Lasik Surgery Ioannis G Pallikaris, Shashi Kapoor, Srinivas K Rao
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Preoperative Assessment for LASIK1

Sathish Prabhu,
Sri Ganesh
2
 
INTRODUCTION
Lasik or Laser in situ keratomileusis is the gold standard in the treatment of refractive errors. It is the most commonly performed laser refractive surgery today.
 
WHY LASIK?
Lasik (Fig. 1.1) has become the most popular procedure among the refractive procedures because it is simple to perform, predictable, stable, and provides early recovery with minimal postoperative care.
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Fig. 1.1: LASIK
 
THE IDEAL LASIK CANDIDATE
The Ideal Lasik Candidate is the one who:
  • Is above 18 years of age and has a stable glass or contact lens prescription for at least one year.
  • Has a sufficient corneal thickness to allow the surgeon to safely create a corneal flap of appropriate depth.
  • Is affected by one of the common refractive errors.
    Myopia:
    −1.0 to −12.0D
    Hypermetropia:
    + 1.0 to + 6.0D
    Astigmatism:
    ± 6.0D
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  • Does not suffer from any eye disease like glaucoma, diabetic retinopathy etc.
 
COMPLETE MEDICAL AND OCULAR HISTORY
 
HISTORY OF WEARING GLASSES
A proper spectacle power history has to be taken to know whether the refractive error has stabilized or not. If the spectacle power is unstable, then it may not be the right time to do the Lasik procedure. An ideal candidate is the one who is at least 18 years of age and has a stable refractive error for at least one year. Higher myopic refractive errors may take a longer time to stabilize. Hence it may be wise to wait for a longer time in these patients.
 
CONTACT LENS HISTORY
Contact lenses may change the corneal curvature, may alter the refractive error and may also interfere with the Pachymetry measurements. It is ideal to ask the patient to stop using the contact lenses for at least 7 days (minimum 3 days) in case of soft contact lenses and for at least 2 weeks in case of RGP lenses. If the contact lens changes (corneal warpage) persist in the serial topographic maps, then the patient may have to be asked to stop the contact lenses for a much longer period till the changes disappear.
 
EYE HEALTH HISTORY
A history of ocular conditions like herpetic keratitis, corneal refractive or ocular surgery, glaucoma, retinal detachment, lattice degeneration, injury, recurrent epithelial erosions should be elicited.
 
MEDICAL HISTORY
A history of medical conditions such as diabetes mellitus, collagen vascular disorders, pregnancy, keloids should be elicited. Diabetes mellitus may increase the risk for dry eyes, infections, and corneal epithelial adherence problems. Moreover, uncontrolled diabetes is a contraindication for Lasik procedure as it may lead to changes in the 4refractive status of the patient. Collagen vascular disorders can cause a severe dry eye, delayed corneal healing and may increase the risk of keratectasia. Pregnancy and its accompanying hormonal changes may lead to an unstable refractive status translating into variable post operative refractive outcome.
 
COMPREHENSIVE EXAMINATION OF THE EYE
  • Determination of uncorrected (UCVA) and best corrected visual acuity (BCVA).
  • Determination of the refractive error of the patient: An accurate refraction is very essential for a good refractive surgery outcome. It is very essential for a refractive surgeon to identify the accommodative component in the patient's refractive error and make necessary corrections for the same. When in doubt, one may perform a cycloplegic refraction using cyclopentolate eyedrops.
  • Corneal topography: It is the computerized contour mapping of the cornea. Topography is used to diagnose or rule out various corneal conditions like keratoconus, (Figs 1.2A and B) posterior cone, irregular astigmatism, and contact lens warpage. Clinical and preclinical Keratoconus are absolute contraindications to any form of refractive procedures. Corneal warpage from contact lens wear results in a central irregular astigmatism that is accompanied clinically by an unstable refraction, decreased visual acuity, spectacle blur and reduced contrast sensitivity. Severe warpage may be a contraindication for going ahead with refractive surgery even if the contact lenses are not worn over a long period. The corneal topography is also very important in patients with a past history of keratorefractive procedures. It is performed in these patients to assess the extent of irregular astigmatism, change in corneal curvature and decentred ablations.
Various topography systems may be used to assess the shape of the cornea. These are:
  • Placido based—EYE SYS, TOMEY
  • Slit scanning—ORBSCAN-II (Figs 1.3A and B)
  • Raster stereography—PAR (Posterior Apical Radius imaging device)
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Fig. 1.2A: Early keratoconus
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Fig. 1.2B: Keratoconus
The ideal topography system is the Orbscan II. Orbscan II is the latest in the state of the art technology for mapping not only the surface of the cornea but also perform a complete anterior segment analysis.
Orbscan II is an integrated system and provides elevation information of the anterior and posterior surface of cornea using scanning slit beam along with Keratometric map using placido system, corneal thickness (Pachymetry) and anterior chamber depth information.
Isolated Placido devices only measure rate of change of curvature from the center to the periphery in a radial fashion.
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Figs 1.3A and B: Orbscan and aberrometer
It has been demonstrated to be a poor measuring tool outside the central zone of the normal eye.
The Orbscan II does a great job of measuring elevation and provides 3 D information from its slit scanning data (Figs 1.4A and B). Using this, Placido can calculate an excellent angle on that 3D surface because it does not have to make an assumption of where any point is on the 3D surface. Therefore, a combination of the data can provide better curvature measurements.
 
INTEGRATED LASIK WORKSTATION: ORBSCAN IIZ AND ZYWAVE ABERROMETER
 
CORNEAL PACHYMETRY
Pachymetry is an important and integral part of any corneal refractive surgery work-up. Corneal thickness will decide the maximum power that can be treated by Lasik. According to FDA regulations, a refractive surgeon must leave a stromal bed of 250 μ or 50% of the corneal thickness after the Lasik procedure. Initial Pachymetry minus calculated flap thickness minus ablation depth must be greater than or equal to 250 μ. Ultrasonic pachymetry or Orbscan pachymetric measurements may be used to assess the corneal thickness. The corneal thickness obtained by Orbscan is thicker than the ultrasonic measurements.
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Fig. 1.4A: Normal topography with this cornea
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Fig. 1.4B: Posterior cone
Hence, an acoustic equivalent of 0.92 to 0.95 is used to obtain a pachymetric value that is equivalent to the ultrasonic value.
 
ABERROMETRY MEASUREMENTS
Aberrometry measurements are done if one is performing a wavefront Lasik or customized Lasik.
 
TEAR FILM ASSESSMENT
An assessment of the tear film is essential to rule out dry eyes. Mild to moderate dry eye is a relative contraindication whereas a severe 8dry eye is an absolute contraindication for the Lasik procedure as the underlying condition increases the risk of corneal thinning and ectasia. Qualitative assessment is done by examining the tear meniscus and the tear BUT and quantitative assessment is done using the Schirmer's test.
 
FUNDUS EXAMINATION
A central and peripheral retinal examination is done in every patient undergoing the Lasik procedure. Prophylactic laser is done in cases with retinal holes or lattice with holes. Lasik procedure is performed about 15 days after the prophylactic laser.
 
CONCLUSION
Every refractive surgeon must follow a protocol during the preoperative assessment of the patient. This is very essential in order to reduce the risk of complications and improve the patient satisfaction.