BASICS
- Congenital stationary night blindness is a hereditary disease that affects night (scotopic) vision and in severe cases, may also affect day (photopic) vision. It is nonprogressive, and persists throughout the horse's life. No morphologic abnormalities are seen ophthalmoscopically or by means of light and electron microscopy
- Systems affected—ophthalmic
- Most common in the Appaloosa, but also described in the Miniature horse, the Thoroughbred, and the Paso Fino
- In Appaloosas and Miniature horses, CSNB is associated with the LP and it is likely that other horse breeds where LP spotting is present may be affected as well
- No sex predilection is known
- Disease is present at birth and persists throughout life
- Visual impairment in dim light and dark conditions, leading to behavioral uneasiness and unpredictability, despite a normal ophthalmoscopic examination
- Vision is usually normal in daylight. However, photopic visual impairment has been reported in some cases, and is believed to be a severe form of the disease
- Dorsomedial strabismus (“star gazing”) and nystagmus may also be present in severely affected animals. Strabismus is not an obvious clinical sign in most cases
- Unlike CSNB in humans, affected horses have refractive error similar to unaffected horses, and do not suffer from severe myopia (nearsightedness)
- Owners may report repeated injuries to the horse during the evening hours
- Most affected horses function very well, and owners may be unaware that their horse is visually impaired. Clinical signs such as apprehension and anxiety in dim light are attributed to behavior and temperament rather than visual impairment
- A defect in the neural transmission between rod photoreceptors and bipolar cells in the mid-retina is responsible for this disease
- The rod photoreceptors are responsible for night vision. TRPM1 is important for rod bipolar cells’ depolarization. Decreased expression of TRPM1 in the retina of affected Appaloosas and Miniature horses prevents the depolarization of the rod bipolar cells, which in turn prevents the propagation of the signal to the optic nerve, resulting in night blindness
- Appaloosas and Miniature horses are affected by CSNB in association with homozygosity of the LP allele (autosomal recessive inheritance)
- Thoroughbreds and Paso Finos are likely affected by a different genetic mechanism
DIAGNOSIS
TREATMENT
MEDICATIONS
FOLLOW-UP
MISCELLANEOUS