An Evaluation Of Excimer Laser Ophthalmic Surgery

 A Photo-Essay for Health Professionals

 

John G. O'Shea MD, Robert B. Harvey FRCSEd

The aim of this article is to provide information and an overview of the potential risks and benefits of excimer laser surgery - a new and promising technique in ophthalmic surgery. Although this review concentrates on the use of the laser for refractive purposes, novel therapeutic techniques will also be discussed.  

The essay is written for general practitioners, optometrists, physicians and other health workers who may wish to understand the basic mechanism of action of the excimer laser and some of the potential risks and benefits of surgery. 

The aim of refractive surgery is to eliminate the need for spectacles or contact lenses or to reduce the strength of the prescription. (1)

The most common cause of visual impairment is  refractive error rather than cataracts or age-related macular degeneration. ( WHO definition of blindness is vision less than 3/60 in the better eye with best available correction; visual disability or impairment is defined as acuity 6/18 to 3/60 with or without spectacle correction. )

Myopia and regular, myopic astigmatism are now the commonest indications for excimer laser procedures, laser treatment of hypermetropia is still at best problematic. Modalities used to treat myopia  include photorefractive keratectomy ( PRK ) and Laser in situ keratomileusis ( LASIK ) (1,3).

Phototherapeutic keratectomy (PTK) utilises the excimer laser for therapeutic, not refractive purposes.  For example,  the treatment of recurrent erosion syndrome or of  an anterior corneal dystrophy.

 

Ametropia ( refractive error ) and  common refractive problems

 

Emmetropia refers to the normal refractive state. Here incoming light rays are focused on the retina. The cornea is the principal refractive component of the eye. (2) The cornea provides two thirds of the eye’s refractive power, the crystalline lens providing the remaining third.  The lens can increase its also focusing power by changing its shape enabling near objects to be seen clearly;  this is called accommodation. (1)

Refractive error (also called ametropia) can be due to hypermetropia, myopia or astigmatism. (1,4)

Hypermetropia

In hypermetropia  ( long sightedness ) parallel light rays from a distant object are not focused on the retina unless there is some degree of accommodation by the crystalline lens. Hypermetropia is often due to the eye being relatively small. (1,4)

Hypermetropes often notice tiredness or aching with prolonged reading or other close work due to excessive accommodation. Hypermetropia is neutralised with a convex spectacle lens or contact lens. (1,4)  Again,  it is stressed that treatment of hypermetropia with the excimer laser is not to be recommended.


Presbyopia

Presbyopia, although not a refractive error, refers to the difficulties experienced with prolonged near vision associated with ageing. (1,4) It is an almost  universal phenomenon- typically noticed by the patient at the age of 40 to 45 years and is due to loss of flexibility and power of the crystalline lens. Presbyopic symptoms can, however, develop at an earlier age particularly after excimer laser treatment in a myopic patient. A convex reading lens which increases the dioptric power of the eye treats presbyopia. (1,4)

 

Myopia

 In myopia ( short sightedness) distance vision is blurred because the light rays are focused anterior to the retina. (1,4) Myopia affects about one quarter of the population;  it is often due to the eye being relatively large. Myopia usually begins in the first or second decade and stabilises by 20 years of age.  Myopia can be neutralised with a concave spectacle lens or contact lens. (1,4)

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Table 1  Myopia

Distance vision is blurred because the light rays are focused anterior to the retina.

Myopia affects approximately one quarter of the population.

Myopia usually begins in the first or second decade and stabilises by 20 years of age.

Myopia can be neutralised with a concave spectacle lens or contact lens. (1,4)

Myopic refractive prescriptions are measured in dioptres, the reciprocal of the far point of the patient’s vision in metres. For example, a four dioptre myope has a far point of 25 centimetres. Beyond this distance light rays will not be focused onto the retina. The patient will not be able to drive without spectacles. ‘High myopia’ is myopia above 6 dioptres. It is often associated with structural abnormalities of the eye, for example, macular degeneration and retinal detachment. The excimer laser does nothing to correct these structural abnormalities.

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Astigmatism

Astigmatism is usually due to a lack of corneal sphericity; circular objects may appear oval. Regular astigmatism is the commonest form. Current refractive surgery aims to treat regular astigmatism.  Astigmatism is neutralised with a cylindrical ( toroidal ) lens. (1,4)  Current refractive surgery techniques also aim to treat regular myopic astigmatism. (1,4)

 

Refractive surgery

Refractive surgery refers to surgical procedures that alter the focusing power of the eye, usually by changing the shape of the cornea. The main techniques are photorefractive keratectomy (PRK) or LASIK using an excimer laser, radial keratotomy, lamellar surgery (such as an intracorneal lens or ring).

