Article

CLINICAL: Glaucoma

CD Ratio Isn’t All That

Perform a qualitative evaluation of the optic nerve

Is an optic nerve with a “0.4” cup-to-disc (CD) ratio glaucomatous? How about a “0.6” or “0.8”? Clinically, there is significant overlap between CD ratios that are glaucomatous or non-glaucomatous. Additionally, the CD ratio does not take into account the nerve size, cup position or the regional variations of the residual rim tissue.1-2 Qualitative factors also play a role in determining diagnosis and treatment. Thus, going just by the numbers can lead to under- or over-diagnosing and under- or over-treatment, respectively.

In our quest to make the right decision, let’s consider (at a minimum) the following diagnostic triad when qualitatively evaluating the optic nerve for glaucoma.

1 GLAUCOMATOUS NEURORETINAL RIM THINNING

Looking closely at the neuroretinal rim (with respect to the size of the optic nerve as demarcated by the scleral ring and inner vessel contour) helps us to determine more accurately the location and the extent of the neuroretinal rim thinning. This rim-to-disc ratio is, therefore, more representative of the stage of the disease than the CD ratio, and if the thinning is to the rim, we should look for and expect correlating VF defects.3 When examining the neuroretinal rim, we should also look for pallor relative to the different sectors within the eye being examined and as compared to the fellow eye. After all, pallor that extends beyond the cupping is more likely nonglaucomatous, needing further workup for proper treatment and an accurate prognosis.

2 GLAUCOMATOUS DISC HEMORRHAGES

After a systematic evaluation of the neuroretinal rim, we should examine this same area for flame-shaped glaucomatous disc hemorrhages. In particular, we should look carefully in the inferior temporal and superior temporal sectors — hot spots for glaucomatous disc hemorrhages in early and moderate glaucoma. These literal “red flags” can be easily missed, unless we are looking for them. As such, I have found it helpful to record disc hemorrhages as at least a pertinent negative when not present (so I consciously look for them). When present, I record the date and location of the disc hemorrhage (ex: Inferior temporal disc hemorrhage, OD, 08/04/20), as it may precede structural and functional changes down the road.

3 GLAUCOMATOUS RETINAL NERVE FIBER LAYER DEFECTS

Not all disc hemorrhages are glaucomatous, but they are more likely to be so if spatially correlated with associated retinal nerve fiber layer defects.4 Whether localized or diffuse, this is the next area to examine, as we extend outward from the optic nerve in a systematic pattern. Such structural defects also are more readily observed in early-to-moderate glaucoma and have a strong association with correlating functional VF defects.

KNOWLEDGE IS POWER

Plato once said, “A good decision is based on knowledge, and not on numbers.” While he wasn’t referring to the CD ratio and the diagnosis of glaucoma, his quote easily applies. Let’s remember the importance of the qualitative evaluation. OM

REFERENCES

  1. Kara-José AC, Melo LAS Jr, Esporcatte BLB, Endo ATNH, Leite MT, Tavares IM. The disc damage likelihood scale: Diagnostic accuracy and correlations with cup-to-disc ratio, structural tests and standard automated perimetry. PLoS One. 2017;12(7):e0181428.
  2. Hong SW, Koenigsman H, Ren R, et al. Glaucoma Specialist Optic Disc Margin, Rim Margin, and Rim Width Discordance in Glaucoma and Glaucoma Suspect Eyes. Am J Ophthalmol. 2018;192:65-76.
  3. Kumar JRH, Seelamantula CS, Kamath YS, Jampala R. Rim-to-Disc Ratio Outperforms Cup-to-Disc Ratio for Glaucoma Prescreening. Sci Rep. 2019;9(1):7099.
  4. Lee EJ, Han JC, Kee C. Location of Disc Hemorrhage and Direction of Progression in Glaucomatous Retinal Nerve Fiber Layer Defects. J Glaucoma. 2018;27(6):504-510.