The superior medullary velum and its contribution to obstructive hydrocephalus: anatomical characterization and surgical targeting considerations in an image-based case review

BACKGROUND: Obstructive hydrocephalus caused by a membranous or synechial occlusion at the level of the superior medullary velum (SMV) is likely under-recognized and may be mislabeled as aqueductal stenosis. We aimed to describe the anatomical pattern of these 'velar obstructions' and discuss their surgical targeting implications. METHODS: We retrospectively re-reviewed 94 patients previously classified as primary aqueductal stenosis (1995-2015) and identified 5 cases (5%) with an obstruction caudal to the aqueductal tectum, at the SMV. A targeted literature search retrieved 18 additional reports with midline sagittal MRI compatible with a velar-level occlusion. In the primary series, morphometrics were obtained from calibrated imaging. Literature cases were used for qualitative/topographic assessment. The SMV was segmented into A (externally free) and B (covered by the lingula), and the occlusion site was classified as A, AB junction, or B. RESULTS: In the combined cohort (n=23), the AB junction was the most frequent site (12/23). In the primary series (n=5; mean age 34 years, range 14-47), the sagittal depth of the dilated cavity averaged 24 mm (range 17-33) and consistently exceeded the length of the aqueductal tectum. Morphometric patterns were heterogeneous. Across cases with reported treatment (20/23), endoscopic third ventriculostomy was the most common strategy, alone or combined with transaqueductal fenestration. CONCLUSIONS: Velar occlusion is a distinct imaging-anatomical pattern that may influence endoscopic planning. Any endoscopic fenestration at the SMV level ('velarplasty') should be framed as investigational and must explicitly account for the nearby trochlear nerve decussation and vermian/lingular relationships.