Because the curves are steeper than for myopia and because
there are more transition points, one can only attempt about a maximum of about
+4 Dioptres in any one axis. More can be done, but it is less accurate and
there can be a lot of optical side effects and regression of effect as detailed
below. The average corneal steepness is about 43-44 Dioptres and you cannot
steepen it beyond about 49 Dioptres. This is because the cornea then has too
much of a steep central cone and is a sort of iatrogenic Keratoconus.
(Keratoconus is a corneal disease where the cornea becomes very steep. Such a
cornea has poor optical performance).
2. Age and accommodation:
People who are longsighted have to focus to
see in the distance and focus more to see close to. This is done by contracting
the ciliary muscle inside the eye to make the lens fatter. As we get older, the
lens gets less elastic and it becomes harder to focus. This is the normal aging
phenomenon of presbyopia and happens to everyone. For a person who is emmetropic
("zero" - being neither short or longsighted), this will mean wearing
reading glasses. For someone who is short-sighted it means you have to take off
your distance glasses to read. Finally, for a longsighted person it will first
of all become blurred for near objects and then also for distance as he/she
becomes older. Eventually a hyperopic person will need glasses all the
time, for distance and near. Even a younger person will have poorer vision when
they are tired if they are more than just a little longsighted.
Hence the people who present for hyperopic eye surgery tend to
be older, usually in their 40's, as this is the age group who are having
problems. Younger patients often have some "latent hyperopia" -
i.e. are more longsighted than they think because the eye is held in a state of
accommodative tone all the time. This can lead to problems as to how much to
treat. Someone may have 2 dioptres of "manifest hyperopia",
which is what their spectacles will correct, but may have another 2 dioptres of
latent hyperopia on top of this. If the surgeon corrects all the 4 dioptres,
then a younger patient may be unable to relax the eye sufficiently and may think
they are now short-sighted and be unhappy. However, if the surgeon corrects only
2 dioptres then as the patient gets older and/or more tired, then they will
complain that they have been under-treated. Hence how much to correct can be
difficult to know. With younger patients many surgeons treat the manifest
hyperopia plus half of the latent hyperopia on top of this. This leaves the
person with a bit of accommodative tone which they are used to having all their
life.
After one eye is treated, there can be problems of
accommodative imbalance between the two eyes and this can lead to vision
blurring intermittently in either eye and headaches. This is usually cured by
treating the second eye or altering the glasses if only one eye is treated.
Hence younger hyperopes should have more caution before having LASIK or PRK and
personally I tend to only treat patients over 40 with smaller
prescriptions. Older patients have little or no latent hyperopia and tend not to
have these problems.
3. Night vision problems:
The optical zones for most lasers are 6.5 to 7.00 mm
diameter for longsight and the transition or "blend" zones are 9 -
10mm. Pupils get smaller with age and are often only 3 to 4mm in diameter. I
tell patients that there are 2 good things about getting older - we get wiser
and we get smaller pupils! Small pupils give a "pin-hole camera"
effect and are more forgiving for night vision. However, as we get older, the
lens inside our eyes gets more spherical and we have more spherical aberration.
This is one of the reasons why older people do not like night driving, and why
modern intraocular lenses are made with a prolate shape to mimic the shape the
lens had when we were 20.
4. Induced astigmatism:
Centration is more important in treating hyperopia than
myopia as the centre is left untreated. If the laser treatment is slightly off
centre or if there is too much "wobble", then there will be some
induced astigmatism as one side of the cornea is steepened more than the other.
To help in centration the surgeon can use a laser with an eye tracker or just
hold the eye during surgery with a small fixation device. There is still a
discussion about where to centre the laser for longsight as some patients have a
visual axis a bit off centre towards the nose. However, with the larger modern
optical zones this is not a major problem in most patients.
5. Dry eyes:
As the cornea is left steeper in the centre, then the
"top of the mountain" has more of a tendency to get dried out. This
degrades the vision as we need a good tear film to see well. The upper eyelid is
initially like a "bent windscreen wiper" and misses the apex of the
cornea slightly. After a few weeks the eyelid molds itself to the new shape of
the cornea and the tear film improves. The eye is also relatively dry initially
as the corneal nerves have been cut by the microkeratome and these take about 12
weeks to re-grow. Both these problems happen in myopia but tend to be worse with
hyperopia. The average age of the patients is also older and our tear film is
often a bit worse as we age. The patient should use artificial tears copiously
if this is a problem. In very severe cases one can put in silicone plugs into
the draining punctae of the lower lids. This is simple to do and they can be
removed. However, they do cost about £25 each.
6 Minification of the image:
Hyperopic glasses make the image size bigger, whereas contact
lenses or refractive surgery do not. Hence after LASIK or PRK for longsight, the
image is smaller and spread over less retinal receptors than with glasses. This
can lead to some loss of sharpness of image. (The opposite is true for myopia).
On the positive side, contact lenses/LASIK reduce the peripheral aberrations
that occur with glasses.
7. Regression:
There is more tendency to regression with treating hyperopia.
Surgeons and laser manufacturers usually allow for this in their
algorithms and initially overtreat a little. Most regression will occur in the
first week. I warn patients that they will be a bit short sighted initially for
up to a couple of weeks.
8. Reading:
One great bonus with hyperopic lasik is that the
"hyperprolate" shape (more steep in the periphery), means that there
is some boost in reading ability. For a given prescription, there is about 0.5D
of extra reading ability over a spectacle lens of the same prescription. This is
a big help for this group of older patients.
İStephen J Doyle
Oct
2008
