Abnormal nasal morphology Pyriform aperture stenosis 2016-08-07T22:32:49Z Congenital nasal pyriform aperture stenosis (CNPAS) is a rare cause of nasal airway obstruction that clinically mimics choanal atresia, but needs to be differentiated from the latter because of the widely divergent modes of management [PMID:21423901]. Narrowing of the anterior nasal aperture (piriform or pyriform aperture), which is a pear-shaped opening in the skull that forms the bony inlet of the nose.