well with ventricular distension and cerebrovascular impedance. The quality of information obtained is very much operator dependent, but can be useful for monitoring patients in an outpatient setting.
CSF sample
In post-subarachnoid and post-meningitic hydrocephalus, CSF samples are useful for cell counts, protein concentration, and to exclude residual infection. A protein concentration greater than 4 g/l will clog up most ventriculo-peritoneal shunt valves.
Psychometric analysis
Although it is highly unlikely that any patients with possible pressure from hydrocephalus would ever stand a chance of seeing a neuropsychologist during an
investigational work up, the right posterior hemisphere has been shown to be most susceptible to functional deterioration from raised ICP. Deterioration in handeye coordination and visuospatial skills may precede classic symptoms of shunt blockage.
The cause of hydrocephalus undoubtably influences the chance of long term success. There is clearly a better success rate in patients with aqueduct stenosis, spina bifida, and tectal, pineal, and posterior fossa tumours. The risk of failure appears to increase with a past or recent history of intracranial infection, presumably as a result of obliteration of cerebrospinal fluid pathways. Long term results have been questioned as follow up is often not more than five years in the literature, but most studies quote an overall success rate between 6575%. The overall complication rates reported in published series varies widely from 4 30%, but the overall rate of serious complications is 9.4% and this includes an average 3% infection rate, 2.3% haemorrhage rate, and 1.3% risk of a permanent neurological deficit. Fortunately the rate of life threatening complications appears to be low and postoperative deaths are rare (0.1%, or 1 per 1000 third ventriculostomies).
Headaches Vomiting Drowsiness Papilloedema with or without failing vision Occasionally failure of upward gaze Neck stiffness Thoracic back pain in patients with spina bifida
CT scan (enlargement of ventricles) Plain x ray of shunt system (lateral skull, anteroposterior (AP) chest and AP abdomen) Palpation of shunt reservoirunreliable Peripheral blood for C reactive protein, white cell count if there has been any recent surgery Shunt reservoir tap ICP monitoring/lumbar infusion test
In selected patients a ventricular access device (otherwise known as an Ommaya reservoir) is placed in the right frontal region for ICP monitoring or treatment of infections (fig 4). These cannot be flushed or assessed in any way by palpation, but provide the facility for potentially life saving percutaneous aspiration of CSF in the event of acutely raised ICP. This can be done by any clinician in this situation and simply involves the passing of a butterfly needle through the skin perpendicular to the surface of the skin at the apex of the dome of the reservoir, until a pop is felt. Elective sampling of reservoirs or shunts should preferably be carried out by a neurosurgeon, unless the clinician looking after the patient has had previous experience in the technique. Differential pressure valves allow the siphoning effect in the upright position and this may lead to excessive CSF drainage from the ventricles. Some systems now incorporate an anti-siphon device to ameliorate this effect. Programmable or adjustable valves allow the closing pressure to be altered externally using a special magnetic adjusting device. Although this is sometimes extremely useful in selected shunted patients with intractable headaches, it can lead to problems following inadvertent change of pressurefor example, by having an MRI scan or, less obviously, by using headphones and certain cordless phones. Flow controlled valves, such as the Orbis-Sigma valve, have a more physiological CSF drainage pattern, but they do not appear to be effective in normal pressure hydrocephalus or where brain compliance is poor (so called brittle ventricles).
COMPLICATIONS OF SHUNTING
Shunt obstruction
Shunt obstruction may occur proximally in the ventricular catheter as a result of choroid plexus, red cells, tumour cells, or a high protein concentration in the CSF. Blockage of the distal catheter can occur as a result of body growth (if the shunt was placed during childhood), adhesions within the abdominal cavity, especially when associated with a low grade infection, pregnancy, and occasionally constipation. Urgent help from the on-call neurosurgeon should be sought for all suspected cases of acute shunt malfunction as patients with little remaining compensatory reserve may deteriorate suddenly as a result of a respiratory arrest, seizures, or simple coning. Shunt blockage may cause death and blindness if there is a combination of sudden onset and delay in treatment. Uncomplicated shunt revisions do not affect long term outcome. However, revision surgery on patients with blocked shunts is occasionally complicated by serious secondary ventricular or intraparenchymal haemorrhage, and any patient who is not quite right soon after a shunt revision should have a follow up CT scan immediately. Most young patients with shunts require a revision operation once or twice every 10 years as the shunt tubing degenerates gradually over the years and flakes of silicone break off (causing subcutaneous granulomas), weakening the wall of the tube. Eventually the tube may fracture or obstruct.