Intraocular surgery (such as corneal graft, clear lens extraction and  phakic lens implantation) is usually reserved for more extreme types of refractive error.

This article specifically reviews excimer laser procedures for myopia and myopic astigmatism. ( PRK )

 Indications for refractive surgery

 Patients often seek refractive surgery because of occupational or cosmetic reasons.

Some patients may have spectacle or contact lens intolerance. For example, high correction spectacles are heavy and produce peripheral image distortion. (1,4)

Occasionally patients require refractive surgery to correct a refractive error that is secondary to previous eye surgery (such as cataract surgery). (1,2,3,4)

Assessment and Counselling

 It is important to determine the patient’s reason for seeking treatment. Patients typically consider undergoing PRK because of intolerance of glasses or contact lenses, occupational reasons, recreational or sporting activities or cosmetic factors. (1,3)

As with all surgical procedures informed consent is required.  Adequate information should be provided regarding possible complications and other treatment options. Patients require the opportunity to ask questions.

The patient needs to be aware that visual acuity will not be improved. A middle aged myope needs to be aware that he or she may require a Presbyopic correction following treatment. (1)

Patients should obtain a letter of referral from their general practitioner. The patient’s refractive error needs to have stabilised.

Counselling should be provided by a refractive surgeon who is experienced with PRK. Educational videos and information leaflets are also  very useful. Some patients may be unsuitable for PRK because they cannot give a legally valid consent to operation. For example, some patients are unable to understand the concept of probability and are therefore unable to give informed consent.

It is inadvisable to ask the patient to sign the consent form during the first consultation. Patients should not be treated immediately  after the first consultation.

 The issue of presbyopia should be outlined to the patient when appropriate. For example, a 50 year old myope may require reading glasses following successful PRK. The optical disadvantage created for such a patient by PRK can be simulated with trial frame lenses. (1,2)

After 60 years of age significant cataract often develops, in which case cataract surgery is more appropriate than corneal refractive surgery.

It is also strongly recommended that other eye should not undergo treatment within a period of 6 months. This time interval, with associated loss of refined stereo vision could, in some patients, lead to occupational difficulties in, for example,  surgeons, dentists or jewellers. (1,5,6)

  The patient should be informed that occupational difficulties also result from the requirement to cease soft contact lens wear for 2 weeks preoperatively and hard contact lens wear for 6 weeks preoperatively.

The Excimer laser 

 

The Excimer laser  is an Argon / fluoride laser which emits coherent ultraviolet C light at 193 nm. It is capable of breaking intermolecular valence bond of proteins and other macromolecules without causing thermal damage to adjacent tissue. The selected wavelength also avoids mutagenesis.  The laser alters the radius of curvature of the cornea very precisely. The surface created by the laser is both remarkably smooth and geometrically precise. (1,5,6) Healing of the cornea is, however, less predictable and cannot as yet be mathematically modelled.

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 Table 3  The Excimer Laser

Used in ophthalmology since 1983

Only about  6 years of  good, clinical follow up at  present.

Subepithelial deposition of extracellular material occurs.

May cause corneal haze. ( i.e. corneal opacification or scarring )

6-8mm ablation Zone now favoured.  ( Zone > 5mm avoids halo effect  with night

vision by avoiding the central visual axis  )

Procedure lasts 20- 160 seconds   (1,2,5,6,7,8)

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  Mechanism of action of the excimer laser

The cornea is usually much steeper in the centre than in the periphery. The central cornea has less refractive power than the peripheral cornea following excimer laser treatment for myopia due to sculpting by the laser. (1,5,6) The cornea absorbs the coherent ultraviolet light and is sculpted in an extremely precise fashion, namely, by amounts of 0.5 microns (the central corneal thickness is about 500 microns). (1,5,6,8)

 

The long-term  clinical significance of corneal shock waves from the excimer laser is unknown. There does not appear to be a significant decrease in the corneal endothelial cell count, critical to the health of the cornea. (1,8)The required depth of corneal sculpting is proportional to the degree of myopia to be and / or astigmatism treated. 

Deep ablation may lead to structural weakness of the cornea. (1,5,6,7,8,9)

 

Laser procedure

The laser procedure is undertaken on an outpatient basis. 

