Behind Blue Eyes: Congenital Stationary Night Blindness

Tommy, can you see me?
Can I help to cheer you?

— Pete Townshend (The Who) from the 1969 album, "Tommy"

One evening soon after Daltrey came home, I was spreading Fly Eliminators in the back pasture. The sun had set, but I could still see clearly. As I headed back to the house, I noticed Daltrey standing close to the culvert that bridges the front and back of the property. It seemed odd, because at that time, he still stuck pretty close to his nannies, who were nowhere to be seen. In any case, he wasn’t grazing, just standing there, about 50 paces away. I greeted him in a normal tone of voice, and the little guy jumped straight up. He obviously hadn’t seen or heard me coming, although he was looking right at me.

If you followed Heather’s genetics posts, you know that Daltrey exhibits a number of color and pattern traits gone wild. Seems like if there was any chance of a pattern gene expressing itself, it did in him. We’ve known since his birth that Daltrey is congenitally hearing impaired to some degree, related to his extreme splashed white pattern (read about splashed white here, and look in the right column for links to all the color/pattern posts). That’s a post for another day. The splashed white pattern is also responsible for Daltrey’s startling blue eye color, which does not affect his sight.

However, Daltrey is also homozygous for leopard-pattern complex (LP/LP), which was directly tied to congenital stationary night blindness (CSNB) in a 2005-2006 study. Appaloosas that are heterozygous for LP (LP/lp) are not affected by CSNB. Horses that are true solid, non-characteristic Appaloosas, with no dominant copies of LP (lp/lp), are also not affected by CSNB.

Click on chart to enlarge.
Daltrey is best represented by the bottom row, third from left.
Poco is bottom row, second from right.

So, coat patterns that indicate the presence of two dominant copies of LP are associated with CSNB. The amount of white patterning they display is not important, except that when they have moderate to large amounts of patterning, they are easier to classify correctly as being homozygous for LP. Horses homozygous for LP born with no coat patterning are also night blind.

There may be a variety of causes for CSNB in horses, but we’re looking only at the type found in Appaloosas. CSNB has been detected in other breeds (Paso Fino, Thoroughbred, Quarter Horse, Belgian), none of which were Paint or Appaloosa patterned.

Night blindness is not a single disorder. A horse can have a form of night blindness and not have CSNB (uveitis is the most common other cause). In general, the term CSNB is used to describe impaired/absent night vision, which is present at birth, inherited, and non-progressive.  Incidentally, the "stationary" part of the name refers to the fact the condition is non-progressive, and has nothing to do with whether or not the horse is standing still. CSNB-affected Appaloosas usually have normal day vision.

Because we typically don’t interact with our horses after dark, we might not ever see some of the symptoms, such as anxiety or closely following another horse at night. Under normal circumstances, affected horses cope with the condition because they're born with it. They just think that's the way the world works: at night, they can't see. The world is completely black, unless the horse looks straight at the moon or some other light source that is bright enough to appear as a light spot in its field of vision. Poco has it, though he is either not impaired to the degree that Daltrey is, or he is just that much more accustomed to it because he's older (and his hearing is fine).

There is nothing structurally wrong with the eyes of horses with CSNB. The problem occurs during the process of transmitting information from the eyes to the horse's brain. Cone-shaped photoreceptors transmit color and day vision, while rod-shaped photoreceptors are responsible for vision in the dark. In horses with CSNB, the information normally carried by the rod-shaped photoreceptors is dropped or not transmitted properly.

The study linked CSNB in Appaloosas to the leopard complex, but that doesn’t explain how two apparently independent genetic features are actually related. Research in 2008 proved both CSNB and leopard pattern in Appaloosas are caused by a gene whose simplified name is TRPM1. Simply, decreased expression of TRPM1 in the eye and the skin alters signaling from the rod-shaped photoreceptors in the retina as well as the function of certain pigment cells in Apps, thus causing both CSNB and leopard complex.

I may not have ever noticed the degree of Daltrey's impairment were it not for having to play Cafeteria Lady each evening. I have seen first hand that he operates by feel and memory. For example, if he raises his head to chew a mouthful, and he either takes a step or two, or I slide the feed pan, he has to feel around to find it again. It doesn't seem to bother him. I have not found him to be anxious or apprehensive at all. And (knock wood), he has not injured himself.

Gives a whole new meaning to Mike's coining of the term daylight pony for Baby D.

Disclaimer: I am not a veterinary professional. This information is presented as entertainment only. I attempted to obtain permission from the authors and publishers to use this information, but after more than 60 days, my efforts have not been acknowledged. Information came from the following sites: www.appaloosaproject.com, www.horsetalk.co/nz, www.genetics.org and www.thehorse.com. Regardless, most of the actual data about CSNB is the result of a 2005-2006 study by Lynne S. Sandmeyer, DVM, DVSc, DACVO; Carrie B. Breaux, DVM; Sheila Archer, BSc Hons; and Bruce H. Grahn DVM, ACBP, DACVO. The information about TRPM1 is from a 2008 study, which also included Dr. Sandmeyer, as well as Rebecca R. Bellone, Samantha A. Brooks, Barbara A. Murphy, George Forsyth, Sheila Archer (of the Appaloosa Project), Ernest Bailey, and Bruce Grahn.