All About Night Myopia or Nocturnal Myopia »
Night Myopia Definition
What is night myopia? Night myopia, also known as nocturnal myopia or twilight myopia, is a correctable cause of decreased visual acuity under low illumination conditions, even though the vision is normal during the daytime. The eyes become near-sighted in dim illumination.
Even though the night myopia has been recognized in the literature for over 200 years, its causes are yet to be fully understood. Chromatic aberration, spherical aberration, and accommodation are supposed to be the causes of nocturnal myopia.
Individuals who are affected by night myopia appear emmetropic with their appropriate refractive correction under high luminance conditions (daytime) but become nearsighted (myopic) as luminance levels are decreased. Particularly, young adults are the victims of nocturnal myopia.
According to Norberto L. et al, uncorrected myopia is less noticeable during the day as the ambient luminance reduces the size of the eye’s pupil, reducing the amount of blur on the retina. With a decline in ambient luminance, the pupil dilates, and retinal blur becomes noticeable subjectively. Thus, the eye appears to have become nearsighted when, in fact, it was always nearsighted.
Under scotopic illuminance lever (reduced-illumination), the nocturnal 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.
Car accident statistics show that in spite of 60 percent less traffic on the roads, more than 40 percent of all fatal car accidents occur at night. According to the National Traffic Highway Safety Administration’s report, the nighttime automobile accident rate was 3.7 percent higher than in the daytime in 1990. Night myopia, the contributor to decreased visual efficiency at night is considered to be the major attributing facto of these statistics.
History of nocturnal Myopia
The phenomenon of night myopia has been known for more than 2 centuries. But the topic is still a subject of great theoretical and clinical debate in the scientific world.
The English astronomer Maskelyne became the first known person to report the phenomenon of night myopia. He found that his eyes became more myopic in dim illumination (at night) when observing stars and he corrected this refractive error by using minus lenses.
A century later, in 1883, Lord Rayleigh hypothesized that the spherical aberration of the eye is responsible for nocturnal myopia when the pupil is dilated. He also found that using -1.00 diopter eyeglass provided him the greatest visual clarity in a dimly illuminated room. Because of this elaboration, the discovery of night myopia is attributed to him.
The phenomenon gained considerable importance during the Second World War because of the crucial need to visually detect points of light at sea or in the night sky.
More recently it has been suggested that night myopia is a potential hindrance to safe driving at night.
A review of the published literature concerning night myopia indicates that several factors are involved: accommodation, spherical aberration, and chromatic aberration. All seem to contribute to nocturnal myopia but in differing degrees in different individuals.
Symptoms of Night Myopia
The following symptoms are common in night myopia
- Blurry vision only in a dimly lit environment
- Feeling discomfort and asthenopia while maintaining fixation in low illumination level
- Halos around light
- Difficulty in night driving.
For the diagnosis of night myopia, the following investigative tests should be performed in a patient complaining about blurry vision at night and at the dim-light condition.
- Visual acuity test in low illumination level
- Contrast sensitivity testing
- Darkroom retinoscopy
- Measurement of pupil diameter
- Open-field optometer to measure objectively the eye’s paraxial refractive state over the central 2.5 mm of the eye’s pupil
- An adaptive optics visual analyzer
What Causes Night Myopia (Nocturnal Myopia)
Chromatic aberration and Purkinje Shift
Chromatic aberration occurs when the shorter wavelengths are focused in front of the retina, rather than on the retina. According to Otero and Duran, the normal eye is basically emmetropic in light of about 550 millimicrons but becomes increasingly more myopic for shorter wavelengths of approximately 510 millimicrons (Purkinje Shift).
In a dark-adapted state, the shift from cone to rod vision causes the eye to become more sensitive to blue light. This is because blue light is predominant in darkness and red light is predominant in daylight. In the dark-adapted eye, the special sensitivity of the retina is shifted toward the shorter wavelengths, thus the eye becomes more myopic for those wavelengths that are most effective in stimulating the retina. Studies on this phenomenon conducted by Otero and Duran indicate that only about 0.25 diopters of myopia could be attributed to chromatic aberration.
According to the study by Ronchi and Wald, chromatic aberration could account for as much as 0.59 diopters of night myopia. As well, Wald and Griffin, from their own studies involving the chromatic aberration of the eye, concluded that it contributed 0.35 to 0.4 diopters of myopia.
The variation in values reported by various researchers can be due to the energy distribution of the testing illumination. However, all the values reported in the literature referring to chromatic aberration and myopia are low in regards to the overall values of night myopia reported.
Night myopia and spherical aberration
Spherical aberration is characterized by peripheral and paraxial rays focusing at different points along the axis. The spherical aberration of the eye is usually undercorrected.
The outer zones of the eye’s optical system have greater optical power or are more myopic relative to the central zones. Under daylight conditions the pupil is constricted, thus light entering the pupil is not affected by the more myopic outer zones of the eye’s optical system. However, under scotopic illumination, the pupil dilates thus exposing the more marginal myopic optical zones.
This creates a reduction in the role of the central optical zone and the more peripheral zones now provide for a majority of the light responsible for image formation. Since the outer zones have greater optical power, the undercorrected spherical aberration manifests itself as myopia in the dark-adapted eye.
