One of the most common misconceptions about glaucoma is that it simply means having high eye pressure.
Eye pressure, also called intraocular pressure or IOP, is extremely important in glaucoma. In fact, lowering eye pressure remains the only treatment proven to slow the disease. But glaucoma is ultimately a disease of the optic nerve, not simply a number on the pressure measurement.
This means that, perhaps surprisingly, glaucoma can sometimes develop or worsen even when eye pressure falls within what is traditionally considered the normal range.
Understanding why requires looking beyond a single pressure measurement.
What Is Considered a Normal Eye Pressure?
Eye pressure is commonly measured in millimeters of mercury (mmHg). Historically, pressures around 10 to 21 mmHg have often been described as falling within the statistically normal range.
But there is an important distinction: a statistically normal pressure is not necessarily a safe pressure for every individual eye.
- Some people can have pressures above 21 mmHg for years without developing optic nerve damage. This condition is called ocular hypertension.
- Other patients develop characteristic glaucoma damage even though their pressures have never been documented above the traditional normal range. This is generally referred to as normal-tension glaucoma.
Large studies have demonstrated that factors beyond the pressure measurement itself, including age, optic nerve appearance, visual field findings, and central corneal thickness, can influence an individual's risk of developing glaucoma.
What Is Normal-Tension Glaucoma?
Normal-tension glaucoma is a form of open-angle glaucoma in which patients develop characteristic optic nerve damage and visual field loss despite having eye pressures that are not unusually high by population standards.
This does not mean that eye pressure is irrelevant.
One of the landmark studies in this area, the Collaborative Normal-Tension Glaucoma Study, evaluated whether lowering pressure could help patients whose glaucoma occurred at relatively low pressures. The study found that substantially reducing eye pressure decreased the likelihood of glaucoma progression in appropriately selected patients.
This helped establish an important principle in glaucoma care. The important question is not simply, is the pressure normal? It is: is this pressure low enough for this particular optic nerve?
The Concept of a Target Eye Pressure
For someone without glaucoma, an eye pressure of 18 mmHg may be perfectly acceptable. For another patient with advanced glaucoma, however, that same pressure may be too high to adequately protect the remaining optic nerve tissue.
Glaucoma specialists therefore often establish an individualized target IOP based on factors such as:
- The severity of existing optic nerve damage
- The patient's previous untreated pressure
- Whether glaucoma appears to be progressing
- Age and anticipated lifetime risk
- Visual field findings
- OCT findings
- Other clinical risk factors
The target pressure may also change over time. If OCT imaging or visual field testing demonstrates convincing progression despite apparently good pressure readings, the target may need to be lowered further.
Lower Pressure Generally Means Lower Risk
The importance of pressure reduction has been demonstrated repeatedly in major glaucoma trials.
In the Early Manifest Glaucoma Trial, treatment reduced the risk of glaucoma progression by approximately half. Researchers also found that each additional millimeter of mercury of pressure reduction was associated with approximately a 10% decrease in the risk of progression within the study population.
The Collaborative Normal-Tension Glaucoma Study similarly demonstrated that lowering pressure can benefit patients even when their starting pressures are already relatively low.
These studies are an important reminder that there is no magical pressure number below which glaucoma becomes impossible. Instead, glaucoma risk generally exists along a continuum.
Why Can Glaucoma Progress at Lower Pressures?
There is probably no single explanation.
1. Some optic nerves may be more susceptible to pressure
Two patients can have the same measured eye pressure but very different optic nerves. An optic nerve that is particularly susceptible to mechanical or vascular stress may develop damage at a pressure that another eye tolerates without difficulty.
2. Eye pressure is not constant
A pressure measurement in the ophthalmologist's office represents only one moment in time. IOP naturally fluctuates, and the pressure measured during an afternoon appointment may not necessarily represent the patient's highest pressure throughout the day or night. For this reason, ophthalmologists generally evaluate pressure trends over time, rather than relying on a single measurement.
3. The pressure measurement itself has limitations
The most commonly used method of measuring eye pressure, Goldmann applanation tonometry, is influenced in part by properties of the cornea. Central corneal thickness is therefore one of several factors ophthalmologists consider when interpreting IOP.
The Ocular Hypertension Treatment Study identified thinner central corneal thickness as an important predictor of glaucoma development among patients with elevated eye pressure. This does not mean that ophthalmologists simply add or subtract a fixed number from an eye-pressure measurement based on corneal thickness. Rather, corneal thickness is incorporated into the overall clinical assessment.
