Every lens extraction patient or presbyopic patient eligible for refractive surgery poses a doctor with the challenge of delivering
them a full range of vision with a lack of accommodative apparatus.
Multifocal or the currently available accommodative intraocular lenses are not a perfect solution for everyone because of
disturbing visual phenomena1 or an insufficient range of accommodation. Multifocality obtained at the corneal plane suffers the same drawbacks as multifocality
at the lens plane. Monovision is a technique where the dominant eye is surgically targeted for the preferred distance of vision
(in general infinity, but it could be a different target, e.g. –0, 50 for TV viewers) and the non-dominant eye is targeted
for near. The brain then provides suppression of the blurred image coming from the non-focused eye.
Two important general rules govern monovision:- Suppression is easier in the non-dominant eye
- The sharper one image the easier the suppression of the blurred one2
Disadvantages of monovision are2:- Insufficient depth of focus if the anisometropia is less than 2 D
- Perceptible loss of stereo acuity if anisometropia exceeds 2 D
- Blurry distance vision or inability to read if adequate suppression of the other image does not take place
The distinct advantage of monovision is that the anisometropia is temporarily reversible with a normal pair of spectacles
used for particular tasks and postoperatively titrable with a laser retouch or lens exchange.
With careful patient selection, the degree of satisfaction with monovision is very high3,4 but in order to be at its best, it has to be designed and delivered in a certain way.
My general approach
Firstly, always discuss the various options for presbyopia compensation with your presbyopic or pre-presbyopic patients. It
may not be obvious but this really is the first step and it should be taken with every over-forty patient who walks into your
office asking for surgery. It's also not always obvious to aim for bilateral emmetropia in cataract surgery. Whilst it may
be the best option in a far-oriented, avid golfer, the more frequent scenario is that of a patient who has developed cataract-induced
myopia and, while suffering from the loss of sharpness for far, would complain bitterly about any post-operative loss of ability
to read without glasses. Moreover, be assured that your 36-year old, minus two diopter patient, has always been used to reading
with his or her glasses on before dialing –2 into your excimer laser. Very often these patients have developed the habit of
taking off their glasses as soon as they do not have to concentrate for far. They soon develop a habit-induced early presbyopia
that is difficult to eradicate post-op. A contact lens trial with full correction of the myopic error is mandatory in this
case and will uncover any need for addressing presbyopia even if the patient's age is not in itself a red flag indicator.
Ophthalmologist turned detective
As a doctor it can sometimes pay dividends to investigate a patient's circumstances a little further. For example, a patient
of mine, a secondary school teacher, with a refractive multifocal IOL implantation, came into my office saying she was unable
to read at all whilst at work. She had to admit that she was able to read fluently J1 at the intermediate level of luminance
from the examination chair in my office but insisted she was near blind whilst at school. What we discovered was that the
position of her desk, by a very large window, meant that on sunny days, (very common in my country) the very bright light
forced her to work in a counter-light environment so that her pupil constricted to shield out the near portion of the lens.
Monovision would have been a better option for that patient.
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