When is the radiology report of posterior fossa containing cyst/cystic-like CSF collection of clinical or surgical significance?

Objective: The nomenclature characterizing posterior fossa (PF) extraventricular (EV) CSF collections in radiological reports can be quite variable, leading to uncertainty about the subsequent clinical course that may result in multiple follow-up imaging studies that may not be needed and occasionally to operative intervention that is not warranted. The important factor is the mass effect of the PF EV CSF collection on adjacent structures, the presence of hydrocephalus, and the likelihood of the CSF collection increasing in size over time.

Methods: The authors respectively reviewed the imaging database at Children's Hospital Los Angeles to identify all radiological reports from 2000 to 2015 indicating the presence of an EV CSF collection in the PF that was characterized as containing an arachnoid cyst, being cystic, or being an abnormal CSF collection.

Results: Of the 332 reports in 65 patients, the PF EV CSF collection was described as an arachnoid cyst or cystic in 306 with 20 different terms being used. In those patients who underwent multiple imaging studies, the PF EV CSF collection was often described differently in each report. Of this group, 47 (72%) patients did not undergo PF surgery. Eighteen (28%) patients did undergo PF surgery, of whom 14 had both hydrocephalus and brainstem displacement, 2 had brainstem displacement but no hydrocephalus, and 2 had neither brainstem displacement nor hydrocephalus and in retrospect did not benefit from PF surgery.

Conclusions: The terminology in radiology reports describing EV PF CSF collections is variable, is inconsistent, and does not correlate well with clinical management or the need for PF surgery. Significant brainstem displacement and hydrocephalus in the presence of EV PF CSF collection is highly correlated with the need for PF surgery. The incidence of a PF EV CSF collection increasing to become symptomatic becomes more remote the older the patient is at the time of diagnosis as compared with those that occur mainly in infancy. There are true EV CSF cysts in the PF, but the ones that are of consequence are those that exert pressure on the brainstem, obstruct CSF flow, or both. Calling any increased amount of CSF in the PF a "cyst" or "cystic" can cause uncertainty, leading to one or more subsequent imaging studies or, in rare cases, unwarranted operative intervention.