Kanski - Corneal and Refractive Surgery (2024) PDF

Chapter Corneal and Refractive Surgery 8 KERATOPLASTY 276 Deep anterior lamellar KERATOPROSTHESES 284 Introduction 276 keratoplasty 281 E...

Chapter Corneal and Refractive Surgery 8 KERATOPLASTY 276 Deep anterior lamellar KERATOPROSTHESES 284 Introduction 276 keratoplasty 281 Endothelial keratoplasty 282 REFRACTIVE PROCEDURES 284 Penetrating keratoplasty 277 Limbal stem cell grafting 282 Introduction 284 Supercial lamellar keratoplasty 281 Laser refractive procedures 287 275 276 Keratoplasty  erapeutic corneal transplantation facilitates removal of KERATOPLASTY infected corneal tissue in eyes unresponsive to antimicrobial therapy (Fig. 8.1D). Introduction  Cosmetic graing may be performed to improve the appear- Corneal transplantation (graing) refers to the replacement of ance of the eye, but is a rare indication. diseased host corneal tissue by healthy donor cornea. A corneal gra (keratoplasty) may be partial-thickness (anterior or posterior Donor tissue lamellar) or full-thickness (penetrating). Donor tissue should be removed within 12–24 hours of death. ere is an attempt to age-match donors and recipients. Corneas General indications from infants (3 years and under) are used only very occasionally,  Optical keratoplasty is performed to improve vision. Impor- even for paediatric transplants, as they are associated with surgical, tant indications include keratoconus, scarring, corneal dys- refractive and rejection problems. Most corneas are stored in coor- trophies (Fig. 8.1A), pseudophakic bullous keratopathy and dinating ‘eye banks’ prior to transplantation, where pre-release corneal degenerations. evaluation includes medical history review and donor blood  Tectonic graing may be carried out to restore or preserve screening to exclude contraindications and microscopic examina- corneal integrity in eyes with severe structural changes such as tion of the cornea including endothelial cell count determination. thinning with descemetocoele (US spelling – descemetocele) Corneas are preserved in hypothermic storage (up to 7–10 days) (Fig. 8.1B) and perforated corneal ulcer (Fig. 8.1C). or organ culture medium (4 weeks) until needed. Culture allows A B C D Fig. 8.1 (A) Optical penetrating keratoplasty for macular corneal dystrophy; (B) tectonic lamel- lar patch graft for descemetocoele; (C) lamellar patch graft for a perforated corneal ulcer; (D) penetrating keratoplasty for pseudomonas keratitis – a dense cataract and posterior synechiae are visible. (Courtesy of S Tuft – g. B.)  CHAPTER Corneal and Refractive Surgery 8 277 extended testing for infective contamination. Contraindications to  Abnormalities of the eyelids, such as blepharitis, ectropion, ocular tissue donation are set out below, though there is interna- entropion and trichiasis. ese conditions should be addressed tional variation and the list is not exhaustive: before surgery.  Death from unknown cause.  Recurrent or progressive forms of conjunctival inammation,  Certain systemic infections such as human immunodeciency such as atopic conjunctivitis and ocular cicatricial pemphigoid. virus (HIV), viral hepatitis, syphilis, congenital rubella, tuber-  Tear lm dysfunction. culosis, septicaemia and active malaria.  Anterior synechiae.  Prior high-risk behaviour for HIV and hepatitis such as sex  Uncontrolled glaucoma. with someone HIV-positive, men who have sex with men,  Uveitis. intravenous drug abuse and prostitution.  Within the last 12 months: sex with someone who has engaged in high-risk behaviour, who has received blood clotting fac- Penetrating keratoplasty tor concentrates or has undergone tattooing, acupuncture, ear/ Component layer graing of the cornea is increasingly utilized, body piercing. but full-thickness keratoplasty remains commonly performed and  Infectious and possibly infectious diseases of the central ner- is the appropriate procedure for disease involving all layers of the vous system, such as Creutzfeldt–Jakob disease, systemic cornea (Fig. 8.3). Key surgical points include: sclerosing panencephalitis, progressive multifocal leuko-  A common gra size is 7.5 mm. Small gras may lead to encephalopathy, encephalitis, Alzheimer disease and other high astigmatism and larger diameters are associated with an dementias, Parkinson disease, multiple sclerosis and motor increasing tendency to peripheral anterior synechiae forma- neurone disease. tion and raised intraocular pressure (IOP).  Receipt of a transplanted organ.  e donor button is usually about 0.25 mm larger in diameter  Receipt of human pituitary-derived growth hormone. than the host site.  