Important steps in the procedure are as follows:

The surgeon checks that the patient is suitable for treatment and that the correct refractive error has been properly determined.

Patients require a detailed refractive assessment. Corneal topography (CVK)

prior to the procedure is now regarded as mandatory. It provides a computer

generated map of the surface of the cornea. It can detect corneal warpage from

contact lens wear. It can also detect early keratoconus in which case PRK may

not be inadvisable. The surgeon then enters the details of the desired refractive

correction into a computer.

   The corneal epithelium is removed with a steel scalpel blade. (1,8 )

The centration of the laser treatment is checked. The patient needs to maintain fixation. The eye not being treated is covered. The laser treatment itself is completed in less than one minute. (8)

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Table 4- Preoperative preparation for excimer Laser

VA + Refraction

Computerised Corneal Topography  (CVK) detects sub-clinical keratoconus and quantifies

astigmatism or other refractive error

Soft contact lenses are removed at least two weeks prior to procedure and hard contact lenses at least eight weeks (1,8 )

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  The laser treatment is undertaken using topical anaesthesia. The patient usually experiences some pain for two or three days. (10) Postoperatively, topical anti-inflammatory agents, antibiotics and oral analgesia are usually prescribed.  An eye patch is worn for one day. (8,10)

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Table 5- Post Operative Regimen

Cycloplegics

Analgesics

Lubricants

Antibiotics until epithelium has healed

The use of post operative topical steroids is controversial. Topical steroids may not  reduce corneal haze. ( Gartry DS, Kerr Muir MG, Lohmann CP, Marshall J. The effect of topical corticosteroids on refractive outcome and corneal haze after photorefractive keratectomy.  Arch Ophthalmol. 1992; 110 : 944-52. )

Re-treatment,  if necessary,  should be delayed for 6-12 months ( 8,9 )

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Excimer Laser Refractive Procedures

 

Photorefractive Keratectomy (PRK)

 

In photorefractive keratectomy an excimer laser makes accurate cuts in the cornea, decreasing the refractive power of the anterior corneal surface. It is used mainly for treating myopia. It can also be used to treat myopic regular astigmatism. (1,8,9,11 )

 

It is important to note that the excimer laser has been used clinically for only a few years; therefore the possible long term effects still remain unknown. (1,8,9,11 )

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Table 6 -  Currently Recommended Indications for PRK

Myopia < 4- 6 D  with little associated astigmatism (<1.5D)

( Theoretically ~20D of myopia is treatable by PTK and LASIK  but results are variable. )

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The results of treatment are less predictable for higher degrees of myopia. For example, a 7 dioptre myope is far less likely to be rendered emmetropic than a 2 dioptre myope. However, the 7 dioptre myope may still be delighted with a postoperative refractive error of 2 dioptres. Results may be also operator-dependent with experienced surgeons producing better results than less experienced surgeons. (1,8,9,11)

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Table 7 - Theoretical and Practical Problems with Treatment High of Myopia  (>6 Dioptre)  by PRK include,

Over or undercorrection, regression of treatment

Persistent corneal opacity

Irregular astigmatism

Decreased contrast sensitivity

Decrease of 1-3 lines of best corrected acuity on Snellen Chart                ( 1,8,9,10,11, )

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Laser in situ keratomileusis (  LASIK )

   

Laser in situ keratomileusis (LASIK) is a technique combining the advantages of lamellar corneal surgery (i.e. under a corneal flap, therefore not disturbing Bowman's layer centrally ) with the precision of excimer laser surgery. It can be used to correct moderate to high myopia between -2.00 to -20.00 D, with fewer refractive complications.

The technique of performing excimer surgery under a raised flap and the term LASIK was coined by Pallikaris and co-workers. ( 3, 12 )  Current surgical algorithms may need modification to improve predictability. ( 13,14,15 )  Stability of refraction after surgery requires further study but results are promising. 

 

LASIK must only be performed by experienced ophthalmic surgeons because of the possible complication of penetration of the globe and very serious intraocular injury. Occaisionally the corneal flap may also become permanently displaced or lost. ( 3,12,13,14,15 )

Phototherapeutic  Keratectomy (PTK)

 Phototherapeutic keratectomy (PTK) is an effective method of treating a variety of corneal disorders involving the corneal epithelium or anterior corneal stroma.  (table 8) Laser ablation of corneal lesions occupying the anterior stroma is a relatively safe means of treating several types of corneal scars and may occasionally offer an alternative to corneal transplantation. (1,8,16,17,18,19,20,21,22,23)