Some unlucky patients run into multiple problems with their shunt, usually as a result of one problem leading to another (fig 5). There is little to be done in such circumstances, other than commiserate with the patient and approach each hurdle in a positive and objective manner. However, when the situation becomes unduly complicated or intractable it may be prudent for the neurologist or neurosurgeon to seek the opinion of a neurosurgeon with a special interest in hydrocephalus.
Infection
Shunt infections are usually caused by the patients own skin organisms (most common is Staphylococcus epidermidis), which gain access to shunt tubing during the shunt procedure. Typically this contamination will cause an internal shunt colonisation where the bacteria settle and grow on the internal wall of the shunt catheter and valve, establishing adherent colonies. However, some bacteria set up a ventriculitis without full colonisation of the shunt, and others (for example, Staphylococcus aureus) cause an external shunt infection (deep wound infection). The most important clinical features of a shunt infection are as follows:
general malaise pyrexia headaches, vomiting, neck stiffness abdominal tenderness or distension recurrent lower end shunt obstruction occasionally pain and erythema around the shunt pulmonary hypertension or shunt nephritis in chronic VA shunt infections recent shunt operation 90% of VP shunt infections present within three months of a shunt operation raised C reactive protein high peripheral and CSF white cell count culture of organism from CSF.
Patients should be reassessed urgently by the relevant neurosurgical team should any of the above symptoms develop within the first few months after a shunt operation. After six months a VP shunt will not become infected unless intraabdominal sepsis occurs (for example, appendicitis, diverticulitis or postgastrostomy feeding tube insertion). The current incidence of shunt infection in most neurosurgical units is about 58%, but many units are now achieving better results as a result of preventative measures and protocols. Details, including infection rates, of most shunt operations performed in the UK are now submitted to the UK Shunt Registry that was set up by the Medical Devices Agency a few years ago. It is hoped that information derived by analysing this huge data set will lead to further improvements in the standards of shunt surgery in the future. Infected shunts will often have to be removed and then replaced after two weeks of antibiotics and temporary drainage. Infections with low grade pathogens can sometimes be treated with intraventricular and intravenous antibiotics alone.
Over-drainage
The perfect shunt valve has yet to be designed and many current models allow overdrainage of CSF due to the siphoning effect. The hydrostatic pressure (2575 cm CSF) caused by the weight of the column of CSF within the distal catheter leads to fluid being sucked out of the ventricles in the upright position. The valve pressure may be set too low for an individual patient leading to over-drainage; this can be remedied by adjustments of the valve pressure either by revision of the valve or by using a programmable valve. ICP monitoring may be required in those patients presenting with possible low pressure headaches without evidence of subdurals on the CT scan. Subdural haematoma can occur during the first six months after a shunt insertion and has been shown to be related to the amount of CSF released at operation. Small collections occur in up to 30% of patients after shunt insertion in the elderly, but symptomatic collections requiring surgery affect only 1015%. The symptoms of a shunt related subdural collection include headaches, confusion, hemiparesis, and drowsiness.
undetected chronic shunt malfunction, and efforts by the Association of Spina Bifida and Hydrocephalus have led to improvements in standards of follow up care. The principle components of good long term care of patients with shunted hydrocephalus are outlined below. All patients should have a baseline CT brain scan 612 months after their initial shunt insertion, while they are well. They should preferably retain a copy of this baseline scan if they travel far from their base neurosurgical unit, and it is our experience that most patients are willing to pay for this copy. Although it is not always possible to detect shunt blockage on a CT scan as a result of a patient having non-compliant ventricles, this is the exception rather than the rule. All patients and carers should be given clear instructions (preferably written) as to what symptoms to look out for and when to contact their doctor. Some documentation of exactly which type of valve has been implanted should be given to the patient. Those with programmable/adjustable valves should always have their shunt re-programmed, or at least checked by their neurosurgeon after any MRI scan has been carried out. They should be made aware of the possible problems of inadvertent valve pressure change from extraneous magnetic sources. All younger (under 60 years) patients should have an annual visual acuity check by an optician or an ophthalmologist if there have been particular concerns about vision. All younger patients with a shunt should probably be encouraged to seek a neurosurgical check up at least every three years, ideally at a dedicated hydrocephalus follow up clinic.