An appraisal of the accuracy of Littmann's method of determining the real dimension of a retinal object.

Estimation of the neuroretinal rim area in absolute units is becoming increasingly desirable in the evaluation of optic discs of glaucoma suspects. There are various methods of optic disc biometry, photogrammetric techniques being the most commonly reported. Any method of estimation of the real dimension of a retinal object must consider magnification due to the patient's eye. Photogrammetric techniques use Littmann's method extensively in this respect, assuming that it is an accurate way of performing the necessary correction. The accuracy of Littmann's method has not yet been assessed in a physiological setting. This study has found that, when the normal distributions of the optical elements eye are considered, in estimating the neuroretinal rim area, the method is at least 86% accurate (95% confidence interval) within an ametropia range -4 to +4 dioptres. This level of accuracy should be considered when drawing conclusions from any study in which Littmann's method is used.