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Table 8- Indications for PTK

Corneal surface abnormalities and anterior stromal opacities within 50-70 micrometers of surface- Useful in

Anterior Stromal Scarring

Reiss Buckler’s Dystrophy

Recurrent Erosion Syndrome

Band Keratopathy

Bullous keratopathy

Cogan’s Fingerprint Dystrophy

Some complications of keratoconus e.g. contact lens intolerance (1,8,16,17,18,19,20,21,22)

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Cogan’s Fingerprint Dystrophy  

Tuunanen and Tervo performed phototherapeutic keratectomy (PTK) in 39 eyes of 38

patients with various corneal diseases. Patients were grouped into four diagnostic

categories: group 1 consisted of 11 eyes with postinfectious or

post traumatic corneal scars; 15 eyes with various corneal dystrophies or

keratopathies comprised group 2; group 3 consisted to two eyes with

recurrent corneal erosion; and 11 eyes with either primary or recurrent

pterygia composed group 4.  (21)

 

Postoperative follow-up ranged from 3 to 15 months ( mean  9 months).

The goals of PTK (visual improvement, ocular comfort, or visibility

for cataract extraction) were set individually for each patient and

were achieved in 19 of 38 eyes (50%); one patient was lost to follow-up

 

Best corrected visual acuity improved two or more Snellen lines in

nine of 31 eyes (29%). Both cases of recurrent corneal erosions were successfully

treated. One eye lost two Snellen lines because of increased irregular

astigmatism after PTK. In two eyes, corneal decompensation

was observed 1 month after the operation,  no other serious

complications were observed.

 

The excimer laser was combined with surgical abrasion or topical EDTA chelation, also with good results.

 

The authors concluded that the excimer laser is  a valuable tool for treating

anterior corneal disease. It can also be combined with other  surgical

methods such as surgical abrasion and chelation. Results could  have been further

improved with better patient selection criteria and by evaluating PTK on the

basis of visual expectations and type and location of corneal pathology. (21)

 

Topical corticosteroids and PRK

The long-term effects of topical corticosteroid use on the final  refractive results are

undocumented. The long-term effect of postoperative topical corticosteroids on

corneal haze is also controversial. According to studies by Gartry , Kerr Muir and co-

workers at St Thomas’s Hospital, London, no long term benefit from topical

corticosteroid use was noted. (11,24)

Results and Complications of PRK, Contraindications

 Problems with PRK include an unpredictable postoperative refractive result, and loss

of the desired refractive outcome ( regression ). (9) Patients tend to become under-

corrected with time, more so with higher degrees of correction. (1,8,911)

Over-correction ,i.e. hypermetropia, can occur- particularly with certain excimer laser

machines. The period of time required for visual rehabilitation depends on the amount

of ametropia corrected and the type of laser machine used.

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Table 9 Contraindications to excimer refractive surgery

 

PRK has well described contraindications, these include;

Monocularity

Irregular Astigmatism

Past or present corneal Herpes Simplex virus infection

Glaucoma

Connective Tissue disease

Ectatic corneal dystrophies ( i.e. keratoconus )

Relative Contraindications

 Pregnancy

Connective Tissue disease (1,8)  

Age younger than 18 years

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Complications of PRK

Complications of PRK include pain, corneal opacification, blurred vision, glare, impaired night vision, night haloes and astigmatism. (1,8,25,26,27,28) Night haloes, concentric blur circles caused by the edge of the excimer ablation zone, may be so troubling as to effect night driving. This is probably the commonest complication of well planned surgery.With higher degrees of myopia there is a greater risk of corneal scarring, blurred vision, ametropia and regression. (1,8,25,26,27,28)

 

Corneal  haze ( opacification ) often occurs following PRK; it is usually asymptomatic. In severe cases it causes blurred vision. “Haze” is a  euphemism that is used when referring to this opacification. The haze increases to a maximum at about 6 months postoperatively and tends to decrease to some extent after 2 or more years.(1,8,29)

 

Astigmatism and a prismatic effect can result from decentration of the laser treatment, this is often secondary to poor patient fixation. If the astigmatism is irregular then it cannot be optically neutralised in which case the vision will be permanently blurred. (1)

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Table 10- Night vision and  PRK

·       In one study, 78% of patients noticed a halo around lights at night. This symptom is less likely to occur with treatment zones larger than 4mm diameter.

Gartry DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive keratectomy 18-month follow-up. Ophthalmology 1992; 99:1209-19.

