Vision loss in all animals can generally occur in one of three areas. These areas include the anterior segment of the eye (cornea or lens), the retina, and the brain (including the optic nerve). Please click here for a general diagram of ocular anatomy.
The parts of the anterior segment of the eye that may contribute to vision loss include the cornea and the lens. Dogs with severe corneal disease including corneal ulceration, pigmentary keratitis, corneal scarring, pannus, corneal endothelial degeneration, and corneal degeneration may have enough opacification of the normally clear cornea to cause vision loss. Many of the corneal causes of vision loss can be treated to improve vision using either medical and/or surgical therapy.
Opacification of the lens (cataract), is also a common cause of vision loss in dogs. Inherited or genetic cataracts are the most common, followed by those secondary to diabetes, secondary to intraocular inflammation, and age. As long as retinal function is normal, the cataracts can usually be removed to restore vision. Please click here for more information about cataracts in dogs.
In general, retinal causes of vision loss include those conditions in which the retina is detached or degenerate. Some of the more common causes of retinal detachment include retinal tears (rhegamatogenous), chorioretinitis, high blood pressure, vitreoretinal or retinal dysplasia, trauma, Collie Eye Anomaly, steroid responsive retinal detachment syndrome, and vitreous degeneration. Depending on the cause of the retinal detachment, medical therapy may be sufficient to allow reattachment and return of vision. In other cases, retinal reattachment surgery may be indicated to help restore vision.
Causes of retinal degeneration include progressive retinal atrophy (PRA, PRCD), sudden acquired retinal degeneration syndrome (SARDS), glaucoma, immune-mediated retinopathy (IMR), chorioretinitis, ivermectin toxicity, vitamin E deficiency, and storage disease. Depending on the underlying cause of the retinal degeneration and its extent, therapy may be available to slow down continued progression. However, retinal degeneration is unfortunately not reversible and tends to cause a permanent vision deficit.
Vision loss secondary to brain disease can involve the optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, optic radiation, and/or visual cortex. The most common optic nerve cause of vision loss is optic neuritis. This is inflammation of the optic nerve and tends to be idiopathic. Early and aggressive therapy for optic neuritis is usually successful at restoring vision. Generalized brain disease may or may not be treatable. Cases where this is suspected are referred to a neurologist for consultation.
**In some cases of vision loss, there may be no visible retinal or optic nerve pathology. In these cases, to differentiate a brain cause of vision loss from a primary retinal cause (such as SARDS), diagnostic testing is available. The diagnostic test of choice is called an electroretinogram. This test measures the retinal electrical activity. Blind dogs with normal retinal electrical activity have either optic nerve or brain disease while blind dogs with absent retinal electrical activity have a retinal cause of vision loss.
**Please click here for information about SARDS.