Glaucoma — Risk Factors and Prevention

Glaucoma isn't a single disease but a group of conditions leading to progressive damage of the optic nerve, usually (though not always) linked to elevated intraocular pressure. The most common form — primary open-angle glaucoma (POAG) — develops slowly, and visual field loss usually starts at the periphery, in areas the brain doesn't consciously process day to day anyway. As a result, the disease is sometimes called "the silent thief of sight": some patients only reach an ophthalmologist once optic nerve damage is already advanced and largely irreversible.

The scale of the problem isn't marginal. The largest meta-analysis to date on the global epidemiology of glaucoma, covering 50 population-based studies, estimated glaucoma prevalence at 3.54% among people aged 40-80, with the number of affected people rising from 64.3 million in 2013 to a projected 111.8 million by 2040 — mainly in Asia and Africa. That's a disease that in practice affects roughly one in a few dozen people in this age group, and its share will keep growing as the population ages.

Intraocular pressure (IOP) is the pressure of the fluid inside the eye, maintained by the balance between the production and drainage of aqueous humor. Elevated IOP is the strongest, and currently the only effectively modifiable, risk factor for developing glaucoma — it's precisely on lowering IOP that all available therapies are based. It's worth immediately noting, though, that the relationship isn't unambiguous in both directions: some people with elevated IOP (so-called ocular hypertension) never develop glaucoma, and some glaucoma patients have IOP within the range considered statistically normal — this is so-called normal-tension glaucoma, in which other factors, described later in this article, play a larger role.

OHTS is one of the most frequently cited studies in all of ophthalmology, because it was the first to show, in such a large, well-controlled trial, a direct causal relationship: lowering IOP genuinely reduces the risk of developing glaucoma, not merely correlates with it in observational data. That's a result of major clinical significance, but it isn't an argument for self-monitoring eye pressure at home — measuring IOP requires specialized equipment (a tonometer) and is performed in an ophthalmologist's office, and treatment decisions always belong to the physician.

The importance of corneal thickness has a dual basis. First, a thinner cornea causes a standard applanation tonometer to underestimate IOP — in someone with a thin cornea, the actual intraocular pressure may be higher than the measurement indicates, which without correction leads to underestimating risk. Second, there's evidence that a thin cornea reflects the overall biomechanical vulnerability of the eye's connective tissue, including the optic nerve's lamina cribrosa, meaning the same IOP level can cause greater structural damage. This is a good example of why glaucoma risk assessment shouldn't be limited to a single pressure reading — a full ophthalmological exam also includes assessment of the optic disc, visual field, and pachymetry (corneal thickness measurement).

Age is the strongest non-modifiable risk factor for glaucoma — its prevalence rises with every decade of life, and markedly nonlinearly. In the classic American population study (the Baltimore Eye Survey, described in more detail in the next section), the age-specific prevalence of primary open-angle glaucoma rose from 0.92% in the 40-49 age group to 2.16% after age 80 among white individuals, and from 1.23% to as high as 11.26% in the same age ranges among Black individuals. Age 40+ is therefore widely accepted as the threshold at which routine glaucoma risk assessment begins as part of a comprehensive eye exam.

The highest odds ratio being for siblings (rather than parents or children) is consistent with what one would expect for a disease with a partly polygenic basis shared within the same generation and often the same environment. In practice, this means someone whose sibling has glaucoma should treat that as a clear signal for earlier and more frequent eye exams — regardless of whether they themselves feel any symptoms, because the absence of symptoms rules nothing out in this disease.

This isn't an isolated observation from three decades ago — the global meta-analysis from 2014 mentioned earlier confirmed the same direction in a much larger, international sample: people of African descent had significantly higher risk of primary open-angle glaucoma than people of European descent, and regionally the highest prevalence was recorded in Africa. The mechanism behind this difference isn't fully explained — it likely involves differences in corneal thickness, optic disc structure, and genetic factors rather than a single cause.

