9th Annual Meeting

Congress Report

The annual conference of the British Society for Refractive Surgery was held recently at Aston University. The conference was highly successful – the largest to date, with over 140 delegates including surgeons, optometrists, nurses and technical representatives. The first guest speaker was Professor Sudi Patel (Glasgow and Alicante)) who discussed the options for presbyopic surgical correction, including multifocal intraocular lenses and excimer laser techniques. Currently, many previously myopic refractive surgery patients experience a delay in the onset of presbyopia due to surgically induced positive spherical aberration. Myopic surgery changes the cornea from a flattening ellipse to a steepening ellipse. The possibility of surgically producing a corneal profile that would allow functional near vision in presbyopic patients was discussed. A corneal eccentricity value (p) of 0.9 gives a reasonable compromise for both distance and near vision. The disadvantage is a reduction in the optical quality of the cornea, resulting in reduced contrast sensitivity.

Presbyopic correction was also considered by David Jory (London) in a case study of ciliary sclerotomy. Deep cuts are made in to the thinnest part of the sclera to weaken the globe and encourage the ciliary body to expand outwards, therefore increasing the distance between the ciliary body and equator of the crystalline lens. However, his patient experienced no change in accommodation, a number of the titanium spacers used to keep the incisions open, fell out. The patient suffered from tenderness and vascularisation over the remaining spacer. David Jory admitted that the procedure almost certainly compromised ocular integrity but nevertheless claimed the technique looked promising.

This year’s international speaker was Lou Probst, the well-known Canadian surgeon, currently based in Michigan. He has performed thousands of LASIK procedures as well as having significant experience in the use of implantable contact lenses (ICL) and bioptics (ICL followed by LASIK). LASIK for myopia >-10.00D is rarely performed nowadays due to the risk of keratectasia, the need for small optic zones and consequently the high risk of night vision problems. An ICL can correct the majority of the spherical error (myopia or hyperopia) very precisely and LASIK is performed 1-6 months afterwards to correct any residual astigmatism. 60% of his patients achieved 6/6 unaided, with 50% of his patients gaining between one and three lines of best-corrected vision, mostly due to the removal of spectacle minification, with only 6% losing one line. The disadvantage is that two procedures mean the potential for two sets of complications. One complication of ICL’s is the risk of mild anterior capsular cataract (12% to some degree), if the crystalline lens is brushed during the ICL insertion process, especially with inexperienced surgeons and hyperopic eyes where the anterior chamber is small. Other complications include pupil block, raised intraocular pressure and pigment dispersion syndrome in a small percentage of patients. In experienced hands, this technique looks promising for patients with significant refractive errors. A study is currently underway to apply for FDA approval. In Lou’s opinion, intra corneal ring segments (ICRS), radial keratotomy (RK) and laser thermokeratoplasty (LTK) are now obsolete.

He also discussed his experience with the Intralase intrastromal femtosecond laser, currently being used in the USA to create a planar flap using photodisruption. This highly accurate method of producing a cleavage plane avoids some of the complications involved in using a microkeratome, such as button-hole flaps. However, it is currently an expensive and inconvenient technique since it requires a second laser in addition to the excimer laser needed for the refractive ablation.

The size of the pupil under low illumination is thought to be associated with night vision symptoms. Sanjay Mantry revealed that it takes up to 6 minutes in a darkened room for the pupil to dilate to maximum. Pascal Annonier (Ultralase) reported on night vision symptoms in 2000 myopes. 9.7% reported some glare but only in a few cases was it associated with a large pupil. The relation between pupil size and symptoms was stronger for the hyperopic group. In almost all cases, symptoms resolved by 6 months.

The conference also included an informative talk from Professor Doughty (Glasgow Caledonian) on corneal thickness and its implications for intraocular pressure (IOP) measurements. There is a weak relationship between pressure measurements and corneal thickness with thinner corneas producing lower IOP measurements. This obviously has implications for refractive surgery patients. Professor Doughty indicated that the average surgical refractive correction reduces the central corneal thickness (CCT) by 10-15% (20% in a few high myopes), causing a 1-2mmHg decrease in measured pressure. He suggested that 2mmHg should be added to IOP measurements for each 10% reduction in CCT. However, it can be difficult for Optometrists to know how much refractive error has been treated and therefore how much the cornea had thinned, since the patient may not know their pre-operative refraction from many years ago and clinics only hold on to surgical records for 8 years.

Another significant point made by Professor Doughty is that the structure of the anterior third of the corneal stroma is fundamentally different from the posterior portion. The anterior portion has lamellae with an average thickness of 0.3m m compared to 1m m in the posterior portion. Consequently, the anterior stroma does not swell as much as the rest of the stroma and has a much greater tensile strength. The LASIK flap separates this portion of the cornea from the rest of the stroma so that it no longer contributes to the strength of the cornea. This may be important in understanding why keratectasia occurs in some patients despite a reasonable residual stromal bed.

