Is the ISNT rule helpful?

This glaucomatous optic nerve head disobeys the ISNT Rule (as evidenced by neuroretinal rim thinning and vessel baring inferiorly). There is also zone beta parapapillary atrophy and temporal RNFL thinning.We’re taught from a very young age that, in science, the laws of nature never change. Then, we learn in college physics that, in fact, these laws actually change a great deal. The hallmark example of this would be the comparison of the laws governing the planets to the laws governing subatomic particles. I’ll admit that watching the reprise of the TV series “Cosmos” has been a guilty pleasure of mine as of late, but enough about that.

Are there any absolute laws for glaucoma other than the broad few that govern its definition? Well, maybe not. Think about it. The word “typically” precedes much of what we say when we describe glaucoma outside of its definition. An example would be the fact that visual field loss is typically nasal at first. However, a temporal wedge defect may, at times, be the first defect to become manifest.

The ISNT rule

So, with an admitted sense of optometric relativism, I’ll turn the conversation to the old glaucoma adage known as the “ISNT rule.” This mnemonic rule states that the inferior neuroretinal rim should be thickest, followed by the superior rim, and so forth. Thinking about the optic nerve head in this respect guides the clinician to be suspicious of optic nerves that disobey this rule. However, several studies have shown the ISNT rule to be of limited use with respect to diagnosing glaucoma. 

Morgan et al showed, through a prospective study, the “limited utility” of this rule in the diagnosis of open-angle glaucoma.¹ More recently, Hwang et al found “limited clinical value in the diagnosis of glaucoma” in a study using the Cirrus OCT (Carl Zeiss Meditec).² Finally, Pogrebniak et al described a particular shortcoming of the ISNT rule with respect to non-glaucomatous large optic cups in a pediatric population.³

Ok, so perhaps the ISNT rule isn’t much of a rule at all. However, I don’t believe anyone ever intended it to be taken literally as a rule, anyway. We all see abnormal optic nerves all the time, and those that disobey the ISNT rule tend to raise suspicion for the presence of glaucoma.

The ISNT guideline

No one has yet been able to elicit an absolute biomarker (be it physical or chemical) for glaucoma or the risk thereof. Maybe such a “God particle” for glaucoma just doesn’t exist. I agree with the aforementioned studies, but that doesn’t mean I don’t think about the ISNT rule when looking at an optic nerve. I would change it up a bit semantically, though, by calling it the “ISNT guideline” instead. I’m a fan of the ISNT rule (guideline) for several reasons. However, the biggest reason I think it has a place in contemporary eye care is the fact that it forces the clinician to think about the optic nerve head in terms other than simply a decimal point with a number after it. I’m not saying cup-to-disc ratios are not of the upmost importance with respect to glaucoma. I’m just saying that if someone were to describe an optic nerve head to me by saying, “point six,” I’d be on the edge of my seat waiting for more information.

So, even if the ISNT rule doesn’t prove to be true (in sensitivity or specificity) a lot of the time, it forces the clinician to really examine the entire tissue of the optic nerve head (any of which may be damaged early on in glaucoma). Further, looking past the cup-to-disc ratio gives the clinician the opportunity to look for other potential aspects of the glaucomatous optic nerve, such as the presence of retinal or optic nerve hemorrhages, areas of zone beta parapapillary atrophy, or retinal nerve fiber layer defects around the optic nerve.

I, for one, find the ISNT rule to be a useful tool of which to be mindful. It certainly isn’t intended to be taken as an absolute truth. It makes me stop and look a little more closely at optic nerves, however, and for that reason, I enjoy keeping the ISNT rule in mind.

Truly, the exciting and highly useful advancements in spectral domain optical coherence tomography (SD-OCT) have given us more ammunition in the battle to diagnose and treat glaucoma earlier, but close, careful, and direct stereoscopic evaluation of the optic nerve head, itself, must not become a lost art. A diagnosis of glaucoma starts at the optic nerve, and adages such as the ISNT rule remind us of this important point.ODT

References

1. Morgan JE, Bourtsouki I, Rajkumar KN, et al. The accuracy of the inferior>superior>nasal>temporal neuroretinal rim area rule for diagnosing glaucomatous optic disc damage. Ophthalmology. 2012 Apr;119(4):723-30.

2. Hwang YH, Kim YY. Application of the ISNT rule to neuroretinal rim thickness determined using Cirrus HD optical coherence tomography. J Glaucoma. 2013 Nov 14.

3. Pogrebniak AE , Wehrung B, Pogrebniak KL, et al. Violation of the ISNT rule in nonglaucomatous pediatric optic disc cupping. Invest Ophthalmol Vis Sci. 2010 Feb;51(2):890-5.