Glaucoma can absolutely affect one eye while leaving the other untouched, and it commonly does. Even in people who develop glaucoma in both eyes, the disease almost always progresses unevenly, with one eye sustaining more damage than the other. Ophthalmologists refer to this as “asymmetric glaucoma,” and it is so common that the difference between the two eyes has become a diagnostic tool in its own right. The reasons range from subtle structural differences in the eye to something as mundane as which side you sleep on.
Why One Eye Often Gets Hit Harder
In open-angle glaucoma, the most common form, both eyes usually have the disease to some degree, but the severity between them can be dramatically different. Researchers studying these lopsided cases have found that the worse eye tends to have measurably different physical properties from its partner. One large study found that the worse eye in asymmetric open-angle glaucoma had lower corneal hysteresis, a measure of how the cornea absorbs and dissipates energy. On multivariate analysis, lower corneal hysteresis was the strongest predictor of which eye was worse, with an odds ratio above 25.1Investigative Ophthalmology & Visual Science. Corneal Hysteresis and Visual Field Asymmetry in Open Angle Glaucoma Traditional risk factors like standard eye pressure measurements and corneal thickness, which eye doctors routinely check, were surprisingly poor at predicting which eye had more damage.
The blood vessels feeding each eye also differ. In patients with asymmetric glaucoma, the more damaged eye shows lower blood flow velocity in the central retinal artery compared to the less affected eye.2PubMed. Color Doppler imaging in patients with asymmetric glaucoma and unilateral visual field loss The retinal blood vessels themselves are measurably narrower in the worse eye. A study of 158 people with asymmetric glaucoma found that retinal vessel caliber was significantly smaller in the eye with more advanced disease, even after adjusting for age, blood pressure, and eye pressure.3PubMed. Retinal vascular caliber between eyes with asymmetric glaucoma Whether these vascular differences cause the asymmetry or result from it remains debated, but they help explain why two eyes sitting inches apart in the same skull can follow very different disease trajectories.
Conditions That Cause Truly One-Sided Glaucoma
While primary open-angle glaucoma tends to eventually show up in both eyes to some extent, several forms of secondary glaucoma can be genuinely unilateral, affecting only the injured or diseased eye.
- Traumatic glaucoma: A blow to one eye can tear the internal drainage structures, a condition called angle recession. This is usually unilateral because only one eye took the hit. The resulting glaucoma can appear months or even years after the original injury.4PubMed Central. Bilateral Angle Recession and Chronic Post-Traumatic Glaucoma: A Review of the Literature and a Case Report
- Neovascular glaucoma: When the retina in one eye loses its blood supply, whether from a vein occlusion, diabetic retinopathy, or other ischemic event, abnormal new blood vessels can grow over the drainage angle and block fluid outflow. The underlying causes, such as central retinal vein occlusion and ocular ischemic syndrome, often strike one eye.5PubMed Central. Neovascular glaucoma: comprehensive evaluation of etiology, treatment modalities, and visual prognosis
- Uveitic glaucoma: Inflammation inside one eye can scar the drainage pathways or cause pressure spikes, either from the inflammation itself or from the steroid drops used to treat it. About a fifth of uveitis patients develop elevated eye pressure, and roughly a third of those eyes progress to glaucoma.6PubMed. Outcome of raised intraocular pressure in uveitic eyes with and without a corticosteroid-induced hypertensive response
- Pigment dispersion: In some people, pigment granules flake off the back of the iris and clog the drainage meshwork. When this is asymmetric between eyes, the anatomy of the iris itself differs: the worse eye tends to have a more concave iris shape and greater iris-lens contact, which physically rubs off more pigment.7JAMA Ophthalmology. Ultrasound Biomicroscopy in Asymmetric Pigment Dispersion Syndrome and Pigmentary Glaucoma
Pseudoexfoliation syndrome deserves a separate mention. This condition deposits flaky protein material inside the eye, and it often appears clinically in just one eye for years before the other eye catches up. What looks like a unilateral problem is more likely subclinical bilateral disease that has not yet become visible in the second eye.8Journal of Glaucoma. The Risk of Glaucoma in Pseudoexfoliation Syndrome This is a good reminder that “one eye only” sometimes means “one eye first.”
