In general, PRAs are characterised by initial loss of rod photoreceptor cell function followed by that of the cones and for this reason night blindness is the first significant clinical sign for most dogs affected with PRA. As other retinal disorders, PRA can be divided into either dysplastic disease, where the cells develop abnormally, and degenerative, where the cells develop normally but then degenerate during the dog's lifetime.

Generalized PRA is the most common type and causes atrophy of all the neural retinal structures. Central progressive retinal atrophy (CPRA) is a different disease from PRA involving the retinal pigment epithelium (RPE), and is also known as retinal pigment epithelial dystrophy (RPED).

Progressive retinal atrophy (PRA) is a group of genetic diseases seen in certain breeds of dogs and, more rarely, cats. Similar to retinitis pigmentosa in humans, it is characterized by the bilateral degeneration of the retina, causing progressive vision loss culminating in blindness. The condition in nearly all breeds is inherited as an autosomal recessive trait, with the exception of the Siberian Husky (inherited as an X chromosome linked trait) and the Bullmastiff (inherited as an autosomal dominant trait). There is no treatment.

The generalized, common presentation for this broad and inclusive group of diseases is painless, bilateral loss of visual acuity and pallor of the optic disc accompanied with varying degrees of dyschromatopsia and central/cecocentral scatomas. On examination the papillary response may be sluggish to light, one would not expect to find an afferent papillary defect. This is because optic neuropathies are often bilateral and symmetric. The optic disc may be mildly hyperemic with small splinter hemorrhages on or around the disc. Optic atrophy may early on be non-existent and only later become mild. In later stages the optic atrophy is severe and this indicates less opportunity for recovery.

The duration of onset can vary between immediate and insidious, owing to the specific etiology. Two key features may be helpful in distinguishing acquired from inherited optic neuropathies: absence of a family history and simultaneous involvement of both eyes; the former more commonly characterized by these two features.

Familial exudative vitreoretinopathy (FEVR) ( ) is a genetic disorder affecting the growth and development of blood vessels in the retina of the eye. This disease can lead to visual impairment and sometimes complete blindness in one or both eyes. FEVR is characterized by exudative leakage and hemorrhage of the blood vessels in the retina, along with incomplete vascularization of the peripheral retina. The disease process can lead to retinal folds, tears, and detachments.

Acute zonal occult outer retinopathy (AZOOR) is an inflammatory retinopathy in the category of white dot syndromes typified by acute loss of one or more zones of outer retinal function associated with photopsia, minimal funduscopic changes and abnormal electroretinography findings.

FEVR is, as its name suggests,

familial and can be inherited in an

autosomal dominant, autosomal

recessive or X-linked recessive pattern.1-3 It is caused by mutations in

FZD4, LRP5, TSPAN12 and NDP

genes, which impact the wingless/

integrated (Wnt) receptor signaling

pathway. 3 Disruption of this path

way leads to abnormalities of vascu-

lar growth in the peripheral retina. 2,3

It is typically bilateral, but asymmetric, with varying degrees of

progression over the individual’s

lifetime. Age of onset varies, and

visual outcome can be strongly

influenced by this factor. Patients

with onset before age three have a

more guarded long-term prognosis

whereas those with later onset are

more likely to have asymmetric

presentation with deterioration of

vision in one eye only. 2-3 However,

because FEVR is a lifelong disease,

these patients are at risk even as

adults.2 Ocular findings and useful

vision typically remain stable if the

patient does not have deterioration

before age 20.2,4 Due to the variability and unpredictability of the

disease course, patients with FEVR

should be followed throughout

their lifetime.

