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Medulloblastomas are tumors arising from embryonal cells in the brain, and occur in various histologic as well as molecular subtypes. In 8 out of 10 children, these tumors arise in the posterior fossa region near the fourth ventricle. However, sonic hedgehog (SHH) tumors arise more often in the lateral hemispheres of the cerebellum, away from the midline.
In tumors that arise in the midline, the tumors cause blockage of the circulation of cerebrospinal fluid (CSF), the fluid secreted by the choroid plexuses in the lateral ventricles of the brain. This fluid normally circulates and nourishes the brain, as well as protecting it against concussive trauma and cushioning it against jolts. When the tumor blocks any of the routes through which the fluid circulates, it builds up and exerts pressure on the brain tissue around it, a condition called hydrocephalus. The symptoms usually present abruptly, and may occur over 2-3 months before the condition is diagnosed.
Typical symptoms due to hydrocephalus include:
The more typical lateral location of SHH medulloblastomas means that appendicular ataxia occurs more commonly than truncal ataxia in these patients. It is possible to localize the side of the tumor because the ataxia occurs on the same side as the tumor. This is manifested as:
Signs of central CSF blockage appear later as the tumor becomes larger and obstructs the communication between the fourth ventricle and the rest of the central nervous system
As the tumor spreads downwards into the spinal cord, the patient may develop back pain, find it difficult to walk, and may become incontinent for both urine and stool.
Another mode of presentation is the catastrophic onset of unconsciousness and weakness due to bleeding into the tumor.
In infants, medulloblastomas may present as: