Increased Axial Length Corresponds to Decreased Retinal Light Dose: A Parsimonious Explanation for Decreasing AMD Risk in Myopia.

Purpose: A recent systematic review indicated that higher sunlight exposure increased risk of AMD. The Beaver Dam study and the Pathologies Oculaires Liées à L'âge study both noted that wearing hats and/or sunglasses significantly decrease some AMD lesions, suggesting that reduced retinal light dose (RLD) may be related to reduced AMD risk. Given that myopes also have reduced AMD risk, we hypothesize its link to decreased RLD. Methods: Using a one-surface schematic eye and ray-tracing, spectacle power, vertex distance, corneal power, anterior chamber depth, and axial length to calculate relative light flux through the pupil and resultant image size on the retina in a randomly selected group of 71 eyes from the Reykjavik Eye Study. Pupil size is unaffected by refractive error; thus, RLD can be calculated. We verified this using a more complete set of ocular biometric variables and ray-tracing included in an optical design software (Opticsoft II). Results: RLD is inversely proportional to axial length. Comparing the two methods for calculating RLD using a Bland-Altman plot demonstrated equivalence. The ray-tracing method indicated that the retina of a hyperope with a 21-mm axial length would always be receiving 1.8× more photons per square millimeter than the retina of a myope with a 27-mm axial length. Conclusions: RLD is inversely proportional to axial length, as is AMD risk. The RLD for our 21-mm axial length wearing a pair of inexpensive commercial sunglasses would be equivalent to the RLD for a 27-mm myope. This may explain the decreased AMD risk in highly myopic individuals.

A new fundus camera technique to help calculate eye-camera magnification: a rapid means to measure disc size.

OBJECTIVE: To find a simple means for calculating eye-camera magnification to permit estimation of true retinal object size from a retinal photograph. METHODS: The position of the focusing knob on 3 different retinal cameras (TRC-50F and TRC-50X; Topcon America Corp, Paramus, NJ; and the CR6-45NM Non-mydriatic Retinal Camera; Canon Inc, Tokyo, Japan) was measured during optic nerve photography and correlated with the refractive error, or spectacle refraction, of the subject (N = 11 for each camera). RESULTS: A strong correlation was found between focusing knob position and spectacle refraction for each of the 3 cameras tested (r = 0.96, r = 0.99, and r = 0.97, respectively). CONCLUSIONS: The focusing knob position reflects the spectacle refraction of the eye being photographed, and spectacle refraction is known to correlate well with eye-camera magnification. Therefore, focusing knob position can be used to help calculate eye-camera magnification and, hence, true retinal object size. CLINICAL RELEVANCE: The true size of the optic nerve head is important for the diagnosis of glaucoma from a retinal photograph. This technique is a simple means to calculate optic nerve head size, which may be especially useful in mass retinal photographic screening programs.

Comparing optic nerve-head-size measurements by the heidelberg retina tomograph with fundus photography performed with a novel focusing technique.

PURPOSE: To compare disc areas as measured with the Heidelberg Retina Tomograph (HRT) to those obtained with a fundus camera. DESIGN: A nonrandomized comparative trial. PARTICIPANTS: Forty-six patients (88 eyes) who were participating in a glaucoma-screening study. METHODS: Disc areas from fundus photographs taken with a Nidek nonmydriatic camera were measured in pixels (Adobe photoshop 7.0). A magnification factor was determined, using a previously described technique that uses the position of the focusing slide on the camera to account for the spherical refractive error. The resulting calculated disc areas were then compared with the disc areas as measured with the HRT. MAIN OUTCOME MEASURES: Disc area. RESULTS: The limits of agreement between the 2 techniques were 0.49 and -0.51 mm2, indicating that the range of differences between these imaging techniques was approximately 1 mm2. Using a Bland-Altman plot, the distribution of residuals indicated that the 2 techniques do not differ in a systematic way. CONCLUSIONS: Disc area measurements can be reasonably determined from digital fundus photography if corrected for focusing slide position when compared with HRT. This may have useful applications for both clinical glaucoma patient care and population-based glaucoma-screening programs.