Imaging in ophthalmology

Introduction

Since Helmholtz first peered inside the human eye more than 150 years ago, the subjective evaluation of the fundus has played a central role in the assessment of eye health and disease. Over the past two decades, technologies have been developing for the objective evaluation of ocular structures. These devices promise quantitative measurement of the retina and optic nerve with high degrees of precision and reproducibility. Optical coherence tomography (OCT) and confocal scanning laser ophthalmoscopy (CSLO) are two methods that have emerged to permit accurate analysis of the internal structures of the eye.1 This chapter will focus on these two imaging techniques. Both are non-con- tact, non-invasive imaging systems.

Confocal scanning laser ophthalmoscopy

CSLO allows real-time, three-dimensional imaging of the retinal nerve fiber layer (RNFL) and optic nerve head (ONH). The confocal scanning system is based on the principle of spot illumination and spot detection. It is designed to allow only a ‘thin’ slice or spot of the retina to be in focus on the image plane. Light rays reflected from higher or lower focal planes are blocked, thus creating highresolution tomographic images. Retinal tissue is illuminated and imaged point by point through a pinhole. The illumination pinhole and the imaging pinhole correspond to the same focal point on the tissue, thereby making the system confocal. A three-dimensional image may be acquired by adjusting the x, y, and z coordinates.

Heidelberg Retina Tomograph

The Heidelberg Retina Tomograph (HRT; Heidelberg Engineering GmbH, Heidelberg, Germany) has two confocal scanning laser ophthalmoscope systems that are commercially available. The original HRT I is an optical scanning system for acquisition and analysis of the posterior pole. HRT is typically used to assess the ONH by using a diode laser of 670 nm to scan a three-dimensional image from a series of optical sections at 32 consecutive focal planes. The confocal scanning optical microscope reconstructs an ONH image by bringing a series of two-dimensional digitized images into registration. The registration process corrects for microsaccades that may occur during image acquisition.2 The topography image is composed of 256 × 256 pixel elements, each of which is a measurement of height at its corresponding location. The optical transverse resolution is approximately 10 µm, whereas the axial resolution is about 300 µm. In HRT I, the transverse field of view can be 10 × 10°, 15 × 15°, or 20 × 20°. In current clinical practice, three scans of each eye are taken and then averaged to create a mean topography image. Images can be obtained through undilated pupil, but dilation will improve image quality in patients with small pupils and cataracts. Reproducibility is best in undilated eyes.3 The printed report displays a topographic image and a reflectivity image of the ONH and its contour line, ONH stereometric parameters, and a mean-height contour of the peripapillary retina (Figs. 1A and B).

The HRT was reported to be more sensitive than clinical assessment in detecting early glaucomatous disc changes in a study that evaluated 72 normal

patients and 51 patients with early glaucoma, using qualitative assessment of stereoscopic optic disc photographs and CSLO imaging.4 In another comparative study, clinicians analyzed 13 ocular hypertensive eyes that subsequently developed reproducible visual field defects and 13 normal eyes that had undergone sequential optic disc images. HRT

was found to detect glaucomatous changes in the ONH before visual field changes occurred.5

The recently-developed HRT II is also designed for topographic ONH analysis; however, the device is small, lightweight, portable, and almost completely automatic. All parameters for image acquisition are fixed or predetermined. The HRT II au-

tomatically acquires 16-64 image planes covering a field of view fixed at 15 × 15° using 384 × 384 pixels per plane. Utilizing an internal fixation target, this system routinely acquires three images with the use of a quality control system that will obtain additional images if one or more of the images cannot be used (e.g., fixation loss). The printed report illustrates a topographic and reflectivity image of the ONH and its contour line, details of the ONH stereometric parameters and classification (Fig. 2).

The HRT I is a research-oriented tool with a wide range of applications. It can be utilized to measure retinal circulation when combined with the Heidel-

berg Retina Flowmeter. The HRT II, however, is restricted to ONH analysis.

