APA Dictionary of Psychology

a congenital anomaly involving a large cyst in the posterior region of the fetal brain. The syndrome is characterized by (a) blockage of the fourth ventricle, resulting in increased volume of brain fluid (see hydrocephalus), and (b) complete or partial agenesis (failure to develop) of the middle portion of the cerebellum (the vermis) because of its displacement by the cyst. [Walter Dandy (1886–1946), U.S. neurosurgeon; Arthur Earl Walker (1907–1995), U.S. neurologist]