It is my understanding that the halos and glare experienced by many LASIK
(and PRK) patients is due to light entering the eye at (and beyond) the
edge of the ablated zone when the pupil is dialated beyond 6mm. (Assuming
a 6mm ablation zone.) This makes sense to me. However, I recently
learned that my soft toric contact lenses have an optical zone of only
about 7mm. My pupils happen to dialate well beyond 6mm, even in low
light, yet I get no halo or glare from my contacts. This night vision
impairment is one of the primary reasons I am currently staying away from
refractive surgery. So…two questions:
1). Why would I not get this effect from my CLs?
2). Is there any reason to think that the effect would be less with LASIK
than with PRK? (If so, I’d love to hear why.)
Thanks for your responses.
In article <19970414044200.AAA19…@ladder01.news.aol.com>
purnell…@aol.com (PurnellRob) writes:
> It is my understanding that the halos and glare experienced by many LASIK
> (and PRK) patients is due to light entering the eye at (and beyond) the
> edge of the ablated zone when the pupil is dialated beyond 6mm.
As a person who has researched LASIK and PRK, I have heard of several
causes for the halo and/or glare that they experience
1. Like you said, the pupil opens past the corrected area, and lets in
out-of-focus light.
2. There is a "haze" in the surface of the cornea. This is a
wound-healing response of the body which actually makes the cornea less
than transparent, sort of cloudy. From what I have heard, this is much
more of a problem with PRK than LASIK. This would be similiar to
wearing a dirty pair of contact lenses that needed to be enzymed.
3. During the healing process of LASIK, the edges of the flap randomly
refract light, and can cause scattering of the light. This usually
heals in a few months after the operation, and the edges of the flap
merge optically with the rest of the cornea. This problem would be
equivalent to if someone scratched a circle around the edge of a
contact lense.
4. There may be uncorrected or irregular astigmatism after the
operation which causes the halos and/or glare.
These problems had really scared me away from LASIK, until I found out
that problems 2 and 3 are completely gone after 6 months for 98+% of
LASIK recipients (note problem #2 hangs around in about 5% of PRK
patients; that is one reason I’m not getting PRK!). Problem 1, the
pupil opening past the ablation zone, just doesn’t seem to be noticed
by most people. Only 2-3% of patients complain about this problem, and
it is possible that they are really talking about problem #4. I think
that the post-processing that the brain does on the retinal image is
probably fairly good at eliminating the out-of-focus image from outside
the OZ, and keeping the in-focus image from inside the OZ.
An interesting study that someone should perform would be to correlate
post-refractive surgery problems with pupil dilation at night versus
the OZ size of pre-op contact lense that exhibits the same problem. So
with that data one could say "If you are able to comfortably wear a
Brand A contact lense with an OZ of X mm at night, then you probably
won’t have any problems with pupil dilation if you have LASIK with a
diameter of Y mm". As it is, there don’t seem to be any reliable tests
to figure out whether you will have problems with pupils opening past
ablation zone, until you actually undergo refractive surgery.
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In article <19970414044200.AAA19…@ladder01.news.aol.com>,
purnell…@aol.com (PurnellRob) wrote:
- Hide quoted text — Show quoted text -
> It is my understanding that the halos and glare experienced by many LASIK
> (and PRK) patients is due to light entering the eye at (and beyond) the
> edge of the ablated zone when the pupil is dialated beyond 6mm. (Assuming
> a 6mm ablation zone.) This makes sense to me. However, I recently
> learned that my soft toric contact lenses have an optical zone of only
> about 7mm. My pupils happen to dialate well beyond 6mm, even in low
> light, yet I get no halo or glare from my contacts. This night vision
> impairment is one of the primary reasons I am currently staying away from
> refractive surgery. So…two questions:
> 1). Why would I not get this effect from my CLs?
> 2). Is there any reason to think that the effect would be less with LASIK
> than with PRK? (If so, I’d love to hear why.)
> Thanks for your responses.
re: halos and glare after RK/PRK/LASIK
It’s not just the ‘ablation zone’. It is actually
a combination of scar formation and abnormal curvature
and thus refraction of the wounds.
For example, an RK with 4 incision scars using a 4
millimeter ‘clear’ central zone would see a starburst
of 4 points around bright light sources. This is permanent
as scar tissue never completely resolves. General glare
is associated with ‘retinal veil’ effect. That is, light
rays passing through the cornea are refracted inconsistently,
reflected back on the cornea, and back onto the retina, etc.
These ‘misbehaved’ rays of light produce the glare effect.
With PRK, the entire central optical zone (5mm or 6mm) is
disturbed. Since the resulting cornea structure –
epithelium/haze-scar layer/stroma – tends to scatter light,
glare would always be some problem. It tends to be
subjective with some postops complaining, others say it’s
not a problem. Likewise, at the edge of the wound ablation zone, more
scarring occurs, and the ‘rough’ transition to the unablated cornea
tissue – epithelium/Bowman’s Membrane/stroma – tends to
cause more intense glare and halos at night.
LASIK is similar with a more intense donut shaped scar
producing halos and glare.
As for why a 7mm CL does not produce halos or glare, you
are lucky. Pupil size at night varies widely from about
5mm to 8mm. Generally, you can add 1 to 2 mm to your
max pupil size to estimate your cornea ‘night zone’
for incoming light rays. It may
be that your contact lenses have an excellent transition
edge that does not produce enough glare or optical
aberrations (halos, starbursts) to bother you.
It’s a good idea to stay away from refractive surgery.
br2…@aol.com
I know why refractive surgeons wear glasses!
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>a 6mm ablation zone.) This makes sense to me. However, I recently
>learned that my soft toric contact lenses have an optical zone of only
>about 7mm. My pupils happen to dialate well beyond 6mm, even in low
>light, yet I get no halo or glare from my contacts. This night vision
>impairment is one of the primary reasons I am currently staying away from
>refractive surgery. So…two questions:
>1). Why would I not get this effect from my CLs?
>2). Is there any reason to think that the effect would be less with LASIK
>than with PRK? (If so, I’d love to hear why.)
Some of this effect can also be attributed to peripheral scarring.
LASIK treats deeper tissues, which are less prone to scarring. In the
next few weeks, there will be some detailed explanations published on
the physiological differences between PRK and LASIK (www.eyesite.net)
including some studies.
Dr. Rozanec
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