Textbook of Neuroanaesthesia and Critical Care - part 1 potx

The anterior circulation is fed by the internal carotid arteries, while the posterior circulation derives from the vertebral arteries the vertebrobasilar circulation.. The segment of the

Textbook of Neuroanaesthesia and Critical Care

Page ii

DEDICATION

To our patients

Textbook of Neuroanaesthesia and Critical Care

Edited by Basil F Matta MB BCh BA BOA DA FRCA

Consultant in Anaesthesia and Neuro-Critical Care Director of Neuroanaesthetic Services Addenbrookes Hospital University of Cambridge

UK

David K Menon PhD MD MBBS FRCP FRCA FmedSci

Lecturer in Anaesthesia University of Cambridge Director of Neurocritical Care Addenbrookes Hospital University of Cambridge

UK

John M Turner MBBS FRCA

Consultant in Anaesthesia and Neuro-intensive Care

Addenbrookes Hospital Associate Lecturer University of Cambridge

UK

The right of Basil F Matta, David K Menon and John M Turner to be identified as editors of this work has been asserted by them in accordance with the Copyright Designs and Patents Acts 1988.

The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made

A catalogue record for this book is available from the British Library

Typeset by Saxon Graphics Ltd, Derby

Printed in Spain by Grafos

Neurosurgical and Neurological Intensive Care

Mark J Abrahams, David K Menon, Basil F Matta

Consultant in Anaesthesia and Neuro-Critical Care

Director of Neuroanaesthetic Services

Specialist Registrar in Neurosurgery

University Department of Neurosurgery

Addenbrookes Hospital

Cambridge, UK

Mark T O'Connell

PhD

Senior Research Associate

MRC Brain Repair Centre

Cambridge, UK

Page xiii

PREFACE

Neuroanaesthesia, perhaps more than any other field of anaesthesia has represented an area where the expertise and competence of the anaesthetist can influence patient outcome Indeed, developments in neurosurgery, neurointensive care and neuroradiology have only been possible due to concomitant advances in neuroanaesthesia These advances in clinical practice have always been

underpinned by experimental research, and there has also been a tradition of high quality clinical research in our specialty The development continues, and the availability of novel drugs and monitoring modalites represent both an opportunity and a challenge They represent an opportunity because the use of modern imaging, novel monitoring modalities and multimodality integration of such data provide exciting insights into disease mechanisms and pathophysiology, and allow the specific selection of appropriate therapies They also represent a challenge because there is a danger that we may confuse the aim of improved clinical management with the technological means of achieving it

We hope therefore, that this book will provide not only an understanding of the science that underpins modern clinical practice in neuroanaesthesia and neurointensive care, but also the clinical context in which such concepts are best applied Wherever possible,

we have attempted to base our recommendations on high quality clinical research with outcome evaluation Where paucity of such data or rapid advances in the understanding of disease makes this inappropriate, we have based our management on personal

experience and data regarding clinical pathophysiology While we expect that future studies addressing clinical pathophysiology and outcome will modify practice, this book is a distillation of current knowledge, experience and prejudices, as viewed from Cambridge

BASIL F MATTADAVID K MENONJOHN M TURNEROCTOBER, 1999

ACKNOWLEDGEMENTS

We would like to thank all our anaesthetic, neurosurgical and nursing colleagues for their help and encouragement during the preparation of this book

Anatomy of the CSF Pathways

The lateral ventricles are C-shaped cavities within the cerebral hemispheres Each drains separately into the third ventricle via the foramen of Monro, which is situated just in front of the anterior pole of the thalamus The third ventricle is a midline slit, bounded laterally by the thalami and inferiorly by the hypothalamus It drains via the narrow aqueduct of Sylvius through the dorsal aspect of the midbrain to open out into the diamond-shaped fourth ventricle This has the cerebellum as its roof and the dorsal aspect of the pons and medulla as its floor The fourth ventricle opens into the basal cisterns via a midline foramen of Magendie, which sits posteriorly between the cerebellar tonsils, and laterally into the cerebellopontine angle via the foraminae of Luschka

Approximatley 80% of the cerebrospinal fluid (CSF) is produced by the choroid plexus in the lateral, third and fourth ventricles The remainder is formed around the cerebral vessels and from the ependymal lining of the ventricular system The rate of CSF production (500–600ml per day in the adult) is independent of intraventricular pressure, until intracranial pressure (ICP) is elevated to the point

at which cerebral blood flow (CBF) is compromised Absorption, however, is largely by bulk flow and is therefore pressure-related Most of the CSF is reabsorbed via the arachnoid villi into the superior sagittal sinus, while some is reabsorbed in the lumbar theca CSF flow across the ventricular wall into the brain extracellular space is not an important mechanism under physiological conditions