An inherent limitation of the HRT technology lies in the reference plane that is required to calculate cup area, rim area, rim volume, cup volume, cup-to-disc ratio, retinal nerve fiber layer thickness, and retinal nerve fiber layer cross-sectional area. The anatomical reference plane used by the current software may change over time, especially in patients with glaucoma who have changing topography.3 Another obstacle is the manual delineation of the optic disc margin by the operator, and the influence of this on ONH parameters.

Published data regarding analysis of ONH pa-

rameters using CSLO show that the slope of the cup (‘the third central moment of the depth distribution’) may be the most significant parameter in the prediction of glaucoma status (Fig. 2).6,7 CSLO appears to have an ability to discriminate between normal and glaucomatous eyes with a sensitivity and specificity of about 85%. However, considerable overlap exists between normal, ocular hypertensive, and glaucomatous eyes.7,8 Authors have claimed the ability to determine RNFL thickness or cross-sectional area using CSLO, by using a reference point in the nasal retina or the macula.9 This indirect method of measuring RNFL thickness is almost certainly not the best technique for RNFL analysis, nor is it particularly accurate given the low axial resolution of CSLO (approximately 300 µm) and that superior technologies exist for the direct measurement of RNFL thickness.10 Chauhan and colleagues recently developed software to detect topographic changes in the optic disc and peripapillary retina that appears to provide the best longitudinal data analysis to date using HRT II. Scanning laser tomography and conventional perimetry were used to follow 77 subjects with early glaucomatous visual field damage. Glaucomatous disc changes revealed with scanning laser tomography were found to occur more frequently than visual field changes. This result implies that glaucomatous damage and progression can be detected earlier using scanning laser polarimetry.11,12

Optical coherence tomography

OCT is an optical diagnostic technology that permits high-resolution cross-sectional imaging of the human retina using low-coherence light that is backscattered by the sample boundaries.13 By performing multiple longitudinal scans at different transverse locations, a two-dimensional scanned image is obtained. Compared with the confocal scanning laser ophthalmoscope, which has a transverse resolution of about 20 µm and an axial resolution of the order of 300 µm, the first commercial OCT (OCT 1) produced by Zeiss-Humphrey has a significantly improved resolution: transverse of 20 µm and axial of about 10 µm in the human eye. Non-contact, non-invasive human eye imaging using OCT proved to advance ophthalmic diagnostics.14 Recently, a third generation commercial OCT (OCT 3) was introduced, which provides high-reso- lution images with 8-µm resolution and 512 A scans per image, acquired in about one second. A comparison between a scanned image obtained with a commercial 10-µm resolution OCT and one obtained with a pre-production 8-µm resolution OCT of a normal eye is shown in Figure 3.

Research studies on ultrahigh resolution OCT (UHR OCT) demonstrate an axial resolution of 3 µm, using a broad bandwidth femtosecond laser light source.15 UHR OCT has image quality mak-

Fig. 3. OCT 1 and OCT 3 images for comparison. The images are obtained with circular scans around the ONH and depict the RNFL of a normal eye. Both images were acquired in the right eye of the patient on the same day. The 8-µm resolution image obtained with the OCT 3 (B) reveals enhanced retina layer demarcation compared to OCT 1 (A). Layers such as the outer plexiform, outer nuclear, and choriocapillaris, and choroid are discernable in the OCT 3 image. These layers cannot be discriminated in the OCT 1 image.

ing possible identification and quantification of the intermediate layers of the retina, such as the ganglion cell, inner plexiform, inner nuclear, outer plexiform, and outer nuclear layers, as well as enabling differentiation of the choriocapillaris and choroid from the retinal pigment epithelium (RPE); these discriminations were not possible with any previous device. Recent UHR OCT images of the retina compared well with histology, as shown in Figure 4. UHR OCT images of the macula to optic nerve area in a normal eye showed a significant improvement compared to images obtained with OCT 1 or 2 and OCT 3, as seen in Figure 5.

