Koret School of Veterinary Medicine, Hebrew University of Jerusalem, Rehovot, Israel
As with any patient, you should begin by taking a history. Inquire whether the blindness is acute, or of gradual onset. Frequently, owners will report sudden loss of vision, even though ophthalmic examination shows signs associated with chronic disease. Careful questioning will reveal that the blindness was noticed when the animal's environment was changed (e.g., visiting friends). We can assume that though the animal has been blind for a long time, the gradual onset of the disease enabled it to learn to navigate at home. The change in environment causes it to bump into objects, misleading the owners into believing that the blindness is acute.
Inquire whether the blindness was associated with preferential loss of night vision. One of the first behavioral signs of inherited, degenerative diseases of the outer retina (commonly known as Progressive Retinal Atrophy, PRA) is loss of night vision as rods are affected before cones. Be careful of how you phrase the question!! Do not 'lead' the owners. Other rod functions (such as detection of moving objects, or objects in the peripheral visual field) are also affected before cone functions (color vision, detection of objects in the central visual field). However, these are usually too subtle to be detected by owners.
Finally, inquire whether the animal is healthy, or are there other signs of illness, besides loss of vision? Blindness may be caused by numerous systemic or neurological diseases. These will frequently be accompanied by additional signs of clinical disease. Therefore, the clinician must take a complete (non-ophthalmic) history of the patient. Furthermore, a complete physical examination and a thorough neurological assessment are part of the workup of every case of blindness.
Examination of the patient
Obviously, every blind patient must undergo a comprehensive ophthalmic examination. This examination, including methods of evaluating the visual system, is described in detail elsewhere in these Proceedings (Ocular Examination - How to Do It?). Retinal function may further be assessed using the electroretinogram (ERG), which records the electrophysiological responses of the retina to visual stimulation. This test is useful in diagnosing early stages of PRA, and in differentiating between optic neuritis and sudden acquired retinal degeneration (SARD).
In addition to the ophthalmic examination, a neurological examination should be conducted in cases of blindness in which CNS involvement is suspected. Attention should be paid to cranial nerve deficits, change in mental status and abnormal gait and body posture. Imaging techniques, such as radiography, CT and MRI, may also be used to workup suspected cases of central blindness.
Localizing the lesion in the blind patient
Based on the results of the ophthalmic examination, the patient may be categorized into one of 4 general categories:
1. Abnormal ophthalmic findings combined with a normal/diminished PLR. Patients in this category suffer either from opacity of the ocular media (e.g., corneal edema, hyphema/hypopyon, cataract, etc.) or from retinal disease (e.g., PRA, chorioretinitis).
2. Abnormal ophthalmic examination and an absence of PLR. Common causes of blindness in this category include glaucoma, retinal detachment and optic neuritis involving the proximal optic nerve.
3. Normal ophthalmic examination and an absence of PLR. Common causes of blindness in this category include Sudden Acquired Retinal Degeneration (SARD), optic neuritis involving the distal optic nerve, and neoplasia compressing the optic nerve or chiasm.
4. Normal ophthalmic examination and absence of PLR. These are usually neurological cases, caused by central lesions affecting the visual pathways from the lateral geniculate nucleus to the contralateral visual cortex. Causes include congenital lesions (hydrocephalus), metabolic disease (hepatic encephalopathy), toxins (lead toxicity), and inflammatory or infectious CNS disease.
Common causes of blindness
Glaucoma, a common cause of blindness, is discussed in Glaucoma--Do I Need a Tonopen--The Role in Diagnosis.
Retinal detachment is a separation between the retina and choroid, at the level of the RPE. If the separation is not quickly resolved, photoreceptors begin dying of ischemia, leading to irreversible blindness.
There are 3 types of detachments, depending on the mechanism of their formation. Serous detachment is caused by accumulation of fluid between the retina and choroid. Traction detachment is caused when the retina is pulled off the choroid. Rhematogenous detachment occurs following formation of retinal holes, combined with vitreous liquefaction. The liquefied aqueous penetrates the subretinal space through the retinal holes, and causes detachment.
Rhematogenous detachment may be caused by senile changes, trauma or inflammation causing vitreal liquefaction and retinal holes. Traction detachment is caused by forward movement of the vitreous (e.g., following anterior lens luxation) or contraction of fibrin clots formed during uveitis.
Serous detachments are caused by inflammation (exudate) or hemorrhage. Exudative serous detachments are caused by chorioretinitis. The inflammation is most commonly caused by an infectious agent, including bacterial, viral (distemper, FIP, FeLV, FIV), rickettsial (Ehrlichia canis), protozoal (Leishmania, Toxoplasma) or fungal (common in North America) organisms.
