Understanding Congenital Stationary Night Blindness

Canadian researchers are investigating the cause of a condition found primarily in Appaloosas that prevents them from seeing in the dark. Congenital stationary night blindness (CSNB) is a hereditary, non-progressive condition for which there is

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Canadian researchers are investigating the cause of a condition found primarily in Appaloosas that prevents them from seeing in the dark. Congenital stationary night blindness (CSNB) is a hereditary, non-progressive condition for which there is no cure, but veterinarians have recommended management techniques that can improve quality of life for affected animals.


Lynne Sandmeyer, DVM, DVSc, Dipl. ACVO, is associate professor of small animal clinical sciences and a veterinary ophthalmologist at the University of Sasketchewan’s Western College of Veterinary Medicine (WCVM). She says CSNB could be caused by a defect in electrical signaling in the retina. In the normal eye, the retina has two types of photoreceptors that respond to light–rods and cones. The cones are responsible for day and color vision, while the rods function best at night. In a horse with CSNB, there appears to be a problem in either the ability of its rods to transfer electrical signals to the next retinal cells in line, or in the retinal cells’ ability to read the signals.


The disorder is diagnosed with the use of an electroretinogram (ERG), which records the electrical response of the retina to a flash of light. “Horses are first kept in the dark for several minutes to dark-adapt them, and basically this primes the rods for activity,” Sandmeyer said. “We then do a series of light flashes of different intensities and measure the retina’s electrical activity.”


Additionally, severely affected CSNB horses can have strabismus, an abnormality in the positioning of the eyes in which one eye might rotate slightly. According to WCVM’s Horse Health Lines, Sandmeyer and colleagues are conducting detailed electroretinographic and anatomical studies of affected horses’ eyes. In the meantime, they are also examining the relationship between CSNB and particular Appaloosa coat patterns. With a better understanding of the pathophysiology and genetics of CSNB, the researchers hope to develop genetic testing to help diagnose the disorder

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Liz Stitt was The Horse’s editorial intern in 2005 and a student majoring in equine science and English at the University of Kentucky.

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