The Science behind the Tip

Optic disc size influences the significance of the cup/disc ratio1,2. Disc size can be estimated using a handheld high power convex lens and the adjustable beam height on the slit lamp3. A small beam is adjusted to the vertical diameter of the optic disc (most accurate in a dilated fundus) and its length is read on the scale of the slit lamp (Figures). This value needs to be modified by a magnification factor depending on lens power and material (Table)4, and may vary slightly with its distance form the cornea and in high refractive disorders (> +/- 8D).

A disc is considered small if = 1.2 mm and large if = 1.8 mm5. Since we mostly use one lens, we can calculate our personal slit lamp mm-range of normal disc size. Regardless of minor inaccuracies, we are able to confirm our clinical impression of abnormal disc size and identify a very small or large disc.

Contributor: Veva De Groot, Antwerp
Co-editors: John Thygesen and Ann Hoste
Peer reviewers: Roger Hitchings and Anders Heijl

References

1. Bengtsson B. The variation and covariation of cup and disc diameters. Acta Ophthalmol (Copenh). 1976;54:804-18.

2. Garway-Heath DF, Ruben ST, Viswanathan A, Hitchings RA. Vertical cup/disc ratio in relation to optic disc size: Its value in the assessment of the glaucoma suspect. Br J Ophthalmol. 1998;82:1118-24.

3. Hoffmann EM, Crowston LM, Zangwill JG, Weinreb RN. Optic disc size and glaucoma. Surv Ophthalmol. 2007;52:32-49.

4. Lachkar Y, Sellem E. La papille glaucomateuse en pratique. Chauvin B&L, France, 2004, 13-20.

5. Crowston JG, Hopley CR, Healey PR, et al. The effect of optic disc diameter on vertical cup to disc ratio percentiles in a population based cohort: Blue mountains eye study. Br J Ophthalmol. 2004; 88:766-70.

Tip Reviewer: Roger Hitchings
Tip Editors: Ann Hoste, John Salmon and John Thygesen