4. Factors other than IOP may influence optic nerve susceptibility
Normal-tension glaucoma has also prompted considerable research into factors involving optic nerve blood flow and vascular regulation. For example, certain studies have found associations between normal-tension glaucoma progression and findings such as optic disc hemorrhages and migraine history.
However, these associations are more complicated than the relationship between IOP and glaucoma. At present, lowering eye pressure remains the principal modifiable treatment strategy with established evidence for slowing glaucomatous damage.
Does Everyone With Normal-Tension Glaucoma Progress?
No.
Another important lesson from the Collaborative Normal-Tension Glaucoma Study is that glaucoma does not progress at the same rate in every patient.
In observations of untreated patients with normal-tension glaucoma, approximately one-third demonstrated localized progression within three years, while roughly half demonstrated progression within five to seven years. Other patients remained relatively stable over the period studied.
This variation is one reason glaucoma management needs to be individualized. A patient with mild disease that has remained unchanged for many years may be managed differently from someone whose OCT or visual field is showing measurable deterioration over several visits.
How Do We Know Whether Glaucoma Is Getting Worse?
This is why glaucoma follow-up involves much more than checking eye pressure. The ophthalmologist may monitor several different measurements, including:
OCT imaging
Optical coherence tomography (OCT) measures structures such as the retinal nerve fiber layer and ganglion cell complex. Repeated scans can help determine whether measurable structural loss is occurring over time.
Visual field testing
A visual field test evaluates how well different areas of one's peripheral and central vision are functioning. Repeated testing can reveal patterns of functional loss typical of glaucoma and help determine whether that loss is progressing.
Examination and imaging of the optic nerve
Changes in the appearance of the optic nerve, including increasing cupping or the development of an optic disc hemorrhage, can provide additional evidence about glaucoma activity. Disc hemorrhages are particularly noteworthy because clinical studies have associated them with an increased risk of glaucoma development or progression.
Eye pressure over time
Rather than focusing on one isolated number, ophthalmologists look at the overall pressure pattern and how that pressure relates to the condition of the optic nerve.
What If My Pressure Is Good, but My Glaucoma Is Progressing?
If convincing progression is detected, one's ophthalmologist may reconsider the patient's target pressure.
Depending on the individual situation, additional pressure reduction might be achieved with:
- Prescription glaucoma eye drops
- Selective laser trabeculoplasty (SLT)
- Minimally invasive glaucoma surgery
- Traditional glaucoma surgery in more advanced or difficult-to-control disease
The appropriate choice depends on the severity of the glaucoma, current pressure, previous treatments, anatomy of the eye, rate of progression, and other individual factors.
Importantly, an isolated abnormal OCT scan or visual field does not always mean that glaucoma is worsening. Imaging artifacts, testing variability, cataracts, refractive factors, and other eye diseases can sometimes produce misleading results. Glaucoma specialists therefore look for reproducible change over time and interpret imaging, visual fields, eye pressure, and the clinical examination together.
The Bottom Line
Yes, glaucoma can worsen even when the eye pressure appears to be within the normal range.
A normal pressure reading does not automatically mean that glaucoma is stable, and a higher-than-average pressure does not automatically mean someone has glaucoma.
The goal of treatment is not simply to produce a pressure number that falls into a standard range. The goal is to find a pressure that is sufficiently low to preserve that particular patient's optic nerve and vision over his or her lifetime.
That is why regular glaucoma follow-up remains important even when one's pressure readings look excellent.
References
- Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Archives of Ophthalmology. 2002.
- Collaborative Normal-Tension Glaucoma Study Group. Comparison of glaucomatous progression between untreated patients with normal-tension glaucoma and patients with therapeutically reduced intraocular pressures. American Journal of Ophthalmology. 1998.
- Leske MC, Heijl A, Hussein M, et al. Factors for glaucoma progression and the effect of treatment: the Early Manifest Glaucoma Trial. Archives of Ophthalmology. 2003.
- Anderson DR, Drance SM, Schulzer M; Collaborative Normal-Tension Glaucoma Study Group. Natural history of normal-tension glaucoma. Ophthalmology. 2001.
- Drance S, Anderson DR, Schulzer M; Collaborative Normal-Tension Glaucoma Study Group. Risk factors for progression of visual field abnormalities in normal-tension glaucoma. American Journal of Ophthalmology. 2001.
Medical Disclaimer
This article is intended for general educational purposes and does not replace an individualized examination or treatment recommendation from your ophthalmologist.