arachnoid cysts are benign , non - neoplastic and extra - axial lesions ; most of them are clinically silent and unchanged in size . in previous studies112131524 ) , the incidence of arachnoid cysts has been estimated to range between 0.3% to 1.7% . however , the advanced neuroimaging techniques have facilitated the diagnosis of arachnoid cysts , which , in turn , has led to an increase in the number of patients2121415 ) . in some cases , an arachnoid cyst arouses symptoms and its clinical presentations vary according to the size and location of the cyst . the most frequent symptom is headache , which is caused by the local mass effect , increased intracranial pressure ( icp ) , and/or hydrocephalus15 ) . above this , seizures , cognitive dysfunction , developmental delay , and intracranial hemorrhage can be also presented . in this report , we describe a case where a patient with a posterior fossa arachnoid cyst complained of visual field defect , visual disturbance , and mild headache ; no ataxia and unsteady gait were reported . the patient 's visual symptoms rapidly deteriorated in a week and , consequently , the patient 's vision got worse . a 39-year - old male patient presented at our emergency room after a traffic accident and the brain computed tomography ( ct ) scan indicated an incidental arachnoid cyst on posterior fossa . at that time , the patient complained only of facial abrasions and the left knee pain , but not of headache or visual disturbance . in eight months , however , he revisited our department of ophthalmology to check the decreased visual acuity , visual field defect , and headache . on the ophthalmologic examination , bilateral papilledema ( fig . 2a ) were found . after the transfer to the department of neurosurgery , we rechecked brain ct scans ( fig . the diameter of the patient 's arachnoid cyst did not change compared to the previous ct scans ; however , the slightly enlarged third ventricle was compressing the optic chiasm and the suprasellar area . since visual complications were expected , we recommended an emergency operation . one week later , he came back to the hospital with nearly complete visual loss . his right eye was completely blind , whereas his left eye recognized only light ( fig . a conventional suboccipital craniectomy was performed on the patient lying on the operating table in the prone position . after the dural incision , the arachnoid cyst was exposed and cystic fluid was expelled . arachnoid membrane was carefully dissected and the arachnoid cyst was fenestrated to cisterna magna for connecting with the cerebrospinal fluid ( csf ) flows . arachnoid membrane which covering the foramen of magendie was also fenestrated and the csf flows was running properly ( fig . 4 ) . excluding the brain tumor , the collected membrane was sent to the pathologic laboratory . in high power field microscopy under the hematoxylin and eosin stain , the cystic membrane was composed by membranous fibrocollagenous tissues without tumor cells . one week afterwards , we checked the brain ct scans and the removal of the cyst and the decreased ventricle size were confirmed . on the ophthalmologic examination , the papilledema and visual field were slightly improved ; however , the patient 's visual acuity did not improve . upon the tenth day after the operation , the patient was discharged and , during the next one month , he was subject to the steroid therapy . in five months after the operation , papilledema ( fig . nevertheless , visual acuity and visual field improved slowly and slightly ( fig . 2c ) . the blind right eye improved to recognize the light and the blurred left eye improved to acknowledge the silhouette of an object ; thus , it did not fully recovered to the prior , normal condition . their prevalence has been reported to amount to approximately 1% of all intracranial mass lesions112152430 ) ; however , advanced neuroimaging techniques and the widespread use of ct and mri imaging have led to an even higher incidence of report of these lesions1249121619203031 ) . one recent study , al - holou et al.1 ) , reviewed 48417 brain images and identified 661 ( 1.4% ) arachnoid cysts . this report found that the prevalence of arachnoid cyst in children was 2.6% , slightly higher than in adults , and men ( 1.8% ) have a high prevalence than women ( 1.1% ) . the most common locations were middle fossa ( 34% ) , retrocerebellar ( 33% ) , and cerebral convexity ( 14%)124 ) . despite the increased identification of arachnoid cysts , the natural history and mechanism of enlargement has not been well defined1131524 ) . nevertheless , when arachnoid cysts give rise to any symptom , headache is the most common complaint . in addition to headache , gait imbalance , seizure , and visual changes can also be presented according to the location and size of the arachnoid cyst . various operation techniques , such as cyst excision1517 ) , stereotactic aspiration1825 ) , endoscopic cyst fenestration512 ) , ventriculocystostomy26 ) , and cystoperitoneal shunt810152728 ) , have been introduced and the main purpose of such operations is similar , namely , a direct decompression of the arachnoid cyst or making the connection between the cyst and the csf flows for spontaneous resolution . generally , visual symptoms occur in the patients with suprasellar arachnoid cysts , because the arachnoid cyst is adjacent to the optic nerves7 ) . the patients with posterior fossa arachnoid cysts usually do not complain of visual symptoms , as the cyst is not adjacent to the optic nerves . however , a posterior fossa arachnoid cyst can indirectly disrupt the csf flows and , in turn , the disruption increases the whole ventricle size and the icp . in our case , the enlarged third ventricle was compressing the optic chiasm with cephalocaudal direction on the ct scan ( fig . the compressed superior part of optic chiasm led to the defect of inferior visual field ( fig . , visual acuity was affected by the papilledema , which was caused by the increased icp . consequently , both visual acuity and visual field were influenced by the posterior fossa arachnoid cyst . when the patient first visited our department of ophthalmology , his visual symptoms and headache were not severe and the patient could walk and drive . besides , the brain ct scan showed no changes as compared to previous ct scans , so we could not predict a risk of rapid deterioration . the patient 's refusal to have an operation and the surgeon 's wrong prediction delayed the appropriate on - time treatment ; ultimately , the patient 's vision was severely damaged during the critical seven days . after the operation , the patient 's visual acuity , visual field , and papilledema were slightly improved ; however , we regard that the patient 's vision will unfortunately not fully recover . unlike gait disturbance and ataxia , visual symptoms do not frequently occur in the patients with posterior fossa arachnoid cysts ; this is so because the cyst is located away from the optic nerves . however , the disruption of csf flows can indirectly induce visual symptoms and these symptoms can rapidly deteriorate . considering the results of our case , it should be emphasized that the patients with posterior fossa arachnoid cysts could develop a rapid neurologic deterioration . therefore , in the event of the development of symptoms , an early intervention or close observation are crucial .
posterior fossa is a site next to the middle fossa where arachnoid cyst frequently occurs . 
 generally , most arachnoid cysts are asymptomatic and are found incidentally in most cases . 
 although arachnoid cysts are benign and asymptomatic lesions , patients with posterior fossa arachnoid cysts often complain of headaches , gait disturbance , and ataxia due to the local mass effects on the cerebellum . 
 we observed a patient with a posterior fossa arachnoid cyst who had visual symptoms and a headache , but did not have gait disturbance and ataxia . 
 we recommended an emergency operation for decompression , but the patient refused for personal reasons . 
 after 7 days , the patient revisited our hospital in a state of near - blindness . 
 we suspected that the arachnoid cyst induced the hydrocephalus and thereby the enlarged third ventricle directly compressed optic nerves . 
 compressed optic nerves were rapidly aggravated during the critical seven days ; consequently , the patient 's vision was damaged despite the operation . 
 considering the results of our case , it is important to keep in mind that the aggravation of symptoms can not be predicted ; therefore , symptomatic arachnoid cysts should be treated without undue delay .