Based on their location it is useful to divide the tumors in discussing their general behavior into supra-tentorial (cerebral) and infra-tentorials( cerebellar, posterior fossa tumors). Both benign and malignant tumors behave as a mass once they reach a certain size. The behavior of supratentorial tumors acting as a mass lesion has been discussed previously as the same principles apply for masses of other origin. Infratentorial or posterior fossa tumors behave somewhat differently due to specific anatomical differences particular to the infratentorial compartment.
The most common tumors in the posterior fossa are meningiomas, neurofibromas, dermoid or epidermoid cysts, gliomas and metastatic tumors. While their specific treatment differs in some and often significant aspects all share some common characteristics. Similarly other mass lesions in this anatomical compartment, such as blood clot, behave in a similar fashion and the same pathological principles apply.
Tumors in the posterior fossa produce different symptoms than those of the supratentorial region. The small compartment of the posterior fossa, confined below the tentorium and above the foramen magnum, contains cerebellum and brain stem with neuronal centers responsible for the maintenance of blood pressure, respiration, heart rate, and other vital functions. All the cranial nerves originate in the posterior fossa with the exception of the olfactory and optic nerves. The cerebrospinal fluid (CSF) pathways in the posterior fossa easily can be compromised at several points where the CSF passes through naturally narrow passageways such as the aqueduct and perimesencephalic cisterns. These anatomical factors are responsible for what is conceptually a biphasic course of the development of clinical symptoms.
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During the first phase, the slowly growing tumors may cause no symptoms except for nonspecific headaches, sometimes referred to the upper neck and back of the head. Given the large number of middle-aged and older persons suffering from cervical discogenic headaches, this symptom is not particularly useful in establishing the diagnosis. On the other hand, the development of cranial nerve palsies is a useful but late localizing sign because slowly growing meningiomas stretch and displace the cranial nerves without causing a deficit. Spasticity with difficulty walking and balance and loss of manual dexterity is a late sign that clinically cannot be distinguished from the more common cervical spondylosis compressing the spinal cord.
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The transition into the second stage is more ominous and represents the development of compromise of the cerebrospinal fluid circulation. The headaches became much more severe and the patient may develop unsteadiness of gait from either direct compression of the cerebello-pontine connections or due to the stretching of the front-ponto-cerebellar fibers secondary to hydrocephalus. This stage is very dangerous because a process that may have taken several years to develop accelerates at a dramatic rate. If not treated immediately, the massive increase in intracranial pressure may lead to a rapid loss of vision or even sudden death due to a cardiorespiratory dysfunction. Once papilledema and hemorrhages in the optic nerve develop, visual impairment may progress even after successful surgical management of the increased intracranial pressure. Other clinical signs such as depression are not readily associated with a posterior fossa tumor and are not part of its classical description although it may be one of the presenting symptoms. Nausea, vomiting, diplopia, nystagmus, decreased hearing, or facial asymmetry are more common and are related to a specific cranial nerve or brain stem dysfunction. A peculiar distribution of bilateral numbness and weakness of hand muscles has been described in patients who harbor a tumor at the foramen magnum.
The paucity of clinical findings should not delay proceeding with appropriate diagnostic studies. The presence of any of the specific signs or persistence of the nonspecific signs for more than two weeks indicate the need for an MRI scan of the head with and without intravenous contrast. As some meningiomas enhance only with contrast, a tumor may be missed if it is too small to produce a significant mass effect on a noncontrast MRI study. The role of cerebral angiography has changed in recent years. and has been replaced with MRA which is obtained, similar to MRI, without puncturing the artery. Only rarely is it necessary to use cerebral angiogram to provide specific information to the surgeon about the position of major feeding vessels and the position and patency of the transverse and sigmoid sinuses.
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The surgical techniques have undergone significant improvement in recent years. In addition to routine use of the surgical microscope, high performance power drills permit more extensive removal of bone, which provides better exposure and minimizes the manipulation of neural structures. The use of specialized pin head holders allows precise positioning of the head based on the detailed preoperative diagnostic studies. Intraoperatively, the newest generation of bipolar coagulators allows pinpoint coagulation of the bleeding vessels. The resection of certain types of tumor is markedly facilitated by their aspiration after emulsification by an ultrasound instrument. This allows removal of even very large and firm tumors without undue traction on the surrounding structures which has been one of the main sources of mortality and morbidity in the past.New anesthetic techniques also have contributed to successful surgical results. The Swan-Ganz catheter placement preoperatively with computerized evaluation of cardiac function and intraoperative monitoring of cardiac output have been helpful particularly in elderly patients with depressed cardiac function who are subjected to intravenous fluid volume loading and cardio-depressant anesthetic agents during surgery.
The results of the treatment of posterior fossa tumors have improved dramatically in the last ten years. For patients in whom tumor attachment to vital structures prevented complete removal, which was quite rare, the residual tumor is treated with post op radiations. The introduction of Gamma or Linac knife has been a major advance in treatment. Its effectiveness is markedly increased by significant "debulking" of the tumor mass which lessens the tumor load.
Figure 1
Figure 1 - Sagittal reformatted image reveals a large foramen magnum lesion (top). Routine axial cut through the foramen magnum (bottom).
Figure 2
Figure 2 - Postoperative CT scan through the level of C1 shows a complete intradural tumor removal (top). A small portion of the tumor attached to the vertebral artery was left in situ. Postoperative reformatted sagittal image shows a complete intradural tumor removal (bottom).
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