Rod Densitomefry in the Aging Human Eye

Abstract

Retinal densitometry is a noninvasivc physiologic technique used to examine the visual pigments in living human eyes. To assess possible age-related disturbances of rod photopigment kinetics, retinal densitometry was done in 44 eyes of 44 healthy subjects (age range, 12-78 yr). With progressing age, a significant but small increase in photopigment density difference (bleached versus dark adapted eye) and an increase in the time constant of rhodopsin regeneration was found. The increased density difference in rods was consistent with morphologic findings of increased rod outer segment diameter and disc content in older subjects. To explain this change in terms of the decreased specular reflections at the level of the inner limiting membrane was inadequate because age effects were independent of wavelength in the region of 450-550 nm. To control for the effects of ocular stray light from the lens, subjects older than 40 yr with a clear crystalline lens were measured and compared with those with pseudophakia. No statistically significant difference was found between the two groups. Increased rod density difference contrasts sharply with an earlier reported decrease in this parameter for foveal cones. The slowing of the regeneration rate is a phenomenon common to rods and cones. It may be a result of a gradual metabolic dysfunction of the retinal pigment epithelium in older subjects. Invest Ophthalmol Vis Sci 32:2676-2682, 1991 In 1722, Anthony van Leeuwenhoek (1632-1723) was probably the first to observe rod photoreceptor cells in the human retina by microscopy, 1 although he was undoubtedly unaware of their functional role. Although much knowledge has been gained on the physiology of vision by rods, age-related changes in rods in the living human eye with regard to their photopigment kinetics have not been documented to our knowledge. We investigated age effects of rod photopigment with the Utrecht fundus reflection densitometer (FC Donders Institute of Ophthalmology, Utrecht, The Netherlands), as a sequel to earlier studies on aging of human cone photopigment kinetics. In addition, the data may be used as a control group for further studies about age-related retinal diseases. Morphologically, rod photoreceptors undergo structural changes with age. A reduction in the number of photoreceptor nuclei (both rods and cones) was reported to be associated with aging, as was a displacement of these nuclei. 7 These authors found evidence for an age-related loss of RPE in both whites and blacks. In whites, photoreceptor loss was significantly and di- rectly correlated with lipofuscin concentration in the opposing RPE but was not related to age. In blacks, however, lipofuscin was not a strong determinant of photoreceptor loss. Detailed histologic information on the morphology of rod outer segments (ROS) was reported. 8 Early studies of aging and dark adaptation 9 " 13 showed a significant decrease in absolute sensitivity with age, but recently insignificant changes in rod sensitivity with age were reported in a study that carefully controlled for preretinal changes. 14 Several investigators 15 " 17 studied age effects to flash stimulation with electroretinography in healthy subjects. They found no significant age correlation for scotopic a-wave amplitude, but the b-wave decreased with age. In summary, histologic data indicate senile rod photoreceptor changes starting after age 40 yr and involving 10-20% of the total rod population, but functional tests show little or no impairment of responses of the photoreceptor ROS. Our densitometric data corroborate these findings. A statistically significant, but small, increase in both rod photopigment density and the time constant of regeneration was found throughout the second to eighth decade of life. Materials and Methods Subjects Healthy subjects (age range, 12-78 yr) participated in the study which was approved by the University Hospital ethics committee. Informed consent was obtained from all participants. All subjects underwent a routine ophthalmologic examination. Patient selection criteria were good physical and mental health, no Downloaded from iovs.arvojournals.org on 07/01/201