The first afternoon of the conference featured an update on wavefront technology and clinical results. A foreword on optical aberrations was provided by a member of the BSRS council, Mr Sheraz Daya from East Grinstead. Currently, conventional PRK and LASIK procedures tend to increase aberrations, particularly spherical aberration, coma and triangular astigmatism, causing a reduction in low contrast acuity, and in severe cases, high contrast acuity. Measuring aberrations is complicated by the fact that the wavefront changes with each blink, accommodation and age. Attempting to correct aberrations surgically is even more problematical: epithelial healing after both PRK and LASIK partially fills ablated areas, we know very little about the biomechanics of the cornea and how it changes after surgery, and the LASIK flap also induces irregularity in a highly unpredictable fashion. PRK and LASEK (epithelial flap) may be more suitable for the correction of aberrations for this reason.

Each of the laser companies involved in the field of wavefront technology had been invited to sponsor a speaker to present clinical results. Mr Julian Stevens (Moorfields) presented on behalf of VISX Instinctive Technologies, Lou Probst spoke on behalf of Bausch and Lomb’s Zyoptix system, Rich Bains for Nidek’s NAVEX system, Thomas Zieger for Wavelight (Surgical Design) and Dan Reinstein on behalf of Asclepion-Meditec. The overall conclusion seemed to be that significant visual improvements could be gained by treating patients with large induced aberrations, such as those who have suffered a decentred ablation during an initial treatment. However, compared to conventional LASIK treatment of low myopia (<–6D) and therefore low aberrations, wavefront technology provides only a tiny increase in unaided vision and reduction in loss of best-corrected visual acuity correction. The role of the LASIK flap in inducing unpredictable aberrations was highlighted.

The final guest speaker was Heather Baldwin, Specialist Registrar (South Thames). She presented details of her research, showing that injury of a single corneal layer (epithelium or stroma alone) does not result in compromised corneal transparency. The problem with PRK is that it injures both the epithelium and stroma. Haze develops as a result of interactions between the two wound healing processes. Epithelial wound healing factors act for around 24 hours and stromal factors for 72 hours therefore haze might be reduced by inhibiting epithelial factors during the first 24 hours using substances such as anti TGF beta 1 and 2. Such a breakthrough would bring PRK back in to favour with potential benefits for wavefront technology.

Minimising damage to the epithelial layer while ablating the stroma may also explain by laser epithelial keratomileusis (LASEK) produces little or no haze. Both Paddy Condon and Stephen Doyle presented results of studies comparing LASEK with LASIK. In LASEK, 20% alcohol is used to create a 8.2mm diameter epithelial flap, which is replaced following stromal ablation. Both studies showed that the two techniques produced very similar refractive outcomes. LASEK resulted in slightly more discomfort but avoided the potentially serious flap complications associated with LASIK.

The second presentation by Professor Patel considered the temporary dry eye problems experienced by some after corneal surgery, related to the reduction in corneal sensitivity. Many practitioners assess the tears using the Schirmer strip. Professor Patel showed that it is the lipid layer, not the aqueous layer that is deficient following LASIK, associated with a reduced blink rate. He recommended examining the height of the tear meniscus to judge the tear quality and claimed that his patients had benefited from the use of systemic antioxidants, such as Vision ACE.

Konrad Pesudovs (Bradford University) presented his work on developing a "Quality of life post refractive surgery questionnaire", which should be available for use in the near future. He also presented an analysis of hyperopic LASIK results from Ultralase. In view of the high risk of losing 2 or 3 lines of best-corrected visual acuity (28 and 8% respectively), he concluded that LASIK is not safe for hyperopes greater than +5.00D. Andy Jarosz talked about the screening of potential refractive surgery patients using a call centre and optometrists. Approximately 30% of patients seen by the Optometrist are deemed unsuitable, mostly due to corneal thickness, refractive error, or unstable refraction.

Professor Neil Charman (UMIST) talked about the effect of the ablation zone margin on periphery retinal image quality. Some light rays passing through periphery of pupil to peripheral retina, pass through the untreated peripheral cornea. This results in doubling of the retinal image at certain eccentricities. As ablation zone increases in size, the angle at which doubling effect occurs, increases. The peripheral image can be smeared up to 2° for a –10.00D myope. He was able to demonstrate a small reduction in detection sensitivity assessed using the Goldmann perimeter in post-PRK patients. This effect also has implications for practitioners since there is a corresponding reduction in our view of peripheral retina.

Catharine Chisholm (City University) reminded the delegates that it is not just surgically induced aberrations that can degrade visual performance post-surgery - forward light scatter is also a factor. Assessing the scatter function using the City University Scatter test, she showed a permanent increase in the spread of forward light scatter post-PRK, associated with a small but statistically significant reduction in visual performance (contrast discrimination thresholds). However, there was no increase in the overall quantity of scattered light. No such effect was seen following LASIK. What causes this change in the distribution of straylight so long after the PRK procedure? The most likely reason is a rough epithelial/stromal interface, which can significantly affect visual performance due to the large difference in refractive index between the epithelium and stroma.