To the Editor.—
In an extensive article in the June 1975 issue of the Archives (32:393, 1975), the authors stated that the cause of the Dandy-Walker and Arnold-Chiari malformations is still unknown and, further, that the many associated defects cannot be explained by "hydrostatic" mechanisms. They stated that only two of their five patients with Dandy-Walker malformations had hydrocephalus, because the lateral ventricles were dilated in only these two. This overlooks the fact that the Dandy-Walker malformation is hydrocephalus that primarily affects the fourth ventricle, aqueduct, and third ventricle, whereas in the Arnold-Chiari malformation the opposite is true since there is compression of the fourth ventricle, aqueduct, and third ventricle, and the hydrocephalus is limited to the lateral ventricles. They suggested that in the Dandy-Walker malformation a primary mesodermal insult may be involved that arrests the descent of the tentorium; that it is not yet known whether the large posterior