Perioperative management of hydrocephalus

Learning objectives.

By reading this article, you should be able to:

• •Distinguish between the clinical features of communicating and non-communicating hydrocephalus.
• •Define the indications for external ventricular drains (EVD), ventriculo-peritoneal shunts and lumbar drains.
• •Explain the practical aspects of EVD management in intensive care.

Key points.

• •Cerebrospinal fluid (CSF) is normally free to flow between cranial and spinal compartments and is a mediator of intracranial compliance.
• •Hydrocephalus, the excess accumulation of CSF in the ventricular system, is caused by obstruction to CSF flow or excess CSF production.
• •The clinical presentation of hydrocephalus depends on the underlying pathology, site of obstruction, speed of onset, and the patient's age.
• •Lumbar spinal, CSF dynamics and imaging studies are useful for diagnosis and management of hydrocephalus.
• •Symptomatic hydrocephalus often requires surgical intervention to shunt excess CSF away from the ventricular system or the spinal thecal space, usually into the peritoneal cavity.

Hydrocephalus, derived from the Greek words hydro (water) and kephalos (head), describes a congenital or acquired condition in which there is an excessive accumulation of cerebrospinal fluid (CSF) within the head. There is a wide range of causes, most of which involve obstructed CSF circulation or impaired absorption.¹

Anatomy and physiology

CSF has both biochemical and physical functions.² It plays an essential role in homeostasis of the brain extracellular fluid and the maintenance of normal neuronal function. This includes the provision of some micronutrients to cerebral tissue, clearance of waste products of nerve metabolism, and maintenance of ionic homeostasis. CSF can act as a mediator of intracranial compliance because of the free flow between the cranial and spinal compartments. It also provides buoyancy, substantially reducing the effective weight of the brain.

The total adult CSF volume is around 150 ml, distributed between the cranial and spinal subarachnoid channels (125 ml) with a smaller, albeit variable volume in the ventricles (average 25 ml). Most CSF is produced in the ependymal cells of the choroid plexus, whilst a small proportion is formed around the cerebral vessels from brain interstitial fluid. The formation of CSF is independent of the cerebral perfusion pressure and intracranial pressure (ICP), up to the point where ICP is increased to such a degree that CSF production is compromised.

CSF flows from the lateral ventricles into the third ventricle, through the interventricular foramen of Monro, and then enters the fourth ventricle through the cerebral aqueduct of Sylvius. CSF exits the fourth ventricle via the foramina of Luschka laterally and Magendie medially. It then flows into the basal cisterns and over the convexities of the cerebral hemisphere (Fig. 1). CSF flow throughout the neuro-axis, including across the foramen magnum, occurs in a pulsatile fashion, related to the cardiac cycle, rather than as a steady stream.³

Fig 1.

Sagittal view of CSF pathway (hand drawn by Graphics Department at University Hospital of Birmingham).

There are two routes of reabsorption of CSF from the subarachnoid channels. Primarily, CSF passes directly into the blood flowing along the cranial venous sinuses, via arachnoid granulations. A secondary conduit is alongside the cranial and spinal nerve roots, particularly the olfactory nerves as they pass through the cribriform plate of the ethmoid bone. The rate of absorption is largely driven by the ICP, with resistance to CSF outflow provided by the arachnoid granulations and the intracranial venous sinus pressure.

The pressure within the cerebral ventricles is governed to some extent by the law of Laplace: P=2T/R.

This tells us that the pressure required to overcome the tension in the surrounding cerebral mantle is less in enlarged ventricles than it is in smaller ventricles. As with inflating a party balloon, high pressure is required initially to produce a modest increase in the ventricular size, whereas large ventricles are likely to be maintained under normal CSF pressures.

Epidemiology

The prevalence of congenital and paediatric hydrocephalus in the USA and Europe is estimated to be 0.5–0.8 per 1000 live births.⁴ Rates may well be higher in resource-poor areas where access to healthcare is limited. In adults, normal pressure hydrocephalus (NPH) is estimated to affect around 5 per 100 000.