The leading mechanistic hypothesis links this risk to the structure of the myopic eye itself: an elongated eyeball has a thinner sclera and a differently shaped lamina cribrosa of the optic nerve, making the nerve tissue more vulnerable to damage at a given IOP level than in an eye of normal axial length. This is practically significant, because myopia is common, well known to patients (unlike, say, subtle visual field changes), and easy to mention to a doctor — someone with high myopia should directly ask their ophthalmologist about glaucoma risk assessment during a routine visit for vision correction, rather than treating it as a separate, optional topic.

The link between diabetes and glaucoma is most often explained by microdamage to the blood vessels supplying the optic nerve and disturbances in ocular blood-flow autoregulation, mechanistically similar to those underlying diabetic retinopathy — though that's a different, separate complication. People with type 2 diabetes, covered in more detail in our knowledge-base entry on diabetes, therefore have double reason for regular eye check-ups: not just because of retinopathy risk, but also glaucoma risk, and one fundus exam doesn't automatically substitute for the other — it's worth explicitly asking your ophthalmologist whether the visit also assesses glaucoma risk, not just retinal status.

This is an area where my own clinical experience as a cardiologist overlaps with ophthalmology — not in the sense of diagnosing or treating glaucoma, which firmly remains the domain of ophthalmologists, but in understanding how systemic blood pressure and its fluctuations can affect tissue blood supply, including the optic nerve. It's worth stressing clearly: the relationship between blood pressure and glaucoma is neither simple nor unidirectional.

At the same time, a growing body of data suggests that excessively low ocular perfusion pressure — the difference between arterial pressure and intraocular pressure — can also be a problem, specifically its nighttime drops. A small, prospective study by a Finnish-Malaysian team, following 65 patients with already-diagnosed normal-tension glaucoma for 5 years, found that low nocturnal diastolic ocular perfusion pressure (below 35 mmHg) was an independent predictor of visual field loss progression, compared with patients whose values stayed above 43.7 mmHg.

The mechanism involves morphological and functional changes in the trabecular meshwork — the structure responsible for draining aqueous humor from the eye — similar to those observed in naturally occurring glaucoma. The practical significance of this fact is often underappreciated: steroids used chronically, not just as eye drops but also inhaled (e.g., for asthma), intranasally (for allergic rhinitis), or systemically, can raise IOP, and patients rarely connect these medications with ophthalmological risk. Anyone taking steroids chronically for any reason, especially with another risk factor from this article, should mention it to their ophthalmologist at a check-up.

Because primary open-angle glaucoma in most cases gives no subjective symptoms for years, the only real prevention tool is a regular, comprehensive eye exam — not just a quick "eye pressure" check at an optical shop's vision test, but a full assessment including the fundus, optic disc, and ideally the visual field. Recommended intervals between exams differ depending on age and the presence of the risk factors described above.

It's also worth mentioning one practical, though secondary, detail: caffeine at doses above roughly 100 mg (in practice, more than one standard cup of coffee) can transiently raise IOP in people with already-diagnosed glaucoma or ocular hypertension, though it doesn't produce this effect in healthy people. This isn't an argument for giving up coffee entirely — we've written more about that in our entry on caffeine — but people already diagnosed with or monitored for elevated IOP might, after consulting their ophthalmologist, consider moderating caffeine intake on the day of an IOP check-up, to avoid a falsely elevated reading.

Glaucoma is one of the few serious eye diseases for which an ordinary reader — without any medical training — can take real, effective preventive action, though not through supplements or home remedies, but through one simple decision: a regular, comprehensive eye exam tailored to their own risk profile. Data from the OHTS trial show that early detection and treatment genuinely reduces the risk of disease progression — but the precondition is precisely early detection, which, given the disease's symptomless course, depends entirely on whether the person actually shows up for an exam before noticeable vision loss appears.

If you have a family member with glaucoma, have diabetes, high myopia, or take steroids chronically, treat that as a concrete signal rather than a general curiosity — and schedule an eye exam, even if your vision subjectively seems fine. It's precisely in this group of people that early detection has the greatest clinical significance.