The nasolacrimal duct usually canalises at eight months of fetal life but there is commonly a delay in this developmental process which can result in residual membrane tissue or stenosis at any level in the nasolacrimal system - from the canaliculi to the extreme end of the nasolacrimal duct underneath the inferior turbinate.
This is a companion discussion topic for the original article at https://patient.info/doctor/paediatrics/congenital-nasolacrimal-duct-obstruction