> > Sandy, about the eye pressure. On my right eye, I have Lasik surgery
> > over previous RK. From morning to about 8:00pm or 9:00pm my right
> > eye has the best vision, but later than that the vision is not as
> > clear as in the morning. My eye doctor told me that it has something
> > to do with the eye pressure which changes from morning to evening.
> > And because I have previous RK, when the eye pressure changes in the
> > evening the shape of my cornea changes, therefore the lights entering
> > through those RK cuts changes in the evening resulting not as good
> > vision as in the day time.
> I'll give it a try.
> The centermost area of the cornea (about 4.0mm) has the best vision
> for most people. This is also the area untouched by your RK
> incisions. The periphery of the cornea tends to have less clear
> vision, and in your case has the RK incisions that distort light
> passing near or through them.
> Not all light passing through your cornea hits the retina and is
> "seen". Some of that light is blocked by the iris. If the pupil is
> small, light passing through the distorted periphery of the cornea is
> blocked by the iris and is not "seen".
> The cornea is not flat, but spherical like a globe. If a straight
> beam of light enters the eye in the center of the cornea, it pretty
> much stays straight all the way through the eye to the retina. If a
> straight beam of light enters the eye at the periphery of the cornea,
> the spherical shape of the cornea bends that light toward the center
> of the retina to be "seen" along with the rays entering the eye from
> the center of the cornea. Combined, these rays provide central and
> peripheral vision.
> The interior of the eye maintains pressure pressing out. If not, the
> eye would collapse from the barometric pressure of the atmosphere
> pushing inward on the outside of the eye. The pressure inside the eye
> (intraocular pressure, IOP) equalizes with the pressure outside the
> eye, thus the eye is relatively stable.
> Combining these issues, you can get your poor night vision effect. If
> the IOP raises, it can press outward on the cornea, causing it to
> change shape. Because the cornea tends to be thinner in the center
> and thicker in the periphery, this could make the center of the cornea
> bulge outward more than the periphery, changing the angle of the light
> rays entering the eye at the periphery of the cornea. In normal IOP,
> the angle of the straight light rays being bent into the eye at the
> perpiphery of the cornea is less, and they hit the iris which blocks
> them from being "seen" by the retina. With high IOP the angle is more
> significant and more toward the middle of the eye. Light rays at the
> edge of the cornea now pass by the iris and through the pupil into the
> eye, thus being "seen".
> In evening, light tends to be lower and the pupil tends to expand.
> The larger pupil allows light entering from the periphery of the
> cornea to pass by the iris, through the pupil, and into the eye to be
> "seen" by the retina.
> The combination of the change in the angle of the light rays passing
> through the distorted periphery of the cornea so they are able to pass
> through the pupil instead of being blocked by the iris and the pupil
> becoming larger in low light environments may be causing the poor
> vision in the evening hours due to light from the distorted periphery
> of your cornea being able to reach the retina to be "seen"..
> This is probably an oversimplification of what is actually happening.
> In addition to this is fatigue by the end of the day, change in
> accommodation due to activities in the evening, dehydration, and other
> factors.
> Glenn Hagele
> Executive Director
> Council for Refractive Surgery Quality Assurance
> http://www.USAeyes.org
> http://www.ComplicatedEyes.org
> glenn dot hagele at usaeyes dot org
> I am not a doctor.

> Sandy, about the eye pressure. On my right eye, I have Lasik surgery
> over previous RK. From morning to about 8:00pm or 9:00pm my right
> eye has the best vision, but later than that the vision is not as
> clear as in the morning. My eye doctor told me that it has something
> to do with the eye pressure which changes from morning to evening.
> And because I have previous RK, when the eye pressure changes in the
> evening the shape of my cornea changes, therefore the lights entering
> through those RK cuts changes in the evening resulting not as good
> vision as in the day time.

> Hi Jack,
> LASIK can affect your normal eye pressure measurements and they may be
> lower than normal, especially at the center of your corneas. Have
> your doctor take some readings around the periphery as well.