OCT has been used to detect glaucoma, retinal diseases such as macular holes and pseudoholes, non-proliferative and proliferative diabetic retinopathy, macular degeneration, retinal and RPE detachment, chorioretinal inflammatory disease, retinal dystrophies, retinal trauma, and diseases of the optic nerve such as optic disc pitting, optic disc swelling.16

Retinal imaging

In the retina, OCT provides a cross-sectional image of optical reflectivity. Retinal thickness is automatically calculated from the anterior and posterior boundaries of the retina. Retinal thickness increases with edema, which can be located in the macula. This is an important and common process in diabetic retinopathy, retinal vein occlusion, and uveitis. A decrease in retinal thickness is associated with atrophy or scarring, as well as with glaucoma. Neurosensory detachments of the retina or RPE, macular lesions such as holes, and fibrosis can be

detected in an OCT image. Recent improvements in resolution facilitate distinguishing RPE and choriocapillaris, providing useful information on agerelated macular degeneration and choroidal neovascularization.

OCT can be useful in the diagnosis and monitoring of macular holes, macular edema, and retinal detachment.17 In eyes with epiretinal membranes, OCT can provide structural assessment of the macula preand postoperatively.18 OCT may be useful

for screening and monitoring patients with diabetic retinopathy.19

Enhancement of the quality of the images obtained with the 8-µm resolution afforded by OCT 3 make clear differentiation of the affected retinal layers and cyst-like spaces in cystoid macular edema possible, as seen in Figure 6.

OCT calculates both retinal and RNFL thickness. A significant correlation exists between macular and RNFL thickness in glaucoma.20 Using the 8-

µm resolution OCT 3, retinal disease imaging and diagnosis may be enhanced; UHR OCT may provide further improvements in reproducibility, sensitivity, and specificity of measurements, due specifically to the higher resolution possible with UHR OCT. Figure 7 demonstrates a case in which a clear demarcation between retinal layers is shown in a normal eye using OCT 3.

Optic nerve head imaging

Alterations in the thickness of the RNFL and the ONH contour are important in the diagnosis and follow-up of glaucoma; these parameters are also

of interest in papilledema and papillitis. OCT calculation of ONH parameters gives useful quantitative information about the degree of ONH injury.

There is a strong correlation between OCT measurements of RNFL thickness in glaucomatous eyes and stereoscopic ONH photography, ophthalmic examination, and functional status, as measured by Humphrey 24-2 visual fields.21-23 OCT measures demonstrated thinning of the RNFL with age, RNFL thinning in glaucoma corresponding to visual field defects, and RNFL focal defects and cupping.22

A study in normal eyes, glaucoma suspects, and early and advanced glaucoma patients demonstrated

that RNFL thickness calculated from the OCT image was the best parameter for discrimination, compared to confocal scanning laser ophthalmoscopy24 or to CSLO, scanning laser polarimetry, short wavelength automated perimetry, or frequency doubling perimetry.22

Improvement in resolution offered by OCT 3 may enhance OCT ONH imaging (Fig. 8), and may allow more accurate quantification of parameters such as rim area, cup-to-disc ratio, and volumetric measurements (Fig. 9).

Summary

The goal of imaging in ophthalmology is to enable the accurate and early diagnosis or tracking of eye disease. The HRT is used to assess the contour of the ONH and posterior pole. OCT imaging provides high (or ultrahigh) resolution images of the retina, permitting reproducible cross-sectional measurements of retinal and RNFL thickness and ONH topography. CSLO and OCT can contribute to earlier and more accurate diagnosis in ophthalmic disease, and may serve to measure change over time. These technologies approach the problem of structural ophthalmic imaging in different ways, and each provides reproducible quantitative information. OCT has the added benefit of utility in multiple areas of ophthalmic imaging; UHR OCT is analogous to in vivo histology.

Conclusions

In conclusion, imaging in ophthalmology is an important tool for the diagnosis and tracking of disease. Imaging techniques such as Heidelberg Retina Tomography and Optical Coherence Tomography provide high (or ultrahigh) resolution images that assess the structure and substructure of the retina. These imaging techniques allow quantitative assessment of retinal tissue and are expected to detect disease and its progression earlier than could be done in any other way.

