Fundoscopy: abnormal findings
- In order to appreciate the abnormal retina, it is essential to become familiar with the normal fundus and its many variations.
- Examination of the fundus is achieved by ophthalmoscopy.
- Direct ophthalmoscopy provides a small field of view of a magnified image, and is useful when examining an area in greater detail Ophthalmoscopy: direct.
- Indirect ophthalmoscopy provides a wide field of view, is the best way of scanning the entire fundus and is possible even with partial opacity of the ocular media Ophthalmoscopy: indirect.
Congenital and early onset conditions
1. Retinal dysplasia
- Rod-cone dysplasia in the Abyssinian Abyssinian is inherited as an autosomal dominant trait, and is visible ophthalmoscopically starting from as early as 4-8 weeks of age Eye: congenital ocular disorders Retina: inherited degenerations.
- Photoreceptor dysplasia in the Persian Persian Longhair is inherited as an autosomal recessive trait, and is visible ophthalmoscopically starting from as early as 2-3 weeks of age.
- Clinical signs include mydriasis, poor night vision followed by total blindness Blindness and loss of menace response, dazzle reflex and pupillary light reflex Eye: examination.
- Ophthalmoscopic signs include increased tapetal hyperreflectivity, mottling of the tapetal area, blood vessel attenuation and eventually optic nerve atrophy .
- Retinal dysplasia can occur as a result of intrauterine infection of the dam with viral infections with panleukopenia Feline panleucopenia virus disease or feline leukemia Feline leukemia virus disease. There can be other accompanying ocular defects such as cataracts Cataract or microphthalmos.
2. Fundus coloboma
- Coloboma refers to a congenital focal absence of part of the ocular tissue.
- So-called ‘typical’ colobomas arise in the area of the optic fissure (approximately 6 to 7 o’clock position) and arise due to its incomplete closure during embryological development. ‘Atypical’ colobomas arise in other areas of the eye.
- Fundus colobomas are very uncommon. When they do arise, they tend to be in the region of the optic disk and affect the peripapillary retina, choroid and sclera.
- Colobomas of the posterior segment may occur in isolation or in conjunction with other congenital ocular abnormalities such as retinal dysplasia, persistent pupillary membrane Eye: persistent pupillary membrane and eyelid agenesis (eyelid coloboma) Eyelid: abnormality .
- Posterior segment colobomas may be an incidental finding with no signs of vision impairment. However, if extensive, they can cause visual deficits and blindness.
- The choroid and retina are intimately associated anatomically. Inflammation of one structure invariably causes concurrent inflammation in the other Eye: chorioretinitis.
- Posterior segment inflammation may occur in isolation, or together with anterior uveal inflammation (panuveitis).
- Active inflammation has a variety of possible presentations. There are regions of tapetal hyporeflectivity due to subretinal exudates, infiltrates and hemorrhages. There may be bullae and regions of retinal detachment. Optic neuritis Optic neuritis may also be present.
- Post-inflammatory retinopathy represents historic inactive lesions arising due to previous chorioretinitis, and presents as multifocal areas of tapetal hyperreflectivity with pigment disruption and greying of the nontapetal fundus. It requires no treatment.
- Causes of chorioretinitis include:
- Infections, eg FIP Feline infectious peritonitis, FIV Feline immunodeficiency virus disease, FeLV Feline leukemia virus disease, any generalized septicemia Shock: septic, mycobacteriosis, bartonellosis Cat scratch disease, toxoplasmosis Toxoplasmosis (Retina: degeneration focal - DSH, 5 years), ophthalmomyiasis interna posterior and various mycotic infections such as cryptococcosis Cryptococcosis, blastomycosis and histoplasmosis Respiratory fungal disease.
- Trauma (blunt and penetrating).
- Intraocular surgery.
- Idiopathic / immune-mediated disease.
2. Hypertensive retinopathy
- Systemic hypertension Hypertension is common in older cats.
- Causes of systemic hypertension include renal disease, hyperthyroidism Hyperthyroidism and hyperadrenocorticism Hyperadrenocorticism, although many cases are idiopathic.
- The eye is a particularly sensitive target organ of systemic hypertension as its precise function depends on vascular integrity.
- Systemic hypertension causes damage to retinal and choroidal capillaries and arterioles which leads to leakage of plasma into the surrounding tissues (retinal and choroidal edema). More severe vascular damage leads to rupture of vessel walls and hemorrhage, and total retinal detachment Retina: detachment is thought to result from choroidal effusion and ischemic damage to the RPE.
- Cats often present in the advanced stages with sudden onset blindness due to retinal detachment and hyphema and/or vitreal hemorrhage.
- Earlier recognizable funduscopic changes include tortuosity of the retinal arterioles and retinal edema with focal bulla.
- As the condition advances, focal subretinal, intraretinal or preretinal hemorrhages occur .
- Bullous retinal detachment can be appreciated with distant direct ophthalmoscopy, with the retinal vessels visible behind the lens, typically accompanied by vitreal hemorrhage .
3. Retinal detachment
- Retinal detachment is separation of the neurosensory retina from the underlying Retinal Pigment Epithelium and choroid.
- Serous retinal detachment is by far the most common type of detachment in cats. It is most often caused by systemic hypertension, but also by exudative disease such as FIP, or other vascular diseases causing anemia, thrombocytopenia, hyperviscosity or coagulopathy.
- Serous retinal detachment causes some ballooning forwards of the retina into the vitreal chamber. It is visible as a floating white, pink or red sheet containing blood vessels and can be easily seen behind the lens even on distant direct ophthalmoscopy or just with a pen-light . Systemic hypertension is the most common cause.