Brain or spinal surgery before 1992.  Preparation of donor cornea should always precede excision  Most haematological malignancies. of host tissue, in case a problem with the former means that the  Ocular disease such as inammation and disease likely to com- surgery cannot be completed. promise gra outcome, some malignant ocular tumours (e.g.  Trephination. Either mechanically guided manual or auto- retinoblastoma) and corneal refractive surgery. mated (including laser) trephination is commonly used.  Sutures. e gra may be secured with either continuous (Fig. Recipient prognostic factors 8.4A) or interrupted (Fig. 8.4B) suture techniques, or a com- e following host factors may adversely aect the prognosis of a bination of both. corneal gra and, if possible, should be optimized prior to surgery. In general, the most favourable cases are keratoconus, localized Postoperative management scars and dystrophies.  Topical steroids (e.g. prednisolone acetate 1%, dexamethasone  Severe stromal vascularization (Fig. 8.2), absence of corneal phosphate 0.1%) are used to decrease the risk of immunologi- sensation, extreme thinning at the proposed host–gra junc- cal gra rejection. Initial administration is typically every 2 tion and active corneal inammation. hours with gradual tapering depending on the likelihood of rejection and clinical progress. Long-term instillation at low intensity, such as once daily for a year or more, is usual.  Other immunosuppressants such as oral azathioprine and topical and systemic ciclosporin are usually reserved for high- risk patients.  Cycloplegia (e.g. homatropine 2% twice daily) is typically used for 1–2 weeks.  Oral aciclovir may be used in the context of pre-existing herpes simplex keratitis to minimize the risk of recurrence.  Monitoring of IOP. Applanation tonometry is relatively unre- liable so measurement is commonly performed during the early postoperative period with a non-applanation method.  Monitoring endothelial function. is can be done indirectly and quickly by measuring the central corneal thickness with a pachymeter.  Removal of sutures is performed when the gra–host junc- tion has healed. is is oen aer 12–18 months, although in elderly patients it may take much longer. Removal of broken or Fig. 8.2 Retroillumination of heavily vascularized cornea resulting from recurrent herpes simplex keratitis. (Courtesy loose individual sutures should be performed as soon as iden- of C Barry.) tied, as this reduces the risk of gra rejection. 278 Keratoplasty A B C D E F G H Fig. 8.3 Penetrating keratoplasty technique. (A) Partial-thickness trephination; (B) incision into the anterior chamber; (C) and (D) completion of excision; (E and F) donor button is placed onto the viscoelastic bed; (G) initial cardinal suture; (H) eight interrupted cardinal sutures in place. (Courtesy of R Fogla.)  CHAPTER Corneal and Refractive Surgery 8 279 A B Fig. 8.4 Penetrating keratoplasty sutures. (A) Secured by continuous sutures; (B) interrupted sutures. (Courtesy of C Barry.) A B Fig. 8.5 (A) Protruding suture with vascular ingrowth; (B) broken sutures within stroma.  Late complications include astigmatism, recurrence of under- TIP In a patient with a corneal graft, broken or loose sutures lying disease, late wound dehiscence, retro-corneal membrane should be removed as this reduces the risk of localized formation, glaucoma, rejection (see below) and failure without vascularization and graft rejection. rejection. Postoperative complications Corneal graft rejection  Early complications include persistent epithelial defects, loose Immunological rejection of any layer of the cornea can occur. or protruding sutures (Fig. 8.5A and B) (risk of infection – Rejection of separate layers (endothelial, stromal and epithelial) marked sterile reaction, papillary hypertrophy), wound leak can occur in isolation, but typically a combination is present. (sometimes with at anterior chamber or iris prolapse), uve- Simple gra failure can occur in the absence of rejection, although itis, elevation of intraocular pressure, traumatic gra rupture rejection is a common contributory factor. (Fig. 8.6A), cystoid macular oedema, microbial keratitis (Fig. 8.6B), endophthalmitis (Fig. 8.6C) and rejection (see below). A rare complication is a xed dilated pupil (Urrets-Zavalia TIP The risk of corneal graft rejection is increased signicantly by the presence of host stromal vascularization. syndrome). 