Background: Low ocular perfusion pressure (two thirds of mean arterial pressure minus intraocular pressure) and myopia have been associated with protection of the retina from clinical diabetic retinopathy. This prompts the question as to whether myopia's protective role could also be a pressure effect, given that pressure could be dissipated in the longer arteriole tree of the myopic eye.Methods: We combined the Ohm, Poiseuille, and Murray laws to derive the following new formulation: the pressure attenuation along a vessel varies directly with its length and inversely with its diameter. A mean pressure attenuation index was calculated for 22 healthy control subjects, 25 patients with axial myopia, and 6 patients with retinitis pigmentosa using digitized fundus images. Results:The myopic arteriolar tree would produce a 16% greater pressure attenuation than that of controls (P = .002), with a linear relationship between mean pres-sure attentuation index and axial length (r = 0.93). Mean pressure attentuation index of the group with retinitis pigmentosa is increased 67% above that of controls, which is calculated to contribute an additional 10 mm Hg of pressure dissipation along their retinal arteriolar system.Conclusions: Pressure attenuation in retinal arterioles is directly proportional to the length and inversely proportional to the diameter of the arteriole segment being measured.Clinical Relevance: A pressure attenuation index may be important in light of the entities known or presumed to protect the retina from diabetic retinopathy. The results support the hypothesis that low-end arteriolar pressure is a common denominator for many protective conditions in diabetic retinopathy.
We examined the on- and off-responses of the photopic electroretinogram in patients with complete congenital stationary night blindness. Standard flash electroretinograms as well as those produced in a ganzfeld modified for long-duration light stimuli (500 msec) permitted the separation of on- and off-responses in four patients and four normal subjects. The amplitude and latency of the elctroretinogram on-response (a- and b-waves) and off-response (d-wave) in addition to the oscillatory potentials of the off-response in normal subjects and patients were compared. The abnormal on-response was demonstrated in all the patients, and the off-response with its oscillatory potentials were preserved. We showed that the second portion of the off-response (of inner retinal origin) is normal. If congenital stationary night blindness is a defect of depolarizing bipolar cells, these results preclude input of the depolarizing bipolar cells and support the hyperpolarizing bipolar cells as the cellular origin of the off-response electroretinogram.
PURPOSE. A recent systematic review indicated that higher sunlight exposure increased risk of AMD. The Beaver Dam study and the Pathologies Oculaires Liéesà 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 1.83 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.Keywords: age-related macular degeneration, axial length, light damage, sunglasses, retinal light dose H igh levels of sunlight exposure have been suggested to be a possible cause of AMD, and indeed, a recent systematic review and meta-analysis on this question 1 indicated that individuals with higher levels of sunlight exposure are at significantly increased risk of AMD. Despite this analysis, the link between AMD and sun exposure remains controversial, as two of the best-known studies included in the review, The Beaver Dam study (BDES) 2 and the Pathologies Oculaires Liées a L'âge (POLA) study, 3 concluded that there were few significant relationships between environmental exposure to light and the 10-year incidence and progression of AMD. Both studies did acknowledge that subjects wearing hats and/or sunglasses, which would decrease retinal light dose (RLD), significantly decreased some of the early retinal lesions seen in AMD. Additionally, when controlling for light exposure, the BDES 2 noted that men exposed to the highest levels of light were protected from some AMD lesions by myopia.Maltzman et al. 4 first noted an association between refractive error and AMD with myopic individuals being at lower risk compared with their hyperopic counterparts. A large population-based study has also shown this to be a dosedependent phenomenon: the greater the hyperopia or the shorter the axial length, the greater the risk for early AMD. 5 Thi...
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