The diagnosis is usually made by the appearance on CT or MRI scan The features which suggest active hydrocephalus rather than ex

vacuo dilatation of the ventricular system secondary to brain atrophy are:

• dilatation of the temporal horns of the lateral ventricles (> 2mm width);

• rounding of the third ventricle or ballooning of the frontal horns of the lateral ventricles;

• low density surrounding the frontal horns of the ventricles This is caused by transependymal flow of CSF and is known as

periventricular lucency (PVL) However, in the elderly this sign may be misleading as it is also seen after multiple cerebral infarcts

Management of Hydrocephalus

If the hydrocephalus is communicating then CSF can be drained from either the lateral ventricles or the lumbar theca Generally, this will involve the insertion of a permanent indwelling shunt unless the cause for the hydrocephalus is likely to be transient, infection is present or blood within the CSF is likely to block the shunt Under such circumstances, either an external ventricular or lumbar drain

or serial lumbar punctures may be appropriate

Obstructive hydrocephalus requires CSF drainage from the ventricular system Lumbar puncture is potentially dangerous because of the risk of coning if a pressure differential is created between the cranial and spinal compartments A single drainage catheter is adequate if the lateral ventricles communicate with each other (the majority of cases) but bilateral catheters are needed if the

blockage lies at the foramen of Monro

Many forms of non-communicating hydrocephalus can now be treated by endoscopic third ventriculostomy, obviating the need for a prosthetic shunt An artificial outlet for CSF is created in the floor of the third ventricle between the mamillary bodies and the infundibulum, via an endoscope introduced through the frontal horn of the lateral ventricle and foramen of Monro This allows CSF

to drain directly from the third ventricle into the basal cisterns, where it emerges between the posterior clinoid processes and the basilar artery

Brain Structure and Function

Disorders of different lobes of the brain produce characteristic clinical syndromes, dependent not only on site but also side Almost all right-handed patients (93%–99%) are left hemisphere dominant, as are the majority of left-handers and those who are

ambidextrous (ranging from 50% to 92% in various studies; for review, see1)

Intracranial mass lesions generally present in one of three ways: focal neurological deficit, symptoms or signs of raised intracranial pressure, or with epilepsy A very simple guide to the neurological deficits which may develop in association with disorders of the cerebral or cerebellar hemispheres is as follows

The Frontal Lobe

The frontal lobe is the hemisphere anterior to the Rolandic fissure (central sulcus; Fig 1.1) Important areas within it are the motor strip, Broca's speech area (in the dominant hemisphere) and the frontal eye fields Patients with bilateral frontal lobe dysfunction typically present with personality disorders, dementia, apathy and disinhibition The anterior 7 cm of one frontal lobe can be resected without significant neurological sequelae, providing that the contralateral hemisphere is normal Resections more posterior than this

in the dominant hemisphere are likely to damage the anterior speech area

Temporal Lobe

The temporal lobe lies anteriorly below the Sylvian fissure and becomes the parietal lobe posteriorly at the angular gyrus (Fig 1.1) The uncus forms its medial border and is of particular clinical importance because it overhangs the tentorial hiatus adjacent to the midbrain When intracranial pressure rises in the supratentorial compartment, it is the uncus of the temporal lobe which transgresses the tentorial hiatus, compressing the third nerve, midbrain and posterior cerebral artery

Figure 1.1 Topography of the brain A = Angular gyrus;

C = Central sulcus (Rolandic fissure);

PC = Pre-central sulcus; S = Sylvian fissure.

There are many functions to the temporal lobe including memory, the cortical representation of olfactory, auditory and vestibular information, some aspects of emotion and behaviour, Wernicke's speech area (in the dominant hemisphere) and parts of the visual field pathway

Like the frontal lobe, lesions in the temporal lobe may present with memory impairment or personality change Seizures are common, with prodromal symptoms linked typically to the function of the temporal lobe (e.g olfactory, auditory or visual hallucinations, unpleasant visceral sensations, bizarre behaviour or déjà vu)

