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 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.

Sandmeyer doesn't know how many horses are affected by CSNB, but she said the disease might be present in up to 25% of Appaloosas. Veterinarians have also reported the disease in a Thoroughbred and a Paso Fino.

Although there is no current treatment for CSNB, Sandmeyer states that the disease is certainly manageable, as many horses know the boundaries of their fields even in the dark. "The owner needs to be aware of the visual deficits in the dark and take precautions to avoid injury to themselves and the horse," Sandmeyer said. "This means avoiding working and riding in the dark, removing hazards from their environment such as low-hanging tree branches, sharp objects on fences and stalls, and holes in the pasture, so that the horse will not injure itself in the dark."

"Other things can be helpful," she added. "One owner of a CSNB-affected horse recommended that if horses have trouble entering a dark trailer, you could paint the inside white."

About the Author

Liz Stitt, Editorial Intern

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

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More