Koret School of Veterinary Medicine, Hebrew University of Jerusalem, Rehovot, Israel
Is the blindness of sudden onset, or gradual onset?
Was deterioration of vision associated with preferential loss of night vision or day vision?
Is the animal healthy? Are there other signs of illness, besides loss of vision?
2. Localizing the Lesion in the Blind Animal
The blind patient, like all patients, should undergo a comprehensive physical examination. This is because in many cases, the cause of blindness is a systemic disease. Likewise, a neurological evaluation should be performed to rule neuro-ophthalmological causes of blindness.
Next, a full ophthalmological examination is conducted. It is described in detail in a separate part of these proceedings ("The Ocular Examination"). Particular attention is paid to methods of assessing vision (e.g., the menace response and obstacle course), and to the subcortical tests (e.g., the pupillary light reflex, dazzle reflex, etc.). Additional tests, such as imaging (ultrasound, CT) or electroretinography (see elsewhere in the Proceedings) may also be conducted.
Based on the results of the ocular examination and the pupillary light reflex (PLR), the blind patient may be categorized into one of 4 general categories:
1. Abnormal ophthalmic findings combined with a normal/diminished PLR:
a. Opacity of the ocular media: severe blepharospasm, corneal edema, hyphema/hypopyon, cataract, vitreal hemorrhage
b. Retinal disease--outer retinal degeneration (PRA), chorioretinitis
2. Abnormal ophthalmic examination and an absence of PLR:
b. Retinal detachment
c. Optic neuritis involving the proximal portion of the optic nerve, and causing papilledema
d. Optic nerve hypoplasia/atrophy
3. Normal ophthalmic examination and an absence of PLR:
a. Sudden Acquired Retinal Degeneration (SARD)
b. Optic neuritis involving the distal portion of the optic nerve
c. Neoplasia of the optic nerve or neoplasia compressing the optic nerve or chiasm
d. Lesions affecting the contralateral optic tract, up to the level of the lateral geniculate nucleus
4. Normal ophthalmic examination and normal PLR: These are usually neurological cases, caused by central lesions affecting the visual pathways from the lateral geniculate nucleus to the contralateral visual cortex.
II. Discussion of Select Causes of Blindness
Many of the diseases listed above, including corneal opacities, cataracts, hereditary retinal diseases and glaucoma, are discussed in other parts of these Proceedings. Below is a discussion of the leading causes of acute blindness (other than glaucoma).
1. Retinal Detachment
Retinal detachment is a separation between the retina and choroid (more specifically, between the retina and retinal pigment epithelium). A result of the separation is ischemia of the photoreceptors. If the separation is not quickly resolved, and blood supply restored, cones & rods will begin dying, 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 in the subretinal space, between the retina and choroid. This fluid, which originates in the choroid, may be blood or exudates. Traction detachment is caused by a force which pulls the retina off the choroid. This force may be generated by forward movement of the vitreous body (for example, following anterior lens luxation) or due to traction by fibrin clots. Rhegmatogenous detachment is due to penetration of liquefied vitreous into the subretinal space, through retinal holes.
Causes of Retinal Detachment
The list of possible causes for retinal detachment depends on the type of detachment:
Rhegmatogenous detachment may be caused by senile changes, trauma or inflammation (see below)
Traction detachment may be caused by lens luxation, or by inflammation (see below)
Serous detachments are caused by bleeding or inflammation
Causes of Exudative (Serous) Detachment
An inflammation that leads to retinal detachment is usually one that involves the choroid and retina (chorioretinitis or retinochoroiditis). As is the case for anterior uveitis, it is conceivable that any systemic or ocular inflammation will lead to chorioretinitis. However, chorioretinitis is usually an inflammation caused by an infectious agent. These can be viral (distemper in the dog; FIP, FeLV and FIV in the cat), rickettsia (Ehrlichia canis), protozoal (Leishmania, Toxoplasma) or fungal infections.
Causes of Hemorrhagic (Serous) Detachment
Any cause of systemic bleeding could result in a hemorrhagic retinal detachment. Common causes include systemic hypertension, thrombocytopenia (Ehrlichia canis), coagulopathies, hyperviscosity, anemia and trauma
Clinical Signs of Retinal Detachment
Blind eye (no menace response).
Fixed dilated pupil. A consensual PLR will be present when stimulating the contralateral eye.
When performing an ophthalmoscopic examination, the clinician will find it difficult to focus on the retina (since it moved from its natural place). It is possible to see a "sheet" floating in the posterior part of the eye. This sheet, which is the retina, may be transparent, white (i.e., edematous), or hemorrhagic, depending on the cause of detachment. Retinal blood vessels may be seen on it even without the use of an ophthalmoscope.
Ultrasound. A 10 MHz probe can image the detached retina. This image is called "the seagull sign", because the detached retina usually remains anchored to the eye in the optic disc and to the ora serrata. An ultrasound examination is particularly useful when an ophthalmoscopic examination can not be conducted due to severe corneal edema, hyphema, etc.
Treatment of Retinal Detachment
It is imperative to diagnose the primary cause of the detachment, and to treat it. Therefore, systemic workup has to be performed. Depending on the type of detachment, this workup should be aimed at diagnosing cardiovascular or infectious diseases.
Lens extraction surgery is indicated in when the detachment is secondary to anterior lens luxation
Fibrin clots and strands can be dissolved by injecting tissue plasminogen activator (TPA) into the eye, thus preventing traction detachments.
Treatment of exudative serous detachments involves draining the subretinal fluid. This may be done using hyperosmotic agents. Systemic carbonic anhydrase inhibitors should also be considered. If the cause of the exudate is inflammatory, systemic steroids should be considered.
Specialized referral centers may perform surgery to re-attach the retina, or to "seal" retinal holes.
2. Sudden Acquired Retinal Degeneration (SARD)
This is an acquired disease of an unknown cause, typically appearing in middle-aged (female) dogs. The history provided is one of sudden onset blindness. The typical patient is "cushingoid". In many dogs, owners will report a history of lethargy, weight gain and PU/PD during the last few months. Blood work is also suggestive of Cushing's disease
Examination will reveal a blind eye with a fixed, dilated pupil. The fundus appears normal during the first few months. Degenerative changes may appear at a later stage (few months). The ERG is flat, indicating lack of retinal activity.
Currently there is no treatment for SARD. Hopefully, once the cause is identified, treatment can be offered.
3. Optic Neuritis
An inflammation of the optic nerve caused by:
Any cause of meningitis
Infectious causes--distemper, fungal diseases (e.g., Cryptococcus), Toxoplasma, bacteremia, etc. In many of the systemic disease, the ocular signs may be the presenting complaint
Neoplasia, trauma or an abscess in regions where the optic nerve passes (especially at the optic chiasm!)
CNS diseases--GME, reticulosis, etc.
Idiopathic--probably the most common cause
Blind eye with a fixed, dilated pupil
ERG is normal, since the retina is not affected (thus distinguishing optic neuritis from SARD)
The optic disc appears normal or inflamed, depending on which part of the nerve is involved. If the proximal part of the optic nerve is involved, papilledema and vascular congestion of the optic disc are seen on examination of the fundus. Atrophy of the optic disc is noticed as the disease resolves. Inflammation of more distal parts of the nerve may present with a normal-looking disc.
Treatment is based on identifying and treating the primary cause. Systemic steroids should be administered if no systemic cause is found. Prognosis is guarded.