3. Diseases of the iris and retina.

What are the components of the uveal tract?

Corpus ciliaris (ciliary body), iris (anteriorly), and choroid (posteriorly)

What are the functions of the uveal apparatus?

Regulation of incoming light through the pupil, production of aqueous humour, supporting the lens/zonular fixation apparatus, changing visual focus (ciliary body muscles), and ensuring eye nutrition

What are the different iris shapes in cats and dogs?

Vertical in cats and round in dogs

What are the muscles of the iris?

Dilator and sphincter muscles

What is the function of the ciliary body?

It is the largest part of the uvea, providing nutrition and removing corneal and lens waste products

What is the iridocorneal angle?

The angle formed anteriorly by the peripheral cornea and posteriorly by the peripheral iris

What is the choroid?

A thin layer of tissue that is part of the middle layer of the wall of the eye, between the sclera and the retina, and connected to the ciliary body

What is the ligamentum pectinatum?

Long fibres anchored on the base of the iris and the inner peripheral portion of the cornea, forming anastomoses of the trabecular network

What is the tapetum lucidum?

A triangular-shaped structure that reflects light

What are examples of diseases of the iris?

1. Developmental diseases

2. Uveitis

What are some developmental diseases of the uvea?

1. Colour variations (subalbinotic/blue iris, heterochromia, merle genes [heterochromia, iris hypoplasia])

2. Persistent pupillary membrane (PPM)

3. Aplasia/hypoplasia (aniridia, notch coloboma, pseudopolycoria, iridodiastasis)

4. Iridociliary cysts

When should persistent pupillary membranes regress?

By 6 weeks of age

What is aniridia?

Lack of iris

What is notch coloboma?

A defect in the pupillary margin

What is pseudopolycoria?

Multiple holes in the iris

What is iridodiastasis?

A defect in the iris base

What are iridociliary cysts?

Accumulations of fluid between the two epithelial layers of the iris and ciliary body

What is uveitis?

Inflammation of the uveal tissue (iris, ciliary body, and choroid)

Can be primary (idiopathic or immune-mediated) or secondary

What are the types of uveitis according to the structures involved?

1. Anterior uveitis

2. Posterior uveitis

3. Panuveitis

4. Chorio-retinitis

What is anterior uveitis?

Inflammation of the iris and ciliary body

What is posterior uveitis?

Inflammation of the choroid and ciliary body

What is panuveitis?

Inflammation of the iris, ciliary body, and choroid

What is chorioretinitis?

Inflammation of the choroid and retina (suppurative or non-suppurative)

What are some aetiologies of uveitis?

1. Lens-induced (small lens proteins pass into the aqueous humour)

2. Immune-mediated (uveodermatologic syndrome/VKH, pigment uveitis)

3. Infectious

4. Traumatic

5. Metabolic (diabetes mellitus)

What are some infectious causes of uveitis in cats?

1. Bacterial: Bartonella

2. Viral: FIV, FeLV, FIP

3. Protozoal: Toxoplasma gondii

4. Fungal: C. albicans, cryptococcus, coccioides, histoplasma

What are some infectious causes of uveitis in dogs?

1. Bacterial: Brucella, Borrelia, Leptospira, septicaemia, diphtheria

2. Viral: Adenovirus, distemper, herpes, rabies

3. Protozoal: Leishmania, toxoplasma, neospora, trypanosoma

4. Parasitic: Dirofilaria, toxocara

5. Fungal: Blastomyces, cryptococcus, coccioides, histoplasma

What are the clinical signs of anterior uveitis?

Aqueous flare, fibrin in the anterior chamber, hypopyon (pus), hyphema (blood), miosis, decreased IOP, pain, corneal oedema, iris swelling, decreased vision, and conjunctival hyperaemia

What are the clinical signs of posterior uveitis?

Vitreous opacity, decreased vision, retinal detachment, retinal haemorrhage, and choroidal effusion

How is uveitis diagnosed?

Ophthalmic examination, fluorescein staining, serum CRP (N: 0-10; bad: >10; uveitis: >20)

What are the differential diagnoses for uveitis?

