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ISNT, IST rules of 'little benefit' in detecting early glaucoma with OCT
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Researchers found the ISNT and IST rules of neural rim thickness used to identify early glaucoma do not clearly distinguish healthy eyes from eyes with glaucoma when used with spectral domain optical coherence tomography.
“ISNT [inferior, superior, nasal, temporal] and IST rules for retinal nerve fiber layer (RNFL) are not universally followed by all normal eyes,” Paaraj Dave, MD, and colleagues wrote in their study published in the British Journal of Ophthalmology.
“On the other hand, eyes with early glaucoma may still follow these rules,” they said. “ISNT rule violation has a higher sensitivity while IST rule violation has a better specificity for diagnosing early glaucoma. However, the [likelihood ratios] associated with these rules are very small and, therefore, of little benefit in differentiating early glaucoma eyes from normal eyes.”
Dave and colleagues identified 76 patients with early glaucoma using the Hodapp-Anderson-Parrish classification and 80 patients with healthy eyes and evaluated their RNFL using SD-OCT, according to the study.
The researchers calculated the likelihood ratio and specificity for rule violation and found that the ISNT rule was followed in 55% of normal eyes and 36.8% of early glaucoma eyes. The sensitivity and specificity for ISNT rule violation was 63.2% and 55%, respectively, and the positive and negative likelihood ratios for rule violation were 1.4 and 0.67, respectively.
Regarding the IST rule, Dave and colleagues found that 60% of normal eyes and 52.6% of early glaucoma eyes violated the rule. Sensitivity and specificity for rule violation was 47.4% and 60%, respectively, while the positive and negative likelihood ratios for IST rules was 1.2 and 0.88, respectively.
“The lack of followability of ISNT/IST rules on the RNFL in our study does not mean that there should be a total disregard for them. These rules may still be useful for those who assess the optic disc by ophthalmoscopy alone, and the same principles may also be applied to the RNFL where OCT is not available,” they concluded. – by Jeffrey Craven
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Scott Anthony, OD, FAAO
Clinical optic nerve evaluation is an essential step in differentiating early glaucoma from healthy optic nerves that mimic the condition. Comparing the thickness of the inferior, superior, nasal and then temporal (ISNT) neuroretinal rims in a stepwise fashion is a common method for identifying glaucomatous structural changes. However, as we know, the ISNT rule has limitations in its ability to assign true glaucoma to optic nerves that do not meet these criteria. This is especially true in early glaucoma where morphometric changes can be minimal, such as focal retinal nerve fiber layer (RNFL) wedge defects.
This study evaluated the clinical utility of applying the ISNT rule to the RNFL thickness in lieu of the neuroretinal rim thickness. Such application could have the benefit of catching early glaucoma before optical coherence tomography analysis flags statistically significant thinning.
The authors clearly show that applying the ISNT rule to the parapapillary RNFL thickness values is inconsistent in its ability to differentiate early glaucoma. Despite this, RNFL OCT analysis remains a highly accurate objective test for detecting early glaucoma using conventional statistical analysis with normative database comparisons.
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