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Laser eye surgery: back to the future.

1 CET POINT

Refractive surgery has evolved significantly in recent years, resulting in safer and improved visual outcome for patients. This article explores the historical development of refractive techniques, current approaches with regard to patient selection and details complications that can arise.

INTRODUCTION

The last two decades have seen a tremendous rise in the popularity of refractive eye surgery as a means for achieving spectacle independence. Refractive correction can be achieved either by altering the refractive power of the cornea or with the help of intraocular surgery.

Corneal refractive surgery includes laser eye surgery (LES) involving corneal ablation by excimer laser; corneal addition procedures such as intracorneal ring segments and epikeratophakia; corneal relaxation procedures such as radial keratotomy (RK), arcuate keratotomy (AK) limbal relaxing incisions; and thermokeratoplasty. Intraocular refractive procedures include clear lens exchange and phakic intraocular lens implantation.

The focus of this article will be on laser refractive surgery, including the history, technological aspects of lasers and a brief overview of the current literature on LES.

THE PAST: HISTORY OF LASER EYE SURGERY

Early refractive surgery procedures were first carried out by Japanese ophthalmologist, Tsutomu Sato in 1936.1 Sato performed anterior and posterior keratotomies to treat myopia after observing that patients with keratoconus who developed breaks in Descemet's membrane had reduction in their refractive error. However, 71% of his patients developed bullous keratopathy resulting from damage to the corneal endothelium, and this procedure was eventually abandoned.

In 1960 a Russian ophthalmologist, Fyodorov, modified Sato's original technique to make it corneal endothelium-friendly by using anterior paracentral incisions rather than peripheral scleral incisions. This formed the basis of a technique known as radial keratotomy (RK), (3) which became popular for a few years before excimer laser eye surgery took the world by storm.

The history of laser eye surgery goes back to the work of Professor Barraquer of Columbia who concluded that the key to altering the refractive status of the eye was changing the curvature of the cornea by adding or removing corneal tissue while preserving corneal layers. (4) He first used the term keratomileusis, derived from two Greek words, keras (hornlike, cornea) and mileusis (carving).

The advent of the excimer laser was the most significant advance in the history of refractive surgery as it significantly improved the safety and efficacy of refractive correction. The term excimer describes ultraviolet lasers that generate nanosecond pulses with wavelengths between 157 and 351nm. The ultraviolet beam of light then vaporises tissues precisely by breaking up molecules without collateral damage to adjacent tissue; this pioneering work was carried out by IBM researcher, Srinivasan, who worked on interaction of 193nm with organic materials and used the term photoablative decompensation (photoablation) to describe this process. (5) The term excimer is derived from 'excited dimer', an interaction of argon and fluoride, emitting UV radiation with a wavelength of 193nm, removing on average, 0.25[micro]m of corneal tissue per pulse.

Stephen Trokel and Francis L' Esperance further expanded on the interaction between excimer laser and the cornea and then filed for separate patents for photorefractive keratectomy (PRK). (6) Theo Seiler of Germany first performed excimer laser phototherapeutic keratectomy in a sighted eye in 1985, whereas Francis L'Esperance performed the first PRK in the US on a blind eye in 1987. (7)

PRK was then followed by the development of LASIK (laser-assisted in situ keratomileusis), which has now become the most commonly performed refractive procedure worldwide. Pallikaris and colleagues coined the term LASIK in 1988, a procedure involving corneal ablation by excimer laser under a hinged corneal flap created by an automated keratome. (8) It was thought that by creating an anterior lamellar corneal flap, the integrity of Bowman's layer and corneal neural net is preserved resulting in better healing and predictability, along with reduced pain and corneal haze. In 1989, the first LASIK was performed on a blind human eye in the University of Crete, and in 1992, Stephen Slade performed the first LASIK in the US. The initial clinical trials on LASIK began in 1996 and culminated in FDA approval for LASIK in the US in 1999, nearly a decade after other parts of the world. (9) In the late 1990s, other variants of PRK were introduced including LASEK (laser assisted sub-epithelial keratomileusis) and EpiLASIK. (10,11)