How Pressure Differences Between Eyes Help Spot Trouble
One of the most useful clinical signs in glaucoma screening is a pressure difference between your two eyes. Healthy eyes tend to have very similar pressures. When a measurable gap opens up, it raises a red flag even if both individual readings fall within the technically “normal” range. A study examining this relationship found that a pressure difference of three points between the two eyes was associated with a 6% probability of glaucoma, and a difference greater than six points pushed that probability to 57%. When there was no meaningful pressure difference, the probability dropped to just 1%.9Journal of Glaucoma. The Value of Intraocular Pressure Asymmetry in Diagnosing Glaucoma This held true even for people whose pressures were both under the traditional threshold of concern, which means a gap between eyes matters more than where either eye’s pressure sits on an absolute scale.
Modern imaging has made detecting asymmetric damage even more precise. OCT scans, which map the nerve fiber layer at the back of the eye, can compare the two eyes directly. One study found that this intereye comparison approach achieved a sensitivity above 97% for detecting glaucoma in the affected eye, including cases where standard visual field tests had not yet picked up damage.10Investigative Ophthalmology & Visual Science. Intereye Comparison of Cirrus OCT in Early Glaucoma Diagnosis and Detecting Photographic Retinal Nerve Fiber Layer Abnormalities Your unaffected eye essentially serves as its own control, making the comparison between the two a powerful early detection tool.
Your Sleeping Side May Play a Role
One of the more surprising findings in asymmetric glaucoma research involves sleep position. When you lie on your side, the eye pressed against the pillow experiences higher pressure than the other eye. In people who habitually sleep on one side, this nightly pressure difference can add up over years. A study found that patients who preferred sleeping on their left side were more likely to have worse glaucoma in their left eye, and those who preferred their right side had worse disease in their right eye. When the cup-to-disc ratio difference between eyes was 0.2 or more, the worse eye was the one pressed into the pillow.11PubMed Central. Effect of body position on the pathogenesis of asymmetric primary open angle glaucoma
A separate study using continuous home sleep monitoring confirmed the pattern: right-sided sleep was about 1.6 times more common overall, and it correlated with worse visual field loss in the left eye.12PubMed. Time Spent in Lateral Sleep Position and Asymmetry in Glaucoma This does not mean side sleeping causes glaucoma, but it may influence which eye deteriorates faster in someone who already has the disease. Some glaucoma specialists now ask their patients about sleep habits and occasionally recommend alternating sides or sleeping slightly elevated.
Blood flow asymmetry feeds into this picture as well. In patients with normal-tension glaucoma, where eye pressure readings are within the normal range but damage still occurs, researchers found that the worse eye had significantly slower blood flow in the ophthalmic artery. The resistance to blood flow was also higher in the worse eye.13PubMed. Retrobulbar hemodynamics in normal-tension glaucoma with asymmetric visual field change and asymmetric ocular perfusion pressure So for some people, the asymmetry isn’t about pressure at all. It’s about one eye receiving less blood than the other, possibly due to differences in the blood vessels feeding each eye or systemic factors like low blood pressure at night.
What Happens to the Second Eye
If you have glaucoma in one eye, a natural worry is whether the other eye will follow. The answer depends heavily on the type of glaucoma. In acute angle-closure glaucoma, the risk to the second eye is substantial. A classic analysis of 200 patients who had an acute angle-closure attack in one eye found that, without preventive treatment, the second eye went on to develop an acute attack in half of all cases. The time gap between the first and second eye varied from days to years.14PubMed Central. Acute Angle-Closure Glaucoma: The Second Eye: An Analysis of 200 Cases This is why a laser iridotomy, a small hole made in the iris to improve fluid drainage, is routinely performed on the unaffected fellow eye as a precaution after a first attack.
For open-angle glaucoma, the picture is less dramatic but still relevant. Most people diagnosed with open-angle glaucoma in one eye already have at least subclinical disease in the other. The fellow eye often catches up over the following years, which is why monitoring both eyes remains essential even when one seems fine. Conditions that start as apparently unilateral, like pseudoexfoliation, tend to become bilateral with enough time, though the severity often remains unequal.
