Ocular Manifestations of Systemic Disease
The eyes may be affected by various systemic diseases, and those with characteristic findings are described below. Click here for Power Point presentation, which includes expanded information and color illustrations of clinical cases.
Inflammation of the uvea (iris, ciliary body, choroids) may manifest as conjunctival hyperemia, blepharospam, corneal edema, aqueous flare, miosis, and ocular hypotension. Clinical signs, diagnosis and treatment of uveitis are discussed here.
Dogs with diabetes mellitus develop cataracts and lose vision usually within 3 months of diagnosis. Elevated blood glucose leads to elevated intraocular glucose levels. In dogs (not cats), aldose reductase in the lens is overwhelmed by excess glucose, leading to lens damage and cataract formation. Aggressive treatment of the diabetes may slow cataract formation. Also, prompt ocular diagnosis and treatment are indicated, since rapid cataract formation can lead to lens induced uveitis and glaucoma. To prevent these secondary issues, topical anti-inflammatory drugs are recommended until surgical cataract extraction can be done.
Sudden Acquired Retinal Degeneration Syndrome (SARDS) is an idiopathic form of retinal atrophy in which the retinal photoreceptor layer becomes acutely necrotic. Vision loss occurs within days or sometimes weeks. Pupillary light reflexes may remain active for some time after vision loss occurs, making diagnosis challenging. Also, the retinas appear clinically normal early in this disease, and only become visibly degenerated months after photoreceptor necrosis. The “gold standard” for diagnosis is made via electroretinography, a diagnostic test which measures retinal electrical activity. With the patient under general anesthesia or heavy sedation, a series of light stimuli is introduced into the eye. Retinal response is recorded and precisely quantified, and a SARDS patient will show no response. Though this condition has been studied extensively, the cause and effective treatment remain unknown. It is not known to occur in cats.
Systemic hypertension often causes retinal detachment and hemorrhage as the intravascular pressure exceeds the ability of the vascular walls to contain plasma and blood cells. Severity of ocular damage depends on degree and duration of blood pressure elevation, and intraocular hemorrhage is usually cause for a guarded prognosis for vision. Prompt diagnosis and treatment of the blood pressure can reverse the ocular signs. Other hematologic conditions (such as anemia or coagulopathy) will cause non-specific ocular signs referable to hemorrhage, usually involving the conjunctiva, and anterior chamber (hyphema), vitreous body, or retina.
Lymphoma and multiple myeloma uncommonly affect the uvea in both dogs and cats; tumor infiltration causes uveitis, typically with focal iridal swelling and inflammation. Systemic signs of illness are usually seen, and referral to an oncologist is indicated.
Malignant intraocular tumors cause uveitis and are often metastatic. Medical work up and tumor staging are indicated. Full physical exam, CBC/chemistry, and thoracic and abdominal imaging provide a good baseline, and referral to an oncologist may be considered. Benign intraocular tumors are usually primary, space occupying, and slowly expansile. While a few benign intraocular tumors can be removed surgically and vision spared, most are monitored and enucleation done if the tumor becomes too large.
Orbital tumors can be challenging to diagnose, and cause globe deviation, strabismus, and limited globe retropulsion. These may be primary or metastatic, benign or malignant. In cats, orbital tumors are more likely malignant with squamous cell carcinoma being most common. Orbital tumors in dogs are more likely to be benign and slowly expansile. Full medical workup is usually indicated, and imaging of the head (radiographs, CT, MR) is very helpful. There are various surgical approaches to the orbit, although enucleation is probably most common in order to remove the tumor most thoroughly.
Intracranial tumors may cause vision loss, pupillary reflex abnormalities, and depressed mentation. The ocular exam is usually normal, and electroretinography is recommended to rule out SARDS. If all ocular findings are normal, referral to a neurologist is indicated.
Horner’s Syndrome occurs when sympathetic innervation to the eye is interrupted. Signs include ptosis of the upper lid, enophthalmos, third eyelid elevation, and miosis. The miotic pupil will characteristically not dilate in the dark. The sympathetic nerve chain which supplies the iris begins in the brain, courses caudally via the spinal cord, diverts into the thoracic cavity, courses anteriorly via the vagosympathetic trunk, synapses at the cranial cervical ganglion, enters the skull, courses through the middle ear, then finally reaches the eye. Damage to any portion of the nerve in any of these locations will cause signs of Horner’s Syndrome. In dogs, the condition is most common in the Golden Retriever, is idiopathic, and nearly always resolves within 1-2 months. In other dog breeds and cats, causes are more varied.