Blindness in dogs can have a variety of causes including glaucoma, cataracts, chorioretinitis, retinal detachments and CNS lesions. Not all causes of vision loss are obvious on ophthalmic examination.
Amaurosis is the term used to describe cases of vision loss with no observable ocular lesions. The three major differentials for amaurosis are:
SARDS (sudden onset retinal degeneration syndrome) – the most common cause of amaurosis. The exact cause of SARDS is unknown but there can sometimes be an association with hyperadrenocorticism-type signs. Risk factors seem to include recent weight gain, small to medium breeds, middle to old age, female gender. Clinically, SARDS cases manifest as sudden-onset dilated pupils, reduced or absent light reflexes and blindness occurring over a period of days to weeks – sometimes even overnight. Diagnosis can be confirmed with an electroretinogram (ERG), which requires sedation and the application of 3 electrodes to measure the amplitude of each retinas electrical response to light flashes (below). Flat ERG traces are seen with SARDS. The vision loss is irreversible and no treatment is effective.
Retrobulbar optic neuritis – this refers to inflammation of the optic nerve posterior to the optic nerve head and therefore unobservable with ophthalmoscopy. The condition is relatively rare compared to more distal optic neuritis observable with ophthalmoscopy. Optic neuritis in dogs is typically presumed to be immune-mediated and a variant of granulomatous meningoencephalitis (GME). ERG traces are normal in dogs with optic neuritis-related blindness and confirmation requires an MRI or highly sensitive orbital ultrasound imaging. Generally, optic neuritis can be controlled and some vision restored with immunosuppressive doses of oral prednisolone.
Intracranial disease – where a focal lesion inside the cranium at or near the optic chiasm and optic tracts leads to amaurosis. Cerebrovascular accidents or focal space-occupying lesions such as pituitary neoplasia or meningiomas are potential causes. ERG traces are normal in dogs with central blindness and MRI is needed for confirmation.