- Retinal detachment with disinsertion from the ora ciliaris retinae is uncommon in cats. The detached neurosensory retina remains attached around the optic nerve, and therefore hangs inferiorly, draped over the optic disk and obscuring it from view. The dorsal tapetal fundus is always hyperreflective because of the absent neurosensory retina.
4. Diabetic retinopathy
- Diabetic retinopathy is not common.
- It has been reported when diabetes mellitus Diabetes mellitus has resulted from prolonged administration of megestrol acetate Megestrol acetate .
- The ophthalmoscopic signs are identical to those of hypertensive retinopathy and thus are more likely explained by systemic hypertension rather than glycemic status per se . Retinal hemorrhages or microaneurysms may be seen.
5. Anemic retinopathy
- Anemia may manifest as a relative pallor of the retinal vessels.
- Profound anemia Anemia: overview, with hematocrit less than 10 % and hemoglobin concentrations less than 5 g/dL, is associated with retinal hemorrhages. It is thought that anemia leads to hypoxic damage of the vascular endothelium with resultant fragility of the vascular walls.
6. Hyperviscosity retinopathy
- Hyperviscosity syndrome most often occurs as a result of an increase in the concentration of blood plasma proteins (hyperproteinemia), which is most commonly associated with multiple myeloma Multiple myeloma and elevated IgG levels (monoclonal gammopathy) but may also occur with chronic systemic inflammatory conditions such as systemic lupus erythematosus Systemic lupus erythematosus and FIP (polyclonal gammopathies).
- Less commonly, it is due to an abnormally elevated concentration of red blood cells (polycythemia). It can either be primary (ie polycythemia vera), or secondary as a consequence of cardiorespiratory disease.
- Systemic signs of hyperviscosity syndrome vary but include lethargy, anorexia, weight loss, neurological disturbance and heart murmur.
- The most common fundoscopic findings are engorged and tortuous retinal vessels, retinal hemorrhages and papilledema although retinal detachment can also occur.
7. Lipemia retinalis
- Lipemia retinalis is the term used to describe ophthalmoscopically visible presence of lipoproteins (specifically chylomicrons or very low density lipoproteins (VLDL)) within retinal vessels.
- It is a diagnostic sign of systemic hyperlipoproteinemia for which there are several causes. Hyperlipoproteinemia is most commonly associated with diabetes mellitus, hypothyroidism and prolonged treatment with corticosteroids and/or megestrol acetate. A primary inherited form of lipoprotein lipase gene mutation has also been reported.
- The retinal vessels become white to cream in color and appear wider than normal but do not seem to exhibit other overt pathology.
- The vessel color is easier to appreciate over the darker nontapetal fundus.
8. Other retinal hemorrhage
- The most common cause of retinal hemorrhage is hypertensive retinopathy.
- There are a variety of other potential causes, including:
- Trauma .
- Anemic retinopathy.
- Migrating parasites.
- Thiamine deficiency Thiamine deficiency.
9. Inherited retinal degeneration
- Photoreceptor degeneration is inherited as an autosomal recessive disorder in the Abyssinian, Somali Somali and Siamese Siamese due mutation in the CEP290 gene Retina: inherited degenerations. Signs develop from 18 months of age, with total retinal degeneration by three to five years.
- Photoreceptor degeneration is inherited as an autosomal recessive disorder in the Bengal Bengal. The causative gene defect has not yet been identified, but the CEP290 or CRX variants genes are not responsible. Signs develop from 9 weeks of age, with total retinal degeneration by one year of age.
- Clinical signs include mydriasis, poor night vision followed by total blindness, and loss of menace response, dazzle reflex and pupillary light reflex.
- On ophthalmoscopy, there is retinal blood vessel attenuation and tapetal hyperreflectivity.
10. Taurine deficiency retinopathy / feline central retinal degeneration
- Taurine is an essential amino acid in the diet of a cat.
- Taurine deficiency Taurine deficiency can lead to retinopathy and cardiac disease.
- The earliest ophthalmoscopic sign of taurine deficiency is a dull, granular appearance in the region of the area centralis, just dorsolateral to the optic nerve head .
- If the dietary deficiency continues, the condition progresses to that region of the tapetal fundus appearing hyperreflective , and a similar lesion develops just dorsomedial to the optic nerve head .
- With progression, these lesions coalesce before more widespread signs of retinal degeneration become apparent .
11. Fluoroquinolone-induced retinal degeneration
- Enrofloxacin Enrofloxacin administered at a systemic dose above the manufacturer’s current dose recommendation can occasionally induce a toxic retinopathy Retina: fluoroquinolone retinopathy.
- Within 3 days of receiving systemic enrofloxacin, the affected cats present with varying degrees of mydriasis, vision loss and neurological signs.
- On ophthalmoscopy, initially there is a granular appearance to the area centralis which progress along the visual streak. Progressively the retinal blood vessels become more attenuated and there is tapetal hyperreflectivity. It can progress to total retinal degeneration and optic nerve atrophy.
12. Retinal neoplasia
- Primary neoplasia is rare. Choroidal melanocytoma has been reported.
- Secondary neoplasia is uncommon. Metastatic lymphoma may occur.
- Angioinvasive pulmonary carcinoma can metastasize to the choroid Lung: pulmonary neoplasia. The neoplastic emboli within the choroidal vessels lead to ischemic necrosis of the choroid and retina which, on funduscopy, appear as wedge-shaped tan lesions within the tapetal fundus.