Laser vision correction surgery

Small Incision Lenticule Extraction (SMILE): A- Lenticular cut and lenticular side cut (arrows); B- Cap cut and SMILE incision (arrow); C- White ring sign described by the author distinguishes anterior from posterior lamellar dissection; D- Intra-stromal lenticule is extracted. Laser vision correction (LVC) is widely accepted and popular among surgeons and patients alike for changing the refractive status of the eye and allowing ability to function without spectacle or contact lens dependency. Primarily performed for occupational, cosmetic or reasons of pure comfort, LVC may also be used to enhance binocular vision, control squint or help treat anisometropic amblyopia. With more than 13 million LASIK procedures having been performed in the United States alone as per 2017 data and more than 40 million worldwide, it is a very commonly performed procedure and is used to correct all refractive errors including myopia, hyperopia, astigmatism and presbyopia. PRE-OPERATIVE EVALUATION: Soft contact lenses should be discontinued for two weeks and semi-soft for four weeks prior to surgery. Pillar stones of pre-operative assessment are proper refraction, clinical examination and corneal tomography. Tomography may be done using slit scanning (Orbscan), Scheimpflug (Pentacam, Galilei, Sirius, TMS-5), reflective LED (Cassini) or OCT tomography (MS39). Anterior and posterior corneal surfaces, keratometric and pachymetric maps are examined for quantitative and qualitative abnormalities. The Belin Ambrosio Enhanced Ectasia Display is useful for detecting early abnormalities that may rule out LVC. Total ocular aberrations are compared with corneal aberrations to detect significant internal eye abnormalities such as early cataract or lens tilt, in which case lens based refractive correction may be preferred. Epithelial thickness maps unmask effects of epithelial remodeling on corneal tomography and are gaining prominence. Other adjunct investigations include assessment for dry eyes, corneal biomechanics etc. Glare and haloes may occur irrespective of pupil size and pupillometry, though previously considered important is not specifically correlated with night vision problems. Special consideration should be given to past history of HSV or HZV, significant dry eye, severe allergy or blepharitis, ptosis, squint, uveitis, glaucoma, retinal abnormalities or corneal diseases such as recurrent erosions, dystrophies, scars, neovascularization, previous surgery etc. Systemic conditions to be looked for include diabetes, connective tissue and immunodeficiency disorders, pregnancy, lactation and use of certain medications. Personality type, expectations and occupational needs of the patient also play a role. Deep set eyes, nystagmus, unstable fixation, uncooperative and pediatric patients need special precautions and techniques. Unstable refraction, inadequate residual stromal bed (RSB), forme fruste keratoconus and ectatic corneas are contraindications. TYPES OF LVC: LVC may be done using excimer laser or via all-femtosecond procedures. Excimer laser procedures include photorefractive keratectomy (PRK), laser epithelial keratomileusis (LASEK), epi-LASIK and LASIK whereas all femtosecond LVC includes Small Incision Lenticule Extraction (SMILE) and Femtosecond Lenticule Extraction (FLEx). Most are equally effective in correcting myopia. Patient characteristics and surgeon preference may dictate choice. LASIK is the most commonly performed LVC worldwide. A flap consisting of epithelium, Bowman’s membrane and stroma of variable thickness is cut with either microkeratome or femtosecond laser followed by excimer laser ablation of stromal bed. The flap is then reflected back and allowed to adhere. Though the flap allows painless, quick recovery and a “wow” effect, it does not contribute to biomechanical strength and can get displaced even years later. RSB is a crucial calculation and too thin RSB can predispose to ectasia. Femtosecond laser flaps allow advantages of predictability, accuracy, repeatability and safety while allowing thinner flaps with better edge profiles, thus decreasing intra- and post-operative complications such as incomplete, torn or buttonholed flaps, free caps, displaced flap, epithelial ingrowth etc. PRK involves epithelial removal followed by excimer laser ablation of underlying stroma. Epithelium is removed using dilute alcohol, hockey-stick, Amoil’s brush or even excimer laser itself. In LASEK and epi-LASIK, the epithelium is reflected as a flap which is replaced after ablation. Epi-LASIK uses an Epikeratome (an oscillating blunt plastic blade) for this purpose. Though debatable, replacing the epithelial flap is proposed to decrease pain, regression and