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....Longsight - eyeball is too short

Treating longsight (Hyperopia) is more problematical than treating myopia for a number of reasons:

1. How much can be treated?

In hyperopia, the laser has to steepen the central cornea as shown in this diagram: (click to enlarge)

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 47 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.

Dec 2001