The anterior 5–6 cm of one temporal lobe (approximately at the junction of the Rolandic and Sylvian fissures) may be resected Usually the upper part of the superior temporal gyrus is preserved to protect the branches of the middle cerebral artery lying in the Sylvian fissure More posterior resection may damage the speech area in the dominant hemisphere Care is needed if resecting the medial aspect of the uncus because of proximity to the optic tract In some patients undergoing temporal lobectomy (the majority of epilepsy cases, for example), it may be appropriate to undertake a sodium amytal test preoperatively, to both confirm laterality of language and establish whether the patient is likely to suffer significant memory impairment as a result of the procedure This investigation, otherwise known as the Wada test, involves selective catheterization of each internal carotid artery in turn Whilst the hemisphere in question is anaesthetized with sodium amytal (effectiveness is confirmed by the onset of contralateral hemiplegia), the patient's ability to speak is evaluated They are then presented with a series of words and images which they are asked to recall once the hemiparesis has recovered, thereby assessing the strength of verbal and non-verbal memory in the contralateral hemisphere

Parietal Lobes

These extend from the Rolandic fissure to the parietooccipital sulcus posteriorly and to the temporal lobe inferiorly The dominant hemisphere shares speech function with the adjacent temporal lobe, while both sides contain the sensory cortex and visual association areas

Page 7Parietal lobe dysfunction may produce cortical sensory loss or sensory inattention In the dominant hemisphere the result is dysphasia and in the non-dominant, dyspraxia (e.g difficulty dressing, using a knife and fork or difficulty with spatial orientation) Involvement of the visual association areas may give rise to visual agnosia (inability to recognize objects) or to alexia (inability to read).

Surgical resections may be undertaken in eloquent parts of the brain either by remaining within the confines of the disease process (intracapsular resection) or by employing some form of cortical mapping This involves either cortical stimulation in awake patients under neuroleptanalgesia or preoperative functional MR which is then linked to an intraoperative image guidance system

ipsilateral limb ataxia Hypotonia, dysarthria and pendular reflexes are other features associated with disorders of the

cerebellum

Surface Markings of the Brain

The precise position of intracranial structures varies, but a rough guide to major landmarks is as follows

Draw an imaginary line in the midline between the nasion and inion (external occipital protuberance) The Sylvian fissure runs in a line from the lateral canthus to three-quarters of the way from nasion to inion The central sulcus (separating the motor from the sensory cortex) lies 2cm behind the midpoint from nasion to inion and joins the Sylvian fissure at a point vertically above the condyle of the mandible

The Cerebral Circulation

Arterial

The arterial anastomosis in the suprasellar cistern is named after Thomas Willis (Fig 1.2), who published his dissections in

1664, with illustrations by the architect Sir Christopher Wren The cerebral circulation is divided into two parts The anterior circulation is fed by the internal carotid arteries, while the posterior circulation derives from the vertebral arteries (the vertebrobasilar circulation) A detailed account of the normal and abnormal anatomy of the cerebral vasculature can be found

CT angiogram of the circle of Willis There is an aneurysm of the middle cerebral artery bifurcation (A).

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Figure 1.3 Subtraction angiogram of the internal carotid circulation

(A) Lateral projection (B) AP projection

from the distal internal carotid artery just beyond the posterior communicating artery The anterior cerebral artery passes over the optic nerve and is connected with the vessel of the opposite side in the interhemispheric fissure by the anterior communicating artery (ACoA) The segment of the anterior cerebral artery proximal to the ACoA is known as the A1 segment and that beyond it as the A2 (until it branches again to form the pericallosal and callosal marginal arteries) The anterior cerebral artery supplies the orbital surface

of the frontal lobe and the medial surface of the hemisphere above the corpus callosum back to the parietooccipital sulcus It extends onto the lateral surface of the hemisphere superiorly, where it meets the territory supplied by the middle cerebral artery The motor and sensory cortex to the lower limb are within its territory of supply

The middle cerebral artery is the largest branch of the circle of Willis It passes laterally behind the sphenoid ridge and up through the Sylvian fissure where it divides into frontal and temporal branches These then turn sharply in a posterosuperior direction to reach the insula The main trunk of the middle cerebral is known as the M1 segment and its first branches at the trifurcation are the M2

segments It supplies much of the lateral aspect of the hemisphere, with the exception of the superior frontal and inferior temporal gyrus and the occipital cortex (Fig 1.4) Within its territory of supply are the internal capsule, speech and auditory areas and the motor and sensory areas for the opposite side, with the exception of the lower limbs

The posterior circulation comprises the vertebral arteries, which join at the clivus to form the basilar artery This gives off multiple branches to the brainstem and cerebellum before bifurcating near the level of the posterior clinoids to become the posterior cerebral arteries (Fig 1.5) The first large branch of the posterior cerebral is the posterior communicating artery (PCoA), thus connecting the anterior and posterior circulations The segment of the posterior cerebral artery proximal to the PCoA is known as the P1 and that which is distal to it as the P2 The P2 then curves posterolaterally around the cerebral peduncle to enter the ambient cistern and cross the tentorial hiatus It is here that the artery may be occluded when intracranial pressure is high (Fig 1.6) Its territory of supply is the inferior and inferolateral surface of the temporal lobe and the inferior and most of the lateral surface of the occipital lobe The contralateral visual field lies entirely within its territory