Classification

Hydrocephalus can be categorised into a number of different types (Table 1).⁵

Table 1.

Types and aetiologies of hydrocephalus.

Communicating
Congenital	Acquired
• Achondroplasia  • Craniofacial syndromes  • Other skull base deformities	• Subarachnoid/intraventricular haemorrhage • Choroid plexus papilloma or carcinoma • Post-infectious
Non-communicating
Congenital	Acquired
• Aqueduct stenosis  • Vein of Galen aneurysm  • Chiari malformations  • X-linked hydrocephalus  • Dandy–Walker malformation	• Tumours • Post-inflammatory adhesions • Cerebellar haematomas/infarcts

Communicating

The term ‘communicating' implies that there is a free flow and transmission of CSF pressure waves, from the ventricles into the basal cisterns and spinal CSF channels. Commonly, this is because of the result of impaired CSF reabsorption into the cerebral venous sinuses.

Intraventricular or subarachnoid blood, shed into the CSF a result of trauma or spontaneous haemorrhage, or sometimes surgery, leads to blockage of the arachnoid granulations by red-cell break-down products. Overproduction of CSF is an uncommon cause of communicating hydrocephalus, caused by a choroid plexus papilloma. In communicating hydrocephalus, the increased intraventricular pressure causes distension of the brain towards the inner table of the skull and simultaneously compresses the periventricular brain tissue against the non-compressible fluid within the ventricles (Fig 2, Fig 3).

Fig 2.

A plain computerised tomography scan demonstrating dilatation of both lateral ventricles with transependymal CSF oedema and lack of visible sulci consistent with acute hydrocephalus.

Fig 3.

A plain computerised tomography scan demonstrating a hyperdense lesion seen in the anterior third ventricle obstructing both foramina of Monro with secondary hydrocephalus.

Non-communicating

Previously designated as an obstructive hydrocephalus, non-communicating hydrocephalus develops when CSF flow is blocked at any point within the ventricular system. This may be the result of tumour growth, intracerebral haemorrhage, or the development of a focal mass lesion. Aqueduct stenosis is one of the more common causes, resulting from a congenital, internal narrowing in the aqueduct of Sylvius, or by external compression of this channel by a tumour in the pineal region. Patients with non-communicating hydrocephalus often have significantly raised ICP; presentation is often acute and emergency intervention is required.

The importance of distinguishing between communicating and non-communicating hydrocephalus is that the former can usually be drained safely by the lumbar route. Sometimes, what began as a communicating hydrocephalus (e.g. after subarachnoid haemorrhage) becomes non-communicating, as the lateral ventricles enlarge progressively and lead to compression of the cerebral aqueduct. High pressure, non-communicating hydrocephalus should be drained directly, with an external ventricular drain (EVD) (see below).

NPH

NPH, first described in 1965 by Hakim and Adams, is a form of communicating hydrocephalus associated with a triad of clinical features: progressive impairment of gait; decline in cognitive function; and episodes of urinary incontinence.⁶

Symptoms typically evolve slowly over years so that the condition is chronic. The principle differential diagnosis is dementia from other causes combined with spondylotic myelopathy. In NPH, the baseline ICP is often normal and patients do not exhibit features of raised ICP. Dilation of the ventricles may be more related to pulse pressure within the ventricles.

Other forms

The term external hydrocephalus is sometimes used to refer to an abnormal accumulation of CSF within the basal cisterns and over the hemispheres of the brain. The cerebral ventricles may or may not be enlarged. Ex vacuo hydrocephalus refers to compensatory enlargement of the cerebral ventricles secondary to atrophy of surrounding brain tissue. This may be a focal distortion of the normal anatomy, for example dilation of a frontal horn after excision of an intracranial tumour, or more generalised ventricular enlargement resulting from senile atrophy of the brain.

Clinical presentation

Clinical features of hydrocephalus depend on the underlying pathology, site of obstruction to CSF flow, its speed of onset, and the age of the patient (Table 2). Acute onset cases usually present with symptoms of raised ICP and the ventricles may not be markedly enlarged, and may even be within normal limits. In chronic forms of hydrocephalus, the ventricles are usually enlarged. In the case of NPH, presenting features will be gait disturbance and cognitive decline, but often combined with headaches.