Clinical presentation can vary

greatly. In mild variations, patients

may experience peripheral vascular

changes, such as peripheral avascular zone, vitreoretinal adhesions,

arteriovenous anastomoses and a

V-shaped area of retinochoroidal

degeneration. 4 Severe forms may

present with neovascularization,

subretinal and intraretinal hemorrhages and exudation. 4 Neovascularization is a poor prognostic

indicator and can lead to retinal

folds, macular ectopia and tractional retinal detachment. 2,4 Widefield FA has been crucial in

helping to understand this disease,

as well as helping to confirm the

diagnosis. An abrupt cessation

of the retinal capillary network

in a scalloped edge posterior to

fibrovascular proliferations can

be made using FA.2,3,5 Patients can

also show delayed transit filling on

FA as well as delayed/patchy choroidal filling, bulbous vascular terminals, capillary dropout, venous/venous shunting and abnormal

branching patterns. 2,3,5 The staging of FEVR is similar

to that of retinopathy of prematurity. The first two stages involve an

avascular retinal periphery with or

without extraretinal vascularization (stage 1 and 2, respectively). 4 Stages three through five delineate

levels of retinal detachment; stage 3

is subtotal without foveal involvement, stage 4 is subtotal with foveal

involvement and stage 5 is a total

detachment, open or closed funnel.4

Because there was neovascularization in the absence of retinal detachment, our patient was

considered to have

stage 2.

LHON, as the name suggests, is an inherited mutation that results in acute or subacute vision loss, displays incomplete penetrance and predominantly affects young males. Onset is usually between the 2nd and 4th decade of life, and usually presents with rapid vision loss in one eye followed by involvement of the second eye (usually within months). Visual acuity often remains stable and poor (below 20/200) with a residual central visual field defect. Patients with the m.14484/ND6 mutation are most likely to have visual recovery.

Symptoms of BCD include:

- Crystals in the cornea (the clear covering of the eye)

- Yellow, shiny deposits on the retina

- Progressive atrophy of the retina, choriocapillaries and choroid (the back layers of the eye). This tends to lead to progressive night blindness and visual field constriction.

These most often occur years after the development of ptosis and ophthalmoplegia. Atrioventricular(abbreviated "AV") block is the most common cardiac conduction deficit. This often progresses to a Third-degree atrioventricular block, which is a complete blockage of the electrical conduction from the atrium to the ventricle. Symptoms of heart block include syncope, exercise intolerance, and bradycardia

KSS results in a pigmentation of the retina, primarily in the posterior fundus. The appearance is described as a "salt-and-pepper" appearance. There is diffuse depigmentation of the retinal pigment epithelium with the greatest effect occurring at the macula. This is in contrast to retinitis pigmentosa where the pigmentation is peripheral. The appearance of the retina in KSS is similar to that seen in myotonic dystrophy type 1 (abbreviated DM1). Modest night-blindness can be seen in patients with KSS. Visual acuity loss is usually mild and only occurs in 40–50% of patients.

In the recessive form corneal clouding is observed at birth or within the neonatal period, nystagmus is often present, but no photophobia or epiphora is seen. In the autosomal dominant type corneal opacification is usually seen in the first or second year of life and progresses slowly, and nystagmus is infrequently seen.

CPEO is a slowly progressing disease. It may begin at any age and progresses over a period of 5–15 years. The first presenting symptom of ptosis is often unnoticed by the patient until the lids droop to the point of producing a visual field defect. Often, patients will tilt the head backwards to adjust for the slowly progressing ptosis of the lids. In addition, as the ptosis becomes complete, the patients will use the frontalis (forehead) muscle to help elevate the lids. The ptosis is typically bilateral, but may be unilateral for a period of months to years before the fellow lid becomes involved.

Ophthalmoplegia or the inability or difficulty to move the eye is usually symmetrical. As such, double vision is sometimes a complaint of these patients. The progressive ophthalmoplegia is often unnoticed till decreased ocular motility limits peripheral vision. Often someone else will point out the ocular disturbance to the patient. Patients will move their heads to adjust for the loss of peripheral vision caused by inability to abduct or adduct the eye. All directions of gaze are affected; however, downward gaze appears to be best spared. This is in contrast to progressive supranuclear palsy (PSP), which typically affects vertical gaze and spares horizontal gaze.

Congenital hereditary corneal dystrophy (CHED) is a form of corneal dystrophy which presents at birth.

A person with photic retinopathy may notice an impairment in their vision, for example a spot that does not go away after a reasonable recovery time, or blurring. They may also have eye pain or headaches. Vision impairment is usually in both eyes, but "can" be in just one. Impairment of a person with 20/20 vision usually ends up being about 20/40 or 20/60, but can be better or far worse.