References

1.Schuman JS, Kim J: Imaging of the optic nerve head and nerve fiber layer in glaucoma. Ophthalmol Clin N Am 8(2):259-279, 1995

2.Echelman DA, Shields MB: Optic nerve imaging. In: Albert DM, Jakobiec FA (eds) Principles and Practice in Ophthalmology, Vol 3, pp 1310-1329. Philadelphia, PA: WB Saunders 1994

3.Mikelberg F, Wijsman K, Schulzer M: Reproducibility of topographic parameters obtained with the Heidelberg Retina Tomograph. J Glaucoma 2:101-103, 1993

4.Zangwill L, Shakiba S, Caprioli J, Weinreb RN: Agreement between clinicians and a confocal scanning laser ophthalmoscope in estimating cup/disk ratios. Am J Ophthalmol 119:415-421, 1994

5.Weinreb RN, Luski M, Bartsch DU, Morsman D: Effect of

repetitive imaging on topographic measurements of the optic nerve head. Arch Ophthalmol 111:636-638, 1993

6.Brigatti L, Caprioli J. Correlation of visual field with scanning confocal laser optic disc measurements in glaucoma. Arch Ophthalmol 113(9):1191-1194, 1995

7.Mikelberg FS, Prafitt CM, Swindale NV et al: Ability of the Heidelberg Retina Tomograph to detect early glaucomatous visual field loss. J Glaucoma 4:242-247, 1995

8.Zangwill L, Horn SV, Lima MDS, Sample PA, Weinreb RN: Optic nerve head topography in ocular hypertensive eyes using confocal scanning laser ophthalmoscopy. Am J Ophthalmol 122:520-525, 1996

9.Weinreb RN, Shakiba S, Sample PA et al: Association between quantitative nerve fiber layer measurement and visual field loss in glaucoma. Am J Ophthalmol 120:732738, 1995

10.Schuman JS, Noecker RJ: Imaging of the optic nerve head and nerve fiber layer in glaucoma. Ophthalmol Clin N Am 8(2):259-279, 1995

11.Chauhan BC, Blanchard JW, Hamilton DC, LeBlanc RP: Technique for detecting serial topographic changes in the optic disc and peripapillary retina using scanning laser tomography. Invest Ophthalmol Vis Sci 41(3):775-782, 2000

12.Chauhan BC, McCormick TA, Nicolela MT, LeBlanc RP: Optic disc and visual field changes in a prospective longitudinal study of patients with glaucoma: comparison of scanning laser tomography with conventional perimetry and optic disc photography. Arch Ophthalmol 119(10):14921499, 2001

13.Huang D, Swanson EA, Lin CP, Schuman JS, Stinson WG, Chang W, Hee MR, Flotte T, Gregory K, Puliafito CA, Fujimoto JG: Optical coherence tomography. Science 254(5035):1178-1181, 1991

14.Swanson E, Izatt, Hee M et al: In vivo retinal imaging by optical coherence tomography. Optics Lett 18:1864-1866, 1993

15.Drexler W, Morgner U, Ghanta RK, Kartner FX, Schuman JS, Fujimoto JG: Ultrahigh-resolution ophthalmic optical coherence tomography. Nature Med 7(4):502-507, 2001

16.Puliafito CA, Hee MR, Lin CP, Reichel E, Schuman JS, Duker JS, Izatt JA, Swanson EA, Fujimoto JG: Imaging of macular diseases with optical coherence tomography. Ophthalmology 102(2):218-229, 1995

17.Wilkins JR, Puliafito CA, Hee MR, Duker JS, Reichel E, Coker JG, Schuman JS, Swanson EA, Fujimoto JS: Characterization of epiretinal membranes using optical coherence tomography. Ophthalmology 103(12):2142-2151, 1996

18.Hee MR, Puliafito CA, Duker JS, Reichel E, Coker JG, Wilkins JR, Schuman JS, Swanson EA, Fujimoto JG: Ophthalmology 105(2):360-369, 1998

19.Guedes V, Schuman JS, Hertzmark E, Correnti A, Mancini R, Wollstein G, Lederer D, Voskanian S, Velazquez L, Parker HM, Pedut-Kloizman T, Fujimoto JG, Matox C: Optical coherence tomography measurement of macular and nerve fiber layer thickness in normal and glaucomatous human eye. Ophthalmology (in press)

