Intracranial tumours Primary intracranial tumours account for some 10% of all neoplasms. The most common tumours are outlined in Table 18.47. See also Fig 18.14. Differences between overall annual incidence rates (Table 18.1) and presentation as clinical problems (Table 18.47) are accounted for by the fact that small symptomless meningiomas are commonly found at postmortem. In general medical practice in the UK, metastases are the most common intracranial tumours. Examples of CT and MRI imaging (a) Cerebral haemorrhage in thalamus (b) MR T2 weighted: Infarction of posterior branches of middle cerebral artery (c) MR T2 weighted: Cervical cord and C5/6 disc causing compression (d) CT: Communicating hydrocephalus (IIIrd, lateral and IVth ventricles) (e) CT: Subarachnoid haemorrhage (blood around brainstem) (f) DSA, lateral: Aneurysm of anterior cerebral artery (g) MR T1 weighted: Bilateral subdural haematomas (h) MR T1 weighted: Glioblastoma multiforme (i) MR T1 weighted: Cerebellar metastases (j) MR T1 weighted: Skull base meningioma Image loading here: Gliomas These malignant, intrinsic tumours originate in neuroglia, usually within the cerebral hemispheres. Their cause is unknown. Glioma is occasionally associated with neurofibromatosis. Primary intracranial malignant tumours virtually never metastasize outside the CNS. They tend to spread only by direct extension. Astrocytomas These gliomas arise from astrocytes. They are classified into grades I–IV, depending on histology. Grade I astrocytomas grow slowly over many years, while grade IV tumours cause death within several months. Cystic astrocytomas occur in childhood, usually within the cerebellum. They are relatively benign. Oligodendrogliomas These gliomas arise from oligodendroglia and grow slowly, usually over several decades. Calcification is common. Meningiomas (see Fig 18.14) These benign tumours 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. Sites of predilection are the parasagittal region, sphenoidal ridge, subfrontal region and skull base. Neurofibromas (Schwannomas) These solid benign tumours arise from Schwann cells and occur principally in the cerebellopontine angle, where they arise from the eighth nerve sheath (see p. 1025). Other neoplasms Other less common neoplasms include: • cerebellar haemangioblastoma • ependymoma of the fourth ventricle • colloid cyst of the third ventricle • pinealoma • chordoma of the skull base • glomus tumour – a vascular neoplasm of the jugular bulb • medulloblastoma – a cerebellar tumour of childhood • craniopharyngioma (p. 908) • cerebral lymphoma. Pituitary tumours These are discussed on p. 904. CLINICAL FEATURES Mass lesions within the cranium produce symptoms and signs by three mechanisms: • by direct mass effect on surrounding structures, which are either destroyed or suffer impairment of function from infiltration, pressure or cerebral oedema • by secondary effects of raised intracranial pressure and shift of intracranial contents (e.g. papilloedema, vomiting, headache) • by provoking either generalized or partial seizures. Although neoplasms, either secondary or primary, are the most common mass lesions in the UK, cerebral abscess, tuberculoma, subdural and intracranial haematoma can also produce symptoms and signs that are clinically indistinguishable. Direct effects of mass lesions The hallmark of a direct effect of a mass lesion is local progressive deterioration of function. Tumours can occur anywhere within the brain, but three examples are given below: •A left frontal meningioma (see Fig 18.14) caused a frontal lobe syndrome – a vague disturbance of personality, apathy and impairment of intellectual function over several years. When the speech area became affected, expressive aphasia developed. As the corticospinal pathway became involved, a progressive right hemiparesis developed. As the mass enlarged further, pressure headaches and papilloedema followed. • A right parietal lobe glioma involving the fibres of the optic radiation caused a left homonymous field defect. Cortical sensory loss in the left limbs and a left hemiparesis followed over three months. Partial seizures causing episodes of tingling of the left limbs developed. • A left eighth-nerve sheath neurofibroma (an acousticneuroma or Schwannoma) growing in the cerebellopontine angle over three years caused progressive perceptive deafness (VIII), vertigo (VIII), numbness of the left side of the face (V) and facial weakness (VII), followed by cerebellar ataxia on the same side, as the cerebellar connections were compressed. Papilloedema was a late sequel. With a hemisphere tumour, epilepsy and the direct effects commonly bring the patient to seek medical attention initially. The rate of tumour progression varies greatly, from a few days or weeks in a highly malignant glioma, to several years in the case of a slowly enlarging mass, such as a meningioma. Cerebral oedema surrounds mass lesions: it is difficult on clinical grounds to distinguish its effect from that of the mass itself. Secondary effects and shift of intracranial contents The secondary effect of a mass, the raised intracranial pressure causing the triad of headache, vomiting and papilloedema, is an important, though relatively unusual, presentation of a mass lesion. These symptoms usually imply obstruction to CSF pathways. Typically this picture is produced early by posterior fossa masses (which obstruct the aqueduct and fourth ventricle to produce hydrocephalus) but later 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 brainstem, as midline structures are displaced either caudally or laterally by a hemisphere mass (see Fig 18.14), causing impairment of consciousness. •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, which are false only because they do not point directly to the site of the mass. Three examples of the false localizing signs are: • A sixth-nerve lesion, first on the side of a mass and later bilaterally, is caused as the nerve during its long intracranial course is compressed by expanded brain. • A third-nerve lesion develops as the uncus of the temporal lobe herniates caudally, compressing the third nerve against the petroclinoid ligament. The first sign is ipsilateral dilatation of the pupil as parasympathetic fibres are compressed. •Hemiparesis on the same side as a hemisphere tumour (i.e. the side you would not expect) is produced by compression of the contralateral cerebral peduncle within the brainstem on the free edge of the tentorium. These false localizing signs, though rare, are of importance in clinical neurology and neurosurgery because their appearance indicates that shift of brain has occurred. Seizures Partial seizures, simple or complex, which may evolve to generalized tonic–clonic seizures, are characteristic features of many hemisphere masses, whether malignant or benign. Their clinical site of origin is of localizing value (see p. 1057). INVESTIGATIONS Imaging is the investigation of choice when a tumour is suspected. CT and MR imaging It is important to emphasize that imaging indicates only the site of a mass and not its absolute nature. Cerebral abscess, cerebral infarction, benign and malignant tumours have characteristic, but not entirely diagnostic, appearances. Contrast enhancement adds to the discriminating ability of imaging. EEG The EEG is rarely of major importance in the study of mass lesions. One exception is in cerebral abscess, where characteristic marked focal slow waves are seen. Technetium brain scan This investigation is only of value in the diagnosis of destructive skull vault or skull base lesions. Skull films See also p. 1037. In hemisphere lesions, plain films are of little screening value, except where the vault lesions are present. In pituitary and parasellar lesions they give important information about changes in the dorsum sellae and clinoid processes. Routine tests Since the proportion of cerebral tumours that prove to be metastases is high, routine tests such as a chest X- ray are of value. More specialized neuroradiology Angiography and volumetric MRI are occasionally used to define the blood supply or changing size of a mass. Lumbar puncture Lumbar puncture is contraindicated when the differential diagnosis includes any mass lesion. Examination of CSF rarely yields diagnostically useful information in this situation, and the procedure may be followed by immediate herniation of the cerebellar tonsils. It should be carried out only after imaging, if at all. Biopsy Stereotactic biopsy via a skull burr-hole is usually carried out to diagnose histologically a hemisphere maligancy. Open exploration at craniotomy is usually carried out when a meningioma is suspected. MANAGEMENT Cerebral oedema surrounding a tumour is rapidly reduced by corticosteroids; intravenous dexamethasone is used in an emergency. Intravenous mannitol is used as an osmotic diuretic to reduce cerebral oedema. Epilepsy is treated with anticonvulsants. Whilst complete surgical removal of a brain tumour is an objective, it is not always possible, nor is surgery always necessary. Follow-up with serial imaging is sometimes preferable initially. At surgical exploration, some but not all benign tumours can be removed in their entirety (e.g. some parasagittal meningiomas). With a brain malignancy it is usually impossible to remove the entire infiltrating mass; this is the most common situation. Biopsy and debulking is carried out. Within the posterior fossa, tumour removal is often necessary because of the effects of raised pressure or the imminent danger of coning. However, overall mortality for posterior fossa exploration remains around 10%. An isolated posterior fossa metastasis can sometimes be excised successfully, with prolonged survival thereafter. Radiotherapy is usually carried out for gliomas and for radiosensitive metastases. Chemotherapy has little real value in the majority of primary or secondary brain tumours. With all malignant brain tumours the overall outlook is poor, with less than 50% survival at two years. Difficult issues surround the management of these patients. Idiopathic intracranial hypertension (benign intracranial hypertension) This syndrome, once called pseudotumor cerebri, is included under brain tumours because marked papilloedema occurs. There is neither a mass lesion nor an increase in ventricular size. The condition occurs mainly in obese young women with vague menstrual irregularities. Headaches and visual blurring (caused by severe papilloedema) are common. A sixth-nerve palsy may be present, as a false localizing sign. The CSF pressure is elevated; the constituents are normal. Imaging is normal. Steroid therapy is sometimes thought to be a cause and many other drugs have occasionally been implicated. Other causes of papilloedema should be excluded. Sagittal sinus thrombosis sometimes causes a similar picture. The condition is benign only in that it is not fatal. Infarction of the optic nerve occurs, with consequent visual loss when papilloedema is severe and longstanding. Thiazide diuretics and acetazolamide appear to reduce the intracranial pressure in this condition. Weight reduction is important. Surgical decompression or shunting is sometimes necessary.
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