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Neurosurgery

C. B. T. ADAMS

INTRODUCTION

The delicate function of the central nervous system, the need for minimal brain retraction, and therefore for exact operative exposure, as well as the overwhelming requirement for good surgical and neurological judgement, have determined the development of the neurosurgery. Despite the advent of modern methods of investigation, a careful history and examination are still paramount in the assessment of patients.

The response of the central nervous system to surgery is essentially the same as it is to other parts of the body, and the neurosurgeon must apply the basic principles of surgical technique: careful handling of tissue, minimal tissue necrosis, elimination of dead space, and avoidance of haematoma formation by careful haemostasis. Like surgery elsewhere neurosurgery is concerned with removing lumps, cysts, and diverticula; with relieving destruction, eliminating sinuses and fistulae; with stopping haemorrhage, removing pus, and improving or altering function. Knowledge of these basic surgical disorders can be applied to all areas of surgery including neurosurgery.

DIAGNOSIS

Structural and functional diagnosis

These are made from the history and examination. When dealing with patients who have disorders of intellect or speech, relatives must be interviewed. The history and examination will indicate whether the patient's weakness, for instance, is upper or lower motor neurone in origin or due to a muscle disorder, and whether disorders such as dysphasia or dysarthria are present. Problems that not have an anatomical or pathological basis, such as migraine or idiopathic epilepsy, are best classified as structural or functional. One must also remember to examine the head for a lump which might indicate an underlying meningioma. Surgeons should be aware of a midline ‘sebaceous cyst’; this may be a dermoid cyst tracking into the posterior fossa. The contents of the cyst may create a fistula in the occipital or lumbar region, and cause meningitis.

Anatomical diagnosis

Intracranial disorders present with either one or more of raised intracranial pressure, meningeal irritation, and focal neurological signs. The absence of these features in the history or examination usually indicates the absence of serious disease. However, caution should be exercised when examining an individual who is receiving high doses of steroids, which may mask raised intracranial pressure.

One decides which of these anatomical disorders is present on the basis of the disorders of structure and function. Diagnosis of focal neurological signs does not usually require a profound knowledge of neuroanatomy (Fig. 1) 2241. It is not difficult to decide whether the focal signs arise from a focal lesion, multiple lesions, or a diffuse lesion.

Pathological diagnosis

Only when the other diagnostic groups have been considered should one consider the pathological diagnosis. The possibilities are congenital, traumatic, inflammatory, neoplastic, vascular, degenerative, metabolic, and toxic. Factors which should be considered in reaching this diagnosis include structural and functional diagnosis, anatomical diagnosis, age and sex of the patient (middle-aged women are prone to meningiomas; children to cerebellar astrocytomas or medulloblastomas), speed of onset of the disorder (sudden or progressive), and the subsequent course of the illness (progressive almost always suggests an enlarging mass; fluctuating suggests a cystic lesion). Multiple lesions are suggestive of metastases, cerebral lymphomas, or multiple sclerosis. The presence of disorders elsewhere, such as primary carcinoma or neurofibromatosis should also be considered.

Raised intracranial pressure

Aetiology

The normal intracranial pressure measured at lumbar puncture is less than 200 mm of water.

Accumulation of CSF (hydrocephalus)

This is caused by obstruction to the normal flow of CSF either in the ventricular system (obstructive hydrocephalus) or in the subarachnoid space (communicating hydrocephalus) secondary to any persistent blood or pus in the subarachnoid space. Occasionally, excessive production of CSF may be responsible, as is seen in patients with a papilloma of the choroid plexus.

Increased bulk

Any space-occupying lesion, tumour, abscess, cyst, or blood clot will raise intracranial pressure by reason of its own bulk or by causing a blockage of CSF pathways. Benign intracranial hypertension due to cerebral swelling has an unknown cause, but is usually seen in overweight young women, and may cause severe papilloedema. This syndrome may also be caused by obstruction to venous drainage of the brain.

