Acuity or quality as yardstick of success in cataract surgery
For many years Snellen visual acuity has been the principal yardstick of success in cataract surgery, but the past decade has seen the introduction of new psychophysical tests and visual quality and quality of life questionnaires that may provide further insight into patients visual experience. The question of whether these new measurements have any bearing on patient satisfaction was the topic of a debate held at the 17th ESCRS Winter Meeting. David Spalton FRCS, St. Thomas’ Hospital, London, UK argued that visual acuity remains the most important yardstick, although different patients may prefer different refractive outcomes. What matters most for the patient is uncorrected visual acuity. Quality has to be at the heart of everything we do but it must be understood from the perspective of the patient. Clinical measures do not always relate to how the patient feels, Dr Spalton commented.
There are a few golden rules in planning surgery that will result in patients having a satisfactory visual acuity for their lifestyle, he said. The first rule is never leave a patient hyperopic. The second rule is to never leave the patient with anisometropia for too long, by planning for the surgery for the second eye. The third rule is to talk to the patients to discover their visual needs. Many patients are entirely happy with emmetropia and readers, while some might prefer a small amount of residual myopia, and yet others might prefer monovision or multifocals. The problem with monovision is that it increases depth of focus at the expense of stereopsis. Patients need to know what to expect from monovision before surgery. Therefore, the best candidates are those who have undergone a contact lens trial. It is important to generally leave the non-dominant eye with no more than 1.5 D of myopia. In patients who undergo monovision surgery only to find that the resulting anisometropia is less satisfactory than they expected, piggy-back IOLs are a useful option and are preferable to IOL exchange. The lenses, available from Rayner, are easy to implant and sit in the sulcus. The main difficulty with the lenses is that they are very thin and fragile and therefore are easily torn on insertion.
Dr Spalton noted that about 20 per cent of patients can read without glasses after routine cataract surgery with monofocal IOLs. In a study that assessed the ocular and optical characteristics of 30 patients implanted with monofocal IOLs who had a visual acuity better than 6/12 for distance and N12 for near, the only significant correlation was with the high level of against-the-rule astigmatism, which had a mean value of 0.76 D, (Nanavaty et al J Cataract Refract Surg 2006 ; 32 : 1091-1097). That may be because Snellen charts are easiest to read when the vertical axis is in focus. Small amounts of astigmatism increase depth of focus for emmetropic patients. In an eye with a monofocal IOL, 0.75 D of astigmatism would increase the depth of focus for distant vision if it was with-the-rule, and increase depth of focus for near if it was against-the-rule, Dr Spalton said. In a similar way, spherical IOLs afford patients greater depth of focus than aspheric lenses, because of associated higher order aberrations.
Subtler aspects of vision also important
Taking the opposing view, Konrad Pesudovs MD, Flinders University, Bedford Park, South Australia, Australia, maintained that although visual acuity is important, it may not be the aspect of vision in which cataract surgery produces the greatest improvements. Vision is very complex and involves more than the ability to read Snellen acuity charts, Prof Pesudovs said. It also involves contrast sensitivity, colour vision, visual fields and binocularity and depth perception. In addition, there are other measures of visual function that are important such as ability to see under different lighting conditions.
Ultimately, the debate must therefore hinge on the question of whether testing these properties of vision measure the loss that occurs with cataracts and the improvement that cataract surgery can provide that are not predictable from visual acuity testing. There are a number of high-quality studies, randomised controlled trials and a Cochrane Review that demonstrate this point. However, what is interesting is that the impact of cataract on visual acuity is fairly small therefore the improvement that we see with cataract surgery is small. Visual quality questionnaires are another way of assessing visual outcomes that numerous studies have validated. Many will measure activity limitation and visual disability and there are some that also measure visual symptoms.
One of the most researched is the Catquest-9SF, which asks questions pertaining to specific aspects of vision. For example, it asks patients about their ability to read the paper, a near vision task. Another question concerns the patient’s ability to recognise faces, a contrast sensitivity task. Questionnaires can tap into these different realms of visual performance without actually having to measure and that makes them a strong tool for your practice, Prof Pesudovs added.