Primary intracranial tumours account for approximately 10% of all neoplasms. The commoner varieties of tumours are outlined in Table 18.46; around one-quarter are metastases (see Fig. 18.20e).
Differences between overall annual incidence rates (see Table 18.1) and presentation as clinical problems (Table 18.46) are accounted for by the fact that small meningiomas and cerebral metastases are commonly found unexpectedly at post mortem.
These are malignant, intrinsic turnours that originate in neuroglia, usually within the cerebral hemispheres. Their cause is unknown. They are occasionally associated with neurofibromatosis. Primary intracranial malignant turnours virtually never metastasize outside the eNS and spread only by direct extension.
ASTROCYTOMAS. These gliomas arise from astrocytes. They are classified into grades I-N, depending on malignancy. Grade I astrocytomas grow slowly over many years while grade N turnours cause death within several months.
Cystic astrocytomas occur in childhood, usually within the cerebellum. They are relatively benign.
OLIGODENDROGLIOMAS. These arise from oligodendroglia and grow slowly, usually over several decades. Calcification is common.
Meningiomas (see Fig. IS.2Od) These benign turnours arise from the arachnoid membrane and may grow to a large size, usually over years. When close to the skull they erode bone. They often occur along the intracranial venous sinuses, which they may invade. They are rare below the tentorium. Neurofibromas
These arise from Schwann cells and occur principally in the cerebellopontine angle, where they arise from the eighth nerve sheath (see p. S90).
Other less common neoplasms include:
• Cerebellar haemangioblastoma
• Ependymoma of the fourth ventricle
• Colloid cyst of the third ventricle
• Chordoma of the skull base
• Glomus turnour-a vascular neoplasm of the jugular bulb
• Medulloblastoma-a cerebellar tumour of childhood
Mass lesions within the cranium produce symptoms and signs in three coexisting ways: 1 By the direct effects of the mass on surrounding structures, which are either destroyed or suffer impairment of function from infiltration, pressure or cerebral oedema
2 By the effects of raised intracranial pressure and the shift of intracranial contents 3 By provoking either generalized or partial seizures Although neoplasms, either secondary or primary, are the
commonest lesions to cause these effects, cerebral abscess, tuberculoma, subdural and intracranial haematoma may also produce symptoms and signs that may be clinically indistinguishable.
Direct effects of mass lesions
These will depend upon the site of the mass and its speed of growth. The hallmark of a mass lesion is a progressive deterioration of function.
Three examples of possible effects are given below:
1 A left frontal meningioma (see Fig. IS.20d) will cause a vague disturbance of personality, apathy and impairment of intellectual function over several months. When the frontal speech area becomes affected, an expressive aphasia will develop. As the corticospinal pathways become involved, a right hemiparesis will follow.2 A right parietal glioma involving the fibres of the optic radiation will cause a left homonymous field defect. Cortial sensory loss and a left hemiparesis may follow. Partial seizures causing episodes of numbness of the left limbs may develop.
3 A left eighth nerve sheath neurofibroma (an ‘acoustic neuroma’) (see Fig. IS.20t) growing in the cerebellopontine angle will cause progressive perceptive deafness (VIII), vertigo (VIII), numbness of the left side of the
face (V) and facial weakness (VII), followed by cerebellar ataxia as the cerebellar connections are compressed. The direct effects are commonly those that first bring the patient to seek medical attention. The rate of progression will vary greatly from a few days or weeks to several yearsin the case of a slowly enlarging mass. Since cerebral oedema surrounds many mass lesions it is often difficult on clinical grounds to distinguish its effect from that of the mass itself.
Raised intracranial pressure
The triad of headache, vomiting and papilloedema is an important, though relatively unusual, presentation of a mass lesion. These symptoms usually imply that obstruction to CSF pathways has occurred. Typically this picture is produced early by masses within the posterior fossa and as a later event with lesions above the tentorium. Shift of the intracranial contents produces symptoms and signs that coexist with the direct effects of an expanding mass:
DISTORTION OF THE UPPER BRAIN STEM, as midline
structures are displaced either caudally or laterally by a hemisphere mass (see Fig. 18.20), leads to impairment of consciousness (drowsiness progressing to stupor and coma).
