An 18-year-old man presenting a 1-month history of headache. At present, he comes to our hospital with seizures. No other accompanying symptoms. He refers to headache and vomiting the day before.
The brain MRI showed an arachnoid cyst located in the middle fossa and mass effect with oedema and displacement of the third ventricle and midbrain. We can see also bilateral subdural hygromas, which communicate with arachnoid cyst.
The signal of hygromas and cyst was like cerebrospinal fluid in all sequences. After paramagnetic intravenous contrast administration, no enhancement was seen.
Arachnoid cysts are the most common type of brain cyst and developed frequently at the base of the skull, the surfaces of the brain or on the arachnoid membrane. Sometimes grow to a large size if they continue to retain cerebrospinal fluid.
Some arachnoid cysts never present a problem, but others can cause symptoms by putting pressure on the brain. Depending on the size and location the symptoms can include headache, nausea and vomiting, seizures or hydrocephalus due to obstruction of normal cerebrospinal fluid circulation. Some arachnoid cysts never present a problem, but others can cause symptoms by putting pressure on the brain.
In juveniles, arachnoid cyst is a risk factor for chronic subdural hematoma.  
It´s important to evaluate a small arachnoid cyst as a risk factor for subdural hematoma in young patients after head injury. Sometimes the arachnoid cyst disappears after rupturing into subdural space.  
Hematoma evacuation is important the first time. If symptoms like headache persist, sometimes is necessary to an additional surgery of the arachnoid cyst.
We can suspect an arachnoid cyst as a cause for chronic subdural hematoma in young patient with head injury.
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