Presbyopia & Monovision

The need for reading glasses is a common problem for most people as they enter their middle years. As people age, the natural lens in their eyes hardens and loses its flexibility, impairing its ability to vary its shape for different ranges of focus. This condition is known as presbyopia and develops in most people in between 45 and 50 years of age.

There are a number of ways to deal with the focusing problems caused by presbyopia. People with a small amount of nearsightedness can simply remove their glasses to read. However, people with previously normal vision, or those who wear contact lenses for nearsightedness may need to use reading glasses for close work such as reading, using a computer, or sewing. Bifocals  or varifocals can also be used to provide both near and far vision without having to constantly put on and take off a pair of glasses or repeatedly switch back and forth between two pairs of glasses. However, some people find it difficult to adjust to these glasses and others consider reading glasses to be an inconvenience. Another option, known as monovision, is available for some people with presbyopia.

Monovision can be achieved through corrective lenses or through refractive surgery. If a person has less than 1.5D  of nearsightedness ,one eye can be surgically corrected to provide good distance vision, and the other eye can be left uncorrected for near vision. People with greater amounts of nearsightedness may have one eye corrected for distance vision, and the other eye undercorrected to provide better close vision. If this option seems desirable, people may wish to consider trying to achieve the similar effect with contact lenses prior to surgery to determine its suitability for their individual needs and their ability to adapt to this situation. People who are zero or "plano" can also have one eye made a little short-sighted to achieve the same result.

The chief advantage of monovision is the freedom it provides from reading glasses. Monovision makes it possible to repeatedly change the range, of focus, without having to constantly remove or add corrective lenses. This can be particularly useful for people who change their focus frequently - particularly teachers, public speakers, salespersons, and people involved in the performing arts.

As with any good thing, monovision comes with some drawbacks. People with monovision may have decreased depth perception without corrective lenses. They may also notice blurred vision in the "near" eye when glancing in the side mirror of their cars or when the, vision in the "distance" eye is blocked by an object. Some people with monovision elect to wear corrective lenses for activities such as driving or prolonged reading so that both eyes are then in focus. If the eyes have less than 1.5 Dioptres between them, then there is not usually a problem in the older patient.

Monovision is most appropriate, for people who answer "yes" to two or more of the following questions:

  • Would it bother me to wear reading glasses and carry them wherever 1 go?
  • Does my lifestyle permit a slight impairment of depth perception for many activities?
  • When I require reading glasses, would 1 need to wear them most of the time?
  • Could I adapt to one eye being out of focus for distances unless glasses are worn?

People who are entering mid-life, and are interested in monovision should discuss the matter with their doctor prior to undergoing refractive surgery. Should they choose monovision and subsequently become unhappy with it, enhancement surgery is an option.


Both Eyes On The Same Day?

Should you have LASIK or LASEK on both eyes on the same day?

The norm in the USA and in much of Europe is to do LASIK on both eyes at the same sitting. i.e. treating both eyes whilst the patient is still on the operating table. There is obviously a lot of financial sense in this for the various clinics as it increases their throughput of eyes treated and hence their profits. However, what is best for the individual patient? The arguments are in 2 groups:

1. Safety:

It goes without saying that if, in a planned bilateral operation, there is any intraoperative complication in the first eye then the second should be abandoned. In practice this usually means any flap problems. I would also include any marked epithelial loss or "slide" due to the passage of the microkeratome. This is because the incidence of the "sands of the Sahara" syndrome is 10 times higher in these cases and will not present until day 1 after the surgery if it occurs. This is a rare occurrence with modern microkeratomes and femtosecond laser.s    Any infection, inflammation, flap wrinkles etc. almost always show up on day 1 post op and by waiting 2-7 days between eyes one should avoid a simultaneous bilateral problem. However, these are unusual and most people opt for same day bilateral surgery.

The commonest complication with surface laser (LASEK) used to be "haze". This is a deposition of material under the epithelium that occurred about 4 weeks after surgery.. We now prevent this from occurring by using dilute Mitomycin-C for about 10-15 secs on a sponge during the surgery.

LASIK/PRK is dependant on the technology of the excimer laser machine. These are gas lasers and are calibrated at the beginning of each day and before every patient. They are complicated machines but are generally reliable and have a lot of safety features. However, if there is a technical problem and both eyes are treated, it could affect both eyes adversely without the surgeon being aware of it at the time of surgery. The most likely fault could be a simple over or under treatment which could be corrected fairly easily in most cases.