Table 2.

Presenting features of hydrocephalus in adults.

Acute	Chronic
Headache Impaired conscious level Vomiting Seizures Reduced upward gaze (Parinaud's syndrome) Diplopia (sixth nerve palsies) Head enlargement (infants) Sudden death (colloid cyst of third ventricle)	Gait disturbance Urinary incontinence Cognitive decline

Investigations

Imaging

Brain imaging can be difficult to interpret when trying to assess patients with suspected hydrocephalus or blocked CSF shunts. Ventricles under high pressure can appear normal in size whereas the pressure within large ventricles may not be high. Enlargement of the ventricles may be because of cerebral atrophy, rather than impaired CSF absorption. Distinguishing between such ‘hydrocephalus ex vacuo’ and the condition of NPH can be difficult on radiological grounds alone.⁷

Sometimes problems are encountered with abnormally small or ‘slit’ ventricles. Such appearances are readily explained when the brain is swollen, for example after trauma. They may also be seen in patients suffering from benign intracranial hypertension, be this idiopathic or secondary to venous sinus occlusion. It may also develop as a result of long-standing over-drainage of the ventricles, the so-called slit ventricle syndrome.⁸ Clinical features, such as severe, persisting headache and an obviously distressed patient should outweigh scan appearances that seem reassuring.

CSF dynamics

Given these difficulties in interpreting standard imaging, other forms of physiological assessments are sought in non-urgent situations. Quantitative measurement of CSF dynamics can help in diagnosis and guide the prognostication of shunt procedures to treat the condition.⁹

Intrathecal or intraventricular saline infusion tests can measure the CSF outflow resistance (R_out) and thereby assess the adequacy of the patient's CSF absorptive capacity. The principle underlying the test is that, although patients may have a CSF pressure within the normal range, abnormalities may be identified in the CSF absorption, after the administration of a fluid challenge. To conduct the study, after local anaesthetic infiltration, the lumbar spinal sac is punctured with an 18 G spinal needle. This is then connected to a pressure monitor and baseline CSF pressures are recorded, for about 10 min. Physiological saline is then infused, at 1 ml min⁻¹, for about 20 min, while CSF pressure is continually monitored.

The unit of measurement for R_out is mm Hg ml⁻¹ min⁻¹. Normal values for R_out are not clearly established, but the authors' practice is to regard values of up to 12 as normal, between 12 and 15 as borderline, and more than 15 mm Hg ml⁻¹ min⁻¹ as abnormal and predictive of a reasonable likelihood of the patient responding to CSF diversion. In addition to being an aid to diagnosing NPH, CSF infusion studies can be used to assess shunt function, particularly with regards to over-drainage, or obstruction.¹⁰

Management

Not all cases of hydrocephalus require or will benefit from surgical treatment. Surgery is not an option for individuals who are asymptomatic and have chronically enlarged ventricles under normal pressure.

Management of symptomatic hydrocephalus requires surgery. Drug treatment, aimed at reducing CSF formation, is unlikely to prove effective in most cases. The operative intervention involves diversion of CSF from the right lateral ventricle, but sometimes the left lateral ventricle or the lumbar spinal theca. Drainage can either be temporary into an external receptacle, as discussed below, or via a permanent indwelling catheter. The most commonly used anatomical location for drained CSF is the peritoneal cavity. If there are contraindications to using this site (i.e. intra-abdominal sepsis or extensive adhesions), other sites can be used including the pleural cavity or right atrium.

External drains

Insertion of an EVD constitutes one of the most frequently performed interventions by neurosurgeons. Lumbar drains are also used for CSF diversion, albeit less frequently. Common indications are given in Table 3. Drains may be inserted in the operating theatre or, if the situation is urgent, in the critical care environment.

Table 3.

Indications for external CSF drains.