Impaired night vision and glare are still common problems, occurring in nearly 60% of patients.  -Kim JH, Sah WJ, Kim MS, Lee YC, Park CK.Three-year results of photorefractive keratectomy for myopia J Refract Surg. 1995; 11: 248-52. (24,25,26)

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One adverse  effect of treatment is an artefactual decrease in intraocular pressure due to the laser induced corneal thinning. 

The diagnosis of glaucoma, a condition more common in myopes,  may be unnecessarily delayed. (8)

 

Subjective results of excimer refractive surgery

A study conducted by H Hamberg-Nystrom and co-workers to evaluate the subjective results of patients who underwent PRK for myopia with excimer laser is typical of results for the procedure quoted in the current literature. (30)

 

One-hundred thirteen patients with preoperative myopia between -1.50 dioptres (D) and -6.50 D with an ablation diameter of 3.5 mm to 5.0 mm answered a questionnaire at the 36-month examination.Sixty-two percent of patients had both eyes treated. Halo problems were experienced always by 34% of patients in the group, 26% sometimes, and 40% never. Permanent night vision problems were experienced by 40%. 30% reported them sometimes and 30% reported no problems at all. Sixty percent never needed to use glasses after treatment, 30% sometimes did, and 10% always wore glasses. 

 

The authors conclude on an optimistic note that seventy percent of patients thought the final results were very good; the remaining 30% experienced varying degrees of halo and night vision problems. (30)

 

Conclusions and summary

Excimer laser surgery is an advancing field of ophthalmology.  Its use will greatly increase in the future. (1,8,31) Many  patients are already happy with the results of PRK. (30) The procedure is well tolerated by patients and is performed as an outpatient procedure. (1,8)  Current problems with  photorefractive procedures include the absence of long-term data, the possibility of blurred vision due to corneal “haze”  (opacification) , impaired night vision and haloes, and the lack of predictable results with high myopia.  (1,8,12,31,32)

 

References

 

 

1. Seiler T, Mc Donnell PJ,  Excimer Laser Photorefactive Keratectomy.  Survey of Ophthalmology 1995 ; 40 : 89-118

 

 2. Abrams D, Duke- Elder’s Practice of Refraction. London, Churchill Livingstone 1978 ; 188-192

 

3. Pallikaris IG,  Siganos DS,  Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia.  Journal of Refractive & Corneal Surgery. 1994 10 : 498-510

 

4. Elkington AR,  Frank HJ, Clinical Optics.  London, Blackwell 1988 ; 116-121

 

5. Epstein D.  Fagerholm P,  Hamberg-Nystrom H.  Tengroth B , Twenty-four-month follow-up of excimer laser photorefractive keratectomy for myopia. Refractive and visual acuity results.  Ophthalmology. 1994 ; 101: 1558-63

 

6. Talley AR,  Hardten DR,  Sher NA,  Kim MS,  Doughman DJ,  Carpel E,  Ostrov CS,  Lane SS,  Parker P,  Lindstrom RL ,  Results one year after using the 193-nm excimer laser for photorefractive  keratectomy in mild to moderate myopia. American Journal of Ophthalmology. 1994 ; 118 : 304-11

 

7. Hersh PS,  Patel R,  Correction of myopia and astigmatism using an ablatable mask Journal of Refractive & Corneal Surgery. 1994 ; 10 : 250-4

 

8. Claoue C, (editor) Laser and conventional refractive surgery London, BMJ Publishing Group1996 2-21, 40-53, 89-127, 269-302, 331-346

 

9. Sutton G,  Kalski RS,  Lawless MA,  Rogers C,  Excimer retreatment for scarring and regression after photorefractive keratectomy for myopia. British Journal of Ophthalmology. 1995 ; 79 : 756-9

 

10. Cherry PM, The treatment of pain following excimer laser photorefractive keratectomy: Ophthalmic Surgery and Lasers. 1996 27 : S477-80

 

11. Gartry DS, Kerr Muir MG, Lohmann CP, Marshall J. The effect of topical corticosteroids on refractive outcome and corneal haze after photorefractive keratectomy.  Arch Ophthalmol. 1992; 110 : 944-52.