Truly one-sided glaucoma, where the fellow eye stays healthy indefinitely, is most common in secondary forms tied to a one-time event: a single eye injury, a vein occlusion, or surgery on one eye that triggered a pressure change. Even then, the fellow eye deserves regular screening, because the same systemic risk factors, like vascular disease or diabetes, affect both sides of the body.
Treating One Eye Differently From the Other
When glaucoma is more severe in one eye, treatment often reflects the asymmetry. Both eyes may receive pressure-lowering drops, but the worse eye might need a higher-intensity regimen or a procedure like selective laser trabeculoplasty or surgery. In some cases, only the affected eye receives medication at all, particularly in secondary glaucomas where the fellow eye is genuinely healthy.
Using drops in just one eye sounds straightforward, but it creates a practical quirk: prostaglandin analogs, the most commonly prescribed class of glaucoma drops, can darken the iris, lengthen the eyelashes, and deepen the crease of the upper eyelid on the treated side.15Dove Press (Clinical Ophthalmology). Managing adverse effects of glaucoma medications When only one eye needs treatment, these cosmetic changes can become visibly asymmetric, with one eye looking subtly different from the other. This can be distressing for patients and is worth discussing with your doctor before starting treatment, especially since these changes are largely reversible if you switch medications.
Early research on prostaglandin drops actually used the fellow eye as a built-in control. In one study, patients applied the drop to only one eye and researchers tracked both. The treated eye showed a pressure reduction of about 6 points more than the untreated eye, confirming the drug worked locally rather than through a systemic effect.16PubMed. Prostaglandin F2 alpha-isopropylester eye drops: effect on intraocular pressure in open-angle glaucoma This is useful knowledge for patients treating only one eye: the drop acts on the eye it goes into, with minimal crossover.
How Lopsided Damage Affects Everyday Vision
You might assume that having glaucoma in just one eye would be less disruptive than having it in both. In many cases that is true: the healthier eye compensates for blind spots in the other, and people with early-to-moderate one-sided damage may not notice anything wrong in daily life. But the compensation has limits, particularly when it comes to depth perception.
Stereopsis, the ability to judge depth using two eyes together, depends on both eyes feeding slightly different but largely intact images to the brain. When glaucoma damages the central visual field in one eye, depth perception suffers disproportionately. Patients with central visual field defects had a median stereoacuity of 60 arc seconds compared to 40 in those without such defects, and only 21% of the group with central damage retained normal stereopsis compared to 61% of those without it.17PubMed Central. Correlation between Central Visual Field Defects and Stereopsis in Patients with Early-to-Moderate Visual Field Loss The same study found that vision-related quality of life was lower in the group with central field defects, with particular drops in near activities, general vision, and mental health subscores.
This matters because people sometimes underestimate one-sided glaucoma. It may not interfere with reading an eye chart, but it can make pouring liquids, judging distances while driving, or navigating stairs harder than expected. The loss sneaks up on people precisely because the good eye masks it during routine activities.
Rare Developmental Causes of One-Sided Glaucoma
Most glaucoma develops in adulthood, but some children are born with or develop glaucoma in one eye due to congenital or developmental conditions. Sturge-Weber syndrome, marked by a port-wine birthmark on the face, can cause glaucoma in the eye on the same side as the birthmark. The mechanism involves elevated pressure in the veins draining the eye, a consequence of the vascular malformation that defines the syndrome.18PubMed Central. Sturge-Weber syndrome secondary glaucoma: From Pathogenesis to Treatment Because the birthmark is usually on one side of the face, the glaucoma is typically unilateral from birth.
Other developmental anomalies, like Axenfeld-Rieger syndrome or aniridia, can also produce glaucoma that varies in severity between eyes depending on the degree of structural abnormality on each side. For parents whose child has been diagnosed with a condition associated with glaucoma, the asymmetry means monitoring both eyes carefully but understanding that intervention may be needed sooner or more aggressively on one side. Pediatric glaucoma specialists tailor surgical approaches to each eye’s anatomy, which in these congenital cases can differ substantially even within the same child.