20.Schuman JS, Hee MR, Puliafito CA, Wong C, PedutKloizman T, Lin CP, Hertzmark E, Izatt JA, Swanson EA, Fujimoto JG: Quantification of nerve fiber layer thickness in normal and glaucomatous eyes using optical coherence tomography: a pilot study. Arch Ophthalmol 113:586-596, 1995

21.Pieroth L, Schuman JS, Hertzmark E, Hee MR, Wilkins JR, Coker J, Mattox C, Pedut-Kloizman T, Puliafito CA, Fujimoto JG, Swanson E: Evaluation of focal defects of the nerve fiber layer using optical coherence tomography. Ophthalmology 106(3):570-579, 1999

22.Bowd C, Zangwill LM, Berry CC, Blumenthal EZ, Vasile C, Sanchez-Galeana C, Bosworth CF, Sample PA, Weinreb RN: Detecting early glaucoma by assessment of retinal nerve fiber layer thickness and visual function. Invest Ophthalmol Vis Sci 42(9):1993-2003, 2001

23.Soliman MA, Van Den Berg TJ, Ismaeil AA, De Jong LA, De Smet MD: Retinal nerve fiber layer analysis: relationship between optical coherence tomography and red-free photography. Am J Ophthalmol 133(2):187-195, 2002

24.Parker HM, Schuman JS, Hertzmark E, Pedut-Kloizman

Tpeiris ID, MacNutt J, Miller VM, So S, Ghanta RK, Drexler W, Fujimoto JG, Puliafito CA, Mattox C, Rasheed ES, Guedes VRF: Optical coherence tomography of the retinal nerve fiber layer, with comparison to Heidelberg retina tomography optic nerve head measurements, in normal and glaucomatous human eyes. In: Lemij HG, Schuman JS (eds) The Shape of Glaucoma, Quantitative Neural Imaging Techniques, pp 149-181. The Hague: Kugler Publications, 2000

In earlier times, having a refractive error in one’s vision was tantamount to having a life-threatening handicap. In prehistoric days, when hunting tribes were dominant, a nearsighted person was at an enormous disadvantage. His possibilities of obtaining food and defending himself were small. By necessity, he became dependent on associated with better vision.

In a more civilized world with an increased differentiation in people’s daily lives, even the individual with a refractive error could find an occupation, doing primarily different types of craft work. Obviously, presbyopia was a handicap for older people. Close, detailed work was impossible to deal with. Reading and writing in poor lighting could be entrusted to nearsighted people. Various aids such as reading stones (magnifying glasses) were used, but these were of such poor quality that it was still impossible to carry out detailed close work.

It was not until the twelfth century that eyeglasses appeared, but they were excessively expensive, and few could afford them. Giving nearsighted people qualified jobs involving fine details in writing, reading, and arts and crafts was common practice.

As optical aids became more advanced, particularly during the twentieth century, with more accurately adapted glasses and contact lenses, individuals with refractive errors began to have the same opportunities for most kinds of work as those with normal vision. There are, however, still some jobs where eyeglasses and contact lenses are a disadvantage.

Freedom from the inconvenience of using optical aids has long been a desire of those who nevertheless need them. When contact lenses began to be used, they were hailed by many as liberating, even though users’ vision did not always become optimal. The risks involved in using them were not

negligible either. Complicated infections and vascular ingrowth in the cornea are not unusual. Soft (hydrophillic) lenses have now been developed for a short period of use, and 24-hour lenses are currently the most popular kind. These has reduced the risks dramatically, but have not eliminated them. However, it has been determined that standard eyeglasses free the user from the risks of both injury and infection.

An otherwise healthy person with nearsightedness (myopia), farsightedness (hyperopia), or astigmatism can be regarded as having a normal variation. Being able surgically to normalize a person’s refraction has long been desired, but it was not until the end of the twentieth century that such an intervention was seriously discussed. The risks involved often presented an obstacle to the development of refractive surgery.

