Mega Cisterna Magna and Arachnoid are both types of cysts that occur in the central nervous system and contain cerebrospinal fluid, or CSF. While they are very similar, it is important to make a correct diagnosis between the two in order to ensure proper planning and treatment and in order to properly diagnosis, a clear differentiation should be made. Both Mega Cisterna Magna and Arachnoid cysts account for less than one percent of all intracranial masses and can develop before or after birth.
How They Are Defined
A Mega cisterna magna refers to patients with enlarged CSF retrocerbellar cisterns in the posterior fossa, usually larger than 10mm in antenatal imaging. There will be normal cerebral morphology, also referred to as no abnormalities in the cerebellar. The cisterna magna is a large space located in the back of the brain, between the medulla and the cerebellum. This is an area where CSF is allowed to drain. With Mega cisterna magna the area has become enlarged but communication between the cerebellar vermis and fourth ventricle is intact. There may be no symptoms with mega cisterna magna and there will be no mass effect. This can be treated with cisternotomy.
An arachnoid cyst is a sac filled with cerebrospinal fluid and located between the brain/spinal cord and the arachnoid membrane, one of three membranes that cover the brain and spinal cord. With Arachnoid cysts there is no communication between the cerebellar vermis and the fourth ventricle, and there will be mass effect. The lack of communication can lead to multiple symptoms, including but not limited to: headaches, nausea, numbness and tingling, seizures, lethargy, hydrocephalus, head bobbing, vomiting, visual impairment, mass protrusion in the skull, and developmental delays in children.
These are the most common type of brain cyst resulting from the splitting of the arachnoid membrane and causing congenital lesions. Most cysts that will become symptomatic do so in childhood. This can be treated with burr hole drainage of the cyst. There is, however, a high rate of recurrence and return of symptoms with this procedure so a craniotomy can also be performed to remove the cyst wall.
Getting Properly Diagnosed
Mega Cisterna Magna and Arachnoid cysts look similar on MRI read outs so in order to correctly diagnosis a ventriculogram should be performed with the injection of dye into the blood or with a catheter into the heart. The Mega cisterna magna will show communication with the subarachnoid space indicating a normal cerebellar vermis and fourth ventricle, the lack of mass effect, and the presence of internal vessels. With the arachnoid cysts mass effect is evident by the displacement of vessels around the cyst itself. The differences are clear when the right tests are used and the necessary information gathered.