Microphthalmos is a congenitally small eye which means that the eye(s) started to form during pregnancy but for some reason stopped, leaving the puppy with small eyes. Sometimes, the microphthalmia condition occurs with coloboma, a structural eye defect when part of the eye does not develop. There is no cure for microphthalmia or coloboma. These conditions can affect one or both eyes. Eye affected by microphthalmia can have a very reduced level of vision. With coloboma, the visual impairment is variable. When one eye is absent the other eye may be completely normal with good sight, or abnormally developed with poor vision. Male dogs are reported to have this condition more often than female dogs. The causes of microphthalmia are considered to be either genetic or environmental, such as toxic substances, toxoplasmosis, viral orbacterial infections which can affect the mother during early pregnancy and damage the fetus.
Microphthalmia and multiple ocular anomalies are consistent with merle ocular dysgenesis (MOD). Merle ocular dysgenesis occurs in breeds affected by the merle gene, such as Australian shepherds, Great Danes, collies, and dachshunds including mixed breeds. The merle gene is a color dilution gene that lightens the coat color. The most severe ocular anomalies occur in homozygous merles with abundant white hair coat involving the head region. The ocular abnormalities that occur with this condition include: microphthalmos, microcornea, iridal abnormalities (coloboma, underdevelopment), asymmetric pupil size, shape, or position, persistent pupillary membranes, lens abnormalities (microphakia, cataract, coloboma, luxation, scleral defects (coloboma, staphyloma) and retinal defects (detachment, retinal dysplasia). Affected dogs also have varying degrees of congenital deafness.
- Diagnostic Ophthalmology Bianca S. Bauer, Lynne S. Sandmeyer, and Bruce H. Grahn Can Vet J. 2015 Jul; 56(7): 767–768.