Glaucoma is classified into three main stages: mild, moderate, and severe. These stages are defined by how much vision has been lost on a standardized visual field test, measured in decibels (dB). The tricky part is that glaucoma can progress significantly before you notice anything wrong, so understanding these stages helps you make sense of what your eye doctor is tracking and why.
How Glaucoma Stages Are Measured
Eye doctors determine your glaucoma stage primarily through a visual field test, which maps how well you can see in different areas of your peripheral and central vision. The test produces a number called the mean deviation (MD), measured in decibels. A score near zero means normal vision. As the number drops further into negatives, more vision has been lost.
The most widely used staging system is the Hodapp-Parrish-Anderson (HPA) classification, which looks at three things: your overall MD score, how many individual test points show reduced sensitivity, and whether the damage is creeping close to the center of your vision. This system divides glaucoma into mild, moderate, and severe categories, and those same three stages are reflected in the medical codes your doctor assigns to your diagnosis.
Mild Stage: Vision Loss You Can’t Feel
Mild glaucoma is defined as a visual field mean deviation better than negative 6 dB. At this stage, the optic nerve shows early signs of damage, and a visual field test picks up small areas of reduced sensitivity. But here’s the critical point: you almost certainly won’t notice any change in your day-to-day vision. Studies confirm that patients with early glaucoma and measurable field defects in their better eye report no visual symptoms at all.
This is why glaucoma is called the “silent thief of sight.” The brain is remarkably good at compensating, and because both eyes work together, the healthy eye fills in gaps the damaged eye misses. Mild glaucoma is most often caught during routine eye exams rather than because something felt wrong. Treatment at this stage typically targets a 30% reduction in eye pressure from your baseline, which significantly slows the risk of progression.
Moderate Stage: Subtle Symptoms Begin
Moderate glaucoma falls between roughly negative 6 dB and negative 12 dB on visual field testing. At this point, more of the nerve fibers in the optic nerve have been lost, and the blind spots on a visual field map are larger and deeper. The damage often shows up as arc-shaped gaps in the upper or lower part of your visual field.
Some patients at this stage start noticing subtle changes, though not the dramatic “tunnel vision” most people associate with glaucoma. In one study of patients with early or moderate disease, the most common complaints were needing more light to see comfortably (58%), blurry vision (52%), and increased glare sensitivity (52%). No patients described tunnel vision or obvious loss of side vision. The symptoms are vague enough that many people attribute them to aging or needing new glasses. Treatment goals shift to a more aggressive 40% reduction in eye pressure to slow further damage.
Severe Stage: Damage Near Central Vision
Severe glaucoma is diagnosed when the mean deviation drops worse than negative 12 dB. At this stage, the HPA system identifies four distinct patterns of advanced loss, all sharing common features: more than half the tested points show significantly reduced sensitivity, and the damage encroaches on the central 5 degrees of vision, which is the area you rely on for reading, recognizing faces, and driving.
Patients with this level of field loss are more likely to report difficulty seeing objects to the side, a sensation of looking through dirty glasses, and trouble distinguishing boundaries between objects or differentiating colors. Daily activities become noticeably harder, particularly in low light or unfamiliar environments. The treatment target at this stage is a 50% or greater reduction in eye pressure, often requiring multiple medications or surgical intervention.
A screening study in southern India found that among 27 previously undiagnosed cases of open-angle glaucoma, 14 already had severe damage at the time of their first diagnosis. This pattern of late detection is common worldwide and underscores why regular eye exams matter even when your vision feels fine.
End-Stage Glaucoma and Legal Blindness
Beyond the three clinical stages, there is a threshold where glaucoma damage qualifies as legal blindness. The U.S. Social Security Administration defines this as a visual field mean deviation of negative 22 dB or worse. At this point, only a small island of central or peripheral vision may remain. The goal of treatment shifts from slowing progression to preserving whatever functional vision is left.
Between 6% and 13% of glaucoma patients eventually go blind from the disease, even with treatment. Most treated eyes never reach that point, but studies consistently find that 3% to 17% of eyes under clinical care progress faster than expected, losing more than 1.5 dB per year. For those fast progressors, moving from mild to severe disease could happen in under a decade without treatment adjustments.
Why Most Damage Happens Before You Know It
The single most important thing to understand about glaucoma staging is that the disease can destroy a significant portion of your optic nerve before you experience any noticeable vision change. The visual field test catches what your brain masks. By the time symptoms become obvious enough to bring you to a doctor on your own, the disease is often already moderate or severe.
This is also why your doctor may label your stage as “indeterminate” early on. If there are signs of elevated eye pressure or suspicious changes to the optic nerve but the visual field test looks normal, you may not yet fit neatly into mild, moderate, or severe. That doesn’t mean nothing is happening. It means you’re being monitored because the damage hasn’t crossed a measurable threshold yet, and catching it when it does is the whole point of follow-up visits.