COMPRESSION OF THE MEDULLA by herniation of the cerebellar tonsils caudally through the foramen magnum (an example of ‘coning’) causes impairment of consciousness, respiratory depression, bradycardia, decerebrate posturing and death.FALSE LOCALIZING SIGNS APPEAR (‘false’ only because they are not related directly to the site of the mass).
Three examples of false localizing signs are: 1 A sixth nerve lesion, first on the side of a mass and later bilaterally, is caused as the nerve is compressed during its long intracranial course.
2 A third nerve lesion develops as the uncus of the temporal lobe herniates caudally, compressing the third nerve against the petroclinoid ligament and stretching it by downward displacement of the posterior communicating artery. The first sign of this is dilatation of
the pupil as the parasympathetic fibres in the nerve are compressed.
3 Hemiparesis on the same side (i.e. ‘the one you don’t expect’) as a hemisphere tumour is caused by compression of the brain stem (the contralateral cerebral peduncle) on the free edge of the tentorium.
These false localizing signs ‘lfe of importance in clinical neurology because their development indicates that a shift of the brain is taking place. Urgent surgical intervention may be necessary.
Partial seizures, simple or complex, which may evolve to generalized tonic-clonic seizures, are characteristic features of many hemisphere masses, whether malignant or benign. The site of origin of a partial seizure is frequently of localizing value (se p. 913). Generalized tonic-clonic seizures with EEG (but not clinical) evidence of a focal onset also occur.
CT scan is the investigation of choice when the diagnosis of a turnour is suspected. cr scan
It is important to emphasize that CT indicates only the site of a mass and not its nature. Cerebral abscess, cerebral infarction, benign and malignant tumours have characteristic, but not entirely diagnostic, appearances. Contrast enhancement adds to the discriminating ability
of CT and should be used when a mass is suspected.
MRI is often more discriminating than other noninvasive tests, particularly for posterior fossa turnours.
The EEG may show electrical abnormalities in the regionof a mass (but it may be normal): it is rarely of major value in management. The main exception is with cerebral abscess, where the EEG shows characteristic marked slow wave changes.
Technetium brain scan
This is sometimes useful to confirm the site of a lesion shown on CT, but the test discriminates poorly and misses many tumours. Very occasionally this test shows a lesion that has been missed by CT.
Skull films Plain X-rays of the skull are discussed on p. 874. In pituitary and parasellar lesions they do give important information about changes in the dorsum sellae and clinoid processes. In hemisphere lesions, plain films are frequently abnormal but the test has little value as a screening process.
Since the proportion of cerebral turnours that prove to be metastases is high, ‘routine’ tests such as a chest X-ray are of great importance.
Specialized neuroradiology Angiography, ventriculography and other contrast studies may sometimes be necessary to define the site or the blood supply of a mass.
Lumbar puncture is contraindicated when the differential diagnosis includes any mass lesion. Examination of CSF rarely yields diagn stically useful information in this situation, and the procedure may be followed by immediate herniation of the cerebellar tonsils.
If there are pressing reasons for the procedure it should be carried out after careful assessment of the consequences and after a CT scan has been performed.
Surgical exploration and either biopsy or removal of the mass is usually carried out to ascertain its nature. Some benign turnours can be removed in their entirety. adiotherapy is usually recommended for gliomas and for radiosensitive metastases. Chemotherapy is of little value in the majority of primary brain turnours.
Cere ral oedema surrounding a turnour is rapidly reduced by corticosteroids; dexamethasone or betamethasone are used by injection in an emergency. Intravenous mannitol may also be used as an osmotic diuretic. Epilepsy is treated with anticonvulsants. With all malignant brain turnours the overall outlook is poor, with less than 50% survival at I year. Meningiomas are, however, often removed in their entirety and