Middle cranial fossa Sylvian fissure arachnoid cysts in infants younger than 1 year: clinical characteristics, preoperative growth patterns, and outcomes of endoscopic fenestration.

BACKGROUND AND PURPOSE: Middle cranial fossa Sylvian fissure arachnoid cysts represent the most common type of intracranial arachnoid cysts in children, yet their natural history and optimal management strategies remain controversial. In infants younger than 1 year, the number of reported cases is limited, and these patients are often analyzed together with older children, limiting our understanding of cyst growth patterns and treatment responses during infancy. This study was aimed at characterizing the clinical features and preoperative growth patterns of middle cranial fossa Sylvian fissure arachnoid cysts in infants younger than 12 months and at evaluating the radiological outcomes and complications associated with neuroendoscopic cyst fenestration. METHODS: We retrospectively reviewed infants with middle cranial fossa Sylvian fissure arachnoid cysts who underwent neuroendoscopic cyst fenestration at the Department of Neurosurgery, Hebei Children's Hospital, between January 2016 and December 2024. Inclusion criteria were age younger than 12 months at the time of surgery. Five patients met the inclusion criteria. Demographic data, clinical and radiological characteristics, surgical details, postoperative radiological outcomes, and procedure-related complications were systematically analyzed. Because clinical symptoms were absent or nonspecific, radiological change was used as the primary outcome measure. Imaging outcomes were semi-quantitatively graded according to the estimated overall cyst reduction as Excellent (more than three quarters), Good (more than one half and up to three quarters), Fair (more than one quarter and up to one half), or Poor (one quarter or less). The duration of follow-up ranged from 4 to 9 years. RESULTS: Five infants (age range, 3-10 months; median age, 6 months) were included. All patients had Galassi type III cysts on preoperative imaging. Three cysts were located on the left side and two on the right. Progressive cyst enlargement before surgery was documented in three patients, including one patient with complete serial imaging demonstrating marked enlargement. Postoperative radiological improvement was graded as Excellent in three patients, Good in one, and Fair in one. All patients developed postoperative subdural effusion; four had bilateral effusions, whereas one had a persistent ipsilateral effusion. In most patients, the effusions gradually regressed during follow-up. Two patients developed subdural hemorrhage, including one with recurrent bilateral hemorrhage requiring two burr-hole drainage procedures. No patient ultimately required shunt placement. CONCLUSION: In this small series of surgically treated infants, progressive cyst enlargement before surgery was observed in selected patients, suggesting that cyst growth may remain active during infancy in some cases. Neuroendoscopic cyst fenestration was associated with substantial long-term radiological reduction in most patients; however, postoperative subdural effusion was frequent, and subdural hemorrhage occasionally required additional surgical treatment. These observations raise the possibility that infancy may be an important period of cyst growth and intracranial remodeling, but the small and highly selected cohort does not allow conclusions regarding a distinct growth stage or the optimal timing of surgery. In infants with established surgical indications, the potential for radiological improvement should therefore be weighed carefully against the risk of postoperative complications.