Refractive Eyecare — November 2010
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When "Dry Eye And Headaches" Are Actually Convergence-Related Eyestrain
Jeffrey P. Krall, OD

Eye alignment may be a significant and unappreciated cause of chronic ocular discomfort that mimics dry eye. In many of these cases, an optimal amount of prism correction can resolve symptoms.


After graduating from optometry school and joining my father’s practice in 1988, I noticed that he sometimes added a small amount of prism to a patient’s lens prescription.“You’re wasting your time,” I scoffed. “It won’t make any difference because patients will simply adapt to it.”

Then I noticed that some of his long-time patients avoided me after I fitted them with spectacles. They wanted to see the “old guy” because with his lenses, their eyes did not burn or get that “gritty” feeling. From my medical model, such symptoms suggested dry eye and called for artificial tears and related medications. But my approach had far less success with these patients than did my father’s addition of just a touch of prism correction in their lenses. He showed me an old method to measure for the needed prism: the same one I use today.

“I don’t know how it works,” he told me. “It just does.” I have spent the last 15 years determining why.

Most Dry Eye Is Dry Eye

Let me be clear. Most dry eye symptoms are the product of meibomian gland dysfunction and/or aqueous deficiency. But there are some patients for whom nothing in our dry eye armamentarium works. There are also patients who suffer headaches And asthenopic symptoms that defy diagnosis or treatment.

Over time, it became clear to me that many cases of stubbornly treatment- resistant “dry eye” are in fact related to a proprioceptive misalignment between the eyes. This is the disparity between where the eyes focus and where they converge, which is different from traditional symptomatic convergence in sufficiencies. I have found that even a slight disparity between focal point and convergence point can produce highly symptomatic eyestrain.

Moreover, I have come to recognize a distinctive pattern in these symptoms. In addition to complaining that their eyes burn when they do close work, many of these patients experience headaches—often quite severe— that generally start in the front or temporal regions and migrate to the back of the head. These headaches tend to start 30 minutes to 2 hours after the onset of near work. In addition, the accompanying ocular irritation is described as more “gritty” or “sandy” than would be typical of ocular surface disease.


Evaluating Convergence Deficiencies and Eyestrain

With best distance correction, place round white target at distance

Slowly initiate alternating cover test

If target “moves” in same direction as the occluder, patient is exophoric

Use handheld prism to neutralize observed movement

Add corresponding amount of prism


Providing Relief

Over the years, I found I could relieve such symptoms with an appropriate “touch of prism,” much as my father did. But I knew this was not enough to prove that the prism was the cure. After all, many of these patients were leaving my clinic with high-end Spectacles that incorporated other enhancements such as anti-glare coating. And the placebo effect is always a consideration. So I did an informal study. I offered my symptomatic patients two pairs of glasses—identical except that one had an appropriate amount of base-in prism. I explained I was testing a new type of glasses and asked them to come back to tell me which they liked better.

By this time, I was in practice with my brother, Joe, and he thought what I was doing was plain crazy. After all, I was giving away a second pair of spectacles. But these patients came back to me saying, “I don’t know what’s in these glasses, but my eyes don’t burn when I wear them.”

I then began consulting and collaborating with ophthalmologist Vance Thompson and neurologist Carol Miles, both of Sioux Falls, SD. I also worked with Walman Optical and Signet Armorlite to better identify the clinical problem and how to address it.

Study Design and Results

Most recently, I collaborated on a double-blind, placebo-controlled study with Drs. Bruce Teite lbaum and Yi Pang of the Illinois College of Optometry. The study, reported Last year in Optometry and Vision Science, assessed the effectiveness of an experimental progressive lens design that incorporated base-in prism to reduce eye fatigue.1 We enrolled 29 pres byopic patients who had scored 16 points or greater on the Convergence Insufficiency Symptom Survey (CISS) (see box).

Each received two pairs of glasses with progressive addition lenses made by the same manufacturer with the same prescription, except that one pair incorporated base-in prism and one did not. In a randomized and blinded sequence, subjects wore one pair of glasses for 3 weeks and retook the CISS, then switched to the second pair For 3 weeks and completed the CISS a final time. From a mean baseline CISS score of 30.21, scores decreased to a mean of 13.38 after 3 weeks of wearing the prism glasses vs 23.62 with the placebo lenses—a statistically significant difference.

Small Deviation/Big Symptoms

One of the study’s most important findings was that a patient’s degree of convergence deviation does not necessarily correlate with symptom severity. This tells us that significant ocular discomfort can result from an alignment deviation that may be missed on standard vision tests. Nor will such subtle misalignment produce the classic convergence insufficiency symptom—double vision.

