Determination of the location of the fovea on the fundus

PURPOSE: To evaluate whether the distance between optic nerve head and fovea in healthy eyes determined by scanning laser ophthalmoscope may facilitate estimation of the location of the fovea relative to the optic disc in patients with macular disease. METHODS: The angular distance was measured, in horizontal and vertical directions, between the center of the optic nerve head and the fovea in 104 eyes of 104 healthy probands. For additional evaluation of intraindividual variation in 70 of these persons the contralateral eye was measured as well. RESULTS: The distance between the optic disc and the fovea differed vertically more than horizontally (-1.5 +/- 0.9 degrees [-3.65 to +0.65 degrees ] vs. 15.5 +/- 1.1 degrees [13.0-17.9 degrees ]). There was a mean angle between the fovea and the center of the optic disc versus the horizon of -5.6 +/- 3.3 degrees. The intraindividual difference between right and left eyes was markedly lower, with average angles being 0.2 +/- 1.3 degrees vertically and 0.0 +/- 1.1 degrees horizontally. CONCLUSIONS: The distance between the optic nerve head and the fovea does not allow for a meaningful determination of the location of the fovea in eyes in which morphologic changes have occurred. The angle of rotation of the fovea relatively to the center of the optic nerve head is relatively stable. Therefore, the size of a central scotoma can be determined by movement of the blind spot according to the change of the preferred retinal locus (PRL). In addition, the knowledge of the location of the fovea enables determination of the position in the contralateral eye of the same patient.     

On the relevance of the long-time follow-up of the cyclocryotherapy

In spite of the availability of cyclocryodestructive laser therapy there still appears to be an indication for cyclocryotherapy in different kinds of glaucoma. PURPOSE: Cyclocryotherapy very often appears to be the operation of choice in cases of progressive functional visual loss if a patient is not sufficiently helped with the maximum tolerated medical therapy. We investigated long-term IOP effects and possible complications in our group of patients undergoing cyclocryotherapy. PATIENTS: Cyclocryotherapy was performed on 109 eyes of 97 patients from 1988 to 1994. Cyclocryotherapy was performed as follows: 67 of 109 eyes were controlled for IOP for an average of 10 months (3-41 months). We applied cryotherapy six times 3-4 mm behind the limbus for an average duration of 50 to 60 s over half of the circumference. RESULTS: We achieved satisfactory IOP control (IOP K 22 mm Hg) in 40 eyes, however, with 30 additional operations. One eye developed phthisis bulbi, and 3 other eyes became blind because of complications in neovascular glaucomas. Neovascular glaucoma cases responded the least. CONCLUSIONS: Cyclocryotherapy produced a statistically significant IOP reduction in eyes with primary open-angle glaucoma or secondary glaucoma. Close long-term follow-up appears advisable in order to check the eyes for possible increase in IOP or complications of surgery.     

On the relevance of the long-time follow-up of the cyclocryotherapy

In spite of the availability of cyclocryodestructive laser therapy there still appears to be an indication for cyclocryotherapy in different kinds of glaucoma.     

Numerical study of the influence of the shell structure of the crystalline lens on the refractive properties of the human eye

In this paper, the numerical model of the refractive properties of the human eye is given. A special account is given of the laminated structure of the crystalline lens. The crystalline lens is presented in the form of hundreds of shells with rotational symmetry, and with the refractive index constant within each shell. The shells have the form of two semi-ellipsoids joined along the equator. The refractive index increases from the cortical shell to the inner one, according to the exponential dependency. The cornea, approximated by two ellipsoidal surfaces, is added in front of the crystalline lens. A ray-tracing procedure is applied to study the refractive properties of such a system: refractive power, spherical aberration and energy distribution. The optical properties of the given model are analysed by changing some parameters such as refractive index profile and number of shells. Calculations show that the gradient of the refractive index inside the crystalline lens results in the generation of many focal planes in such an optical system.     

Electrophysiological determination of the refractive state of the eye of the opossum

The refractive state of the eye of the South American opossum Didelphis marsupialis aurita was investigated with electrophysiological techniques. Using adult specimens, trapped from the wild, averaged cortical evoked responses were recorded from the region of projection of the central visual field. Stimuli consisted of a phase reversal of a square wave grating generated on a CRO screen, with luminance of 2.4 cd/m2 and contrast 0.84. The refractive state of the eye was altered by means of trial lenses and the amplitude of the cortical responses thus obtained compared to those obtained with no lens (control values). Refraction "tuning curves" were determined for each animal. The average refractive state was found to be -2.27 D indicating that this species when raised in its habitat shows, at low ambient luminosity, some degree of myopia. Determination of the Contrast Sensitivity Function indicate that induced ametropias lead to a reduction of the cut-off value of the spatial frequency and a loss of contrast sensitivity.