 

12. Pallikaris I, Papatzanaki ME, Siganos D, A corneal flap technique of laser in situ keratomileusis Archives of Ophthalmology 1991; 145 : 1699-1702

 

13. Salah T, Waring GO,  el Maghraby A,  Moadel K,  Grimm SB,  Excimer laser in-situ keratomileusis (LASIK) under a corneal flap for myopia of 2 to 20 D.  Transactions of the American Ophthalmological Society. 1995 ;   93 : 163-83

 

14. Helmy SA,  Salah A,  Badawy TT, Sidky AN, Photorefractive keratectomy and laser in situ keratomileusis for myopia between 6.00 and 10.00 diopters. Journal of Refractive Surgery. 1996 ; 12:417-21

 

15. Guell JL,  Muller A, Laser in situ keratomileusis (LASIK) for myopia from -7 to -18 diopters. Journal of Refractive Surgery. 1996 ; 12: 222-8

 

16. Cameron JA,  Antonios SR,  Badr IA, Excimer laser phototherapeutic keratectomy for shield ulcers and corneal plaques in vernal keratoconjunctivitis. Journal of Refractive Surgery. 1995 ;  11 :31-5

 

17. O'Brart DP,  Muir MG,  Marshall J, Phototherapeutic keratectomy for recurrent corneal abrasions Eye. 1994.  8 : 378-83

 

18. Moodaley L,  Liu C, Woodward EG, O'Brart D,  Muir MK,Buckley R, Excimer laser superficial keratectomy for proud nebulae in keratoconus.  British Journal of Ophthalmology. 1994 ; 78 : 454-7

 

19. Ward MA, Artunduaga G, Thompson KP, Wilson LA, Stulting W, Phototherapeutic keratectomy for the treatment of nodular subepithelial corneal scars in patients with keratoconus who are contact lens intolerant. CLAO Journal. 1995 ; 21(2) ; 130-2

 

20. Thomann U,  Meier-Gibbons F,  Schipper I, Phototherapeutic keratectomy for bullous keratopathy  British Journal of Ophthalmology.  ; 79 : 335-8 1995

 

21. Tuunanen TH, Tervo TM, Excimer laser phototherapeutic keratectomy for corneal diseases: a follow-up study.  CLAO Journal. ;  21 : 67-72, 1995.


22. Orndahl M,  Fagerholm P,  Fitzsimmons T,  Tengroth B, Treatment of corneal dystrophies with excimer laser.  Acta Ophthalmologica. 1994 ;  72 : 235-40

 

23. Migden M,  Elkins BS,  Clinch TE , Phototherapeutic keratectomy for corneal scars. Ophthalmic Surgery and Lasers.  27  ; 1996 : S503-7

 

24. Gartry DS, Kerr Muir MG, Marshall J. The effect of topical steroids on refraction and corneal haze following excimer laser treatment of myopia: an update. A prospective randomised double-masked study.  Eye 1993; 7: 584-90.

 

25. Kim JH, Sah WJ, Kim MS, Lee YC, Park CK, Three-year results of photorefractive keratectomy for myopia J Refract Surg. 1995; 11: 248-52

 

26. Gartry DS, Kerr Muir MG, Marshall J,.Excimer laser photorefractive keratectomy 18-month follow-up. Ophthalmology 1992; 99:1209-19.

 

27. O'Brart DP,  Lohmann CP,  Fitzke FW,  Smith SE,  Kerr-Muir MG,  Marshall J, Night vision after excimer laser photorefractive keratectomy: haze and halos.  European Journal of Ophthalmology. 1994 ; 4 : 43-51

 

28. O'Brart DP.  Corbett MC.  Lohmann CP.  Kerr Muir MG.  Marshall J, The effects of ablation diameter on the outcome of excimer laserphotorefractive keratectomy. A prospective, randomized, double-blind study. Archives of Ophthalmology. 1995 113:438-43,

 

29. Cherry PM, Removal of epithelium and scraping the underlying stroma as treatment for photorefractive keratectomy overcorrection or undercorrection of myopia. Ophthalmic Surgery and Lasers. ; 27 : S487-92 1996

 

30. Hamberg-Nystrom H,  Tengroth B,  Fagerholm P, Epstein D, Patient satisfaction following photorefractive keratectomy for myopia.  Journal of Refractive Surgery. 1995 ; 11: S335-6

 

31. Carson CA,  Taylor HR, Excimer laser treatment for high and extreme myopia. Archives of Ophthalmology 1995 ; 113: 431-6

 

32.  Taylor HR,  Guest CS,  Kelly P,  Alpins NA, Comparison of excimer laser treatment of astigmatism and myopia.  Archives of Ophthalmology. 1993 ;  111:1621-

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John G. O'Shea MD

  Illustrations: Robert Harvey FRCSEd (from Practical Ophthalmology,  2002 Palmtrees Publishing)

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