In 1942, Otero and Duran6 concluded that spherical aberration could only account for about 0.50 diopters of night myopia. Le Grand, in 1942, calculated that spherical aberration combined with chromatic aberration could cause as much as 1.5 diopters of myopia.
The majority of experimental data available seems to indicate that spherical aberration accounts for only a very small amount of the night myopia found in most individuals affected.
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The major factor responsible for causing night myopia is the change in accommodation. Some researchers found that cycloplegics reduced night myopia which suggested that accommodation played a significant role in night myopia.
In 1942, Otero and Duran were the first to publish data supporting the role of accommodation in night myopia. They found that after cyclopleging their subjects the magnitude of night myopia decreased.
They also found that a presbyope with only 0.5 diopters amplitude of accommodation had only 0.5 diopters of night myopia. They concluded that accommodation is a causative factor of night myopia.
In 1954, Schober introduced the resting state hypothesis to explain the role of accommodation in inducing night myopia. Schober believed that when the eye is relaxed the accommodative focus is not at infinity but rather at an intermediate distance. If no stimulus for an accommodation exists, such as in darkness, the eye returns to its resting state placing the focus at an intermediate distance resulting in myopia.
A major reason that has made it difficult to correlate accommodation with night myopia is that most techniques used to measure the accommodative response under scotopic conditions actually stimulated accommodation. Within the past thirty years, most research into accommodation and its correlation with night myopia has included the use of the ‘laser speckle’ optometer.
Nocturnal Myopia (Night Myopia) Treatment
(Useful for Night Driving, Aviation, Astronomy)
Recent studies have cited an increased incidence of road traffic accidents at night and a prevalence of visual difficulty on the road after dusk. Eye care professionals should suspect night myopia as one of the factors of vision problems encountered during twilight or night time.
According to some authors, the correction of night myopia can be done by supplying a minus power clip-on over the normal distance correction. To find out the refractive error, a minus prescription should be determined by first establishing the individual’s dark focus, but because the road at night is not at scotopic illuminance levels, the optimal correction should be one-half the dark focus value.
However, other researchers have found that when drivers were refracted under simulated road lighting levels in order to find an appropriate night driving correction, only about one-third of the sample studied found the prescription helpful.
According to Leibowitz and Owens, there are two approaches to measure the patient’s dark focus, and hence to correct night myopia.
- Laser Spectacle Optometer
It is the best device to measure the dark focus. When light from a laser is reflected from a granular surface, a speckled pattern becomes visible. If a subject observing this pattern moves his head, the ‘speckles’ appear to move.
The ‘speckles’ will move in the same direction as the head movement if the subject is hyperopic; they will move opposite to the head movement if the subject is myopic for this distance.
The laser speckle optometer is simple to operate and allows for rapid, accurate measurements. These characteristics now allow larger groups of subjects to be studied quickly and efficiently. But these optometers are not available for commercial use. The feasible alternatives to these optometers would be laser Badal Optometers.
- Dark Retinoscopy
Ownes’s Dark Retinoscopy is a readily available approach in a clinical setting. The retinoscopy is performed at 25 to 100 cm in a completely darkened room and with the distance correction in place. We can use the following formula to calculate dark focus based on the dark retinoscopy value.
Dark Focus = (Dark Retinoscopy – 0.25D)/0.64
Now the night myopia glasses prescription can be determined by dividing the dark focus in half.
Due to the loss of accommodation during presbyopia, the dark focus shows a gradual reduction throughout life. If a presbyopic patient experiences night myopia, then ocular aberrations might be the cause rather than accommodation.
Other Factors to Consider
Glare is another major contributor to visual difficulties at night. A study found that when the eye and face are brightly illuminated and a target of low luminance is being viewed, the reflections from the back and front surfaces of the lenses can significantly degrade vision.
Visual acuity was measured under these conditions by recording the loss of contrast sensitivity. When an anti-reflective coat was applied to these same lenses, contrast sensitivity greatly increased.
Nighttime drivers also should consider the tilt of the windshield for a better visual experience at night driving. According to Waetjen and Schiefer, extreme windshield tilt should be avoided. Their study confirmed that an extreme windshield tilt can further reduce the performance of the eye in distance recognition during nighttime driving.
Furthermore, scratched or dirty windshields and any window tints should be avoided. Patients should be counseled to keep both their windshields and spectacle lenses clean when driving at night. Dust, film, and deposits can increase scatter and thus degrade images.
Broods and Borisch explain that every 10% of tint reduces a person’s distance vision by 10%. They report that with normal headlights, we can see 350 ft at night but by wearing a 10% tinted lens, a person’s distance vision is reduced to only 315 ft. Therefore, increased tint in spectacle or windshield will only degrade distance vision at night.
Accommodation is the major contributor to night myopia. Other factors such as chromatic and spherical aberration, Purkinje shift also play minor roles in nocturnal myopia, aberration being the sole cause of night myopia in presbyopia.
If presbyopic patients are suffering from night myopia (although it is rare), the best approaches to improving a presbyopic’s nighttime vision would include anti-reflective coats on spectacle lenses, careful cleaning of both spectacles, and windshields to minimize scattered light, and discourage the use of tints.
A measured minus lens correction derived from dark focus calculation can be of great benefit to a young patient suffering from night myopia although much more research needs to be given to this topic.
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