By the end of the fifth week of gestation, the nasolacrimal groove forms as a furrow lying between the nasal and maxillary prominences. In the floor of this groove, the NLD develops from a linear thickening of the ectoderm. A solid cord separates from adjacent ectoderm and sinks into the mesenchyme. The cord canalizes, forming the NLD and the lacrimal sac at its cranial end. The canalicular system is an outgrowth of the lacrimal sac. Caudally, the developing duct extends intranasally, exiting within the inferior meatus. The central tissue of this cord eventually breaks down, forming a lumen. Canalization of the NLD is usually complete around the time of birth. Incomplete development of the distal aspect of the duct with obstruction at the valve of Hasner represents the most common cause of congenital NLD obstruction (NLDO); it is symptomatic in approximately 5% of infants at birth. Patency usually occurs spontaneously within the first few months of life. As noted earlier, lacrimation does not function normally until around age 6 weeks; therefore, excessive tearing, which is often associated with this obstruction, may not be immediately obvious even if an obstruction exists.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.