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Journal of Postgraduate Medicine, Vol. 48, Issue 2, 2002 pp. 153-154 View Point Tumour Induced Hydrocephalus and Oedema: Pathology or Natural Defence Goel A Department of Neurosurgery, King Edward VII Memorial Hospital and Seth G. S. Medical College, Mumbai, Indi Code Number: jp02048 The ventricular and subarachnoid cerebrospinal fluid (CSF) is the principal buffer system of the brain assisting in accommodating a mass lesion. An intra-axial tumour and its associated brain reaction in the form of cerebral oedema result in flattening of the gyri, obliteration of the subarachnoid cisterns and narrowing and displacement of the ventricles. In situations where the tumour obstructs the pathway of CSF flow `obstructive hydrocephalus' is a result. Preoperative shunt surgery, temporary drainage of the CSF or diverting its flow has been advised in various situations. This procedure is done in an attempt to reduce the raised intracranial pressure and relieve the patient's symptoms, to normalise the altered cerebral blood flow and to `relax' the brain, which would assist the surgeon in the definitive surgical procedure on the tumour. Although complications following the insertion of a shunt in supra- and infra-tentorial tumours have been occasionally reported, this issue has not been adequately discussed in the literature. In our opinion tumour-obstructive hydrocephalus is a protective response of the brain. The increase in supra-tentorial intracranial pressure is an attempt by the body to push the tumour away from vital structures like the brainstem and hypothalamus. The use of ventricular drainage or any other artificial alteration in CSF pressure prior to surgical resection of the tumour appears to be an irrational strategy of management. Some degree of hydrocephalus is present in almost all cases of medium to large posterior fossa tumours. The initial symptoms in such cases are usually due to hydrocephalus and increased supratentorial pressure. These symptoms more often precede those primarily due to local tumour invasion and compression of the brain stem by a significant length of time. It has been recognised that hydrocephalus and consequent raised intracranial pressure make surgery for these tumours difficult. Initially ventricular taps were employed to deal with hydrocephalus prior to surgery on the tumour. This was then replaced by external ventricular drains and later by shunts. The initial enthusiasm for the routine use of a preoperative shunt procedure has gradually waned with the passage of time. There have been various reports in favour of a preoperative shunt and also against its use.
Cause of Hydrocephalus In the initial phases, tumour growth is compensated for by subsystem responses of the central nervous system. The growing tumour distorts the adjacent brain, which is gradually pushed away. This process results in movement of the brain matter into the adjoining subarachnoid space. Patency of the foramina of Magendie and Luschka and free flow of CSF assure an isopressure system within and around the entire central nervous system so that no single structure is at a greater pressure with regard to any other. Presence of tumour resulting in obstruction to the path of CSF flow is the most accepted and logical explanation of development of hydrocephalus. The concept that the natural response of the brain in the form of enlargement of the ventricles is pathological seems to be inappropriate. It is seen that in most pathologic states the wisdom of the body swings into action to minimise the damage on any tissue. In cases of injury to the knee there occurs swelling of the region, resulting in pain and restriction of the movements of the joint. The swelling of the knee is the response of the body and pain and restriction of movement of the knee joint are the symptoms, which limit the activities of the knee, thus providing the best opportunity for healing. Isolation of an inflamed appendix by the tightly overlayed omentum is another example to show how nature protects the rest of the abdomen from the spreading infection. In the case of brain, it is difficult to imagine that nature would commit a major folly. It needs to be reemphasised that nature has chosen to make the vital centres of the brainstem and hypothalamus in particular more resistant to endogenous or exogenous effects as a part of its survival strategy. The whole science of anaesthesia is dependant on the fact the vital centres in the brain are active and kicking when the rest of the sensory and motor areas of the brain are knocked down by anaesthesia. It is our assumption, based on a large series of cases, that hydrocephalus secondary to obstruction in the path of CSF flow by the tumour is a part of nature's defence mechanism. The hydrocephalus accompanying the brain tumour is obviously a hydrodynamic system operative both within and around the CNS because of the patency of foramen Magendie and foramina of Luschka. We observed that hydrocephalus and increased supratentorial pressure are mechanisms, which tend to push the tumour away from the more sensitive areas of the brain. The net effect achieved by the above homeostatic strategy is that the tumour is denied the right to pressurise adjacent centres or blood vessels by forcibly kept at bay by the CSF push. This was more eloquently observed in cases of posterior fossa vermian and suprasellar lesions. As the tumour increases in size, the supratentorial pressure build-up increases and prevents as much as possible the encroachment of the brainstem and hypothalamus. Symptoms of local pressure are late, while symptoms of increased intracranial pressure secondary to hydrocephalus are the principal presenting symptoms. The ventricular enlargement and rise in intracranial pressure correspond to the size of the tumour. The larger the tumour, the more severe is the ventricular dilatation and the rise in pressure. This suggests that there is an increasing effort on the part of the body to keep the tumour away from vital structures. Whenever local symptoms arise, the situation is urgent and early treatment should be instituted. The pathways of CSF flow are only rarely completely blocked and alternative channels for CSF outflow are almost always present. In situations where the pathways of CSF flow are completely blocked the ventricular pressure will show an acute rise, which could result in transtentorial herniation and other such sequel. Such a severe rise in the intraventricular pressure is rarely seen in the presence of a tumour. In other words, the hydrocephalus per se seldom needs emergency treatment. If symptoms are compelling, decongestive drugs may be used to temporarily tide over the crisis.
