A colleague recently asked me to take a look at a friend of hers, a professional pilot for private jet services. One year prior, the pilot needed his glasses remade three times by yet another optometric colleague, and he was still unhappy. 

He had his first symptom of double vision in 1994 and recalled getting his first prism in 1996 after suffering a concussion as a firefighter. He was given what he was told were “muscle-building glasses.” 

We were immediately sworn to secrecy to not call up the FAA because he had been flying a plane for 23 of the past 24 hours. Upon final approach to the airport, he clearly saw two runways, one next the other, and wondered on which one should he land the plane. How to pick? He closed one eye and said, “That must be the one.” When he opened his other eye, he looked harder and harder to move the second one over to the first one, and then brought the plane down. 

He knew that he had pushed himself beyond his limits and never wanted to do that again. 

In October 1999, he had sustained another concussion in a motorcycle accident. In February 2000, he had fusion of his C4-C6 vertebrae. In 2010, he underwent fusion of L4-L5-S1 vertebrae. 

Our patient, a professional pilot, saw two runways while trying to land a plane.

We laughed when he proudly stated that he was “as flexible as an oak tree,” but this actually raised a red flag because of the likely connection between posture and movement with vision and refractive conditions.1,2 With his restricted back movement, we expected some interesting asymmetries in his visual system—and we weren’t disappointed.

We have to admit that we don’t neutralize a new patient’s glasses until we’re done with the exam because we don’t want to be biased by the prior data. And this patient had optometric data going back to 2009 for comparison.

Diagnostic Data
Visual acuity with his current glasses was quite good at 20/19 OD, 20/14 OS and 20/13 OU. A cover test over his glasses, which he said had prism in them, revealed 10Δ exophoria, 3Δ right hyper at distance and 4Δ exophoria, 3Δ right hyper at near.

His refraction was pretty straightforward: +2.75 OD and +3.00 -0.75 x 180 OS. With his glasses, near point of convergence was 4”/6” OD out, and he reported diplopia spontaneously. 

The Worth 4-dot test was quite revealing. He saw five dots, which indicates that his eyes are out of alignment relative to the lights whenever he looked 20° or more downward or 35° to his right or left. This clearly showed that his binocularity was fragile, to say the least. 

In the phoropter, with his new refraction, we performed vertical ranges with prism moving in front of the left eye with the right eye as the reference eye and found: 

  • Left infra duction showed a break at 13.5Δ and a recovery of 12.0Δ
  • Left supra duction showed a break at -10.0Δ and a recovery of -11.5Δ

This means that he has a total vertical range of half a prism diopter at distance and needed between 11.5Δ and 12.0Δ of vertical prism. He also needed at least 3Δ more over his glasses. At near, he had the same half a prism diopter of range, but this range increased to 14.0Δ to 14.5Δ of vertical prism.

Although patients with these verticals can function with less prism in their glasses, they’ll compensate with a head tilt. In some cases, the glasses themselves have gotten out of adjustment. Induced vertical prism can occur when one lens is high and the other is low relative to the visual axes. This patient’s glasses were slightly out of kilter and he had a small head tilt, but nothing extraordinary.

What about his horizontal ranges, you ask? At distance, his base-in range was a respectable break at 9Δ with recovery at 6Δ, but his base-out was non-existent. At -4Δ or 4Δ of base-in, he broke into seeing double vision while moving toward base-out. He had to go back to 5Δ of base-in to recover fusion again. So, based on the recoveries, his zone of comfortable binocular vision should be a range of 1Δ horizontally and a half a prism diopter vertically. Yes indeed, as flexible as an oak tree! 

Previous Data
Now it was time to see what was in his glasses as determined by his previous eye care providers. His habitual glasses were:

  • +3.00 -0.50 x 30 +2.50 add 3Δ base-down
  • +3.00 -1.00 x 167 +2.50 add 3Δ base-up

He had been prescribed only 6Δ of vertical prism, yet his testing showed he needed a minimum of 11.5Δ to not have to tilt his head or “work” at it. 

Most concerning was that his binocular testing from the previous optometric records was limited to a cover test and, in one or two instances, a phoria measure, which is only a central tendency measurement. This is not sufficient, so be sure to investigate vergence ranges. 

By digging deeper than the basic sight-based examination, you’ll find the source of the patient’s visual issue, which will allow you to finally address it. In this patient’s case, giving more and more prism, or giving the prism he had before simply because he had it before without doing the requisite testing, was just not solving his problem. 

Fortunately, he was no longer flying commercially, so we didn’t have to “ground” him. He will begin vision therapy soon after receiving his new glasses with a goal to reduce the amount of the prism needed for fusion and to improve his control over his visual system.3,4 

1. Harmon DB. Notes on a Dynamic Theory of Vision. Santa Ana, CA: Optometric Extension Program Foundation; 1958. 
2. Kraskin RA. Lens Power in Action. Santa Ana, CA: Optometric Extension Program Foundation; 1982.
3. Robertson KM, Kuhn L. Effect of visual training on the vertical vergence amplitude. Am J Optom Physiol Opt. 1985 Oct;62(10):659-68.
4. Cooper J. Orthoptic treatment of vertical deviations. J Am Optom Assoc. 1988 Jun;59(6):463-8.