Functional quantification of diaphany

History of ocular straylight measurement: A review

The earliest studies on 'disability glare' date from the early 20(th) century. The condition was defined as the negative effect on visual function of a bright light located at some distance in the visual field. It was found that for larger angles (>1 degree) the functional effect corresponded precisely to the effect of a light with a luminosity equal to that of the light that is perceived spreading around such a bright source. This perceived spreading of light was called straylight and by international standard disability glare was defined as identical to straylight. The phenomenon was recognized in the ophthalmological community as an important aspect of the quality of vision and attempts were made to design instruments to measure it. This must not be confused with instruments that assess light spreading over small distances (<1 degree), as originating from (higher order) aberrations and defocus. In recent years a new instrument has gained acceptance (C-Quant) for objective and controllable assessment of straylight in the clinical setting. This overview provides a sketch of the historical development of straylight measurement, as well as the results of studies on the origins of straylight (or disability glare) in the normal eye, and on findings on cataract (surgery) and corneal conditions.

Primary megalocornea: clinical features for differentiation from infantile glaucoma

PURPOSE

To describe the ocular findings in megalocornea to assist in its differentiation from infantile glaucoma in the evaluation of children with abnormally enlarged corneas.

METHODS

The clinical findings of 4 boys found to have megalocornea following referral for evaluation of large corneas and suspected glaucoma were reviewed.

RESULTS

Three of the 4 patients had photophobia. Clear and enlarged corneas were observed associated with deep anterior chambers, posterior bowing of the irides, and normal intraocular pressures (IOPs) in all eyes. Transillumination of the irides was found in 6 of 8 eyes and pigment dispersion was seen in 4 of 8 eyes. Pigment dispersion appeared to be acquired over time, and the youngest patient in this series who had pigment dispersion detected on slit-lamp examination was 15 years; the youngest patient with the condition detected on gonioscopy was 8 years. No breaks in Descemet's membrane were present. Family history obtained from 3 of the 4 patients revealed evidence of sex-linked recessive inheritance. These findings are distinct from the clinical features of infantile glaucoma characterized by elevated IOP, breaks in Descemet's membrane, corneal edema, a generally flat iris profile, less pronounced enlargement of the anterior segment, the absence of iris transillumination and pigment dispersion, and autosomal recessive inheritance. Three patients had corneal size asymmetry, a finding that has not been previously reported.

CONCLUSION

Hereditary megalocornea has defining clinical findings that help to identify and differentiate it from other causes of enlarged corneas. Asymmetry in corneal size does not preclude its diagnosis.

Fuch's heterochromic cyclitis and HLA histocompatibility antigens

Genetic typing of class I major histocompatibility complex (MHC) antigens (HLA-A, -B and -C) was performed in 22 patients with Fuch's heterochromic cyclitis (FHC) and in a control group of 339 healthy subjects in order to ascertain whether there are any genetic factors that protect against or increase the risk of developing this disease. The frequency of only one HLA-antigen was found to differ significantly in the patients with FHC as compared with the normal subjects: HLA-A2 had a negative association with FHC (9.09% in FHC verus 47.47% in the control group; p < 0.01, Fisher's exact test with correction for the number of antigens studied) with an odds ratio of 0.11 (95% confidence limits 0.03 and 0.50). Other HLA antigens also showed a negative or positive association with FHC but not to a statistically significant level. Our results suggest a possible role for HLA genetic factors in the pathogenesis of FHC.

An improved mathematical description of the foveal visual point spread function with parameters for age, pupil size and pigmentation

An analytical description of the point spread function (PSF) for human foveal vision, together with its closed form two-dimensional Fourier transform, the modulation transfer function (MTF), is proposed. It also possesses an analytical line spread function (LSF) counterpart. It fits to both PSF and MTF experimental data and holds over the full angular and spatial frequency domain. Variation in the PSF and MTF descriptions with pupil size, age and iris/skin pigmentation are accounted for by analytical expressions in the parameters.

Dependence of intraocular straylight on pigmentation and light transmission through the ocular wall

The straylight function of the human eye depends on eye color, especially at larger angles of scattering. As a potential cause for this dependence, transmission of light through the ocular wall was measured, using a psychophysical method. For a light-blue eye effective transmission of the iris was 1% for red and 0.2% for green light. Also the eyewall around the iris transmits a significant amount of light. For the dark-brown eyes of pigmented individuals transmission is lower by two orders of magnitude. Although important, transmission proved to be only partly responsible for the pigmentation dependence, the other cause probably being reflection from the fundus.