What Is Glaucoma?
Glaucoma is a group of optic neuropathies characterized by progressive damage to retinal ganglion cells and their axons, leading to characteristic optic nerve head changes and visual field loss. While elevated intraocular pressure (IOP) is the most important modifiable risk factor, glaucoma is defined by the structural and functional damage, not by a specific IOP number.
Primary Open-Angle Glaucoma (POAG)
POAG is the most common form of glaucoma. The anterior chamber angle is open and appears normal on gonioscopy, but the trabecular meshwork provides increased resistance to aqueous outflow, leading to elevated IOP in most (but not all) cases.
POAG is often called the "silent thief of sight" because:
- It progresses slowly over years
- Peripheral vision is lost first, which patients often do not notice
- Central vision is preserved until late in the disease
- There is no pain or redness in most cases
By the time a patient notices vision loss, significant and irreversible damage has already occurred.
Primary Angle-Closure Glaucoma (PACG)
PACG occurs when the peripheral iris physically blocks the trabecular meshwork, preventing aqueous drainage and causing IOP to rise. It can present as:
- Acute angle closure: A sudden, dramatic IOP spike (often 40-80 mmHg) causing severe eye pain, headache, nausea/vomiting, halos around lights, and a red eye with a fixed mid-dilated pupil. This is an ophthalmic emergency.
- Chronic angle closure: Gradual closure of the angle without acute symptoms, causing progressive damage similar to POAG
Risk factors for angle closure include hyperopia (short eyes), shallow anterior chambers, thicker lenses (advancing age), and Asian or Inuit ethnicity.
Optic Nerve Assessment
The hallmark of glaucoma is optic nerve cupping. The optic disc has a central depression (the cup) surrounded by neural tissue (the neuroretinal rim). As glaucoma damages retinal ganglion cell axons, the rim thins and the cup enlarges.
The cup-to-disc ratio (C/D ratio) compares the cup diameter to the total disc diameter. Normal is approximately 0.3 or less in most individuals. Asymmetry of 0.2 or more between the two eyes is suspicious even if both ratios are within the "normal" range.
Risk Factors for Glaucoma
- Elevated IOP: The primary modifiable risk factor
- Age: Risk increases significantly after age 40
- Family history: First-degree relatives have 4-9x increased risk
- Race/ethnicity: Higher prevalence in African Americans (POAG) and Asian populations (angle closure)
- Thin central corneal thickness: Independent risk factor beyond its effect on IOP measurement
- Myopia: Increased risk for POAG
Treatment Goals
The only proven treatment for glaucoma is lowering IOP. Even in normal-tension glaucoma, reducing IOP slows progression. Treatment modalities include:
- Topical medications: Prostaglandin analogs (first-line), beta-blockers, alpha agonists, CAIs
- Laser: SLT for open-angle glaucoma, peripheral iridotomy for angle closure
- Surgery: Trabeculectomy, tube shunt, MIGS (minimally invasive glaucoma surgery) for cases uncontrolled by medications and laser
The target IOP is individualized based on the severity of damage, the rate of progression, and the patient's life expectancy. A common initial target is a 20-30% reduction from baseline.
Key Takeaways
- Glaucoma is defined by progressive optic nerve damage, not by a specific IOP value
- POAG is the most common type: painless, slow, and often undetected until significant vision is lost
- Acute angle closure is an emergency with severe pain, very high IOP, and a fixed mid-dilated pupil
- Optic nerve cupping, rim thinning, and cup-to-disc ratio asymmetry are key diagnostic findings
- Lowering IOP is the only proven treatment; target IOP is individualized for each patient