Absence of Persistent Complete Membranous Obstruction at the Foramen of Magendie in Pediatric Blake's Pouch Cyst With Hydrocephalus: Evidence for a Delayed Perforation Theory

BACKGROUND AND OBJECTIVES: Blake's pouch cyst (BPC) is traditionally attributed to failure of perforation of the posterior membranous area, resulting in persistent membranous obstruction at the fourth ventricular outlet. However, direct evidence of this membrane in postnatal cases remains limited and controversial. This research aimed to investigate the presence of membranous obstruction at the foramen of Magendie in pediatric patients with BPC and hydrocephalus using multimodal imaging and direct surgical inspection. METHODS: We retrospectively analyzed 20 pediatric patients with BPC who underwent surgery for associated hydrocephalus at our institution from October 2015 to February 2026. Patients underwent detailed imaging including T2 constructive interference in steady state sequence and cine phase-contrast MRI sequences to study cerebrospinal fluid flow. Direct surgical inspection was performed in 4 patients who underwent flexible endoscopy or open surgery. RESULTS: Filiform structures in the posterior fossa were visualized on T2 constructive interference in steady state MRI in 100% (14/14) of the evaluated cases. Cerebrospinal fluid flow studies in all 16 tested patients demonstrated patent flow through the foramen of Magendie with no evidence of complete membranous obstruction. Direct visualization revealed no complete obstruction but filiform remnants in the posterior fossa in 4 patients. Primary treatment with endoscopic third ventriculostomy was attempted in 10 patients, and posterior fossa exploration was attempted in 1 patient, but all the patients required ventriculoperitoneal shunt placement (except for 1 patient who was lost to follow-up). CONCLUSION: Our findings challenge the concept of persistent complete membranous obstruction at the foramen of Magendie in postnatal pediatric patients with BPC and hydrocephalus. On the basis of these observations, we propose a delayed perforation theory. In our cohort, endoscopic third ventriculostomy showed limited efficacy in treating hydrocephalus associated with BPC. Careful evaluation of the posterior fossa anatomy is therefore essential when planning treatment.