When Sato’s idea1 was taken up by Fyodorov2 in the Soviet Union, in what came to be called radial keratotomy (RK), the risks of injury and infection were regarded as relatively minor. Experience has been gained from several decades of using this method, during which millions of patients have undergone RK surgery. Even though most of those with low-grade myopia have had an acceptable visual improvement, there have been a few unfortunate cases in which perforations occurred, and postoperative infections followed. There have also been reported of hyperopia.3 It is also know that people who have had this operation are at higher risk of injury from blows to the eye. Considering all the disadvantages now known, it is understandable that RK surgery has been abandoned in most places.

Replacing RK with surgery involving less risk and more precision led to the development of the excimer laser for this purpose. At first, attempts were made to replace the diamond knives then being used to

make the radial incisions with the excimer laser, in order to eliminate perforations. However, the incisions were too broad.

The idea of improving the technique of using radial incisions to flatten the cornea, and being able to ‘plane away’ a portion of the corneal stroma over a larger area appeared at the beginning of the 1980s.4-8 However, this method involved working in the optical zone, which many ophthalmologists considered hazardous.

Several companies in Germany and the USA developed excimer lasers for this purpose. They treated surfaces of between 3 and 4 mm in diameter with excimer pulses that removed between 0.2 and 0.3 µm per pulse. Generally, treatment was done to a depth of less than 100 µm. The larger the diameter of the treated area, the deeper you had to go into the stroma. This technique, photorefractive keratectomy (PRK) quickly became popular in Europe, where there were no major restrictions against carrying out a procedure of this nature. In the USA, the procedure was not accepted until after seeing the results of a multicenter study with a few cases in each study, and this was not until November 1995.8 In Europe, and especially in Sweden, thousands of people underwent PRK surgery in the early 1990s.9 The advantages of this method were that there was no risk of perforations, of reduction of stability in the cornea, or of infection. The disadvantages were that surgery was still being performed in the optical zone, and that there was postoperative pain and a slow healing process in ten percent of the cases. There could also be corneal haze in the postoperative period, which could nevertheless be regarded as a temporary scarring that always disappeared with time. Some regression also occurred in a number of eyes, and the precision was not perfect.

Even though this method involved less risk when compared to RK, and produced good results (20/40 UCVA in 95% of cases, and stability after about three months), some patients had to undergo re-operation. Most patients had problems with night vision (halo phenomenon) and reduced sensitivity to contrast, primarily during the first year after surgery. The halo phenomenon was caused by the small diameter of the operated area, often smaller than the diameter of the pupil in weak light. By improving the technique with larger operation zones (>7 mm), and by using different treatments with antiphlogistic medications such as steroids, some of the postoperative complications could be reduced.9

In the USA, PRK was not permitted at first. Before the appearance of the excimer laser, a number of refractive procedures had been done on corneas, primarily in South America. Epikeratophakia and keratomileusis has been performed in Bogota.10 The latter technique could be modified so that, instead of implanting small discs made from donor corneas, the excimer laser was used.11,12 This was how laserassisted in situ keratomileusis (LASIK) was developed. LASIK became very popular, especially in the USA.

One procedure which became popular is the one in which an intraocular lens is implanted after extraction of the clear lens. However, this method has a very long history.13,14 Another method with a more modern background, which became equally popular, involves implantation of anterior chamber lenses in front of the regular lens.15,16

In LASIK, a flap of the cornea with a diameter of about 9 mm and a thickness of about 150 µm is cut from the corneal surface. Excimer laser treatment removes tissue under this flap, which is normally fastened by a hinge at the 12 o’clock position in the corneal incision. After the operation, the flap is replaced, and a bandage lens is placed over it.