Ventricular drains

1) Acute, new-onset hydrocephalus

• Subarachnoid/intraventricular haemorrhage

• Cerebellar haemorrhage/infarction

• Cerebellar/brain stem tumours

• Pineal tumour

• Colloid cyst of third ventricle

2) Removal of infected apparatus in shunt-dependent patient

3) Raised ICP with normal/small ventricles

• Acute head injury

• Administration of intraventricular drugs

Lumbar drains

• Acute head injury with patent basal cisterns

• Communicating hydrocephalus with infected CSF

• During intracranial procedures, to facilitate surgical exposure

• Decompressing post-operative sub-galeal scalp collections

• Following repair of skull base CSF leaks

• Spinal cord protection during aortic aneurysm surgery

One end of the EVD catheter is placed intracranially in the frontal horn of the lateral ventricle and the other end is tunnelled under the skin of the scalp. This end is then connected to an extension tube that leads onto the drainage burette, which itself empties into a collecting bag (Fig. 4).

Fig 4.

External ventricular drainage system.

The height of the burette and scale are adjusted so that the ‘zero’ on the scale corresponds with the foramen of Monro. When the patient is supine, the external auditory meatus approximates to this point. When the patient is being nursed in the lateral position, the reference point lies between the eyebrows, above the nasion. Lumbar drains are inserted via a Tuohy needle, in much the same way as an epidural catheter, except that the theca is penetrated deliberately. Either the drain insertion point or right atrium is usually chosen for zero reference point.¹¹

Complications associated with EVD include haemorrhage, epileptic seizures, and sub-optimal placement or displacement of the catheter. The most feared complication of an EVD is infection.¹² The incidence of EVD-associated infections ranges from 5 to 20%.

Practical points

The prescribed level at which the top of the collecting burette on the scale is set depends upon the opening pressure at drain insertion and the size of the ventricles. In most cases, the initial aim will be to control pressure, but with low pressure hydrocephalus the burette height may need to be set lower, in order to drain a fixed amount per hour to match the rate of CSF production—normally about 10 ml but sometimes up to 20 ml per hour.

Whenever the patient's position is changed, the burette and scale need to be rezeroed to the appropriate reference point. During patient transport, a dedicated pole should be used to mount the system. Any decision to clamp the drain device needs to take into account the dependence of the patient upon the drain and, in the authors' hospital, a drain is not routinely clamped during the transport.

Sampling

CSF samples should only be obtained when there is a clear need and always using a full aseptic technique. Sample collection should be from the proximal port (nearer to the head) and CSF should be aspirated very slowly. Samples should never be collected from the drain bag because rapid degeneration of any cells therein may produce misleading laboratory results. The same aseptic principles apply when medications are introduced into the system, after which the drain is usually clamped for an hour, before being re-opened.

Ventricular shunts

Cerebral ventricular shunts divert the CSF on a long-term basis. The proximal, ventricular catheter is fed into one or other lateral ventricle, most usually the right, unless there is a specific reason for choosing the left. The distal end is tunnelled subcutaneously and fed into the final receptacle, which in most cases is the peritoneum.

Lumbar shunts

These may be chosen as an alternative to ventricular shunts in cases of communicating hydrocephalus and idiopathic intracranial hypertension.¹³ The advantage of these shunts is they do not penetrate cerebral mantle so there is a less risk of seizures and intracerebral haemorrhage. Disadvantages are that they are not as controllable and reliable in the long-term compared with ventricular shunts.

Endoscopic third ventriculostomy

An alternative treatment for non-communicating hydrocephalus is endoscopic third ventriculostomy.¹⁴ This bypasses any obstructions to the CSF flow at the level of the aqueduct, caused by conditions such as congenital aqueduct stenosis or pineal region tumours. When successful, ventriculostomy avoids the commoner complications of shunt infection and blockage and does not leave the patient shunt-dependent.

Anaesthesia

Preoperative assessment should include a standard anaesthetic history and examination.¹⁵ More specific questioning should relate to ICP, such as alteration in the level of consciousness compared with baseline or increased seizure activity. Standard monitoring is appropriate and invasive vascular monitoring is not normally required, unless there is another indication related to the patient's comorbidities.

