Optic disc size can be quickly assessed at the slit lamp
May 31, 2007

Optic disc size can be quickly assessed at the slit lamp

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


  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

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