Recent advances in customising cataract surgery

Introduction

Cataract is an important cause of visual impairment worldwide. In the United Kingdom, 30% of people aged over 65 have visually impairing cataract (that is, Snellen visual acuity of less than 6/12 attributable to a lens opacity) in one eye or both eyes.1 (See table A on bmj.com for risk factors.) A visual acuity of 6/12 is below the legal vision requirement to drive in the United Kingdom, which approximates to a visual acuity of 6/10, and evidence indicates that cataract surgery may even decrease the incidence of road traffic crashes among people over 65.2 The NHS does approximately 200 000 cataract operations annually, making this one of the most common surgical procedures in the country.3 Despite this, 88% of people with treatable visual impairment from cataract are not in contact with any eye healthcare services, which represents a very large potential healthcare need.1

The fundamental aim of cataract surgery, the removal of the opacified natural lens to improve vision, has remained the same for hundreds of years. However, the way in which this is achieved and the expectations of the people having the surgery have altered drastically (table 1). Here we give an overview of some of the advances that allow cataract surgery to be customised for each patient. We discuss the process that patients go through from their first meeting with an ophthalmologist, through the preoperative assessment, to the actual surgery itself and beyond to the appraisal of the results of that surgery. The visual potential of the patient can be realised in many different ways, and when looked at in this context very few cataract operations are the same.

Sources and selection criteria

Most of the information used in this review came from our day to day experience as surgeons doing cataract extractions. We identified the papers used in the review by a computerised literature search of the PubMed database with the keywords phacoemulsification, extracapsular cataract extraction, intraocular lens, and biometry. We gathered other information from lens manufacturers' data sheets.

Discussion of expectations and risks

Having a cataract is not in itself an indication for surgery. In general, if people are still able to do all the things they enjoy with their vision then surgery is not needed. Informed consent is vital to allow patients to assess the risk for themselves. One particular indication is achieving the legal vision requirement to drive.

Preoperative assessment

Most cataract extractions can be done under local anaesthetic, injected or applied topically as drops, and as a day case. However, attention still needs to be paid to the patient's general medical condition, and this requires multidisciplinary cooperation.

Operative considerations

Astigmatism is a condition in which the surface of the cornea is not spherical but is irregularly shaped like the back of a spoon. This prevents light images from being focused clearly, causing a blurred image. An incision in the cornea causes a change in its shape. It relaxes the curvature, making the cornea flatter. In cataract surgery the smaller the corneal wound the less flattening (astigmatic change) is produced by the incision. Large incisions heal with added tissue, which leads to considerable flattening of the cornea in that axis, causing substantial astigmatism.11 With the advent of phacoemulsification and foldable intraocular lenses, small clear corneal incisions became practicable. Incisions of less than 3 mm, if prepared correctly, can be astigmatically neutral (that is, not flatten the natural corneal curvature).12

The surgeon has several options for reducing or even attempting to eradicate a person's pre-existing astigmatism. The position and size of the corneal incision can be varied to counteract the patient's preoperative astigmatism. Alternatively, the shape of the cornea can be deliberately changed by making partial thickness relaxing incisions (cuts to allow the cornea to gape slightly), called astigmatic keratotomies, in it (fig 1).

Fig 1

Schematic diagram of astigmatic keratotomies

• Download figure
• Open in new tab
• Download powerpoint

Lens implantation

The first intraocular lenses needed a relatively large incision (about 6 mm) to get them into the eye. Modern lens technology has advanced to the development of foldable silicone and acrylic lenses that can be placed in the eye through very small incisions that are self sealing and do not even need suturing (fig 2).

Fig 2

Implantation of the lens

• Download figure
• Open in new tab
• Download powerpoint

Standard intraocular lenses are monofocal—that is, focused in one position (usually distance)—but patients still need glasses for near and intermediate distances. The development of multifocal intraocular lenses offers a further option for patients (fig 3). The lenses are made up of multiple concentric rings that focus for either near or distance vision. This allows the eye to be in focus for near and far simultaneously, and the brain decides which image it will ignore. The use of this sort of lens needs extremely accurate biometry for the patient to be able to benefit from unaided near and distance vision. As the whole lens is not devoted to the generation of each image, the quality of the vision is inevitably somewhat reduced compared with a standard monofocal lens, and the risk of halos and glare at night also exists.

An advance that overcomes these problems is the accommodating intraocular lens, which can move in the same way as the natural crystalline lens does before it hardens with age. Another type of intraocular lens available is the toric lens, which is shaped in such away as to overcome astigmatism without the need to resort to surgical manipulation of the cornea (box 2).

