What Is Glaucoma and How Does It Damage Vision?

The Silent Thief of Sight

Glaucoma is one of the most common causes of permanent blindness in the world, yet most people who have it do not know it. The condition earns its nickname — "the silent thief of sight" — because it destroys vision so gradually and painlessly that significant, irreversible damage can occur long before a person notices anything is wrong. According to the BrightFocus Foundation, more than 60 million people worldwide are affected, a number projected to exceed 110 million by 2040.

What Exactly Is Glaucoma?

Glaucoma is not a single disease but a group of eye conditions that progressively damage the optic nerve — the cable that transmits visual information from the eye to the brain. The most common driver of this damage is elevated pressure inside the eye, known as intraocular pressure (IOP).

Inside every healthy eye, a clear fluid called aqueous humor is continuously produced and drained through a mesh-like structure called the trabecular meshwork. When this drainage system becomes partially blocked or less efficient, fluid builds up, pressure rises, and the optic nerve bears the brunt. As the National Eye Institute explains, the mechanical strain from high IOP causes retinal ganglion cells — the nerve cells that relay visual signals — to degenerate and die. Because these cells cannot regenerate, any vision lost to glaucoma is permanent.

Types of Glaucoma

Not all glaucoma behaves the same way. The two main forms are:

• Primary open-angle glaucoma (POAG) — by far the most common type, accounting for roughly 90% of cases. The drainage angle between the iris and cornea remains open, but the trabecular meshwork gradually loses efficiency, causing pressure to rise slowly over years. Peripheral (side) vision is typically the first to go, which is why most patients do not notice early changes.
• Acute angle-closure glaucoma — a sudden, dramatic blockage of the drainage angle that causes a rapid spike in eye pressure. Symptoms can include severe eye pain, headache, nausea, and blurred vision with halos around lights. This is a medical emergency requiring immediate treatment.

Other less common variants include normal-tension glaucoma — where optic nerve damage occurs even when IOP is within the normal range — and secondary glaucomas triggered by injury, medication, or other eye diseases.

Who Is at Risk?

According to the American Academy of Ophthalmology, several factors raise a person's risk significantly:

• Age — prevalence rises sharply after 60; POAG affects around 10% of people over 75
• Family history — having a parent or sibling with glaucoma substantially increases risk
• Elevated eye pressure — IOP above 30 mm Hg carries a 40-fold greater risk versus readings below 15 mm Hg
• Ethnicity — people of African descent face up to a 15-fold increased risk of blindness from open-angle glaucoma; angle-closure glaucoma is more prevalent among those of Asian descent
• Medical conditions — diabetes, high blood pressure, and prolonged use of corticosteroid medications are all associated with higher risk

Diagnosis: Why Regular Eye Exams Matter

Because glaucoma causes no pain and no early visual symptoms in its most common form, a comprehensive dilated eye exam is the only reliable way to detect it early. During such an exam, an ophthalmologist measures IOP, examines the optic nerve for signs of thinning or cupping, and performs a visual field test to map peripheral vision. The Mayo Clinic recommends baseline eye exams every few years from age 40, and annual checks for those with risk factors.

Treatment: Slowing the Damage

There is currently no cure for glaucoma and no way to restore lost vision — but treatment can halt or dramatically slow its progression. The main options, in increasing order of intervention, are:

• Prescription eye drops — most patients begin with medicated drops that either reduce aqueous humor production or improve drainage, lowering IOP
• Laser treatment — Selective Laser Trabeculoplasty (SLT) uses short pulses of energy to improve drainage through the trabecular meshwork; it is increasingly recommended as a first-line option
• Surgery — traditional trabeculectomy creates a new drainage channel; newer minimally invasive glaucoma surgery (MIGS) procedures implant microscopic devices with smaller incisions and faster recovery times

The Bottom Line

Glaucoma's greatest danger lies in its silence. By the time vision loss becomes noticeable, up to 40% of optic nerve fibers may already be gone. Regular screening — especially for those over 40 or with known risk factors — remains the single most effective strategy for preserving sight. Early detection cannot reverse damage, but it can stop the clock.