Increased volume of intracranial blood

Accumulation of carbon dioxide due to respiratory insufficiency causes vasodilation of cerebral blood vessels and raised intracranial pressure—hence the importance of maintaining the airway in head-injured patients. Obstruction of the large venous sinuses will cause impaired cerebral venous drainage and brain swelling. The ventricular size will be normal, unlike the situation found in hydrocephalus. Benign intracranial hypertension is characterized by severe papilloedema with retinal haemorrhages in a patient who is otherwise well, apart from possibly showing a sixth nerve palsy and an extensor plantar response. Benign is a misnomer: this condition may cause blindness unless treated early.

Relative diminution in the size of the skull

Craniosynostosis is characterized by premature fusion of the cranial sutures, which prevents normal enlargement of the skull pari passu with brain growth. The treatment of this condition is considered elsewhere (Chapter 45.2) 308.

Patients not infrequently present with raised intracranial pressure without focal signs. Hydrocephalus due to an intraventricular mass such as a colloid cyst of the third ventricle or a tumour of the fourth ventricle should be considered. A tumour in the silent part of the brain (the non-dominant frontal lobe or in the region of the tentorium) is also a possibility.

Multiple tumours, typically metastases surrounded by oedema, may cause raised pressure but little in the way of focal signs. Bilateral subdural haematomata can present with raised pressure without focal signs. Finally benign intracranial hypertension due to venous sinus obstruction or cerebral oedema should be considered if there is no evidence of a mass or hydrocephalus, though a diffusely infiltrating astrocytoma can mimic this condition quite closely.

Pathology

The most important effect of raised intracranial pressure is the distortion and shift of the brain-stem. Supratentorial lesions force the ipsilateral temporal lobe through the tentorial notch, applying pressure on the horizontally directed ipsilateral third cranial nerve. This causes a dilated pupil, and the pressure on the midbrain causes drowsiness. Eventually the contralateral cerebral peduncle is pressed against the opposite margin of the tentorial notch, causing pyramidal signs on the side of the lesion and eventually damage to the contralateral third cranial nerve and bilateral fixed dilated pupils. Severe distortion of the brain-stem causes decerebrate posturing, characterized by extension of the limbs, hyperpronation of the hands and wrists and hyperventilation in response to any stimulus. The delicate perforating arteries entering the brain-stem become stretched and eventually rupture, causing irreversible damage and death.

The posterior cerebral artery may also become compressed as it passes around the midbrain by the ipsilateral temporal lobe, distorting the artery against the tentorium. (Figs. 5 and 6) 2245,2246. This produces occipital lobe infarction and a homonymous hemianopia, which is permanent if the patient recovers.

Bilateral lesions, such as bilateral chronic subdural haematomata, force both temporal lobes through the tentorial notch, compressing the brain-stem and pushing it downwards. This causes bilateral ptosis and limitation of upward gaze due to compression of the superior colliculi, as well as eventually, dilated and fixed pupils. More slowly progressive lesions cause bilateral sixth nerve palsies as these vertically directed nerves become stretched across the petrous bone when the brain-stem descends.

Posterior fossa lumps cause ‘tonsillar coning’ when the cerebellar tonsils are forced through the foramen magnum, thus squeezing the medulla oblongata. This produces neck stiffness with slowing, irregularity, and finally cessation of respiration. The latter may occur very suddenly, although the blood pressure is maintained. The correct treatment is an emergency burr hole and ventricular drainage to relieve the hydrocephalus.

Any sudden rise in intracranial pressure causes reflex slowing of the pulse and elevation of blood pressure (Cushing reflex). Patients with long-standing hydrocephalus (i.e. aqueduct stenosis) show gross dilatation of the lateral ventricles. This causes compression on the skull vault, which becomes thin, particularly over the cerebral gyri (Fig. 7) 2247. The infundibular recess of the third ventricle becomes dilated and herniates into the pituitary fossa. Hypopituitarism may develop with stunted growth and incomplete sexual development.

Features of raised intracranial pressure

The most important symptom of raised intracranial pressure is headache that is worse in the morning and may wake the patient up at night. The site and quality of the headache are less helpful, but any severe unusual headache should be taken seriously. The headache is often associated with vomiting and the diagnosis of migraine is made all too often. ‘Acute migraine’—that is, headache of only a few months duration, is an extremely dangerous diagnosis; it is usually associated with raised intracranial pressure. Vomiting unassociated with or preceding the headache may be due to a lesion (such as an ependymoma) of the floor of the fourth ventricle and may well defy gastrointestinal diagnostic efforts.

