Anatomy of nasolacrimal canal in congenital nasolacrimal duct obstruction – 18 cases retrospective study

Congenital nasolacrimal duct obstruction is one of the most common ocular disorders in infants with an estimated prevalence ranging from 1.8% to 20% (MacEwen & Young 1991). It is usually congenital in origin resulting from a failure of canalization of the nasolacrimal duct. Asymmetrical enlargement of the nasolacrimal canal in the absence of associated neoplasm is uncommon. It has sparsely been reported in the literature (Rheeman & Meyer 1998). In this study, CT results of unilateral congenital nasolacrimal duct obstruction were investigated after excluding the possibility of congenital nasolacrimal canal agenesis. The transverse diameters, anteroposterior diameters and areas of bilateral nasolacrimal canal at different levels were surveyed and compared to demonstrate whether there was relationship between nasolacrimal canal enlargement and congenital nasolacrimal duct obstruction. We retrospectively reviewed the CT results of 18 children with unilateral congenital nasolacrimal duct obstruction seen in the Department of Ophthalmology of Beijing Children's Hospital from June 2011 to June 2012. This study followed the Declaration of Helsinki and was approved by the Ethics Committee of Beijing Children's Hospital. We followed the low-dose CT protocol for children throughout the study. The traverse diameters, vertical diameters and areas of bony nasolacrimal duct at three levels were measured on the CT workstation. We selected initial segment, midpoint and the lowest point of nasolacrimal canal as three measurement levels (Fig. 1). The results showed that the transverse diameters of bony nasolacrimal duct increased significantly on the affected sides compared with the normal sides in unilateral congenital nasolacrimal duct obstruction (p = 0.000). The anteroposterior diameters of bony nasolacrimal duct were much larger on the affected sides than the normal sides (p = 0.000). And the areas of bony nasolacrimal duct were also larger on the affected sides than the normal sides (p = 0.000). In recent years, many researchers have sought to study the characteristics of nasolacrimal canal in normal children and congenital nasolacrimal duct obstruction by CT scans (Moscato et al. 2010; Weiss et al. 2012). However, few studies have addressed the issue of bony nasolacrimal duct enlargement. Our results showed that the transverse diameters, vertical diameters and areas of bony nasolacrimal duct on the affected side increased significantly compared with those on the normal side in unilateral congenital nasolacrimal duct obstruction. It is currently believed that bony nasolacrimal duct enlargement is due to the fact that lacrimal duct obstruction leads to high pressure in the lacrimal duct on the affected side. The bony lacrimal duct is plastic in childhood, which results in significantly expanded bony nasolacrimal duct on the affected side compared with the normal side. As liquid is the major substance present in the lacrimal duct on the affected side, the pressure inside the nasolacrimal duct derives mainly from hydrostatic pressure. The equation is P = ρgh, where P = hydrostatic pressure, ρ = density (g/cm3), g = gravitational acceleration and h = height. Liquid density generally changes little, and gravitational acceleration is constant. Thus, hydrostatic pressure is primarily dependent on liquid height. The height of bony nasolacrimal duct was linearly correlated with children's age. This means that the height of bony nasolacrimal duct increases with the age of children (Moscato et al. 2010). Pressure in the nasolacrimal duct also increases with age. Accordingly, hydrostatic pressure within the lacrimal duct plays a positive role in the self-healing of congenital nasolacrimal duct obstruction (Moscato et al. 2010). Meanwhile, hydrostatic pressure within the lacrimal duct also presents an equal pressure on the wall of nasolacrimal canal, which results in the enlargement of bony nasolacrimal duct on the affected side. In conclusion, this study found that bony nasolacrimal duct enlarged significantly on the affected side compared with normal side in children with unilateral congenital nasolacrimal duct obstruction. This phenomenon is caused by sustained increase in hydrostatic pressure within the lacrimal duct.