What are the limitations of the isnt rule?

While the isnt rule is highly validated, it should not replace clinical judgment. OpiCalc provides the latest evidence-based limitations for the isnt rule to ensure safe bedside use and optimal diagnostic accuracy.

How does the isnt rule compare to other Ophthalmology scores?

The isnt rule is a gold-standard diagnostic tool. OpiCalc provides side-by-side clinical guidance to help clinicians choose the most specific evidence-based tool for their patient's presentation.

Is the isnt rule free to use?

Yes. OpiCalc provides the isnt rule completely ad-free and optimized for mobile bedside use, ensuring you can calculate medical risk swiftly and without distraction.

ISNT Rule

Inferior Rim

Superior Rim

Nasal Rim

Temporal Rim

Guidelines & Evidence

Clinical Details

Section 1

Formula & Logic

Neuroretinal Rim Anatomy

Inferior (Thickest)

Superior (Next thickest)

Nasal (Thinner)

Temporal (Thinnest)

Rule Violation

In early glaucoma, the inferior and superior rims typically thin first. If the Inferior rim is thinner than the Temporal rim, the ISNT rule is violated, raising suspicion for glaucomatous damage.

Section 2

Pearls/Pitfalls

Caveats

The ISNT rule is highly specific for average-sized discs but less reliable in very large (macro-discs) or very small (micro-discs) optic nerves, where temporal thinning may be physiologic.

Last Comprehensive Review: 2026

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