Glaucoma, keratitis, conjunctivitis, and corneal ulcers

How is uveitis treated?

1. Topical anti-inflammatories (corticosteroids like prednisolone or dexamethasone are primary therapy, contraindicated with corneal ulcer)

2. Topical NSAIDs

3. Mydriatics (tropicamide; less potent than atropine)

4. Immunosuppressive (azathioprine)

5. Systemic ATB (amoxicillin)

Are uveal tract tumours more common in cats or dogs?

Cats

What are the categories of uveal tract tumours?

1. Primary (melanocytic, iridociliary, medulloepithelioma, mixed)

2. Secondary (lymphoma, haemangiosarcoma, melanoma)

How are uveal tract tumours diagnosed?

Ophthalmic examination and fine needle aspirate (FNA)

What are the differential diagnoses for uveal tract tumours?

Uveitis, ocular cysts, and glaucoma

How are uveal tract tumours treated?

Surgery (partial iridectomy), radiation therapy, laser therapy, and chemotherapy

What are examples of diseases of the retina?

1. Retinal detachment

2. Retinal dysplasia

3. Collie eye anomaly

4. Progressive retinal atrophy/hereditary retinal degeneration

5. Retinal toxicity

6. Sudden acquired retinal degeneration syndrome (SARDS)

What is retinal detachment?

Separation of the retina from the underlying retinal pigment epithelium (RPE) layer containing blood vessels that provide oxygen and nutrients

What can cause retinal detachment?

Systemic hypertension, coagulopathy, infections, trauma, uveitis, cataracts, and lens luxation

What are the clinical signs of retinal detachment?

Visual loss (if partial detachment), blindness (if complete detachment), red eyes (vitreous haemorrhage), pus, and a dilated pupil not responding to light

How is retinal detachment diagnosed?

Ophthalmoscopy/fundoscopy (visualising the detached retina; wrinkled) or ultrasound (USG)

What are the differential diagnoses for retinal detachment?

Posterior vitreous detachment (PVD), retinal artery/vein occlusion, macular degeneration, and optic neuropathy

How is retinal detachment treated?

Treatment depends on the underlying cause and extent of detachment. Surgery aims to reattach the retina (scleral buckle surgery, vitrectomy, pneumatic retinopexy)

Which surgeries can be used to treat retinal detachment?

1. Scleral buckle surgery: Placement of silicone band around eye to provide external support & counteract traction on retina.

2. Vitrectomy: Removal of vitreous gel & any tractional membranes then intraocular gas or silicone oil injection to tamponade retina.

3. Pneumatic retinopexy: Injection of gas bubble into vitreous cavity followed by laser or cryotherapy to seal retinal breaks, combined w/ postoperative positioning.

What is retinal dysplasia?

An inherited condition where the retina develops abnormally, with unusual growth or changes in one or more of its parts

What can cause retinal dysplasia?

1. Viral infections

2. Vitamin A deficiency

3. X-ray irradiation

4. Certain drugs

5. Intrauterine trauma

6. Hereditary factors

What are the clinical signs of retinal dysplasia?

Retinal folds, retinal detachment, focal retinal degeneration, leukocoria (white reflection from the retina), microphthalmia, cataracts, nystagmus, and blindness

What are the pathological findings in retinal dysplasia?

1. Retinal Folds: Areas where retina has developed into abnormal folds.

2. Retinal Rosettes: Clusters of disorganised retinal cells.

3. Geographic Areas of Dysplasia: Larger, irregular areas of retinal malformation.

4. Retinal Detachment: severe cases: can cause retina to detach from underlying retinal pigment epithelium (RPE)

How is retinal dysplasia diagnosed?

Ophthalmoscopy (diagnosed in puppyhood), and genetic testing (possible)

What are the differential diagnoses for retinal dysplasia?

Progressive retinal atrophy (PRA), retinal detachment, Collie eye anomaly (CEA), and optic nerve hypoplasia

Is there a cure for retinal dysplasia?

No, as it is a developmental defect

What is Collie eye anomaly (CEA)?

