NIGHT MYOPIA - is a tendency for eyes to become near-sighted in dim illumination. There are multiple theories explaining the phenomenon, most of them are related to accommodation or chromatic shift in scotopic light conditions when to light is focused before the fovea thus worsening the existing myopia.
In dark adaptation the eye becomes more sensitive to shorter wave
lengths (Purkinje shift- In the light-adapted eye the region of
maximal brightness is in the yellow; in the dark adapted eye, the
region of maximal brightness is in the green),and visual acuity
depends on parafoveal blue cones. Shorter wavelengths come into focus
in front of the retina, and this chromatic aberration accounts for
some of the relative myopia that a normal eye experiences at night;
much of the remainder is due to an increase in accommodative tone in
the dark. It is also called dark focus of accommodation, or tonic
accommodation, or resting state of accommodation. Consequently, a
measurement of the refraction (i.e. the determination of appropriate
optical glasses to obtain good vision on these distant objects) gives
the result that the eye is myopic. Night myopia can reach values of up
to about - 4.0 D. Night myopia decreases with age31 and therefore it
can be of special importance for young drivers at night. In one
experiment with people aged 16 to 25 years, 38% had night myopia of -
0.75 D or more, and 4% had – 2.50 D or more. Cause of Night Myopia:
Spherical aberration (The optical effect of the larger pupil decreases
the depth of focus) Chromatic aberration. Parkinje shift Ciliary spasm
At low light levels there is a change of the biochemistry of imaging
on the retina: in bright light the receptors in the retina are cones
with higher image acuity, and imaging of colors, whereas in low light
the receptors in the retina are rods with reduced image acuity, and
imaging of black and white only. The transition between both states,
i.e. to get maximum sensitivity when moving from the bright to the
dark, doesn't happen immediately, but takes some minutes33. This is a
problem e.g. when driving a car in sunlight and entering a tunnel. The
low light imaging by rods in the retina can be further reduced by a
lack of vitamin A and zinc Symptoms:
1. Blur vision only in low luminance.
2. Feeling discomfort while maintaining fixation in low illumination level.
3. Difficulty in night driving.
4. Hallows around light.