Dear Editors,

We read with interest the letter to the editor sent by Goergen S et al. [1]. In this letter, the authors report an additional case of vermian cyst and propose an alternative explanation to that given in our article [2] for the nature of this new type of posterior fossa cyst.

In our series, we showed that all cysts were in the horizontal fissure, between the two vermian lobules. The inner borders of the cysts were thick (corresponding to the lobules displaced by the cysts) while the outer border was thin. The same pattern is observed in the case presented by Goergen et al. In all cases (including Goergen’s), the vermian parenchyma has been normal and remained normal even after complete postnatal regression, as was observed in eight out of the 16 cases reported in our study. No irregularity of the parenchyma lining the cysts was observed, which is an additional argument against a porencephalic cyst. We are not aware of germinolytic cysts being present in the vermis. We do not agree that extrinsic compression by an arachnoid cyst should necessarily compress and narrow the primary fissure. As in megacystis where the inferior part of the bladder is smaller than the dome due to bony constraints [3], we can assume that the tension outside the vermis is probably lower than that inside, due to absence of parenchyma, which favors greater development of the cyst outside the vermis.

Fourteen out of the 16 children in our series showed excellent psychomotor development. One child also had an inner ear malformation, which may account for the slight delay in his walking. Another child showed isolated walking ataxia at the age of 5 years. Except for the vermian cyst, no other abnormality was observed on magnetic resonance imaging. The temporal subependymal pseudocysts which were observed in three fetuses showed a usual pattern without any parenchymal abnormality and without concern for porencephalic cysts. In the case presented by Goergen et al., the supratentorial white matter is T2 hyperintense. We do not share their subjective impression of general vermian volume loss insofar as the normal ratio of the anterior to posterior vermis appears maintained in their case (the vermian measurements can be objectively evaluated and compared to reference charts [4]).

To conclude, we are not convinced by the alternative explanation proposed by Goergen et al. regarding the nature of this new type of posterior fossa cyst. We think that their patient presents two distinct findings: (1) a vermian cyst identical to those we have described and (2) supratentorial parenchymal T2 hyperintensity.

In our opinion, the mild hemiplegic cerebral palsy is not related to the vermian cyst; rather, we believe that it may be related to the parenchymal abnormalities.