A congenital ocular syndrome involving defects of the posterior vascular and fibrous tunics of the eye (choroid, optic disc, sclera)

What is the cause of CEA?

Abnormal mesodermal differentiation, affecting the sclera, choroid, optic disc, retina, and retinal vasculature. It is bilateral and has no sex predisposition.

What are the clinical signs of CEA?

Varying severity from no visual deficit to total blindness. Choroidal hypoplasia, posterior polar colobomas, retinal detachment, and intraocular haemorrhage (hyphema) can occur.

How is CEA diagnosed?

Causal gene testing (4-8 weeks). Diagnosis can be difficult.

What are the differential diagnoses for CEA?

Progressive retinal atrophy (PRA), retinal dysplasia, congenital optic nerve defects, and ocular trauma

What is the treatment for CEA?

None

What are some surgical procedures that might be used to preserve vision in CEA?

Scleral buckling and vitrectomy

What is progressive retinal atrophy (PRA)/hereditary retinal degeneration?

Inherited retinal photoreceptor dysplasia or degeneration causing progressive vision loss

What are the clinical signs of PRA?

Always bilateral, leading to blindness. Early signs include impaired vision in dim light/darkness (night blindness). Progressive form: total blindness, slow pupillary light reflex, cataracts

How is PRA diagnosed?

Behavioural signs, maze test, ophthalmoscopy, and electroretinography (ERG)

What are the differential diagnoses for PRA?

Sudden acquired retinal degeneration syndrome (SARDS), retinal detachment, glaucoma, and cataracts

Is there a treatment for PRA?

No, treatment is supportive/symptomatic

What is retinal toxicity?

Damage to the retina caused by certain drugs, either directly (damaging neuroretinal or RPE cells) or indirectly (through inflammatory reactions). It is usually bilaterally symmetrical.

What are some drugs that can cause retinal toxicity?

1. Vasodilating drugs → Cause tapetal colour change

2. Ethambutol → Discoloration of the tapetal fundus

3. Diphenylthiocarbazone → Fundic alterations

4. Quinine → Vasoconstriction of retinal arterioles and pallor of optic disc

5. Closantel → Changes to the fundus

6. Ivermectin and enrofloxacin can also cause blindness

What are the clinical signs of retinal toxicity?

Loss of vision

How is retinal toxicity diagnosed?

History, clinical signs, and eye examination

How is retinal toxicity treated?

Cessation of the offending drug

What is sudden acquired retinal degeneration syndrome (SARDS)?

A unique retinal disorder in dogs characterised by acute onset, permanent blindness, and occurrence in middle-aged, often overweight dogs

What are the clinical signs of SARDS?

Sudden vision loss (within days or 1-2 weeks), pupillary dilatation, and unresponsive pupils (although some may retain a PLR). Typically, there are no ophthalmoscopic fundus abnormalities in the early stages

How is SARDS diagnosed?

Electroretinography (ERG), which is non-recordable in SARDS, differentiating it from central causes of sudden vision loss (where the ERG is relatively normal)

What are the differential diagnoses for SARDS?

Retinal detachment, optic neuritis, glaucoma, and progressive retinal atrophy (PRA)

What is the treatment for SARDS?

None

How many layers are there in the retina?

10

What are examples of congenital diseases of the retina?

1. Collie eye anomaly: choroidal hypoplasia

2. Primary retinal dysplasia: disorderly proliferation and disorganisation of retinal layers. Focal, multifocal or total

3. Feline progressive retinal atrophy: autosomal, early-onset retinal degeneration

4. Canine multifocal retinopathy: multifocal, well-circumscribed grey to tan lesions throughout the fundus of puppies and young dogs. Rarely have significant impact of vision

What is electroretinography?

Electroretinography is a test to measure the electrical response of the eye's light-sensitive cells, called rods and cones

What is the difference between hyperlacrimation and epiphora?

• Hyperlacrimation: excessive watering due to reflex irritation of the corneal and conjunctival surface as in cases of dry eye, corneal abrasion, corneal foreign body, etc.

• Epiphora: watering due to obstruction in the lacrimal outflow pathway