People over the age of 40 sometimes develop farsightedness, or presbyopia. Monovision is a treatment where the contact lens in the non-dominant eye is set for reading while the lens in the dominant eye is used for perceiving objects at a distance. When both distance and near vision correction are needed, people who wear contact lenses have three options: multifocal contacts that change power from the bottom to the top of the lenses; reading glasses placed over lenses; and, the Monovision system.
Side Effects During Adjustment Period
According to Eyezone Medical, some patients grapple with a period of adjustment in the early stages of Monovision. Because the brain has to bring into line dissimilar visual input from each eye, patients may endure blurred or double vision, nausea, headaches, dizziness and general discomfort. Eyes may tire quickly when reading. Some patients may observe halos or glare around lights during the evening hours. This period of adjustment typically lasts one to three weeks. During this time, both eyes should be used simultaneously in order for the full adjustment to take place. Symptoms should subside within three months.
Loss of Depth Perception
Optometrist Dr. Larry Bickford says, "For those with normal stereo vision, Monovision could cause a potentially significant loss of depth perception." While some people can hold back visual information from one eye while using the other, people who are firmly binocular find it difficult to use Monovision. The attenuation of stereoscopic depth perception can put drivers at risk, particularly while travelling at night. Practitioners counsel patients who frequently operate vehicles, such as cab drivers and pilots, against Monovision. The loss of depth perception can be mitigated to a degree with a "modified Monovision" treatment, according to Bickford. By replacing the near vision lens with a multifocal lens in the non-dominant eye, the patient can recover depth perception for distance viewing. However, people who perform tasks that require three-dimensional near vision---typesetting, bioengineering laboratory work, electronic assembly---should avoid Monovision.
Plane Crash and Monovision
In 1996, a McDonnell Douglas MD-88 crash-landed at La Guardia Airport in New York. It collided with an embankment and went into a skid on the runway. While only three people bore minor injuries, it was an unusual incident. In a follow-up report by the National Transportation Safety Board, the cause of the crash was the pilot's loss of depth perception due to Monovision contact lenses. During his approach to the runway, the captain was unable to surmount visual distortion in restricted light conditions over water. To date, regulations stipulate that pilots are prohibited from flying with Monovision.