With a parasagittal mass, the ipsilateral cingulate gyrus may herniate beneath the free edge of the falx (Fig. 24.3A). The anterior cerebral artery may be compressed sufficiently to cause medial hemispheric infarction, but otherwise there are no obvious clinical signs except deteriorating conscious level.

Fig. 24.3 Herniation syndromes.

A Coronal diagram of the dynamics of an intracranial mass lesion. Tumour (T) causes mass effect that compresses the midline and the ventricles (unnumbered arrows), provoking subfalcine or cingulated herniation (arrow 1), and transtentorial herniation (arrow 2). If the mass effect is uncontrolled, tonsillar herniation may also occur (arrows 3). B Transtentorial herniation in the axial plane (plane of dotted line in Fig. 24.3A). C Foraminal herniation.

Transtentorial (uncal) herniation

With large ipsilateral brain lesions, the medial part of the temporal lobe is pushed down through the tentorial notch to become wedged between the tentorial edge and the midbrain (Fig. 24.3B). The opposite cerebral peduncle is pushed against the sharp tentorial edge, and the midbrain and uncus become wedged at the tentorium. The aqueduct is compressed, obstructing CSF flow, and venous obstruction leads to midbrain haemorrhage. The clinical features of an uncal herniation, most often due to a traumatic intracranial haematoma, are:

Table 24.1 Glasgow Coma Scale

Eyes open • Spontaneously	4
• To verbal command	3
• To pain	2
• No response	1
Best motor response To verbal command
• Obeys verbal command	6
To painful stimulus
• Localizes pain	5
• Flexion withdrawal	4
• Abnormal flexion (decorticate rigidity)	3
• Extension (decerebrate rigidity)	2
• No responses	1
Best verbal response
• Orientated and converses	5
• Disorientated and converses	4
• Inappropriate words	3
• Incomprehensible sounds	2
• No response	1
Total number of points (minimum 3, maximum 15)

Foraminal (tonsillar) herniation

With mass lesions of the posterior cranial fossa, the cerebellar tonsils and medulla are displaced downwards through the foramen magnum (Fig. 24.3A and C). Cerebellar impaction leads to medullary compression. Following traumatic or spontaneous haematomas, this can lead to a dramatic decrease in the GCS, acute hypertension, bilateral extensor responses and bilateral fixed dilated pupils, followed by sudden respiratory arrest. A similar syndrome may occur following the removal of CSF at lumbar puncture in patients with raised ICP due to a posterior fossa tumour, and is also known as ‘coning’. There is a rapid deterioration in conscious level, with decerebration. Lumbar puncture must not be performed in patients suspected of having raised ICP due to a mass lesion.

Summary Box 24.1 Intracranial pressure

• The rigid bony framework enclosing the central nervous system means that any increase in mass content increases intracranial pressure (ICP)

• Acute increases in ICP lower perfusion pressure and, if unrelieved, lead progressively to decreased Coma Score, herniation syndromes, bradycardia, hypertension, respiratory abnormalities (e.g. apnoea), vasoparalysis and death

• The principal symptoms of chronic raised ICP are headache, vomiting and visual disturbance (blurring of vision). Papilloedema may be apparent

• If ICP is due to a unilateral mass lesion, intracranial structures may be displaced. There are three major forms of herniation: subfalcine, transtentorial and foraminal.

Investigations

Plain X-ray

Plain X-rays of the skull and spine may reveal evidence of metastatic tumour spread, narrowing of the intervertebral discs, congenital abnormalities, bony erosion due to tumour (e.g. of the pituitary fossa) or abnormal

Summary Box 24.2 Vascular disorders and the central nervous system

• Symptoms due to occlusive vascular disease most often originate from blockage of extracranial vessels by atherosclerosis. Intracranial occlusion can be thrombotic or embolic

• Transient ischaemic attacks are associated with a high risk of major stroke within 5 years unless treatment is instituted (e.g. by carotid endarterectomy or aspirin therapy)

• Intracranial haemorrhage may be extradural, subdural, subarachnoid or intracerebral. Extradural and subdural haemorrhage are usually the result of trauma; subarachnoid bleeding is due to rupture of an aneurysm in around 70% of cases; and intracerebral bleeding is frequently associated with hypertension and amyloid angiopathy

• Patients with subarachnoid haemorrhage from an aneurysm should be considered for either clipping or coiling of the aneurysm to avoid recurrent bleeding.

vascular markings. Calcification of the pineal gland or choroid plexus may allow displacement to be detected, and abnormal calcification can develop in certain cysts and tumours. This investigation has, however, largely been superseded by computed tomography and magnetic resonance imaging.

