Flat floor of fourth ventricle sign

Last revised by Andrew Murphy on 25 Apr 2019

The flat floor of fourth ventricle sign is useful in detecting a pontine mass and is a sign of mass effect. The normal floor of the fourth ventricle (remember that the floor is anterior) normally slopes upwards towards the midline, with the facial colliculi visible on either side. 

It is a non-specific sign of a pontine mass, or process with mass effect but in children is most frequently seen in brainstem gliomas.