The most important sign of raised intracranial pressure is drowsiness; this is much more important than papilloedema, though the latter, when present, is of great importance. However patients may lapse into coma and die from raised intracranial pressure before papilloedema has time to develop (Fig. 8) 2248: its absence does not exclude the diagnosis. Severe and sudden raised intracranial pressure may cause retinal haemorrhages, which may rupture into the subhyloid space, producing a subhyloid haematoma. The Glasgow coma scale (Table 2) 590 is a well established clinical method of observing and recording the depth of coma. This scale should be used rather than vague terms such as stupor, semicoma, and coma.

Brain-stem distortion causes slowing of the pulse, elevation of the blood pressure, and so-called ‘false localizing signs’ of tentorial herniation or foramen magnum coning, depending on the site of the mass.

Infants show vomiting and drowsiness, as well as an increase in circumference of the head, which has a ‘crackpot’ sound when percussed, due to spreading of the sutures (Fig. 9) 2249. Bulging of the anterior fontanelle is an important sign. Papilloedema is less common, and although retinal haemorrhages may occur, they usually indicate a subdural haematoma in this age group.

Treatment

This is directed to the cause. Any space-occupying lesion is removed if possible; obstructions to CSF circulation are removed or bypassed. Any sudden rise in pressure may be countered by administration of high dose steroids, or in a more acute situation, 20 per cent mannitol intravenously. Of course it is essential to maintain the airway at all times, particularly when the patient is drowsy.

Meningeal irritation

Meningeal irritation is a useful clinical entity similar to peritoneal or pleural irritation, with the same causes, including blood, pus, cancer cells, or chemicals. The features of meningeal irritation are headache, which may be associated with neck ache, backache, and pain extending down the backs of the legs. These pains reflect the distribution of blood or pus within the subarachnoid space. Vomiting is common, and photophobia is more severe than one would expect with the degree of headache. The signs are those of limitation of neck flexion (also caused by foramen magnum ‘coning’ from a posterior fossa mass and, of course, by something wrong with the neck) together with limitation of straight leg raising.

The latter sign is also a feature of any lump pressing on a lumbar or sacral nerve root, usually a prolapsed lumbar disc. The limitation of straight leg raising is then usually unilateral. A fever may occur with meningeal irritation of any cause. A lumber puncture and examination of the cerebrospinal fluid is necessary to elucidate the cause.

Subarachnoid haemorrhage

This is one diagnosis which can be made over the telephone. The headache has two characteristic features: it is sudden, and unusual. It is important to be alert to this diagnosis, since these patients require urgent hospital admission even if the symptoms are mild or improving, (although the headache usually lasts 24 h or more).

A lumbar puncture is necessary to establish the diagnosis. CT scanning will show subarachnoid blood but will not exclude a minor haemorrhage: if a CT scan is normal then a lumbar puncture is mandatory. Cerebrospinal fluid must be spun down: xanthochromia of the supernatant fluid demonstrates the presence of blood in the cerebrospinal fluid for at least a few hours before the lumbar puncture, differentiating it from bloodstained fluid due to a traumatic puncture. Subarachnoid haemorrhage needs full investigation by carotid and vertebral angiography. The most common cause is a ruptured aneurysm: ruptured arteriovenous malformation, a primary cerebral haemorrhage, and bleeding tumours are other possibilities. Sometimes no cause is found, even after full investigation.

Meningitis

This diagnosis indicated by an increased white cell count in the cerebrospinal fluid in association with meningeal irritation. It is of surgical importance as it may occur after head injuries, particularly a fracture involving the paranasal air sinuses. This cause should be considered with any case of pneumococcal meningitis, even if it occurs some years after a head injury. Such meningitis may cause coma within a few hours. Lumbar puncture is a better method of investigating meningeal irritation than is CT scanning, since the latter will never diagnose meningitis. Meningitis may also be associated with a dermoid fistula (often marked by a lump or dimple over the vertex of the skull) connecting the scalp with the posterior fossa. A skull radiograph shows a bony groove and channel in the region of the torcular; such an examination should be undertaken before excision of a midline ‘sebaceous cyst’ from the back of the head.

