microcephalus vs microcephaly

microcephalus and microcephaly both are nouns.

Word	Noun	Adjective	Verb	Adverb
microcephalus	Yes	No	No	No
microcephaly	Yes	No	No	No

As nouns, microcephalus and microcephaly are synonyms defined as:

microcephalus and microcephaly: an abnormally small head and underdeveloped brain

Other synonyms of microcephalus include nanocephaly.

microcephalus (noun)	microcephaly (noun)
an abnormally small head and underdeveloped brain	an abnormally small head and underdeveloped brain