A doctor examining an eye with retinopathy may be able to see no signs at all, or a slight macular edema, which is a sort of blister on or under the macula, an oval colored spot normally visible to an eye doctor on each person's retina.

But while even that edema goes away, within a few days the patient will generally develop a discoloration of the retina at the injured point, often yellow or white, turning red over the next few weeks.

Weakness of extraocular muscle groups including, the orbicularis oculi muscle as well as facial and limb muscles may be present in up to 25% of patients with CPEO. As a result of the orbicularis oculi weakness, patients may suffer from exposure keratopathy (damage to cornea) from the inability to close the eyes tightly. Frontalis muscle weakness may exacerbate the ptotic lids with the inability to compensate for the ptosis. Facial muscles may be involved which lead to atrophy of facial muscle groups producing a thin, expressionless face with some having difficulty with chewing. Neck, shoulder and extremity weakness with atrophy may affect some patients and can be mild or severe.

Mild visual impairment was seen in 95% of patients that were evaluated using the Visual Function Index (VF-14).

The ciliary muscles that control the lens shape and the iris muscles are often unaffected by CPEO.

Additional symptoms are variable, and may include exercise intolerance, cataracts, hearing loss, sensory axonal neuropathy, ataxia, clinical depression, hypogonadism, and parkinsonism.

Kearns–Sayre syndrome is characterized by onset before 15 years of age of CPEO, heart block and pigmentary retinopathy.

Affected males develop generalized reticular hyper pigmentation in early childhood.

Hair often looks bedraggled or brushed backwards, hanging low on the forehead.

Among the associated extracutaneous manifestations are described:

- Respiratory infections

- Dyskeratosis corneal photophobia

- Hypohidrosis with large deficit of thermoregulation

- Growth retardation

- Gastrointestinal disorders

- Kidney disease

- Kidney stones

- Urinary infections

- Webbed feet or hands

- Electrolyte imbalance

- Retinitis pigmentosa

- Lymphoedema

- Thyroid abnormalities

Each patient shows some of the symptoms listed above. Not every sick person will show all of the listed symptoms.

In females the disease is characterized by skin rashes linear hyper pigmentation following the Blaschko's lines, morphologically similar to stage 3 pigment incontinence. There are no systemic manifestations associated with XLPDR in females.

The most common malformation in patients with the syndrome is kidney hypodysplasia, which are small and underdeveloped kidneys, often leading to end-stage renal disease (ESRD). Estimates show approximately 10% of children with hypoplastic kidneys are linked to the disease. Many different histological abnormalities have been noted, including:

- decrease in nephron number associated with hypertrophy

- focal segmental glomerulosclerosis

- interstitial fibrosis and tubular atrophy

- multicystic dysplastic kidney

Up to one-third of diagnosed patients develop end stage kidney disease, which may lead to complete kidney failure.

Many people often do not have symptoms until very late in their disease course. Patients often become symptomatic when there is irreversible damage. Symptoms are usually not painful and can include:

- Vitreous hemorrhage

- Floaters, or small objects that drift through the field of vision

- Decreased visual acuity

- "Curtain falling" over eyes

Diabetic retinopathy often has no early warning signs. Even macular edema, which can cause rapid vision loss, may not have any warning signs for some time. In general, however, a person with macular edema is likely to have blurred vision, making it hard to do things like read or drive. In some cases, the vision will get better or worse during the day.

In the first stage which is called non-proliferative diabetic retinopathy (NPDR) there are no symptoms, the signs are not visible to the eye and patients will have 20/20 vision. The only way to detect NPDR is by fundus photography, in which microaneurysms (microscopic blood-filled bulges in the artery walls) can be seen. If there is reduced vision, fluorescein angiography can be done to see the back of the eye. Narrowing or blocked retinal blood vessels can be seen clearly and this is called retinal ischemia (lack of blood flow).

Macular edema in which blood vessels leak their contents into the macular region can occur at any stage of NPDR. The symptoms of macular edema are blurred vision and darkened or distorted images that are not the same in both eyes. Ten percent (10%) of diabetic patients will have vision loss related to macular edema. Optical Coherence Tomography can show the areas of

retinal thickening (due to fluid accumulation) of macular edema.