Computed tomography (CT)

Although CT does not image brain tissue as well as magnetic resonance imaging, it is particularly good for visualizing bony tissue and has the advantage that images can be acquired more simply and rapidly. It is the investigation of choice following trauma and for imaging spontaneous intracerebral haematomas. Modern spiral CT scanners allow the craniofacial skeleton and blood vessels (CT-angiography, or CT-A) to be reconstructed in remarkable detail.

Magnetic resonance imaging (MRI)

Magnetic resonance images can be reconstructed in axial, coronal or sagittal planes. As the water in bony tissue is tightly bound, the cranium is not as well visualized as it is with CT. However, as the cortical grey matter comprises approximately 78% water and white matter approximately 68% water, these two areas, as well as subnuclei within the brain, can be clearly distinguished. CSF is also well imaged. By using different magnetic pulse sequencing techniques, the anatomy of the normal and abnormal brain can be imaged in millimetre detail. With both CT and MRI, an intravenous enhancing agent can be administered (iodine-based compounds with CT and gadolinium-based for MRI). Both of these compounds are normally excluded from the brain parenchyma by the blood–brain barrier (BBB). However, with either neoplastic, inflammatory or ischaemic breakdown of BBB integrity or the development of tumour neovascularity, enhancement will be seen in areas involved with the pathological process. MRI is limited by certain conditions (cardiac pacemakers, early pregnancy, some metallic aneurysm clips).

CT and MR angiography

By employing certain contrast agents and rapid sequence imaging, both MRI and CT can be used to perform angiography (MRA, CT-A). In some circumstances, these non-invasive techniques are replacing conventional intra-arterial digital subtraction angiography (IA-DSA). More complex imaging sequences can be used to show white matter disorders, subtle changes in brain water (diffusion tensor imaging) and chemical signals from the brain (magnetic resonance spectroscopy). The function of brain tissues can also be evaluated, since active areas of the brain will have increased blood flow (functional MRI).

Cerebrovascular disease

Stroke is a common major clinical disorder in clinical neuroscience practice. Occlusive disease often affects the extracranial vessels and is a common cause of stroke and transient ischaemic attack, usually managed by vascular surgeons (Ch. 25). Most forms of embolic and ischaemic stroke are dealt with by medical neurologists, whereas many large primary intracerebral haemorrhages and, more importantly, subarachnoid haemorrhage (SAH) are dealt with by the neurosurgeon. Brain tissue metabolism is vitally dependent on a consistent delivery of oxygen and glucose substrates for energy. If there is cessation of substrate delivery, the brain tissue will either die (if CBF is below a threshold of 12–15 ml/ 100 g/min) or stop functioning (if CBF is between 15 and 25 ml/100 g/min). These ischaemic thresholds are very important in terms of the extent of stroke (i.e. the amount of tissue that will die) and the penumbra (i.e. tissue that is damaged but still able to recover from these acute events). Urgent thrombolysis in ischaemic stroke is possible if the patient reaches hospital within 4 hours of onset of symptoms and can improve prognosis. Decompressive craniectomy can also save life in selected cases if raised intracranial pressure due to ischaemic brain swelling is a problem.

Subarachnoid haemorrhage

Spontaneous SAH affects 100 persons per million per year and most frequently (70%) results from the rupture of an intracranial ‘berry’ aneurysm. Other causes include arteriovenous malformation (AVM), cavernoma, tumour, infection and trauma. Typically, the patient complains of a sudden onset of severe headache that peaks in intensity within 1 minute. Patients often describe it as like being ‘hit on the head with a hammer’ or as ‘the worst headache they have ever had’. There is usually associated neck stiffness and photophobia. A positive Kernig’s sign denotes meningism. In some cases, a small ‘herald’ bleed may go unnoticed, or is only remembered when a major bleed occurs. Nausea and vomiting are common. The patient’s conscious level is variably affected, ranging from mild disorientation to coma to rapid death.

Grading of SAH depends on the coma score of the patient at time of presentation. The most widely used system, and also the easiest to use, is the World Federation of Neurosurgical Societies’ (WFNS) grading, which goes from grade 1 to grade 5 (Table 24.2). When the SAH has produced a syndrome of grade 2–5, it is quite apparent that some neurological catastrophe has occurred. However, when the syndrome is of a grade 1 haemorrhage, differential diagnosis is quite extensive and not infrequently the primary event is overlooked. SAH can mimic atypical migraines, thunderclap headache, coital cephalgia, pituitary apoplexy and meningitic-like syndromes. Sudden death is not uncommon when an aneurysm ruptures into the brain substance rather than the subarachnoid space. The focal signs depend upon the vessel affected. When symptoms and signs are mild, the differential diagnosis is quite extensive and diagnosis depends on having a high index of suspicion.