Types of Tumours and Hydrocephalus It is generally agreed that any tumour, which obstructs the pathway of CSF flow, will result in hydrocephalus. Large intra-axial brainstem and hypothalamic tumours do not cause hydrocephalus despite their presence in the pathway of flow of CSF. The consensus favours that the growth pattern of the tumour is responsible for the non-development of hydrocep-halus. It may be possible that when structures like the hypothalamus and brainstem are already affected, the brain needs no additional protective mechanical phenomenon in the form of ventricular enlargement and allows the local growth of the tumour.
Effects of Ventricular Drainage Procedures The drainage of CSF from the ventricles will result in immediate relief from the symptoms of increased intracranial pressure. However, secondary to the release of supratentorial pressure there may occur subtle movements in the tumour towards the direction of CSF drainage. Such movements in the tumour and in the adjoining brain have been reported to be the cause of reverse tentorial herniation in cases of large posterior fossa tumour. It was observed that following the release of lateral ventricular CSF, the initial phase of surgical procedures like dural incisions, cortical manipulation and retraction of the brain become relatively safer. However, as movements of the tumour are towards the brainstem or hypothalamus, surgical dissection of the tumour from these structures could become difficult and dangerous. Dilatation of the contralateral ventricle in hemispheric tumour could be an attempt to reduce the movement of the brain that may result in subfalcine herniation and anterior cerebral-pericallosal arterial stretch and compromise, and damage to the corpus callosum and medial surface of the hemisphere. An attempt to drain the dilated lateral ventricle of the contralateral side may exaggerate the subfalcine herniation and result in neurological worsening.
Role of Cerebral Oedema in Brain Protection The extent of tumour-reactive cerebral oedema varies with the type of tumour. In our experience, the cerebral oedema was much greater in more malignant, more vascular and more infective varieties of tumours. Cerebral oedema is a reaction to the presence of the tumour. It is possible that the reaction of the brain, in the form of oedema, to the presence of the tumour is a protective mechanism of the brain as well. Cerebral oedema and the resultant local and generalised rise in intracranial pressure could help in limiting the spread of malignant tumour cells and infection. Disproportionately extensive cerebral oedema around small abscesses lends some support to the hypothesis. It is possible that circumferential cerebral oedema around the tumour could provide support to delicate and abnormal tumour blood vessels and prevent them from rupturing. The cerebral oedema and the resultant rise in intracranial pressure are the cause of the symptoms. On the basis of these observations it appears that unless the symptoms of raised intracranial pressure are crippling, the primary aim of the surgeon should be directed towards the treatment of the pathological lesion and not towards the treatment of cerebral oedema. The treatment of cerebral oedema should be only in the perioperative phase and aimed towards obtaining a relatively lax brain, which may facilitate safe tumour resection and avoid cortical damage due to retraction and manipulation. Treatment directed to the sole aim of treating the natural brain reaction or cerebral oedema and raised intracranial pressure by decongestant drugs and decompressive craniectomies could be disastrous.
Goel A Department of Neurosurgery, King Edward VII Memorial Hospital and Seth G. S. Medical College, Mumbai, India. This article is also available in full-text from http://www.jpgmonline.com/
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