Today, I firmly believe that unrecognized proprioceptive disparity is a leading cause of eyestrain symptoms including the kind of ocular irritation that mimics ocular surface disease. A number of progressive lens makers now recognize this dynamic and are incorporating base-in prism in what they call “anti-fatigue” lenses. These manufacturers include Signet Armorlite (San Marcos, CA), which developed the KODAK Lenses used in the Illinois College of Optometry study.

In addition, I am working with Essilor’s Stereo Optical Co., which has developed a device that can measure proprioceptive disparity. In essence, it will be as easy to use as an auto refractor, with the readout specifying the disparity between the focal plane and the convergence location of the patient’s eyes. A double blind study with this device is scheduled to start this year. Meanwhile I continue to share my basic methods of diagnosis, evaluation, and treatment with interested clinicians.

A Clinical Checklist

When it comes to symptomatic patients with subtle convergence insufficiency, the most important diagnostic task is to listen carefully to their complaints and ask For elaboration. For instance, do they have headaches? If so, when do these headaches occur? If the patient wakes up with them, eyestrain related to proprioceptive disparity is not likely to be the cause. By contrast, headaches associated with near work represent a red flag. When this is the case, I ask where the headaches begin. As mentioned, those associated with eyestrain tend to start frontally and move to the back of the head. Given that description, the chances are good that the clinician will be able to resolve the symptoms. I also ask about ocular dryness associated with near work or computer work. Here, the distinguishing characteristic is a “gritty” or “sandy” sensation along with burning sensation. Other associated symptoms can include difficulty focusing, a continual need to blink, and, sometimes, photophobia. With the latter tell-tale symptom, patients often describe the need to shut one eye when they pass an oncoming car at night.

Given the above symptoms, the following evaluation can help determine the optimal amount of prism correction needed to resolve eyestrain.

Patient Evaluation

The most effective way to measure the required prism is to set up a condition in which the central vision is monocular, while the peripheral vision remains binocular (Turville infinity binocular balance). Admittedly, this is difficult to replicate in most offices. The following Method does not measure the proprioceptive disparity directly, but helps determine the maximum prism a patient can tolerate.

First, with best distance correction, place a non-accommodative target such as a white spot at distance. Slowly initiate an alternating-cover test, asking the patient to report the direction of any target movement. Patients who see the target moving in the same direction as the occluder are exophoric.

Next, use a hand held prism to neutralize the observed movement. Although you are neutralizing the movement at distance, in most cases, this amount of prism will provide adequate correction at near.

I generally split the required prism between both lenses, but one may add slightly more prism in either eye to compensate for edge thickness when there is significant difference between the correction of the two eyes. Keep in mind that patients can rarely tolerate more than 3 D of prism change at any one time (1.5 D in each lens). Even 2 D is significant enough to require some adaptation.

Patients who are also presbyopic may gain additional benefit from the KODAK Lens with Anti-Fatigue Progressive Technology which incorporates prism that varies with power (Figure 1). Its 2.00 D add lens, for example, provides an additional 0.75 D of base-in prism at near. Also remember that accurate pupillary distance is extremely important.

Importantly, resist the temptation to equate the amount of prism a patient needs to the magnitude of Associated eyestrain symptoms. As mentioned, even subtle misalignment between the eyes can cause severe symptoms in some patients, while others do not appear to be bothered by far more significant divergence.


It is hard to underestimate the patient gratitude that results when this simple remedy resolves the headaches and eye discomfort that have been plaguing them for years. I have testimonials from patients who describe how their lives have utterly changed, as well as from neurologists who have sent me some of their most frustrating cases.

Certainly, I am not the only clinician who is helping patients in this manner. Many, like my father, have discovered this “secret,” sometimes without fully understanding it. I hope that increased awareness will enable many more clinicians to help their patients in this way.


Even subtle convergence disparities can cause significant eyestrain, resulting in headache and ocular discomfort that can mimic the symptoms of dry eye disease. Close attention to the pattern of a patient’s symptoms can reveal the likelihood that proprioceptive disparity is behind their discomfort, which can be resolved with the addition of the appropriate amount of base-in prism.


Jeffrey P. Krall, OD, is the coowner of Krall Eye Clinic, in Mitchell, SD, where the Krall family has been providing optometry services for over a century. Refractive Eye care senior editor Jessica Sachs assisted in the preparation of this manuscript.


1. Teitelbaum B, Pang Y, Krall J. Effectiveness of base in prism for pres byopes with convergence insufficiency. Optom Vis Sci. 2009;86(2):153-6.