Growing Posterior Fossa Arachnoid Cyst Causing Tonsillar Herniation and Hydrocephalus

Abstract

A25-YEAR-OLDWOMAN without any significantmedicalortraumatichistorypresented with a 1-month episodeofrecurrentandprogressive bioccipitalheadachesandnausea.At age18years,shehadbeeninvestigated foroccasionalbitemporalheadachein alocalclinic,anda11-cmretrocerebellarcysthadbeenincidentallydetectedonmagneticresonanceimaging (Figure, A). The lesion was consideredtobeofnosignificancewithregardtohersymptomsatthattimeand thus was left untreated. Neurologic examination findings were normal other than the presence of ataxia on the Romberg test. Fundus examination documented bilateral papilledema. Magnetic resonance imaging revealed a 45-cm retrocerebellar cyst compressing the cerebellum anteroinferiorly, resulting in tonsillar herniation and hydrocephalus (Figure, B and C). The patient underwent an urgentsuboccipitalcraniotomy.Intraoperatively, a bulging cyst containing a xanthochromic fluid was documented (Figure, D). Chemical analysis of the cyst fluid content revealed an elevated ferritin level of 21.4 ng/mL (to convert to picomoles per liter, multiply by 2.247), whereas the level of ferritin in the cerebrospinal fluid, which was independently collected from the cisterna magna, was normal at 6.8 ng/mL (12.0 is pathological). A wide resection of the cyst membranewasperformed,andthepathologicaldiagnosiswasconsistentwith an arachnoid cyst. The patient showed a full recovery after the operation. Follow-up magnetic resonance imaging confirmed shrinkage of the arachnoid cyst with resolution of the tonsillar herniation and hydrocephalus, and fundus examination confirmed the disappearance of bilateral papilledema 3 months after the operation. COMMENT Arachnoid cysts are incidentally detected in 2.6% of the pediatric population (aged 18 years). 1 Because progressive neurological signs secondary to growth of the lesion are rare, radiological follow-up and prophylactic surgery for asymptomatic arachnoid cysts are considered unnecessary. We are unaware of any published cases where both tonsillar herniation and hydrocephalus resulted from the growth of a posterior fossa arachnoid cyst. Examination of the ferritin concentration in the cerebrospinal fluid as a potential marker of previous subarachnoid bleeding showed a 3-fold elevation in the arachnoid cyst fluid compared with the cerebrospinal fluid in our case, which may reflect the preceding intracystic hemorrhage that can cause arachnoid cyst growth via an osmotic gradient. 2 The xanthochromic arachnoid cyst fluid in our case also seems to be suggestive of the previous intracystic bleeding.