haze that are potential complications of PRK. Surface ablation procedures are preferred when there is inadequate thickness for LASIK, dry eyes, glaucoma, basement membrane disease, recurrent epithelial erosions and occupational hazard of blows to the face. PRK allows larger optical zone and may be useful in larger pupils. It may also be a better option for enhancements and in wavefront guided ablations to avoid effect of iatrogenic flap aberrations. Surface ablation should be avoided when post-operative pain is an issue, rapid visual recovery is required or if there is predisposition to haze and regression. Intra-operatively, mitomicin-C is used off-label to decrease haze, especially with high corrections and with prior corneal surgery. Post-operatively, steroids are used for much longer. PRK-Xtra combines accelerated cross-linking in eyes with suspicious topographies. SMILE and FLEx use femtosecond laser to carve an intra-stromal lenticule that is extracted either via a small incision (SMILE) or after lifting a flap (FLEx), thus giving smooth and fast visual recovery. While SMILE has advantages of smaller incision, less flap/ cap related complications, better biomechanical strength and better preservation of corneal nerves leading to less dry eyes, FLEx loses these advantages. The vertical flap cut that weakens the cornea is avoided as vertical entry incision in SMILE is just 2mm. SMILE may be offered as first line treatment to patients with normal topography and may also preferentially be used in thin corneas, high powers and those with occupational risk of blows to face. Hyperopic SMILE is not yet commercially available. SMILE also has advantage of smoother work-flow. Unlike excimer ablation which is affected by stromal bed hydration and environmental conditions such as temperature and humidity, SMILE and FLEX are not affected by these. SMILE can have unique complications such as torn retained lenticule, adherent lenticule, epithelial implantation etc as also complications common to other LVC such as suction loss, opaque bubble layer, rough dissection, decentered ablation, diffuse lamellar keratitis, infectious keratitis, under- or over-correction, decreased contrast sensitivity, loss of BCVA, night vision symptoms, ectasia etc. The White Ring Sign and the ‘sequential, segmental, terminal lenticular-side-cut dissection’ techniques described by the author help avoid difficult dissection and is especially important with thin lenticules. EXCIMER LASER ABLATION: Excimer LVC can be done for myopia, hypermetropia, regular and irregular astigmatism. Centration of ablation on pupillary axis is sufficient in myopes, however in hyperopes with larger angle kappa, centration is done on visual axis or in between the two. Centration over entrance pupil increases risk of decentration of optical zone from visual axis in large angle kappa. The Munnerlyn formula (t = S2 D/3) gives tissue ablation depth (t); s= optical zone diameter (mm); D= dioptric correction. At least 300 microns of RSB is advisable. Percentage tissue altered {PTA = (flap thickness + ablation depth)/central corneal thickness} below 40% has been recommended. Excimer laser LVC may employ various ablation profiles such as wavefront optimized, aspheric, wavefront guided, topography guided, tissue saving and so on. Conventional ablation profile induces spherical aberration because of decreased peripheral corneal laser ablation due to angle of incident light on peripheral cornea. Wavefront optimized ablation compensates for this by applying additional pulses in the periphery. Wavefront guided ablations attempt to treat both lower and higher order aberrations of the patient’s cornea by applying complex ablation patterns, however these are limited to dilated pupil size. Topography guided ablations treat a wider zone and with much more detailed image of the corneal refractive surface. All commercial excimer lasers have integrated eye tracking technology. SPECIAL SITUATIONS: Enhancements are done after minimum three months and only after confirming refractive stability. Presbyopia may be treated with monovision correction or special multifocal ablation patterns. Patients with ocular hypertension and glaucoma need correction factor to be applied to IOP measurements post LVC. The possibility of pressure-induced stromal keratitis, interface fluid syndrome and false low IOPs should be kept in mind and treated. Post LASIK IOL power calculations need special attention because of altered anterior to posterior corneal curvature ratio. Dr. Soosan Jacob is Director and Chief of Dr. Agarwal's Refractive and Cornea Foundation at Dr. Agarwal's Eye Hospital, Chennai, India and can be reached at dr_soosanj@hotmail.com.

Tags: laser vision correction