Monovision: Probable Cause?

BY JOSEPH T. BARR, O.D., M.S., EDITOR
AUG. 1997

Investigators at the National Transportation Safety Board have decided that reduced stereovision was the probable cause of the crash of Delta Flight 554 [see News Spectrum p. 3]. They said that because of the captain's monovision contact lens system (along with heavy rain and fog and non-standard runway light spacing), he was unable to overcome the illusion that he was farther from and higher above the runway than he actually was.

NTSB medical officer and accident investigator Mitchell Garber, M.D., correctly told the board that stereopsis is most critical for viewing objects within about 20 to 25 feet, but he added it is "probably functionally useful to about 1,000 feet and theoretically useful beyond 4,000 feet."

At increasing distances, stereovision is reduced and monocular cues to depth are more important even with normal, clear singular binocular vision. So, in a jet going 150 knots well over 20 feet in the air, I don't believe the captain's reduced stereoacuity from monovision is the likely cause for a landing accident.

Testing performed after the accident revealed that the captain's near stereopsis was 80 seconds of arc with monovision. Distance visual acuity was 20/80 OS (near eye), 20/20 OD. At near, OS was 20/20, OD was 20/60. So, it's more reasonable to say that blur in one eye, rather than reduced stereopsis, may be a probable cause.

FAA rules require 20/20 vision in each eye and forbid monovision and bifocal contact lenses. However, the FAA does allow monocular individuals to obtain medical certification after a six-month adaptation period. This policy may seem reasonable in and of itself, but it makes little sense in light of the fact that more visual information is available to a pilot successfully wearing monovision contact lenses than for one who is blind in one eye. Despite the fact that stereovision is reduced in all monovision (anisometropia) situations, it's not completely absent as it is in monocular patients.

The captain of Delta Flight 554 claims he didn't know monovision is forbidden. It's important that your patients who must follow FAA regulations are aware of the regulations regarding monovision. It sounds like two distance lenses and reading spectacles would make more sense.

In 1990, Josephson, Erickson, Back, Holden, Harris, Tomlinson, Caffrey, Finnemore and Silbert published an article in the Journal of the American Optometric Association called "Monovision." They reviewed the literature, most of which is very favorable, and discussed the limitations of the modality and the need for informed consent for patients. More recently, Classé has written about the scarcity of any cases of legal action involving monovision. The FDA has recognized monovision as an acceptable modality for contact lenses. Numerous recent presidents used it. Of course, they weren't pilots at the time.

We need to get this and other monovision stories straight and inform patients of the limitations and FAA regulations. Monovision is very effective and safe if used judiciously. Monovision flight simulator studies are called for. Many patients do well with monovision, so let's be careful with it but not use it as a scapegoat.