Patau syndrome (trisomy 13 syndrome): A syndrome characterized by multiple malformations, commonly including scalp defects, hemangiomas (blood vessel malformations) of the face and nape of the neck, cleft lip and palate, malformations of the heart and abdominal organs, and flexed fingers with extra digits. Patients with Patau syndrome are also profoundly mentally retarded. This syndrome is due to the presence of an extra chromosome 13 (trisomy 13). The majority of trisomy 13 babies die soon after birth or in infancy. Also known as trisomy 13 syndrome.