Canine Glaucoma Study Guide

Overview and Clinical Importance

Glaucoma is defined as a group of optic neuropathies characterized by progressive retinal ganglion cell death and optic nerve degeneration, typically associated with elevated intraocular pressure (IOP). It is one of the leading causes of irreversible blindness in dogs, affecting approximately 0.9% of purebred dogs in North America. Glaucoma is also a major cause of ocular pain and is the most common reason for enucleation in dogs.

Understanding glaucoma pathophysiology, classification, clinical signs, and treatment options is essential for the NAVLE examination. The disease represents a true ophthalmic emergency when presenting acutely, as irreversible vision loss can occur within hours of IOP elevation.

Aqueous Humor Dynamics and Pathophysiology

Aqueous humor is a transparent fluid produced by the ciliary body epithelium that provides nutrients to the avascular lens and cornea while maintaining IOP and globe shape. Understanding aqueous humor production and drainage is fundamental to understanding glaucoma.

Production and Flow

Aqueous humor is produced by the ciliary processes through active secretion (80%) and ultrafiltration (20%). The fluid flows from the posterior chamber, through the pupil, into the anterior chamber. It circulates in the anterior chamber, supplying the metabolic requirements of the lens and cornea, before draining through the iridocorneal angle (ICA).

Drainage Pathways

• Conventional (trabecular) outflow: 85% of drainage in dogs. Aqueous flows through the pectinate ligaments, trabecular meshwork, and into the angular aqueous plexus and scleral venous plexus.
• Unconventional (uveoscleral) outflow: 15% of drainage. Aqueous diffuses through the iris root and ciliary body into the suprachoroidal space.

Normal Intraocular Pressure Parameters

Classification of Canine Glaucoma

Glaucoma is classified based on cause (congenital, primary, or secondary) and iridocorneal angle status (open, narrow, or closed). This classification has important implications for treatment and prognosis.

Congenital Glaucoma

Develops early in life (less than 1 year of age) due to severe abnormalities in iridocorneal angle development. Puppies present with unilateral or bilateral rapidly enlarging globes. The young sclera is highly elastic, accounting for marked buphthalmos. Prognosis for vision is typically poor.

Primary Glaucoma

Occurs without antecedent ocular disease, resulting from inherited anatomical abnormalities of the drainage angle. Primary angle-closure glaucoma (PACG) is the most common form in dogs, comprising approximately 87% of primary glaucoma cases. It is strongly associated with pectinate ligament dysplasia (PLD), a developmental abnormality where sheets of tissue replace the normal thin, widely separated pectinate ligaments.

Breed Predispositions for Primary Glaucoma

Secondary Glaucoma

Results from physical obstruction of aqueous outflow caused by another ocular disease. Secondary glaucoma is 2-3 times more common than primary glaucoma in dogs. May be unilateral or bilateral depending on the underlying cause.

Common Causes of Secondary Glaucoma

Clinical Signs

Clinical signs are classically divided into acute and chronic presentations, though most cases of acute glaucoma are superimposed on underlying chronic changes.

Acute Glaucoma - The Red, Painful Eye

Chronic Glaucoma - End-Stage Disease

Diagnosis

Tonometry

Measurement of IOP is ESSENTIAL for diagnosis. All patients presenting with a red, painful eye should have tonometry performed. The two primary methods used in veterinary practice are:

• Applanation tonometry (Tono-Pen): Measures pressure required to flatten corneal surface. Requires topical anesthesia. Accurate when used correctly with perpendicular, light contact.
• Rebound tonometry (TonoVet): Measures probe rebound characteristics. Does NOT require topical anesthesia. Probe must be held horizontally. Equally accurate and increasingly popular.

Gonioscopy

Visualization of the iridocorneal angle using a special goniolens placed on the cornea. Essential for:

• Classifying angle status (open, narrow, closed)
• Identifying pectinate ligament dysplasia (PLD)
• Assessing the contralateral eye of patients with unilateral glaucoma
• Screening at-risk breeds

Treatment

Treatment goals depend on whether vision is salvageable:

• Visual eye: Rapidly reduce IOP to preserve or restore vision
• Blind eye: Control pain and provide comfort (surgery of comfort)

Emergency Medical Treatment

Maintenance Medical Therapy

Surgical Treatment

Surgery is indicated when medical therapy fails or when IOP remains greater than 25 mmHg despite maximum medical management. Surgical options depend on whether vision is present.

Vision-Sparing Procedures

Surgeries of Comfort (Blind, Painful Eyes)

• Enucleation: Removal of the globe. Most definitive procedure. Submit for histopathology (rule out neoplasia).
• Evisceration with intrascleral prosthesis: Cosmetic alternative. Remove intraocular contents and place silicone sphere within scleral shell.
• Intravitreal gentamicin injection: Chemical ablation of ciliary body (25 mg gentamicin + 1 mg dexamethasone). Requires only sedation. NOT recommended for cats (risk of malignant ocular tumors).

Prognosis

Prognosis depends on the type of glaucoma, timing of diagnosis, and response to treatment:

• Primary glaucoma: Guarded to poor. Disease is bilateral and progressive. Most dogs eventually become blind despite treatment.
• Secondary glaucoma: Variable. May be curable if underlying cause is treatable (e.g., lens extraction for anterior luxation).
• Acute glaucoma treated within 24-48 hours: Better prognosis for vision recovery.
• Chronic glaucoma with buphthalmos: Vision permanently lost. Treatment focused on comfort.