Position of lens

Depending on the complexity of the case, several different options exist for where to place the intraocular lens within the eye (box 3). The posterior chamber is the ideal position for the lens in terms of both optical performance and potential complications.

Postoperative assessment

Patients often notice a useful improvement in their vision the day after phacoemulsification, compared with several weeks in those having large incision surgery. However, before the final outcome can be ascertained the eye must be allowed to settle down for three to four weeks. An optometrist in the community usually obtains the final correction with glasses, but this information is vital to monitor outcomes. The postoperative assessment is also vital for the diagnosis and management of potential complications resulting from the surgery (table C on bmj.com).

Fine tuning

The cataract surgery is not necessarily the end of the story. The surgery itself may go very well and the patient may be capable of seeing very clearly but only with an appropriate correction by glasses. If a patient gets an unexpected postoperative glasses error, many things can be done to improve matters (box 4). If the error is very large, usually as a result of inaccurate biometry, the best option may be to proceed straight to a lens exchange or to implant a second “piggyback” lens.

Corneal refractive surgery provides another option. Photorefractive keratectomy, laser assisted epithelial keratomileusis, and laser in situ keratomileusis are different techniques that use an excimer (excited dimer) laser to change the shape of the central cornea and induce a change in the glasses correction of the eye. Astigmatic keratotomies may also be of use after the operation in situations in which unexpected or unacceptable postoperative corneal astigmatism occurs. All of the above treatments are available on the NHS, although postoperative laser refractive surgery is available only at a few specialist centres owing to lack of availability of equipment and experience of surgeons.

Audit

Postoperative data must be constantly audited to ensure that appropriate standards are maintained and also to identify areas for potential improvement. Computerised databases can be of great use in this sort of audit, especially when integrated into the day to day running of a unit. For example, we can identify whether the preoperative measurements are continuing to be taken accurately enough and whether our incisions are causing excessive flattening of the corneas.

What is next?

Commonly implanted intraocular lenses block or reduce the transmission of light only in the low frequency ultraviolet spectrum. A new lens is now being evaluated that also blocks the “higher” frequency blue light (up to 450 nm) that may damage the retina and accelerate the development of age related macular degeneration.

Corneal incision sizes are continuing to reduce through a combination of advances in ultrasonic probes and design of intraocular lenses. New phacoemulsification probes and intraocular lens injection devices can be used in 1 mm corneal incisions, reducing the astigmatic impact on the cornea of the incision and the chance of wound leakage.

The material science of intraocular lens technology has also advanced. New lenses are becoming available that can cancel out some of the optical aberrations occurring naturally in the eye (for example, the positive spherical aberration of the cornea) to enhance contrast sensitivity for improved visual function.

Another approach that may come into common practice is two stage cataract surgery. The first stage would involve standard cataract surgery, and a second stage refractive procedure would then use a wavefront guided excimer laser. Wavefront technology now gives us the ability to map the higher optical aberrations of the eye accurately. These aberrations and any residual postoperative glasses errors would then be neutralised by accurately adjusting the shape of the cornea with wavefront guided excimer laser treatment.

Intraocular lenses of which the focusing strength can be adjusted after cataract surgery have also been developed. The lens is composed of a material whose shape can be changed by the application of the appropriate wavelength of light. Variables in the healing process and errors arising from inaccurate biometry or previous refractive procedures can be neutralised by postoperative adjustment of the lens with non-invasive light. Some of these lenses may also allow the eye to focus for near and distance (accommodate), removing the need for glasses.

Conclusion

The treatment of cataracts has progressed enormously since the days when the “couchers” used to roam from town to town dislocating cataractous lenses with needles, and it continues to evolve in the 21st century with an increasing trend towards customisation to the individual patient's needs (fig 4). What is clear, however, is that the provision of cataract surgery customised to the individual patient relies on far more than just the skill of the surgeon removing the cataract. Customised cataract surgery needs a multidisciplinary approach at several levels. Industry has a vital role in the development of new technologies, as do health services in the provision of appropriate eye care to populations. At the level of the individual patient such surgery involves the close cooperation of several professional groups, including physicians, anaesthetists, surgeons, opticians, and nurses, sharing information to carefully plan, carry out, and assess the results of every procedure. The future for people with cataracts is bright.

Fig 4

Customised cataract surgery

• Download figure
• Open in new tab
• Download powerpoint