Any cause of systemic bleeding could result in hemorrhagic serous detachment. Common causes include systemic hypertension, thrombocytopenia, coagulopathies, hyperviscosity, anemia and trauma.
Patients typically present with acute blindness and fixed dilated pupils. Ophthalmoscopically, it is difficult to focus on the retina (since it moved from its natural place). The clinician will see a "sheet" floating in the posterior part of the eye. This sheet (i.e., the retina) may be transparent, white (i.e., edematous), or hemorrhagic. Retinal blood vessels may be seen on its surface, behind the lens.
Ultrasound may be used to image the detached retina. It is particularly useful when ophthalmoscopy can not be conducted due to severe corneal edema, hyphema, etc.
Treatment of retinal detachment is based on diagnosis and treatment of the primary cause of the detachment. Therefore, systemic workup should be performed. Depending on the type of detachment, this workup could include cardiovascular evaluation, workup for infectious diseases, etc.
Some types of serous detachments are responsive to steroids. Hyperosmotic agents and carbonic anhydrase inhibitors may also be considered, to drain subretinal fluid. Fibrin clots are dissolved by injecting tissue plasminogen activator into the eye, thus preventing traction detachments.
Surgical intervention includes lens extraction in detachments secondary to anterior lens luxation. Specialized referral centers perform surgery to re-attach the retina.
Optic neuritis can be caused by non-infectious (GME, reticulosis, neoplasia) or infectious (distemper, toxoplasmosis, or fungal) diseases of the CNS. Most cases of optic neuritis, however, are idiopathic, as a primary cause can not be diagnosed (or treated).
Patients present with acute blindness and fixed, dilated pupils. Ophthalmoscopically, edema and vascular congestion of the optic disc are seen in inflammations of the proximal optic nerve. However, inflammation of the distal nerve present with a normal-looking disc. In such cases, an electroretinogram (ERG) is required to distinguish between optic neuritis and SARD (see below). The ERG is normal in optic neuritis, since the retina is not affected.
Treatment and prognosis
Treatment is based on identifying and treating the primary cause. Many of the systemic causes (e.g., distemper) carry a grave prognosis, and may be lethal. Systemic steroids can be considered if no systemic cause is found. Even idiopathic optic neuritis often leads to irreversible blindness. In a study of 12 dogs with optic neuritis, only 7 survived, among which 5 remained blind.
Sudden acquired retinal degeneration (SARD)
This is an acquired disease of unknown causes. Patients (typically middle-aged female dogs) present with acute blindness; the pupils may be fixed dilated, or sluggish. Ophthalmoscopically, the fundus appears normal, though degenerative changes appear at later stages. Conclusive diagnosis is based on a flat ERG, indicating lack of retinal activity.
For many years, an endocrine cause was suspected as many patients are 'Cushingoid'. Owners report lethargy, weight gain and PU/PD preceding blindness, and bloodwork is frequently suggestive of Cushing's disease. Toxicity and autoimmune inflammations have also been suspected causes. As the cause remains unknown, there is no treatment for SARD.
Progressive retinal atrophy/rod-cone degeneration
This is the most common inherited retinal disease and cause of blindness in dogs. It is caused by an inherited defect in one of the phototransduction enzymes of the retinal photoreceptors. Different canine breeds have different forms of the disease, depending on the precise mutation. However, regardless of the specific mutation, the final pathway of all forms of the disease is progressive atrophy of rods and cones. Particular forms of the disease in different breeds will be discussed.
The initial behavioral sign is usually loss of night vision, reflecting damage to the rods. Therefore, behavioral evaluation of the dog (menace response, maze test) must be performed under varying light conditions. The disease is progressive, and with time cone function is also affected, leading to blindness.
Initial ophthalmoscopic signs are observed in the peripheral tapetum. These include vascular attenuation (especially of arterioles) and gray discoloration. Moderate and advanced cases present with progressive tapetal hyperreflectivity, blood vessel attenuation and mottling of the non-tapetum. Optic nerve atrophy is seen in the final stages. PLRs are usually present, but may be slow and incomplete.
The ERG is useful in the diagnosis, as it provides an objective and accurate assessment of outer retinal function, more reliable than subjective ophthalmoscopic and behavioral signs. It is particularly useful in early diagnosis, as in many breeds ERG abnormalities may be detected long before behavioral or funduscopic signs.
Recently, commercial DNA tests for PRA/prcd have become available. The tests can be conducted at any age, and can also identify heterozygous carriers. However, some tests identify genetic markers, rather than the mutation itself, so their accuracy has been questioned.
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