Trisomy 18 (Edwards Syndrome; Trisomy E) By Nina N Powell-Hamilton, MD, Sidney Kimmel Medical College at Thomas Jefferson University Reviewed / Revised Dec 2021 | Modified Sep 2022 View Patient Education Symptoms and Signs Diagnosis Treatment More Information Topic Resources Rocker-Bottom Feet (Congenital... Clenched Fist With Polydactyly Low-Set Ear and Clenched.. Rocker-Bottom Feet in Trisomy... Clenched Fist in Trisomy 18 Trisomy 18 Karyotype Surveillance Protocol for... Trisomy 18 is caused by an extra chromosome 18 and is usually associated with intellectual disability, small birth size, and various congenital anomalies, including severe microcephaly, heart defects, prominent occiput, low-set malformed ears, and a characteristic pinched facial appearance.