In the second stage, abnormal new blood vessels (neovascularisation) form at the back of the eye as part of "proliferative diabetic retinopathy" (PDR); these can burst and bleed (vitreous hemorrhage) and blur the vision, because these new blood vessels are fragile. The first time this bleeding occurs, it may not be very severe. In most cases, it will leave just a few specks of blood, or spots floating in a person's visual field, though the spots often go away after a few hours.

These spots are often followed within a few days or weeks by a much greater leakage of blood, which blurs the vision. In extreme cases, a person may only be able to tell light from dark in that eye. It may take the blood anywhere from a few days to months or even years to clear from the inside of the eye, and in some cases the blood will not clear. These types of large hemorrhages tend to happen more than once, often during sleep.

On funduscopic exam, a doctor will see cotton wool spots, flame hemorrhages (similar lesions are also caused by the alpha-toxin of "Clostridium novyi"), and dot-blot hemorrhages.

Ocular melanosis (OM), also known as ocular melanocytosis or melanosis oculi, is a congenital disease of the eye which affects about 1 in every 5000 people and is a risk factor for uveal melanoma. In dogs is found almost exclusively in the Cairn Terrier, where until recently it was known as pigmentary glaucoma. The disease is caused by an increase of melanocytes in the iris, choroid, and surrounding structures. Overproduction of pigment by these cells can block the trabecular meshwork through which fluid drains from the eye. The increased fluid in the eye leads to increased pressure, which can lead to glaucoma. In humans, this is sometimes known as pigment dispersion syndrome.

Onset : Early childhood

Progression: Chronic progressive

Clinical: Cerebellar ataxia plus syndrome / Optic Atrophy Plus Syndrome

Ocular: Optic atrophy, nystagmus, scotoma, and bilateral retrobulbar neuritis.

Other: Mental retardation, myoclonic epilepsy, spasticity, and posterior column sensory loss. Tremor in some cases.

Musculoskeletal

Contractures, lower limbs, Achilles tendon contractures, Hamstring contractures, Adductor longus contractures

Systemic

Hypogonadotrophic hypogonadism.

Vision loss due to solar retinopathy is typically reversible, lasting for as short as one month to over one year. The fundus changes are variable and usually bilateral, mild cases often show no alteration and moderate to severe cases show a foveal yellow spot on the first days after exposure. After a few days it is replaced by a reddish dot often surrounded by pigment.

Permanent holes and lesions are possible; prognosis worsens with dilated pupils or prolonged exposure.

Putscher's retinopathy is a disease where part of the eye (retina) is damaged. Usually associated with severe head injuries, it may also occur with other types of trauma, such as long bone fractures, or with several non-traumatic systemic diseases. However, the exact cause of the disease is not well understood. There are no treatments specific for Purtscher's retinopathy, and the prognosis varies. The disease can threaten vision, sometimes causing temporary or permanent blindness.

It is named for the Austrian ophthalmologist, Othmar Purtscher (1852–1927), who detected it in 1910 and described it fully in 1912.

Bietti's crystalline dystrophy (BCD), also called Bietti crystalline corneoretinal dystrophy, is a rare autosomal recessive eye disease named after Dr. G. B. Bietti.

BCD is a rare disease and appears to be more common in people with Asian ancestry.

The severity varies, but the most severe form results in an enlarged disc where vessels exit from the periphery instead of the center. Redundant fibroglial tissue also is seen in severe cases. Milder forms of dysplasia exhibit missing portions of the optic disc located in the optic nerve pit. The least severe form of papillorenal disease shown in the eye is the exiting of blood vessels from the periphery that do not disturb the shape of the eye. Other eye malformations include scleral staphyloma, which is the bulging of the eye wall. There can also be retinal thinning and myopia. Additionally, there can be an optic nerve cyst, which is dilation of the optic nerve posterior to the globe; which most likely results from incomplete regression of the primordial optic stalk and the filling of this area with fluid. Retinal coloboma is also common, which is characterized by the absence of retinal tissue in the nasal ventral portion of the retina. However, this is an extremely rare finding.