Arterial Anomalies

In postmortem series, a fully developed arterial circle of Willis exists in about 96% of cadavers, although the communicating arteries will be small in some.3 Because haemodynamic anomalies are associated with an increased risk of berry aneurysm formation, an incomplete circle of Willis is likely to be more common in neurosurgical patients than in the general population

Hypoplasia or absence of one or more of the communicating arteries can be particularly important at times

Figure 1.4

CT head scan showing infarction of the right middle cerebral artery territory Midline structures are slightly displaced towards the side of the lesion, indicating loss of brain volume and that the infarct is therefore long-standing

when one of the major feeding arteries is temporarily occluded, for example during carotid endarterectomy or when gaining proximal control of a ruptured intracranial aneurysm Under such conditions the anastomotic circle cannot be relied upon to maintain adequate perfusion to parts of the ipsilateral or contralateral hemisphere This situation will be compounded by atherosclerotic narrowing of the vessels or by systemic hypotension The areas particularly vulnerable to ischaemia are the watersheds between vascular

territories Some estimate of flow across the ACoA can be obtained angiographically by the cross-compression test During contrast injection, the contralateral carotid is compressed in the neck, thereby reducing distal perfusion and encouraging flow of contrast from the ipsilateral side (Fig 1.7) Transcranial Doppler provides a more quantitative assessment Trial balloon occlusion in the conscious patient is a further method of evaluating crossflow and tolerance of permanent occlusion

The A1 segments are frequently disparate in size (60–80% of patients) In approximately 5% of the population one A1 sement will be severely hypoplastic or aplastic The ACoA is very variable in nature, having developed embryologically from a vascular network It exists as a single channel in 75% of subjects but may be duplicated or occasionally absent (2%) The PCoA is less than 1mm

diameter in approximately 20% of patients In almost 25% of people the PCoA is larger than the P1 segment and the posterior cerebral arteries are therefore supplied primarily (or entirely) by the internal carotid rather than the vertebral arteries Because the posterior cerebral artery derives embryologically from the internal carotid artery, this anatomical variant is known as a persistent foetal-type posterior circulation

If both the ACoA and PCoA arteries are hypoplastic then the middle cerebral territory is supplied only by the ipsilateral internal carotid artery (the so-called 'isolated middle cerebral artery'; Fig 1.8) Such a patient will be very vulnerable to ischaemia if the internal carotid is temporarily occluded during surgery Should it be necessary to occlude the internal

Figure 1.5 Subtraction vertebral angiogram

(A) AP projection (B) Lateral projection

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Figure 1.6

CT head scan showing extensive infarction (low density)

in the territory of the posterior cerebral artery (open arrow)

This was the result of compression of the vessel where it crossed the tentorial hiatus due to raised intracranial pressure

carotid artery permanently, for example in a patient with an intracavernous aneurysm, some form of bypass graft will be required Usually this is between the superficial temporal artery and a branch of the middle cerebral artery (an extracranial–intracranial artery [EC-IC] bypass) The small perforating vessels which arise from the circle of Willis to enter the base of the brain are known as the central rami Those from the anterior and middle cerebral arteries supply the lentiform and caudate nuclei and internal capsule, whilst those from the communicating arteries and posterior cerebrals supply the thalamus, hypothalamus and mesencephalon Damage to any of these small perforators at surgery may result in significant neurological deficit

Microscopic Anatomy

An understanding of the histology of cerebral arteries is particularly relevant to subarachnoid haemorrhage Unlike systemic

muscular arteries, cerebral vessels possess only a rudimentary tunica adventitia Whereas clot surrounding a systemic artery will not result in the development of delayed vasospasm, it is likely that the lack of an adventitia allows blood breakdown products access to

The cross-compression test Contrast has been injected into the left internal carotid artery whilst the right is occluded by external compression in the neck This shows that the distal vessels on the right will fill from the left if the right ICA is occluded and that the ACoA and A1 segments are therefore patent

However, this test alone is not a reliable way of determining that neurological deficit will not ensue if the contralateral carotid artery is permanently occluded

The tunica media of both large and small cerebral arteries has its muscle fibres orientated circumferentially This results in a point of potential weakness at the apex of vessel branches and may lead to aneurysm formation Approximately 85% of berry aneurysms develop in the anterior circulation