The advantages of LASIK surgery include a painless postoperative period, rapid healing, and functional vision almost immediately after the procedure. Disadvantages are that, in a very few cases, the incision can go askew or be made too deeply, which can cause permanent injury. By cutting through the epithelium and into the cornea, it can happen that there is epithelial growth in under the flap. This must be removed by re-lifting the flap. This is not a problem, since the flap never grows together with the base. In the same manner, folds can appear in the flap, and re-operation is necessitated. When the results are not satisfactory, the flap must be lifted and a new treatment performed. Since the flap never reattaches, i.e., heals, an injury to the eye with a sharp object can cause the entire flap to detach. This is, of course, an injury that can be difficult to repair. When the flap is cut, a considerable reduction in the cornea’s stability is unavoidable. Keratectasia has been reported in a fair number of these cases.17 An increased hyperopia with irregular astigmatism is the consequence. The LASIK technique’s popularity has decreased somewhat in recent years, to be replaced by the ELSA technique described below.

Experiments have been made with the implantation of flexible plastic bands into the corneal stroma.18,19 A plastic band is inserted into the corneal stroma about 4 mm from the center of the cornea, and by tightening this band, a certain amount of alteration can be made in the shape of the cornea. The procedure is not performed near the optical zone, and the band can be removed at any time. However, the precision of this procedure is not good, and, at present, it does not appear that this method has much of a future. Some infection has also been reported.

Since cataract surgery is performed frequently, eye surgeons from all over the world have extensive experience in operating on cataracts by implanting an intraocular lens. Removing a completely clear lens and replacing it with an appropriate intraocular lens has become a popular technique. Since the risk of infection is very small, although still present, it has been recommended that this procedure be reserved for patients with higher degrees of refractive error.

Another method involving the placement of an intraocular lens in front of the existing lens has been used successfully in several countries. There is a risk

of very serious infection here, as well as the likelihood of the patient’s own lens rubbing against the implanted lens, which can lead to cataract formation over time. The precision of the two last-mentioned procedures is not as advanced as that of LASIK.

Laser thermokeratoplasty (LTK) is a technique in which an attempt is made to increase the curvature of the cornea. It has been used in cases of farsightedness and presbyopia. In this procedure, a special laser beam is used to heat up a spot deep down in the corneal stroma.20 Repeated burns are made around the entire circumference, sometimes in one circle and sometimes in several concentric circles. This method is completely painless, and results in an immediate correction. However, after three to six months, regression sets in and within 12 to 18 months, the refraction returns to its preoperative condition. This technique is still used in some places, but its use can hardly be defended when there are better techniques currently in existence.

In recent years, a new method has been developed and used. This is called excimer laser subepithelial ablation (ELSA) or laser subepithelial keratectomy (LASEK).21,22 The latter name should be avoided, as it can easily be confused with LASIK. In this procedure, the epithelial layer is completely removed, but about 90° of the circumference is allowed to remain as a short of hinge. After the laser treatment, which is equivalent to PRK, the epithelium is replaced. In order to loosen the epithelium effectively, a solution of 20% alcohol is applied to the cornea for 25-35 seconds before the dissection of the epithelial flap is made. The results of this procedure are very good, and show ELSA’s advantages without any of its disadvantages. In general, the patient experiences very little postoperative pain, and healing usually takes place rapidly, so that the patient has useable vision within a day or so. There is no haze, and no long-term healing process. Since the technique is more or less the same as PRK, there is great precision. ELSA can in principle be regarded as riskfree in comparison with all the other methods. We have 15 years of experience with PRK, and the two methods can be seen as so similar that we expect no long-term reactions.

There are several excimer laser apparatuses available at present. In principle, there are two different methods. The first involves placing various kinds of apertures in the excimer laser’s beam, and with the second, a small spot of 1-2 mm in diameter is programmed to move in a certain pattern over the area to be treated. The laser’s computer is programmed to treat the specific refraction. There has been no evidence of any long-term differences in results.

There are several kinds of lasers on the market, which operate on somewhat different principles. No evaluation has yet been made, and no-one has been able to show any clear advantages with any one type of laser. This is a serious drawback. All the articles that tout the advantages of one method over another either contain statistics that are not well-qualified,

or are written by authors who are associated with the companies that procedure the new instruments.

A number of the additions mentioned above have not been adequately documented either. There is a danger that both patients and doctors will find distorted and biased information with Internet web searches. Many doctors use the various new instruments and auxiliary equipment for advertising purposes, which is hardly beneficial to refractive surgery in general.

