Etiological differences between isolated lateral ventricle and the isolated fourth ventricle.

Dear Editor: I read with interest the article by Ang et al. [1] “Etiological differences between isolated lateral ventricle and the isolated fourth ventricle” comparing the differences in the factors leading to the creation of separate compartments of cerebrospinal fluid. They describe 34 children with sufficient isolation of these compartments to require some form of treatment. The conclusions regarding the isolated fourth ventricle as primarily a function of an early infectious process are correct and widely understood. The discussion regarding the isolation of the lateral ventricles, however, deserves further discussion. The authors quoted Kaufman et al. [2] who described this phenomenon in the context of chronic shunting. They found that in these chronically shunted patients whose ventricles become small and who have an intact septum pellucidum, the ventricle with the ventricular catheter becomes smaller than the contralateral ventricle. This asymmetry does not appear until the size of both lateral ventricles has decreased substantially. That pulse pressure in the shunted ventricle is lower than in the contralateral ventricle suggests the importance of choroidal pulsations as the underlying etiology. Kaufman et al. [2], however, did not deal with the need to treat the isolation of the ventricles. This finding, reasonably called “postshunt ventricular asymmetry,” is common and rarely symptomatic. This asymmetry was also studied at length by Linder et al. [3] who confirmed the findings of Kaufman. Ang et al. [1] did not upgrade the valve mechanism in any of their patients. Rather, they treated the process as a fixed obstruction by placing a second ventricular catheter or by placing a ventricular catheter from one lateral ventricle to the other with extra holes placed on both sides. Unfortunately, scans or ventricular volume measurements of their patients were not reported. It is conceivable that some of their patients may have had atresia or postinflammatory closure of the foramen of Monro. However, it is most likely that they had postshunt ventricular asymmetry as discussed above. We have studied this asymmetry in some detail and found that significant pressure differentials (to negative values) can be measured between the lateral ventricles when one of the lateral ventricles is drained externally. The anatomy underlying the differential pressure relates to movement of the septum pellucidum, which eventually lies against the head of the caudate nucleus, thereby closing the drainage pathway. The asymmetry and pressure differential do not develop in ventricles with positive intraventricular pressure. For the most part, the size of the lateral ventricles will balance if positive pressure can be maintained within them [4]. In patients with Chiari II malformation, an enlarged massa intermedia fixes the head of the caudate nucleus. Therefore, the likelihood of developing this asymmetry increases in patients with spina bifida whose septum pellucidum is intact. The obstruction leading to the asymmetry is indeed “functional” and can always be treated with a valve upgrade.