has part has part characteristic of inheres in has modifier Abnormal choanae morphology Choanal atresia Blockage of the rear opening of the nasal cavity SNOMEDCT_US:204508009 HP:0004496 Fyler:4203 UMLS:C0008297 HP:0004503 HP:0000453 HP:0000416 Absence or abnormal closure of the choana (the posterior nasal aperture). Most embryologists believe that posterior choanal atresia results from a failure of rupture between the 35th and 38th day of fetal life of the partition which separates the bucconasal or buccopharyngeal membranes. The resultant choanal atresia may be unilateral or bilateral, bony or membranous, complete or incomplete. In over 90 per cent of cases the obstruction is bony, while in the remainder it is membranous. The bony type of atresia is commonly located 1-2 mm. anterior to the posterior edge of the hard palate, and the osseous septum varies in thickness from 1 to 10 mm. In the membranous form of choanal atresia the obstruction usually occurs further posteriorly. In approximately one third of cases the atresia is bilateral. abnormal atretic posterior nasal aperture