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020 7490 7222 Book an appointment
020 7490 7222 Book an appointment
We often measure the standard of vision achieved with or without glasses using the familiar eye test (Snellen) chart, which is viewed from a distance of 6 metres (20 feet). It should be noted that this is not a particularly accurate way of recording visual standards – other methods are used in laboratory and research settings. The high contrast lettering on a Snellen chart is also unrepresentative of more difficult visual tasks, such as driving at night or in conditions of poor visibility.
The top line on the chart is designated 6/60. The second line up from the bottom (on most charts) is 6/6 and represents ‘normal’ sight – commonly known as 20/20 vision, particularly in the USA, as 6 metres is equivalent to 20 feet. In the UK, the minimum standard of vision permissible for driving is between 6/9 and 6/12, equating to the ability to read a number plate from a distance of 20 metres (20.5 metres for older-style plates). The Royal Air Force requires its pilots to have an unaided vision of 6/5 in each eye (usually the bottom line on the chart).
LASEK (surface laser treatment with no flap) tends to be reserved for patients with higher myopic prescriptions and/or below average corneal thickness (a measurement known as pachymetry).
Although it takes much longer to settle down (and patients are advised to take one week off work) the results are very good indeed with around 85% of patients beyond -6 dioptres (limit around -10) achieving a final result within plus or minus 0.5 dioptre of emmetropia (zero prescription). It is therefore very unusual to have to repeat LASEK (less than 1% statistically).
The other group of patients who benefit from LASEK are those who take part in sports or occupations with significant physical contact (martial arts, competitive boxing, rugby, police, military, etc.). These patients will not necessarily have high prescriptions and, as can be seen from the above histograms, the accuracy of LASEK when treating more modest prescriptions is extremely high. The main drawbacks of LASEK remain the discomfort in the first few days following the surgery along with the delayed recovery time, with patients having to take one week off work.
There are a number of intraocular lenses (IOLs) available that can be tailored to suit a range of visual requirements.
The most frequently implanted lens is a monofocal IOL (standard IOL). This lens offers excellent image quality (especially in aspheric form) and is primarily used to achieve good distance vision. Monofocal lenses of different powers can be implanted so that one eye is corrected for distance and the other for close vision, helping the patient to minimise reliance on spectacles for reading and other close work. This is known as monovision. Some patients acclimatise to monovision more easily than others; in some cases reading glasses may still be preferred for fine print, and a distance prescription may help with some activities, particularly in poor light conditions.
Multifocal IOLs have grown in popularity in recent years allowing for an element of close work vision not present with standard monofocal lenses. The more modern designs have now overcome some of the limitations of earlier examples. These lenses can be well tolerated and can give reasonably good distance and close vision, with a greater depth of focus than monofocal IOLs. However, this is sometimes at the expense of image quality, and glare and haloes may be troublesome. We would not recommend multifocal IOLs for those who drive frequently at night, for example, or those with very exacting visual requirements. These lenses can also be used in the treatment of presbyopia (the loss of near vision as we move into our mid-40s).
Toric lens implants are for patients with moderate amounts of corneal astigmatism (more than 1 or 1.5 dioptres). Reducing your astigmatism will reduce your dependence upon glasses as compared to a non-toric IOL. A standard IOL can be set to make you either far sighted or near sighted but if you have astigmatism your vision without glasses will still be blurred to some degree because this astigmatism has not been corrected by the standard IOL. You could wear glasses in order to correct the astigmatism or undergo laser eye surgery at a later date (Bioptics). However, if it is desirable for you to have better vision when not wearing your glasses, you may want to consider a toric lens implant.
These lens implants are the same as the above Toric IOLs but have a +1.5 dioptre reading addition built in to aid near vision.
With cataract and refractive lens exchange surgery the eye’s natural lens is removed and replaced with an artificial implant. Some ophthalmic surgeons perform an alternative procedure – phakic implant surgery – where the natural lens remains in place while the phakic IOL is positioned immediately in front of it.
The combined focusing power of the two lenses can be effective at treating severe short and long-sight, and the artificial lens can also reduce or eliminate astigmatism. David Gartry does not perform phakic implant surgery, as it is a relatively new procedure and he is unconvinced as to its long-term effectiveness and safety. These lenses tend to be used more for younger patients who can still accommodate (focus for close work) who are contact lens intolerant and have high prescriptions. Beyond mid-40s a better option is refractive lens exchange using the new femtosecond laser technology. Phakic implants have been known to cause cataract and glaucoma and, since the technique is intraocular (placing a lens inside the eye), rarely serious problems such as infection or retinal detachment can follow. If at all possible David Gartry encourages these younger patients, who are well beyond the normal laser range, to persevere with contact lenses – perhaps seeking the advice of an experienced contact lens specialist.
Around 20% of patients who have cataract surgery will start to experience PCO (Posterior Capsular Opacification), a gradual clouding of their vision that can develop weeks, months or even several years after surgery.
This condition is treated with a procedure known as a YAG laser capsulotomy, where a laser is used to remove the clouded-over part of the lens capsule to allow a pathway for the light rays to focus on the retina, restoring visual clarity.
