Applicability of the ISNT and IST rules on retinal nerve fiber layer measurement on spectral-domain optical coherence tomography in normal Indian children.

Background: To determine the applicability of the ISNT (inferior>superior>nasal>temporal) and IST (inferior>superior>temporal) rules on retinal nerve fiber layer (RNFL) measurement on spectral-domain optical coherence tomography (SD-OCT) in normal children.

Methods: A prospective, cross-sectional study including consecutive subjects between the ages of 5-18 years who were born at term (≥37 weeks gestational age) and with a normal birth weight (≥2500 g) presenting to the out-patient department for refractive error examination. RNFL measurement was done on Spectralis SD-OCT. Exclusion criteria were best-corrected visual acuity less than 20/20, spherical equivalent (SE) > ± 5 diopter (D), applanation IOP >21 mmHg, cup-to-disc (C/D) ratio of >0.5, C/D ratio asymmetry of >0.2 between eyes and any retinal or optic disc anomaly as determined by mydriatic fundus examination. Subjects with amblyopia, strabismus, or family history of optic nerve or retinal disease were excluded. Poor cooperation for SDOCT imaging and lack of consent were other exclusion criteria.

Results: The ISNT rule on the RNFL was followed only by 30 eyes (23.8 %), while the IST rule was followed by 66 eyes (52.4 %) (p < 0.001). The superior RNFL was thicker than the inferior in 57 eyes (45.2 %) while the temporal RNFL was thicker than the nasal in 63 eyes (50 %). The age, gender, spherical equivalent, and disc size did not predict the followability of the ISNT and IST rules (p > 0.05).

Conclusions: The ISNT and the IST rules for RNFL are not universally followed by all normal eyes in children. All deviations should therefore not be considered pathological.