In an infant presenting with progressive macrocephaly, hydrocephalus is suspected. Brain MRI shows features of arrested marked hydrocephalus involving supra-tentorial ventricular systems and normal cortical sulci. Flow void is noted at the aqueduct denoting patency. 

A large cystic area occupies the posterior fossa communicating directly with the fourth ventricle. Normal vermis cannot be identified, with a small vermian remnant appearing displaced and rotated superiorly. Features are consistent with a Dandy Walker malformation with associated marked arrested hydrocephalus, mostly communicating type. Patients usually present with macrocephaly in the first year of life.

Dandy-Walker malformation classical triad consists of:

• hypoplasia of the vermis and cephalad rotation of the vermian remnant
• cystic dilatation of the fourth ventricle extending posteriorly 
• enlarged posterior fossa with torcular-lambdoid inversion (the torcula lying above the level of the lambdoid due to abnormally high tentorium)