The latest advance in refractive surgery has been the use of femtosecond (FL) laser to create lamellar corneal flaps with a degree of precision and safety not possible with the automated microkeratome used in traditional LASIK. Dr Juhsaz and Dr Kurtz at the University of Michigan designed the first femtosecond laser system for use in ophthalmology in the early 1990s. (12) FL is increasingly used in LES to create anterior corneal flaps; 30% of LASIK procedures used a FL in 2006 with the number increasing to more than 55% by 2009. (13,14)

THE PRESENT: CURRENT STATUS OF LASER EYE SURGERY

LES currently involves the use of both excimer laser and femtosecond laser to achieve the desired refractive correction. The frequency of the beam determines which biological tissues it will target, with lasers used in LES having a wavelength of around 193nm in the UV-C range of the electromagnetic spectrum. Their main target in the cornea is the peptide bond, linking the adjacent amino acids in collagen. When the laser beam strikes the cornea, it breaks these bonds and imparts kinetic energy to the resulting fragments. This results in high-speed ejection of material from the corneal surface, a process referred to as photochemical ablation. (15)

There are three main approaches to photochemical ablation; wide area, scanning slit and flying spot:

* Wide area ablation uses a wide diameter beam and treats the entire operating field simultaneously, obviating the need for eye tracking. However, creating a wide diameter stable laser beam is costly and technically difficult

* Scanning slit ablation uses a red blood cells under the flap, and bacterial endotoxins. The patient presents with decreased visual acuity, pain and alterations in corneal topography including a hyperopic shift. Patients with mild DLK are treated with intensive topical steroids with antibiotic prophylaxis. Severe DLK requires lifting and irrigation of the flap with balanced salt solution, in addition to intensive topical steroids. (42)

Infectious keratitis under the flap is rare, but potentially vision threatening, with incidence varying from 0-1.5%. (43) Early infectious keratitis within two weeks is caused by common bacterial pathogens such as Staphylococcus and Streptococcus, whereas late-onset infection is caused by opportunistic organisms like fungi, Nocardia and mycobacteria. (44) The treatment involves intensive topical antimicrobial antibiotics with discontinuation of topical steroids.

Corneal haze after LASIK is minimal or absent especially when compared to PRK. (45) The addition of antimetabolite mitomycin C to LES, applied after excimer laser ablation at a concentration of 0.02% for 10 to 30 seconds, can further reduce the incidence of corneal haze as well as treatment regression. (46)

Late postoperative complications

Post-LES dry eye is a common problem, especially after LASIK, due to damage to the surface corneal nerves resulting in a neurotrophic cornea, which in turn leads to reduced lacrimal gland secretion. (47)

A serious complication of LASIK is corneal ectasia due to weakening of the collagen lamellar structure of cornea. The major risk factors for corneal ectasia are thin cornea preoperatively, high myopic correction and leaving a postoperative stromal bed of less than 250[micro]m. (48) The treatment of progressive corneal ectasia is complex and involves the use of rigid gas permeable contact lenses, intrastromal corneal rings and collagen cross-linking. (49,50)

Problems with night vision including glare and haloes are also seen after LES. Glare is caused by optical aberrations and haloes can occur when the pupil size in the dark is greater than the treatment zone. The use of FL and WFG excimer laser has reduced the incidence of these complications.

