Glaucoma - Comprehensive Overview

Definition

• Goldman-Cecil Medicine, p. 612

Aqueous Humor Dynamics (Pathophysiology)

1. Aqueous is produced by the non-pigmented ciliary epithelium of the ciliary body (posterior chamber)
2. It flows through the pupil into the anterior chamber
3. It drains via the trabecular meshwork → Schlemm's canal → venous circulation (major pathway), plus the uveoscleral pathway (minor)
4. If drainage fails to match production → IOP rises → retinal ganglion cell death → optic cup enlargement → visual field loss

Classification

1. Primary Open-Angle Glaucoma (POAG)

• The anterior chamber angle is anatomically open, but outflow resistance is increased within the trabecular meshwork
• Chronic, insidious, asymptomatic until late stages
• Peripheral fields are lost first; central vision and acuity preserved until advanced disease
• Often detected during routine eye exam (high cup-to-disc ratio, elevated IOP)

• Retinal nerve fibre layer thinning
• Optic nerve head cupping (elevated cup-to-disc ratio)
• Characteristic arcuate visual field defects
• Open anterior chamber angle on gonioscopy
• IOP typically >21 mmHg (though normal in ~30% of cases)
• Kanski's Clinical Ophthalmology 10e, p. 364

2. Primary Angle-Closure Glaucoma (PACG)

• Commonest in hyperopic (far-sighted), Asian, and older women
• The iris apposes the lens (pupillary block), blocking aqueous flow from posterior to anterior chamber
• Pressure builds in posterior chamber → iris bows forward (iris bombé) → angle physically closes
• Can be acute (ophthalmic emergency) or chronic/subacute

• Sudden severe eye pain, blurred vision, halos around lights
• Red eye, corneal oedema, mid-dilated fixed pupil
• Headache, nausea, vomiting
• IOP may exceed 50-70 mmHg

3. Secondary Glaucomas

• Secondary open-angle: debris in angle (red cells from trauma, pigment, pseudoexfoliation material, necrotic tumour cells)
• Secondary angle-closure: neovascular glaucoma (fibrovascular membrane from ischaemia in diabetic retinopathy or CRVO), anterior synechiae from uveitis
• Congenital/developmental: trabeculodysgenesis; associations include Sturge-Weber syndrome (~30% develop glaucoma), aniridia, neurofibromatosis type 1
• Drug-induced: prolonged corticosteroid use (topical > systemic) → steroid-induced open-angle glaucoma

4. Normal-Tension Glaucoma (NTG)

• All features of POAG but IOP consistently ≤21 mmHg
• Suggests vascular insufficiency or increased optic nerve susceptibility is the primary driver
• More common in Japanese populations

Risk Factors for POAG

• Kanski's Clinical Ophthalmology 10e

Diagnosis

1. Tonometry

• Goldmann applanation tonometry = gold standard for IOP measurement
• Results affected by central corneal thickness (CCT) - thick corneas overestimate, thin corneas underestimate IOP

2. Gonioscopy

• Direct visualisation of the anterior chamber angle to distinguish open vs. closed angle
• Essential for classification

3. Optic Disc Evaluation

• Cup-to-disc (C:D) ratio: normal ≤0.5; suspicious >0.6; any asymmetry between eyes is significant
• Focal rim notching, disc haemorrhages, RNFL defects

4. Visual Field Testing (Perimetry)

• Automated static perimetry (Humphrey Field Analyzer) is standard
• 24-2 pattern - most commonly used for glaucoma (tests central 24° + 2°); detects defects within 30° of fixation
• 10-2 pattern - for advanced glaucoma threatening central vision
• Classic defects: arcuate scotomas, nasal steps, paracentral scotomas

5. OCT (Optical Coherence Tomography)

• Measures retinal nerve fibre layer (RNFL) thickness and optic disc parameters objectively
• Can detect structural damage before visual field loss is apparent

Treatment

Medical (First-line)

Laser

• Laser trabeculoplasty (LTP/SLT): applies energy to trabecular meshwork, improves outflow. Can be considered first-line therapy - maintains IOP control for years without medications.
• Laser peripheral iridotomy (LPI): treatment for angle-closure/acute attack - creates a hole in peripheral iris to relieve pupillary block
• Diode laser cycloablation: destroys ciliary body; reserved for refractory cases

Surgery

• Trabeculectomy: creates a fistula between the anterior chamber and episcleral space (bleb filtration). Augmented with mitomycin C (antifibrotic) to reduce scarring
• Tube-shunt (glaucoma drainage device): synthetic tube drains aqueous to an equatorial reservoir; used in refractory cases or after failed trabeculectomy
• Minimally invasive glaucoma surgery (MIGS): e.g., iStent, Hydrus - lower risk but more modest IOP reduction

• Immediate IOP lowering: IV acetazolamide, IV mannitol (hyperosmotic), topical beta-blocker + alpha agonist + pilocarpine (miotic to pull iris away)
• Definitive: Laser peripheral iridotomy to both eyes (prophylactic in the fellow eye)

Optic Nerve Changes (Key Exam Point)

• Normal C:D ratio: 0.3-0.5
• Glaucoma: focal notching (especially inferior > superior > nasal > temporal - "ISNT rule"), pallor, RNFL haemorrhages
• Baring of circumlinear vessels, bayoneting of vessels at disc margin

Prognosis

• Glaucoma is the second leading cause of blindness worldwide (after cataracts) and the leading cause of irreversible blindness
• Vision loss is permanent; treatment halts or slows progression but does not restore lost vision
• Early detection and sustained IOP reduction are the only proven strategies to preserve vision

• Goldman-Cecil Medicine, International Edition (p. 612-650)
• Kanski's Clinical Ophthalmology: A Systematic Approach, 10th Edition
• Robbins & Kumar Basic Pathology (p. 868-869)
• Textbook of Family Medicine, 9th Edition