142 CHAPTER FIVE and in persons blind in both eyes from two separate causes. Approximately 15 percent of visually impaired persons had two or more disorders that contributed to their loss of vision (e.g., cor- neal scars from trachoma as well as cataracts). A coding system was developed to combine ·primary and secondary etiologies of ocular disorder and major and other causes of visual impairment into a single diagnostic Category of Ocular Disease (COD). The major cause of both unilateral and bilateral blindness in Nepal is cataract. Of the estimated 233,612 cases of unilateral blindness in the country, cataract accounts for 79,886 (34.2%) and cataract sequelae account for another 15,098 cases (6.5%). Trauma is the second leading cause of unilateral blindness, accounting for 31,870 estimated cases (13.6% of the total). The remaining causes of unilateral blindness are infection (22,920 cases and 9.8% of all unilateral blindness); corneal scars (13,039 cases and 5.G%); phthisis of undetermined origin (11,731 cases and 5.0%); and smallpox (10,089 cases and 4.3%). Although trachoma was the most prevalent potentially blinding ocular disorder, only a small percentage of trachoma cases are blind (1.35%); trachoma was low on the list of causes of blindness, accounting for 3.9 percent of unilateral blindness in Nepal. Nutritional blindness was a rare cause of unilateral blindness (2,124 estimated cases and 0.9%). Of the 335 cases of bilateral blindness detected in the NBS, 26 (7.8%) had a different major cause of blindness in each eye. In or- der to summarize the causes of blindness in persons, rather than in eyes, a summary measure was developed called mutually exclusive cause of bilateral blindness (MECOBB). This measure was programme- oriented. The decision rule was to allocate whichever of two causes of blindness was the most avoidable, i.e., preventable or curable. A person with cataract in one eye and phthisis due to trauma in the other was called cataract, etc. Using this measure, the percentage of blindness attributed to the ten major blinding dis- eases was estimated for the population as a whole, for men and women separately and for childhood blindness. Out of an estimated 117,623 people who are blind, cataract ac- counts for two-thirds of Nepal's blindness, an estimated 78,605 cases (66.8% of all blindness) and the sequelae of cataract account for another 6,195 cases (5.3%). Retinal disease follows, with an es- timated 3,849 cases (3.3%); glaucoma, with 3,820 cases (3.2%); other infections, 3,305 cases (2.8%); trachoma, 2,822 cases (2.4%); trauma, 2,853 cases (2.4%); smallpox, 2,610 cases (2.2%); amblyopia, 1,476 cases (1.3%); and nutritional blindness, 1,095 cases (0.9%). Seen another way, cataract and its sequelae account