Table 24.2 World Federation of Neurosurgical Societies (WFNS) grading system for subarachnoid haemorrhage

WFNS grade	Glasgow Coma Score	Focal neurological deficits
1	15	No
2	13–14	No
3	13–14	Yes
4	9–12	–
5	3–8	–

Saccular intracranial aneurysms

Risk factors include smoking, hypertension, polycystic disease of the kidney and female gender. The median age of affected patients is 47 years, although familial aneurysms may rupture earlier. Most aneurysms (85%) affect the anterior circulation and 15% of patients have more than one. The majority have no symptoms until rupture occurs, although a few may suffer compressive symptoms, and ectatic aneurysms may not rupture at all but cause symptoms by embolic phenomena. The most common compressive clinical signs are features of oculomotor nerve (CN III) palsy due to an aneurysm of the posterior communicating artery. Many intracranial aneurysms are now discovered as incidental findings when the brain is imaged.

Investigations

The standard investigation is CT, which characteristically shows blood in the CSF basal cisterns in the acute phase (Fig. 24.4). As CSF blood is broken down, the CT detection rate falls after the first 72 hours. If the diagnosis is in doubt, then a lumbar puncture should be performed, but only in patients whose clinical condition is good and in whom CT has excluded an intracranial mass lesion or midline shift. If this is performed early, the CSF will be uniformly blood-stained; later it will contain haem pigments that will be apparent on naked-eye inspection (xanthochromia) or can be detected by spectrophotometry.

Fig. 24.4 Axial CT scan of a patient following subarachnoid haemorrhage.

Blood is seen in the basal cisterns as a white lesion (solid arrow), and also in the fourth ventricle (broken arrow). There is early enlargement of all ventricles. Courtesy of Dr R Gibson.

Next, a search must be made for the site of the bleeding. About 80% of aneurysms involve the anterior circulation. Conventionally, carotid or vertebral angiography has been performed (Fig. 24.5). However, CT and MR angiography are equally good at revealing aneurysms greater than 5 mm, and are non-invasive and so associated with less morbidity. As the consequences of missing a diagnosis of SAH are serious, there is a tendency to perform angiography in patients in whom the diagnosis is equivocal. For that reason, a source of bleeding will be identified in only 70% of angiograms. This is usually an aneurysm, less commonly an AVM or a cavernoma. In 30% of cases, no source is found. In the majority, this is a ‘true’ negative, and many of these patients will have a condition called perimesencephalic SAH, which is of unknown aetiology.

Fig. 24.5 Intra-arterial digital subtraction angiogram outlining the cerebral vasculature and showing an aneurysm of the anterior communicating artery (arrow).

Management of aneurysmal SAH

The medical management of SAH includes intravenous fluids; the calcium antagonist, nimodipine (EBM 24.1); analgesia; and antiemetics. Patients in coma will usually be intubated and managed in a neurointensive care unit. Having initially been quite well, many patients begin to exhibit signs of focal or global cerebral ischaemia 4–10 days following an SAH. This has been attributed primarily to vasospasm and can be ameliorated by the prophylactic use of nimodipine; however, clinical deterioration may occur due to hydrocephalus, seizures, metabolic abnormalities and systemic infections.

24.1 Nimodipine and subarachnoid haemorrhage (SAH)

‘Nimodipine, a calcium channel antagonist, significantly reduces both death and stroke following SAH.’

Pickard JD, et al. BMJ 1989; 298:636–642.

Rebleeding is a major cause of morbidity and mortality following aneurysm rupture. Until recently, standard management was occlusion of the aneurysm from the cerebral circulation by surgically clipping its neck (Fig. 24.6 and EBM 24.2). However, it is now possible to place detachable coils within the aneurysm via a catheter passed from the femoral artery into the cerebral circulation (Fig. 24.7). The coils unwind in the aneurysm and induce thrombosis. Coiling is a much less invasive procedure than open surgery and a recent prospective randomized controlled trial showed that, when an aneurysm can be treated by surgery or coiling, the latter is safer (EBM 24.3). Previously, only a proportion of aneurysms were suitable for coiling. However, improvements in coil, stent and basket technology mean most aneurysms can now be coiled.