Distal monosomy 13q is a rare chromosomal anomaly syndrome, resulting from a partial deletion of the long arm of chromosome 13, with a highly variable phenotype typically characterized by varying degrees of intellectual disability and developmental delay, as well as CNS malformations (e.g. holoprosencephaly, anencephaly, ventriculomegaly, Dandy-Walker malformation), ocular abnormalities (e.g. hypertelorism, microphthalmia, strabismus, aniridia, retinal dysplasia) and craniofacial dysmorphism (microcephaly, trigonocephaly, large and malformed ears, broad prominent nasal bridge, micrognathia). Cardiac, genitourinary, gastrointestinal and skeletal manifestations have also been reported.

A congenital brain malformation typically characterized by incomplete formation of the cerebellar vermis, dilation of the fourth ventricle, and enlargement of the posterior fossa. In layman's terms, Dandy Walker malformation is a cyst in the cerebellum (typically symmetrical) that is involved with the fourth ventricle. This may interfere with the ability to drain cerebrospinal fluid from the brain, resulting in hydrocephalus. Dandy Walker cysts are formed during early embryonic development, while the brain forms. The cyst in the cerebellum typically has several blood vessels running through it connecting to the brain, thereby prohibiting surgical removal.