Retinal detachment has been reported after LASIK, either due to peripheral retinal breaks in myopic patients or by pressure on the vitreous base caused by sudden compression and decompression of the eye due to the placement of suction ring. (51)

THE FUTURE OF LES

The major future goal of LES is to achieve vision better than 6/6 and to improve the quality of vision by completely eliminating optical aberrations. The increasing use of custom ablation to remove optical aberrations and FL to reduce flap-associated complications has helped refractive surgery take a giant leap forwards, and research is continuing in this area. Small incision lenticule extraction (SMILE) is a promising new flapless keratomileusis technique to correct myopia, and early results are encouraging. (52) It involves using a femtosecond laser to cut a lenticule of corneal stromal tissue, which is then removed through a small pocket incision made by the FL. Increasing developments in the field of corneal imaging are helping in preoperative selection of patients who want to undergo LES, hence reducing the incidence of post-LASIK ectasia.

Another field of interest is the use of collagen cross-linking and intrastromal corneal rings in combination with LASIK. Collagen cross-linking strengthens the cornea. When combined with refractive surgery it may reduce the risk of post-LASIK ectasia in patients with thin corneas.

A concept of photoablative inlay has been introduced called PAI-LASIK, where a hydrogel inlay is sculpted and placed between the flap and stroma. (53) This technique can be used to treat high degrees of refractive error without the risk of corneal ectasia, and is reversible. Early animal studies have shown that this material is biocompatible.

CONCLUSION

LES surgery has come a long way from RK to intraLASIK, greatly increasing in efficiency, predictability and safety. At the same time, patient expectations have dramatically increased and in this respect, refractive surgery is a victim of its own success. On-going developments in the field of corneal imaging and laser technology will lead to further improvements in the technique of LES in this rapidly evolving field.
Course code: C-42217 Deadline: October 10, 2015

LEARNING OBJECTIVES

To be able to explain to patients about the options for refractive
surgery (Group 1.2.4)

To be able to advise patients on their suitability for refractive
surgery based upon clinical findings (Group 2.2.5)

To be able to identify post-operative symptoms following refractive
surgery (Group 6.1.2)

LEARNING OBJECTIVES

To be able to explain to patients about different types of refractive
surgery (Group 1.2.4)

To understand the potential post-operative complications that can
arise following refractive surgery (Group 8.1.5)

LEARNING OBJECTIVES

To understand the typical post-operative symptoms following
refractive surgery (Group 1.1.1)

To be able to identify post-operative symptoms following
refractive surgery (Group 2.1.3)


Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams. Please complete online by midnight on October 10, 2015. You will be unable to submit exams after this date. Answers will be published on www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

References

Visit www.optometry.co.uk/clinical, click on the article title and then on 'references' to download

Nadia Chaudhry FRCOphth, Alex Hamilton MPHTM and Arun Brahma MD, FRCOphth

Nadia Chaudhry graduated from King Edward Medical College, Lahore, Pakistan in 2002 before coming to the UK for postgraduate training. She has attained FRCOphth from Royal College of Ophthalmologists, London and is currently working as a specialist trainee in ophthalmology at Manchester Royal Eye Hospital.

Alex Hamilton undertook his ophthalmology specialist training in Sydney. He is currently a corneal and refractive surgery fellow at Manchester Royal Eye Hospital.

Arun Brahma is a consultant ophthalmic surgeon at Manchester Royal Eye Hospital and specialises in corneal, cataract and refractive surgery.
Table 1
The degree of refractive correction offered
by various procedures

([dagger]) Intraocular surgery is the option of choice
for refractive errors falling outside of
the treatable range for PRK/LASEK/LASIK but is also
suitable for lower prescriptions

Refractive procedure    Myopia     Hyperopia   Astigmatism

PRK/LASEK               <-6.00D    <+4.00D     <4.00D

LASIK                   <-10.00D   <+6.00D     <4.00D

Intraocular             >-10.00D   >+6.00D     <6.00D
refractive
surgery ([dagger])
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Article Details
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Author:Chaudhry, Nadia; Hamilton, Alex; Brahma, Arun
Publication:Optometry Today
Geographic Code:4EUUK
Date:Sep 12, 2015
Words:2042
Previous Article:Keratoconus: the Athens protoco.
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