Night Myopia: Etiology And Correction

Night myopia is an increase in myopia that tends to develop whenever the light level of an individual's environment is substantially reduced. This myopic shift has been
reported to be as high as 3.0 to 4.0 diopters but has an average magnitude of 1.0 to 1.5
diopters. These values refer to a young adult eye under scotopic illuminance levels.
Individuals who are affected by night myopia appear emmetropic with their appropriate
refractive correction under high luminance conditions but become nearsighted as
luminance levels are decreased. The eye does not focus on infinity but rather on an
intermediate distance under dim lighting conditions. The blur created by this myopic
shift can create significant problems for individuals performing activities in a dimly lit
environment. In 1990 the National Traffic Highway Safety Administration found that
the nightime automobile accident rate was 3. 7 times higher than in daytime. Statistics
released by the National Safety Council attribute 5.4 deaths per million vehicle miles
traveled at night versus 1.6 deaths per million vehicle miles traveled during the day. It is
important that decreased visual efficiency be considered as a contributing factor to these
statistics. It is the purpose of this article to examine the possible causes of night myopia
and describe techniques for its evaluation and correction.