Congenital achiasma is a rare, genetic, non-syndromic cranial nerve and nuclear aplasia malformation characterized by the congenital absence of the optic chiasm, resulting from the failure of the optic nerve fibers to cross over and decussate to the contralateral hemisphere, leading to decreased vision, strabismus and congenital nystagmus in infancy. The disease is Monarch Disease Ontology id MONDO_0017929 (congenital achiasma).