The fundal cerebrospinal fluid cap in vestibular schwannoma surgery: a predictor of outcome after the retrosigmoid approach

The fundal cerebrospinal fluid (CSF) cap is a radiological finding correlating to a pocket of CSF lateral to vestibular schwannomas in the fundus of the internal acoustic meatus. Its presence may increase the likelihood of good facial nerve outcome and hearing preservation after microsurgical resection. A systematic review of the literature was performed. Studies that reported the association of a fundal fluid cap with postoperative outcomes including facial nerve outcome, hearing preservation and extent of resection were included. A total of 17 studies were included, comprising 2370 patients. Studies were generally at high risk of bias. The presence of a fundal cap was associated with significantly higher rate of good (HB I-II) facial nerve outcome after retrosigmoid approaches (OR 6.04; 95%CI 2.79-13.11), but not after translabyrinthine and middle fossa approaches. A fundal cap was associated with an increased rate of gross total resection (OR 2.13; CI: 1.51-3.00) and hearing preservation after retrosigmoid (OR 3.37; 95% CI: 2.32-4.90), but not middle fossa approaches (OR 1.47; 95% CI: 0.89-2.44). A fundal cap was also predictive of hearing preservation after radiosurgery. The fundal CSF cap is an important predictor of facial nerve function and hearing preservation after retrosigmoid craniotomy for vestibular schwannoma. Its importance in middle fossa and translabyrinthine surgery is less clear, which reflects the anatomical considerations of each approach. The presence or absence of a fundal cap should be documented preoperatively and used to guide more nuanced risk assessment for preoperative patient counselling.