2. Accuracy:

Both eyes tend to behave the same way when lasered. Hence, in theory, it is more accurate to do one eye at a time, so that, if the first eye behaves in an unusual way, then you can modify the laser settings on the second. However, in practice, this is generally such a small advantage that most people tend to have both eyes on the same day.  Also, I have seen a number of cases where I have treated both eyes on the same day and they have different results. This must only mean that each eye has a sort of unique microenvironment that responds to the laser energy and heals in its own particular way.

3. Speed of recovery:

High myopic prescriptions, over about -8D, take longer for clear vision to develop with surface laser (LASEK). Hence, in these patients, it is sometimes better to wait a couple of weeks between eyes so that the patient does not have both eyes blurred for a few days.

LASIK or LASEK

Many patients can have either LASIK or LASEK and want help to decide which one to have.

Essentially, the 2 operations achieve the same results and cover the same range of prescriptions (+4 to -8 approx.)

However, LASIK is a nicer surgical experience for the patient as there is quick visual recovery and no pain apart from a little grittiness.

Fairly clear vision is achieved from about 3 to 4 hours post op and both eyes can be treated on the same day even in high prescriptions. On the downside, there is more risk than any variety of surface laser (PRK. LASEK, Epiflap, Epi-LASIK) as in LASIK a flap is cut with a mechanical microkeratome or with a femtosecond laser.

Most LASIK complications are flap complications and there is the same risk in treating a small prescription such as a -1 and a large one such as -10. The cut edge of the flap is covered over by epithelium by the next day but the interface always has a potential space. It is useful to think of the analogy of a self-sealing envelope flap - you throw it in the post and it travels across the country without falling to bits. However, at the other end one can lift up the stuck flap with a letter-opener. Although we surgeons find it quite easy, in the first few months especially, to peel open a flap with a similar sort of "letter-opener" , in real life this seems to happen only infrequently. It really requires a direct oblique physical blow onto the eyeball to dislodge the flap.

However, we advise patients who indulge in physically risky situations to have surface laser rather then LASIK. Such would be rugby players, boxers, karate etc. The American army now accepts both LASIK and LASEK. Recently astronauts have been allowed to have LASIK with the flap cut with a femtosecond laser. It is not known if a femtosecond flap is more robust than one cut with a mechanical microkeratome.  A femtosecond flap with a backward cut edge might well be the strongest option and this is just coming into normal practice.

LASEK has a slower visual recovery and is also more painful. Pain is a lot less than it used to be with PRK because of the epiflap technique, silicone contact lenses post op, anaesthetic drops and better oral medication. Most patients have some pain the first few hours for days 2 and 3 usually just a little grittiness and photophobia. We give patients anaesthetic drops to use during this period. Patients will see at day 1 post op with LASIK what they will see at day 4-5 with LASEK, but at 1-2 weeks post op there is no difference in vision even with large prescriptions. In fact there is a quicker recovery of low light vision with surface laser than LASIK.

LASEK patients are back to the baseline pre-operative values by 1 month post-op, whereas with LASIK this normally takes up to 3 months and occasionally longer. You need longer off work with LASEK than LASIK as the special "bandage" contact lenses come out at 3 to 4 days post op. If I operate on a Friday, then the contact lenses come out on Monday and the patients are back to work on Tuesday or Wednesday with a small prescription. The bigger the initial prescription, the slower the visual recovery. It is reasonable to do a bilateral same day LASEK up to about -5. For higher prescriptions than this, I usually do one eye at a time unless the patient has got a week free. One is not blind during this time but may not be up to driving standard.

LASEK has less risk that LASIK because there is no flap cut. However, complications are rare with either operation. LASIK flap complications are around 1 in 1000. The major risk with LASEK used to be "haze", but we now use prophylactic Mitomycin-C during surgery, which has almost completely eliminated this problem. LASIK seems to be more prone to causing dry eye problems than LASEK.
 
For longsight (hyperopia), it is normally better to have LASIK.


Spherical Aberration

Spherical Aberration ImageA spherical surface has a "Q value" of 0. A surface which is a parabola has the peripheral part of the lens relatively flatter than the centre and so bends the peripheral light rays less, eliminating this spherical aberration. Such a cornea has a negative Q value and has a prolate shape A parabola has a Q value of  -0.5. 

The human eye of a young person has a Q value of -0.5, which is made up of the cornea (Q= -0.25) and the lens of the eye (Q= -0.25) added up together. The over 40y age group has a rounding out of the lens, so its Q value becomes near O. Hence older people have more natural spherical aberration as their Q value is only that of the cornea i.e. -0.25.

