Isolated inferior vermian hypoplasia (IIVH) is a congenital malformation characterized by partial absence of the inferior portion of the cerebellar vermis.

Terminology

Previously the term Dandy-Walker variant was used to describe cystic malformations of the posterior fossa that did not meet the criteria for Dandy-Walker malformation. This is now discouraged as it is ambiguous. Instead, a more detailed anatomic description like IIVH is recommended ^1,2. 

In a review on this subject, Robinson proposed that the term isolated inferior vermian hypoplasia should also be abandoned in favor of simply vermian hypoplasia or vermian dysplasia unless it can be proved that the inferior vermis is deficient ⁴. 

Clinical presentation

There is some controversy about the clinical impact. Some studies have reported normal cognitive, language, social and behavioral outcomes. Others have described mild functional deficits in fine motor activity and receptive language in ~25% of patients ^1-3. 

Pathology

Macroscopically, there is partial inferior vermian hypoplasia. The remainder of the vermis, the cerebellar hemispheres, the 4^th ventricle, and the posterior fossa, have a normal size and architecture ¹.

Radiographic features

Ultrasound

• the inferior vermis should have formed by the 18^th week of gestation, so antenatal diagnosis of an IIVH may be possible beyond this point ^1,3

Differential diagnosis

• Dandy-Walker malformation

  • cephalad rotation of the vermian remnant

  • cystic dilatation of the fourth ventricle extending posteriorly 

  • enlarged posterior fossa with torcular-lambdoid inversion 

• Blake pouch cyst

  • no vermian hypoplasia or rotation