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Meningiomas
Meningiomas are tumors which arise from the so-called arachnoidal cap cells. You can forget that bit of trivia right now. For all practical purposes, meningiomas arise from the dura. The dura mater is a membrane which covers the entire interior of the skull. In addition, it creates compartments within the skull: the falx which separates the right side of the brain from the left side and the tentorium which forms a shelf which separates the cerebrum from the cerebellum and brainstem. The later structures are housed in the posterior fossa below the tentorium - the infratentorial space.
Dura also encloses a space on the inside of the skull located on each side of the sella where the pituitary gland sits. This space is called the cavernous sinus. In this space runs the carotid arteries which supply the front 2/3rds of the brain, a large network of veins which drain the fron 2/3rds of the brain and several cranial nerves which go to the muscles which move the eyeball and get sensation from the upper face. This space is called the cavernous sinus.
The reason for discussing the parts of the dura mater is that meningiomas are usually named for the part of the dura they come from. Hence, a meningioma starting in the falx is called a falx meningioma, a meningioma in the tentorium is called a tentorial meningioma, a meningioma in the cavernous sinus is called a cavernous sinus meningiomaand so on. Meningiomas located over the outer surface of the brain are called convexity meningiomas if they do not involve the dura covering the midline vein called the sagittal sinus. When they do involve the sagittal sinus, they are called parasagittal meningiomas.
Some meningiomas are named for the bone which the tumorous dura covers. The sphenoid wings, for example, are bones located behing the eye socket. There are two parts; greater wing(which forms the back of the eye socket) and lesser wing(which forms the back roof of the eye socket). Meningiomas from the dura covering these are called sphenoid meningiomas. The place where the greater and lesser wings meet is called the sphenoid ridge and is a favorite spot for meningiomas. Surgeons like to classify sphenoid ridge tumors still further: the lateral third, middle third and medial third of the sphenoid ridge. This is important in assessing surgical risk. Lateral and middle third meningiomas are usually less complicated to remove than medial third meningiomas which are complicated by the juxtaposition of the carotid artery and optic nerve.
Most meningiomas are benign and slow growing. They can grow for years without producing any symptoms. When they do produce symptoms the symptoms are due to irritation of compressed brain tissue (seizures), damage to brain tissue (neurological deficit such as paralysis) or elevation of intracranial pressure from the mass of the tumor itself. Olfactory groove meningiomas which grow from beneath both frontal lobes can become very large before a diagnosis is made. The first actual symptoms relate to a loss of the sense of smell which is usually not noticed or blamed on allergies.
Treatment of Meningiomas
Meningiomas, when symptomatic, almost always require surgery. This means a craniotomy with removal of the tumor and the dura to which it is attached. The latter step - the removal of the dura may not be possible at the base of the skull. That's why these tumors can back more frequently than those removed from the convexity where a nargin of dura can be taken with the tumor. Lately, surgeons have been using stereotactic techniques to reduce the invasiveness of the surgery.
Stereotactic radiosurgery can be used to treat small meningiomas, especially in elderly patients or those in whom surgery may carry a high risk. In addition, radiosurgery is sometimes used to treat the place where the tumor started after it has been removed. This is thought to "sterilize" the dura and may prevent or delay recurrances.
Not all meningiomas need to be removed. Some die off spontaneously and become replaced with calcium. They can remain the same size for years when followed by serial CT and MRI examinations. A small tumor which does not produce symptoms in an elderly person can simply be observed over time. If growth is noted on a follow-up CT or MRI, surgery is then recommended.
Not all meningiomas can be cured by surgery. Bear in mind that a meningioma starts because of some inherent defect in the dura that causes it to form the tumor in the first place. Neither surgery nor any other known therapy can change or correct this defect. Therefore, patients who have had a meningioma removed should get an MRI scan every 1-2 years in order to find a recurrance or a new tumor as early as possible. It is much easier and safer to remove a small tumor than a large one.
Malignant Meningiomas
Some meningiomas are more aggressive than others and tend to recur faster. The so-called "atypical meningioma" have mitotic figures and other features suggestive of malignant behavior. Occasionally radiation therapy is recommended for these tumors following surgery. In addition, the so-called hemangiopericytoma can be a bad actor and can recur quickly and even metastasize to other parts of the central nervous system and to other parts of the body.
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