A lens system is always impaired by certain optical errors, such as spherical and chromatic aberrations, as well as coma.23,24 With the aid of a device called an aberrometer, these errors can be calculated. Aberrations located both in the cornea and in the eye’s lens vary with the size of the pupil and the accommodation status of the lens, and also with the composition of the light. In calculation, this is always done in the new technology with a dilated pupil and with accommodation paralyzed. A monochromatic light is also used. Thus, these calculations give information about a very special situation that never occurs in everyday life. The numbers obtained are only to be used in the laser algorithms, and the value of this can be discussed. So far, no satisfactory investigations have been done that allow us to say that these calculations are sufficiently significant to be used in treatment.

Some of the lasers can be partially programmed with a topographical image of the cornea, and this ought to be better results. No scientific report has yet been published that shows the advantages. The topography of the cornea can change postoperatively in such a way that an irregular astigmatism appears. In a case like this, a treatment controlled by a topographical image would have had a great advantage. No such construction has yet appeared on the horizon; this is probably due to the laser algorithm’s inability to handle a more complicated topographic image.

The use of refractive surgery has now spread over most of the industrial world. Millions of people have had operations. The development and manufacture of instruments for refractive surgery have become a major industry; the investment for each clinic is about $500,000 to $1,000,000. At conventions and meetings, as well as in professional publications, the subject of refractive treatment claims more and more space. The optical branch is starting to feel threatening, especially the contact lens market.

This is the first time that ophthalmologists have entered an area with clearly mercantile objectives

– an area in which the focus is not on diseases, but rather on variations within the normal range of people’s vision errors. We have started a process that digresses from the ethical rules that have guided us since the Hippocratic oath was instituted.

We doctors are here to help without injuring. Are we moving in the right direction? Seen from our patients’ viewpoint, refractive error is a troublesome handicap. They come to us for help. In today’s world,

most of our patients are very well informed, especially by the Internet. Our homepages are a form of advertisement, but are they objective? Are we duping our patients into believing that we have the right solutions to their problems?

There are huge economic interests at stake here, especially for the eye surgeon. These are normal variations that we are treating, and not diseases! It should be the duty of every eye doctor to weight the risks in relation to the refractive surgery we are going, without casting sideways glances at the financial gain. Of course, our patients can see the many advantages of a successful treatment. The operation is also financially rewarding for the patient in the long run; the operation’s cost is written off in a few years. But, have we given them the proper information about the risks? Several of the methods described above do indeed carry significant risks. Obtaining vision that is precisely as good as the patient has with eyeglasses is actually rare in cases of mild myopia. Night vision and contrast sensitivity are impaired, and aberrations increase, even though visual acuity is good.25

In cases in which there is a greater refraction error, visual acuity usually improves by one or two lines on the eye chart, but the errors mentioned above remain. There are some individuals whose jobs or other situations prevent them from wearing glasses or contact lenses, and it is of course suitable to operate on these individuals – but what about the others?

The patient’s motivation is the most important factor in carrying out a treatment, but his motivation should not be influenced by either erroneous information or withheld information. If he is satisfied with his contact lenses or glasses, he would do well to avoid refractive surgery as it is being handled today! The future may bring us even better and safer methods that may change these conditions, but until then, the ophthalmologist’s approach to refractive surgery should be characterized by a certain amount of restrictiveness.

Conclusions

Refractive surgery has come to stay. The methods will improve and so also the lasers and other equipment. However we have to be observant of side effects, as infections especially in intraocular surgery, unexpected regression and keratoectasia. So far we have a relatively long experience from PRK which of course can be applied even for LASEK/ ELSA. The long term results from LASIK (>10 years) we do not know but the risk for keratoectasia should not be overlooked as more and more cases are reported.

No method can be totally safe when compared to spectacles. The doctor should be aware of the fact that the eyes treated are normal and no risks should be taken that can result in loss of vision. On the other

hand all our patients will benefit if they can get rid of spectacles and contact lenses in their work and daily life but they have to be totally informed and well motivated prior to any kind of treatment.

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