Carcinomatous meningitis

This occurs when a secondary deposit adjacent to the ventricle seeds into the cerebrospinal fluid. Malignant gliomas, particularly medulloblastomas in children, may also behave in a similar fashion. The patient is ill and often in severe pain due to deposits on the nerve roots, and the cerebrospinal fluid contains malignant cells, high levels of protein, and a low level of sugar. The diagnosis is best made by myelography.

Chemical meningitis

This is usually iatrogenic, and follows introduction of substances into the subarachnoid space during lumbar puncture. Occasionally a ‘cholesterol’ meningitis may occur secondary to leakage of cholesterol from a dermoid or an epidermoid cyst and very occasionally, from a craniopharyngioma. This may follow surgery, and very occasionally occurs spontaneously.

INVESTIGATIONS

Investigations should only be undertaken after careful clinical assessment has produced a likely clinical diagnosis. Talking to the patient may have profound implications for the surgeon and may determine, for instance, a more cautious excision of a tumour to cause less risk of disability in certain domestic circumstances.

Patients with intracranial lesions should routinely undergo chest radiography to exclude carcinoma of the lung, as well as routine blood and urine tests.

Plain radiography of the skull

This is still a useful investigation and may show evidence of raised intracranial pressure by erosion of the dorsum sella or thinning of the skull vault by cerebral convolutions (copper beating). In children and infants, spreading of the sutures and enlargement of the head may be apparent (Figs. 7, 9) 2247,2249. Local bone erosion, bony metastases, hyperostosis due to a meningioma, or expansion of the sella turcica indicating a pituitary tumour can also be seen on the plain radiographs of the skull. Shifting of a calcified pineal gland, vertically or horizontally; is useful information, and calcification may be apparent in tumours. Radiographs of the spine may show bone collapse or erosion of the pedicles of the vertebral bodies by secondary deposits, while scalloping of the posterior surface of a vertebral body or enlargement of an intravertebral foramen usually indicates a benign neoplasm such as neurofibroma.

Lumbar puncture

This is indicated when there is ‘meningeal irritation’, but is contraindicated in patients with evidence of raised intracranial pressure, especially if a posterior fossa mass lesion is likely. Evidence of cord compression requires a lumbar puncture and myelography, but this investigation is best done in a neurosurgical unit as function may deteriorate following this investigation and may require urgent surgery. Lumbar punctures are also performed to obtain cerebrospinal fluid for assessment of protein levels, for instance, in patients with suspected multiple sclerosis.

CT scanning

The introduction of CT scanning has dramatically changed the ease with which neurosurgical investigation can be performed. CT scanning demonstrates intracranial blood and calcium (Figs. 10, 11) 2250,2251, and the ventricles as well as the brain and skull. It demonstrates most space-occupying lesions (Figs. 12, 13, 14) 2252,2253,2254, although an isodense subdural haematoma may be misinterpreted as brain swelling. CT scanning is not as sensitive as is MRI for showing small or subtle intracerebral lesions.

Angiography

The main indication for angiography is the investigation of subarachnoid haemorrhage following its confirmation by CT scanning or lumbar puncture. Angiography demonstrates intracranial aneurysms, arteriovenous malformations, and the pathological circulation of some tumours which, if very vascular, can be embolized preoperatively.

MRI

This method of imaging depends on exciting molecules in a magnetic field. Its inability to show calcification, bone, or blood clot is offset by the lack of bone artefact. It is the diagnostic method of choice for small acoustic neuromas within the internal auditory meatus or small pituitary tumours in the pituitary fossa. It is also superior to CT scanning in demonstrating small lesions in the hemisphere, brain-stem, and spinal cord. Cavernous angiomas are particularly well shown, as are brain-stem or spinal cord (intramedullary) gliomas. The multiple paraventricular lesions of multiple sclerosis are well demonstrated by MRI. MRI has disclosed for the first time multifocal low density lesions associated with comparatively mild head injuries. These are not seen on CT scanning but confirm that mild injuries can have profound effects on brain function. Affected patients require careful sympathetic counselling.

Electroencephalography (EEG)

This has little value in neurosurgery except in the investigation of patients with epilepsy. Neurophysiological peroperative monitoring of cranial nerve or spinal cord function is likely to be used more often in the future.