Glaucoma Module 1: Aetiology and Pathology

• The course consists of four modules on:
  • A tutorial
  • A quiz
  • Further reading articles
• The aim of the course is to review the aetiology, pathology, and management of glaucoma and highlight the role of the veterinary nurse

Module Overview

• Module 1 covers the aetiology and pathology of glaucoma.
• Module 2 covers clinical examination and diagnostics for glaucoma patients.
• Module 3 covers medical and surgical intervention for glaucoma patients.
• Module 4 covers long-term nursing care for glaucoma patients.

Module 1 Aims

• Recap ocular anatomy and the role/function of the eye
• Explore the aetiology and pathology of canine glaucoma.

Ocular Anatomy and Physiology

• The eyeball, a globe-shaped structure within the orbit, comprises three tissue layers:
  • Sclera
  • Uveal Tract
  • Retina
• Accessory structures of the eye include:
  • Upper and lower eyelids
  • Nictitating membrane
  • Lacrimal glands
  • Meibomian glands
  • Nasolacrimal ducts

Aetiology of Glaucoma

• Glaucoma is a notable disease where RVNs can have a significant impact.
• Glaucoma features elevated intraocular pressure (IOP) accompanied by optic neuropathy and retinal damage.
• Glaucoma is not a single disease entity; rather 'the glaucomas' represent a group of disorders.
• Glaucoma has a unified final pathway of optic neuropathy and retinal damage.
• Without accurate medical or surgical management, patients will experience irreversible vision loss and ophthalmic pain.
• Glaucoma is classified by impaired aqueous humor drainage (primary or secondary) and the duration of glaucoma (acute or chronic).

Aetiology of Canine Glaucoma

• Canine Glaucoma is either Primary (inherited) or Secondary (non-inherited).
• Primary Glaucoma is separated into:
  • Primary Closed Angle Glaucoma (PCAG)
  • Primary Open Angle Glaucoma (POAG)
• The BVA Canine Health Schemes:
  • A useful reference point when exploring canine glaucoma
  • Several breeds are predisposed to both PCAG and POAG
  • Provides advise on diagnosis and breeding

Primary Open Angle Glaucoma (POAG)

• POAG is an inherited condition in dogs.
• The increase in intraocular pressure is much more gradual than in PCAG.
• The condition can take years to develop.
• Breeds certified for primary open angle glaucoma (POAG) under the Eye Scheme in the UK include:
  • Petit Basset Griffon Vendeen
  • Basset Hound
  • Shar Pei
• Testing is available under the Eye Scheme
• The UK Kennel Club and Laboratories such as Labkolin provide information for RVNs and Owners.
• Genetically predisposed patients should routinely measure intraocular pressure with tonometry to allow for early interception.
• The mutated genes in affected breeds cause a gradual increase in eye pressure.
• Untreated POAG can lead to progressive retinal damage, lens luxation, and retinal haemorrhage, resulting in painful and blind eyes.
• As the condition is gradual and rarely presents as an acute emergency, medical management is much more successful compared to PCAG

Primary Closed Angle Glaucoma (PCAG)

• PCAG or Goniodysgenesis is the more severe and common form of primary glaucoma in dogs.
• Goniodysgenesis is the defective development of the drainage angle, which can worsen and prevents aqueous humor flow.
• Diagnosed via gonioscopy and is graded from 0–3 based on its severity.
• The higher the grade, the higher the risk of glaucoma.
• PCAG has been shown to be significantly associated with Pectinate Ligament Abnormalities/Dysplasia (PLA/PLD) which is an abnormality affecting the iridocorneal angle (Goniodysgenesis).
• Pectinate ligaments are usually thin and widely separated tissues.
• PLA/PLD results in thickening of the ligaments, which allows for intermittent flow of aqueous humor only.
• Goniodysgenesis is thought to progress with time, and medications can become less effective.

Canine Secondary (Non-inherited) Glaucoma

• Secondary glaucoma results from a physical obstruction to aqueous humor drainage.
• The iridocorneal angle is obstructed by cellular debris or inflammatory proteins.
• Pupillary flow obstructs by iris adhesions to the lens (Posterior synechia) or anterior lens luxation.
• Secondary glaucoma presents acutely (usually due to anterior lens luxation) or chronically.

Aetiology of Feline Glaucoma

• Primary glaucoma/inherited glaucoma in cats is rare.
• Burmese and Siamese cats are thought to be predisposed to primary glaucoma.
• Secondary glaucoma is more common in cats:
  • Caused by physical obstruction to the flow of aqueous humor
  • Examples include lens luxation, iris adhesions/tumors, uveal tumors, chronic uveitis
  • Complications from ocular surgery and ophthalmic damage due to systemic hypertension can also cause secondary glaucoma
• The most common causes of feline glaucoma are chronic uveitis and hypertensive retinopathy.

Chronic Uveitis Syndrome

• Uveitis that continues to persist and/or reoccurs frequently in cats is referred to as chronic uveitis syndrome.
• The most common cause is viral infection, particularly Feline Immunodeficiency Virus (FIV) but also Feline Leukaemia Virus (FeLV) and FHV-1 infection.
• Treating the underlying cause is preferable but long-term management of symptoms may be the only option to prevent glaucoma.
• Severe uveitis creates proteins that block the drainage angle, increasing intraocular pressure.

Hypertensive Retinopathy

• Hypertensive retinopathy is relatively common in geriatric cats.
• Consider for any cat with concurrent chronic kidney disease or hyperthyroidism.
• Hypertensive retinopathy is particularly prevalent in cats with a systolic blood pressure above 170mmHg.
• The eye is particularly sensitive to blood pressure changes.
• Prolonged hypertension results in retinal and choroidal oedema, intraocular haemorrhage, and retinal detachment.
• Secondary glaucoma occurs due to damage to the intraocular tissues and intraocular hemorrhage.

Glaucoma - Unified Pathology

• 'The Glaucomas' share a final pathway of optic neuropathy and retinal damage.
• If the aqueous humor is not adequately drained, intraocular pressure (IOP) will rise.
• Normal IOP for cats/dogs is 15–25mmHg; anything above 25mmHg is indicative of glaucoma.
• Prolonged increased IOP damages retinal ganglion cells (RGCs) at the optic nerve head, resulting in blindness.
• Increased pressure also leads to Buphthalmos, lens luxation, and continued damage to intraocular structures.