Arachnoid cysts (AC) are benign lesions that are formed between the inner and outer layers of the arachnoid. They are filled with clear cerebral spinal fluid (CSF) and they do not have direct communication with the ventricular system. The origin of these cysts is thought to be congenital by dysgenesis in embryological development of subarachnoid space. Other causes have also been attributed to trauma,
mastoiditis,
meningitis,
and subarachnoid hemorrhage.
AC account for 1% of all intracranial space occupying lesions. The size varies,
from small and incidental to a large lesions that cause raised intracranial pressure and / or direct compression of cranial structures.
The most common associated symptoms include headache,
epilepsy,
motor and sensory disorders and endocrine disorders. They can cause bone remodeling and macrocrania.
About 25% of cases are found incidentally on CT and MRI of asymptomatic patients.
The incidence is higher in men than in women.
Most are detected in less than 16 years.
In its evolution over time can increase in size,
decrease or remain stable.
Cases of spontaneous resolution have been reported,
some of them after a head injury.
Two thirds of the AC are located in the supratentorial space,
half of which are in the middle cranial fossa.
Other locations include the suprasellar region,
convexity, interhemispheric,
and intraventricular.
The remaining third are located in the posterior fossa.
Fig. 1
In 1980 Galassi et al classified arachnoid cysts of middle cranial fossa into three types according to their size and effect on neighboring structures.
Fig. 2: Galassi classification
Type I: small size,
located in the anterior temporal lobe. No mass effect
Type II: medium-size,
located in the anterior and middle temporal fossa.
Temporal lobe displaced.
Type III: constitutes a large oval or round cyst that fills the entire temporal fossa.
Large mass effect.