There is a nice demonstration of spherical aberration at the Olympus web site. The Hubble Space Telescope suffered from spherical aberration when first launched. This was solved by using "adaptive optics", similar to that now being used in excimer lasers. See the Hubble page on this web site.

Early lasers had errors in their mathematical algorithms and also in their physical optics, After a myopic laser treatment the Q value became positive with increased spherical aberration. The cornea then had an oblate shape No normal human cornea is oblate or has a positive Q value.

Modern lasers have adjustments to put more shots in the periphery and so preserve this prolate shape. Most of the value of so called "wavefront" treatments is  that they keep the cornea prolate.


Spherical aberration is one of the most important problems that can occur after laser eye surgery, in particular with high myopic corrections.

For lenses made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point.  The peripheral light rays are bent more than the central ones as in the following diagram:



A spherical surface has a "Q value" of 0. A surface which is a parabola has the peripheral part of the lens relatively flatter than the centre and so bends the peripheral light rays less, eliminating this spherical aberration. Such a cornea has a negative Q value and has a prolate shape A parabola has a Q value of  -0.5.  The human eye of a young person has a Q value of -0.5, which is made up of the cornea (Q= -0.25) and the lens of the eye (Q= -0.25) added up together. The over 40y age group has a rounding out of the lens, so its Q value becomes near O. Hence older people have more natural spherical aberration as their Q value is only that of the cornea i.e. -0.25.

There is a nice demonstration of spherical aberration at the Olympus web site. The Hubble Space Telescope suffered from spherical aberration when first launched. This was solved by using "adaptive optics", similar to that now being used in excimer lasers. See the Hubble page on this web site.

After a myopic PRK or LASIK, the Q value becomes positive with increased spherical aberration. The cornea then has an oblate shape No normal human cornea is oblate or has a positive Q value. However, all the modern lasers have "blend zones" that smooth off the mid-peripheral "knee" that has a high local Q value and this lessens the induced spherical aberration. e.g. The Technolas 217 laser has true optical zones up to 7mm with a  blend zone at least 3mm bigger than this. (the cornea is only about 12.5mm diameter). Similarly the Nidek EC5000 has optical zone up to 6.5mm and the blend zone is adjustable up to 10mm.

Spherical aberration is not really a problem with low myopic corrections but can be a problem with some patients having higher corrections e.g. about about -5 D. The laser manufacturers are tying to improve the shape of the ablation profile to lessen this problem. All the "custom ablations" done by various lasers have totally "aspheric" profiles that have, in theory, no aberrations. However, they can take off more tissue, which can again be a problem with higher corrections as there may not be much to spare. Spherical aberration is not normally a problem in good light but in low light. See night vision and lasik complications

Laser Technical Details

Excited Dimer = Excimer

The excimer laser is an ultraviolet Argon-Fluoride laser at 193 nano metres (nm) It is a gas laser and requires refilling on a regular basis. The argon is an inert gas whereas the fluorine is highly excitable.

Photo 1: The excimer laser has a photon energy of 6.4 electron volts. The carbon-carbon links of the corneal structures are 3.5 electron volts. Each laser pulse lasts 18 nano seconds and takes off about 0.25 microns of corneal tissue with each pulse ( 1000 microns in 1 mm).

Photo 2: Hence, when the laser hits the cornea, it "blows apart" the carbon-carbon links and the noise that one hears is the particles from the cornea breaking the sound barrier as they come off the cornea in a mushroom cloud shape similar to an atomic bomb.

Photo 3: For a -3 Dioptre laser correction, the laser will take off about 40 microns. (The average cornea is about 540mm thick) Most modern lasers these days use a so- called "flying spot" At present I use the WaveLight 400 Hz laser which fires at 400 times a second. This enables one to sculpt fine details for more customised treatments. The fact that all the energy of the ablation is taken off the cornea in this supersonic plume means that the temperature of the cornea hardly rises at all and this means that there is virtually no "collateral damage" due to thermal effects on adjacent tissue. Hence the excimer laser gives a very keen cut. Below is a lasered hair of Professor Marshall from St Thomas's Hospital in London, who did some of the pioneer work with Professor Steven Trokel, the inventor of PRK.

Photo 4: This is a graph of corneal temperature change during a 20 second ablation of a human cornea. Normal corneal temperature is about 30-33 C and it rises to about 37 C, which is blood heat.

Photo 5: Finally, a cartoon sent by an optometrist friend of mine!