Third Ventricular Diameter is Inversely Related to Thalamic Massa Intermedia Thickness in Hydrocephalus Caused by Congenital Aqueductal Stenosis

Abstract

Background and purpose: In fetuses with lateral ventriculomegaly and normal posterior fossa cerebrospinal spaces, third ventricular distention is a compelling clue that supports a diagnosis of aqueductal stenosis. However, this association assumes normal ventricular anatomy. Structural constraints can impair pressure-induced compliance. We aimed to determine how thalamic massa intermedia (TMI) size alterations may impact the size of the 3rd ventricle in the setting of congenital aqueductal stenosis (CAS).

Materials and methods: This retrospective study was performed at a single academic pediatric hospital after IRB approval. We searched our brain MRI reports for all exams describing "aqueductal stenosis" and included all the patients who had both fetal and postnatal exams. Patients with interhypothalamic adhesions and hydrocephalus unrelated to CAS were excluded from this study. We evaluated all the MRIs for the presence of TMI and documented third ventricle diameters (supraoptic recess, central, and suprapineal recess) and the TMI circumference. Spearman correlation was used to identify the potential relationship between the TMI circumference and 3rd ventricle size in fetal and postnatal MRIs. Patients were also stratified into two groups based on the presence or absence of TMI. Mann-Whitney U tests were used to compare third ventricle diameters between these groups.

Results: The study included both fetal and postnatal studies from 59 patients. The overall third ventricular diameter was inversely proportional to the circumference of the TMI in both groups (fetal: p=0.001, rho=-0.422, CI=[-0.628-0.181]; postnatal: p<0.001, rho=-0.653, CI= [-0.782-0.479]). Nonetheless, dilation of anterior and posterior recesses still occurred when the mid third ventricle was non-dilated or less severely dilated in patients with an enlarged TMI. Third ventricular dilation was most severe in patients lacking a TMI compared to patients with a TMI (p<0.001).

Conclusions: In patients with suspected congenital aqueductal stenosis, lack of significant third ventriculomegaly as conventionally measured can sometimes be explained by thickening of the TMI. In this circumstance, it is important to evaluate the extreme recesses of the 3rd ventricle for evidence of dilatation on fetal MRI.ABBREVIATIONS: TMI = Thalamic massa intermedia; CAS = Congenital aqueductal stenosis.