The refractive state of the eye in Icelandic horses heterozygous for the Silver mutation

Abstract: Background: The syndrome Multiple Congenital Ocular Anomalies (MCOA) is a congenital eye disease in horses. Previous studies have shown that both the MCOA syndrome and the silver coat color in horses are caused by the same missense mutation in PMEL17. Horses homozygous for the Silver mutation (TT) are affected by multiple eye defects causing visual impairment or even blindness. Horses heterozygous for the Silver mutation (CT) usually have iridociliary cysts, occasionally extending into the peripheral retina. Clinical signs of visual impairment seem to be very rare in these horses. However, a recent study reported that Comtois horses carrying the Silver mutation had deeper anterior chambers of the eye compared to Wild-type horses. This could potentially cause refractive errors. The purpose of the present study was to study refraction of the eye in horses heterozygous for the Silver mutation to investigate if they have refractive errors. Ninety-four Icelandic horses were examined. All horses were genotyped for the missense mutation in PMEL17. Each of the 47 CT horses was matched by a Wild-type (CC) horse of the same age ± 1 year. A brief ophthalmic examination was performed and questionnaires regarding signs of visual impairment were filled out by the owners. Skiascopy was performed using a streak retinoscope (Heine Beta 200) and Trousseau racks without dilation of the pupils. Association between refraction and age, eye, genotype and sex was tested by linear mixed-effect model analysis. Pairwise interactions between each fixed factor (age, eye, genotype and sex) were included in the model. Results: The mean refraction value ± SE was 0.00 ±0.12D for the right eye and 0.03 ±0.10D for the left eye in CT horses compared to 0.22 ±0.05D for both right and left eye in Wild-type horses. The interaction between age and genotype had a significant impact on the refractive state (P=0.001). A deviation towards myopia (nearsightedness) could be observed in CT horses older than 200 months (16.7 years). In the questionnaire, forty-one CT horse owners (87.2%) reported that their horse seemed to have no impaired vision at all. Conclusions: The refractive state of young and adult CT horses were not significantly different from Wild-type horses. Although, CT horses older than 200 months (16.7 years) were more likely to develop myopia than Wild-type horses. This suggests that the Silver mutation exerts a slowly progressive effect on the optics of the eye.