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A. E. Pogrebniak, C. G. Zaleski; The ISNT Rule in Pediatric Optic Disc Cupping. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3621. doi: https://doi.org/.
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In adults, large optic disc cups are a hallmark of glaucoma. In children, glaucoma is relatively less common than in adults, and optic disc cupping of non-glaucomatous origin is seen in higher proportion. The ISNT rule (which states that for normal optic discs the neuroretinal rim thickness is greatest in the order inferior ≥ superior ≥ nasal ≥ temporal) is violated in glaucoma and has been used in the clinical evaluation of optic discs for detection of glaucoma. The purpose of the study was to determine if the ISNT rule is valid for non-glaucomatous optic disc cupping in children.
Digital ocular fundus photographs existing within a random cohort of children with large optic disc cups were analyzed in masked fashion using computer graphic software. The diameter and perimeter of each optic disc and optic cup and the width of the neuroretinal rims was drawn and measured. Vessels within the cup were not counted as part of the neuroretinal rim.
Healthy non-premature eyes (without glaucoma) in children with cupping totaled 57 (55 eyes with cupping and 2 fellow eyes); eyes of children with a past history of premature birth (gestational age at birth 32 weeks or less) with non-glaucomatous cupping totaled 28. Children ranged in age from 1 year, 4 months to 16 years, 5 months. Twenty-three eyes were myopic (3 eyes between -4 and -5 diopters, 4 eyes between -3 and -4 diopters, and the remaining 16 eyes at -2 diopters or less by spherical equivalent). Sixty-two eyes were plano or hyperopic. The ratio of vertical cup diameter to horizontal cup diameter correlated positively with the ratio of vertical disc diameter to horizontal disc diameter (correlation coefficient 0.711). The ISNT rule did not hold for the population of the study (intact 16% for non-premature eyes and 21% for premature eyes).
Violation of the ISNT rule does not predict glaucoma in the pediatric population with large cups. Whereas in discs with small cups neuroretinal rim thickness would be influenced by the overall shape of the disc (the normal vertical orientation of the disc), large cups create greater variability of relative neuroretinal rim thickness around the disc and are associated with lower predictability of the ISNT rule.
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