Mary Thompson, a vertebrate paleontologist at Idaho Museum of Natural History, has found evidence in many species of early horses and concludes, "The results of this study strongly suggest that man’s intervention (whether by increased usage or improper breeding practices) may not be the sole cause of the syndrome", although she cautions that her results are preliminary. Work
Working on steep hills, galloping, and jumping all contribute to navicular syndrome, as they place greater stress on the DDF tendons, and may cause overextension of the pastern and coffin joints. Regular exercise on hard or irregular ground increases concussion on the hoof, thus increasing the risk of navicular syndrome. It is possible that standing can also increase the chance of navicular disease (such as a horse that spends most of the day in a stall with little turnout, as with some racehorses and show horses). Blood flow to the hoof decreases when the horse is not in motion. The horse is also constantly applying pressure to the navicular bones (which is intermittent as the horse moves). Body weight
Horses with a high weight-to-foot-size ratio may have an increased chance of exhibiting symptoms of navicular syndrome, since the relative load on the foot increases. This might explain why the syndrome is seen more frequently in Thoroughbreds, American Quarter Horses, and Warmbloods as opposed to ponies and Arabians. Signs
Heel pain is very common in horses with navicular syndrome. Lameness may begin as mild and intermittent, and progress to severe.
Abnormal coloring
Redness of the sclera is typically caused by eye irritation causing blood vessels to expand, such as in conjunctivitis ("pink eye"). Episcleritis is a generally benign condition of the episclera causing eye redness. Scleritis is a serious inflammatory disease of the sclera causing redness of the sclera often progressing to purple. Yellowing or a light green color of the sclera is a visual symptom of jaundice. In cases of osteogenesis imperfecta, the sclera may appear to have a blue tint, more pronounced than the slight blue tint seen in children. The blue tint is caused by the showing of the underlying uveal tract (choroid and retinal pigment epithelium). In very rare but severe cases of kidney failure and liver failure, the sclera may turn black. See also
Extraocular implant
Scleral tattooing
Sclerotomy

Notes
References
External links

Histology image: 08008loa – Histology Learning System at Boston University
Atlas image: eye_1 at the University of Michigan Health System—"Sagittal Section Through the Eyeball"
MedlinePlus Encyclopedia: 002295