Antibiotic prophylaxis is usually used to reduce the incidence of wound infection. Antibiotics are less likely to be effective in preventing the establishment of infection in the CSF pathways, in part because of the limited CSF penetration of many antibiotics. Instead, now there is a widespread use of antibiotic- or silver-impregnated catheters.

Ventriculo-peritoneal (VP) shunt insertion can be performed as an emergency or an elective procedure. The duration of surgery varies, typically between 45 min and up to 2 h.

Tracheal intubation and lung ventilation are preferred because of limited access to the airway during surgery. When the distal catheter is being passed, this can be a particularly stimulating part of the procedure for the anaesthetised patient. Postoperative pain after shunting procedures is usually mild-to-moderate. Usually, paracetamol and COX-2 inhibitors are sufficient, with oral opioids for breakthrough pain, usually required for no more than the first 72 h.

Other forms of surgery in patients with a CSF shunt

There is no absolute contraindication to spinal anaesthesia in patients with CSF shunts, although it is often avoided because of concerns about the risk of shunt contamination or CNS infection. There is no evidence that dural puncture after a spinal anaesthetic compromises the function of the shunt.¹⁶

Questions are often raised when a patient with a peritoneal shunt is to undergo laparoscopic surgery.¹⁷ One concern is regarding whether or not the shunt system might fail under the pressure of the pneumoperitoneum. Good practice is observation by the surgeon of continuous CSF flow from the distal end of the catheter at the start, during, and at the end of the procedure. Minimising the duration and the pressure of pneumoperitoneum are also suggested.

Considerations in paediatrics

The normal range of ICP in neonates and infants is 0–6 mm Hg. Children with congenital hydrocephalus are likely to have extensive multisystem disorders, such as congenital heart disease or major spinal defects linked to premature birth. Children often present for repeated revision surgeries and may have other comorbidities, especially cerebral palsy and epilepsy. In neonates, the clinical signs of hydrocephalus include vomiting, irritability, drowsiness, downward gaze, bulging fontanelles, and an increased head circumference. An important sign of raised ICP in neonates is a propensity to apnoeic episodes.¹⁸

Sites of ventricular shunt insertion are similar to adults; however, VP shunts are avoided if further surgery is likely to be required for any concurrent abdominal pathology. Ventriculo-atrial shunts are avoided where possible because of the risk of shunt nephritis, caused by immune complex deposition in the kidneys.

As with adults, neurological status should be part of routine preoperative assessment. It is important to remember that vomiting, secondary to raised ICP in neonates and children, is more likely to cause dehydration than in an adult, with resultant electrolyte imbalance. Such metabolic disturbances should be corrected before surgery.

Summary

Acute and chronic hydrocephalus in all age groups present potential challenges to the anaesthetist. Neurological examination before surgery and an appropriate postoperative discharge location are important considerations. Short-term or long-term CSF diversion procedures are the neurosurgical operations performed most frequently. When an EVD is used, the transducer must be zeroed at the level of external auditory meatus with the patient supine. If the patient's position is changed, the drainage system should be clamped briefly and re-levelled and re-zeroed before unclamping. The EVD should be kept unclamped during transport of the patient.

Declaration of interest

None declared.

MCQs

The associated MCQs (to support CME/CPD activity) will be accessible at www.bjaed.org/cme/home by subscribers to BJA Education.

Biographies

Hari Krovvidi MD FRCA is a Consultant Anaesthetist with a subspecialty interest in neuroanaesthesia who edited the Neuroanaesthesia section of a recent book: ‘Anesthesiology—A practical approach’.

Graham Flint FRCS is a Consultant Neurosurgeon at the Queen Elizabeth Hospital Birmingham whose special interests include disorders of cerebrospinal fluid circulation. He has written a chapter on spinal and cerebrospinal fluid dynamics for the forthcoming Oxford Textbook of Neurosurgery.

Anna Williams FRCA PGCME is a Specialty Trainee in Anaesthesia in the Birmingham School of Anaesthesia.

Matrix codes: 1A01, 2A07, 3F00