280 Keratoplasty Important predisposing factors for rejection include eccentric or larger gras (over 8 mm in diameter), infection (particu- larly herpetic), glaucoma and previous keratoplasty. If the host becomes sensitized to histocompatibility antigens present in the donor cornea, rejection may result. Human leukocyte antigen (HLA) matching has a small benecial eect on gra survival. Gender incompatibility has recently emerged as an important risk factor for gra rejection and failure. Whereas the cornea of a female donor can be used in either male or female recipients, the cornea of a male donor should not be allocated to a female recipient.  Symptoms. Blurred vision, redness, photophobia and pain are typical, but many cases are asymptomatic until rejection is established. e timing of onset is very variable, occurring A from days to years aer keratoplasty.  Signs vary depending on the type of gra. { Ciliary injection associated with anterior uveitis is an early manifestation (Fig. 8.7A). { Epithelial rejection may be accompanied by an elevated line of abnormal epithelium (Fig. 8.7B) in a quiet or mildly inamed eye, occurring at an average of 3 months. { Subepithelial rejection is characterized by subepithelial inltrates, reminiscent of adenoviral infection (Krach- mer spots – Fig. 8.7C) on the donor cornea, with deeper oedema and inltrative opacication. { Stromal rejection features deeper haze. It can be chronic or hyperacute, the latter in association with endothelial rejection. { Endothelial rejection is characterized by a linear pattern of keratic precipitates (Khodadoust line – Fig. 8.7D) associ- B ated with an area of inammation at the gra margin. { Stromal oedema is indicative of endothelial failure.  Management. Early intensive treatment greatly improves the likelihood of reversing the rejection. e most aggressive regi- men is generally required for endothelial rejection, followed in order of severity by stromal, subepithelial and epithelial. Intra- ocular pressure monitoring is critical. { Preservative-free topical steroids hourly for 24 hours are the mainstay of therapy. e frequency is reduced gradu- ally over several weeks. Steroid ointment can be used at bedtime as the regimen is tapered. High-risk patients can be maintained on the highest tolerated topical dose (e.g. prednisolone acetate 1% four times daily) for an extended period. { Topical cycloplegia (e.g. homatropine 2% or atropine 1% once or twice daily). C { Topical ciclosporin 0.05% to 2% may be of benet, but the onset of action is delayed. Fig. 8.6 Postoperative complications. (A) Traumatic graft rup- { Systemic steroids. Oral prednisolone 1 mg/kg daily for 1–2 ture and extrusion of intraocular lens implant; (B) microbial weeks with subsequent tapering. If given within 8 days of keratitis; (C) endophthalmitis. (R Bates – g. A; S Tuft – g. C.) onset, intravenous methylprednisolone 500 mg daily for up to 3 days may be particularly eective, suppressing rejection and reducing the risk of further episodes.  Pathogenesis. e corneal gra is immunologically privi- { Subconjunctival steroid injection (e.g. 0.5 ml of 4 mg/ml leged, with an absence of blood vessels and lymphatics and the dexamethasone) is sometimes used. presence of relatively few antigen-presenting cells. Inamma- { Other systemic immunosuppressants such as ciclosporin, tion and neovascularization contribute to loss of this privilege. tacrolimus or azathioprine can be considered.  CHAPTER Corneal and Refractive Surgery 8 281 A B C D Fig. 8.7 Allograft rejection. (A) Ciliary injection; (B) elevated epithelial line in epithelial rejec- tion (arrow); (C) Krachmer spots; (D) endothelial rejection with Khodadoust line (arrow). Note intense peripheral vascularization. (Courtesy of S Tuft – gs A, B and C.)  Dierential diagnosis includes gra failure (no inamma- { Marginal corneal thinning or inltration as in recurrent tion), infective keratitis including fungal and herpetic, uveitis, pterygium, Terrien marginal degeneration and limbal der- sterile suture reaction, raised IOP and epithelial ingrowth. moid or other tumours. { Localized thinning or descemetocoele formation (see Fig. 8.1B). TIP In a patient with acute corneal graft rejection, early intensive treatment greatly improves the likelihood of reversing the episode of rejection. Deep anterior lamellar keratoplasty Deep anterior lamellar keratoplasty (DALK) is a technique in which corneal tissue is removed almost to the level of Descemet Supercial lamellar keratoplasty membrane. A theoretical advantage is the decreased risk of rejec- is involves partial-thickness excision of the corneal epithelium tion because the endothelium, a major target for rejection, is not and stroma so that the endothelium and part of the deep stroma transplanted. e major technical diculty lies in judging the are le behind as a bed for appropriately partial-thickness donor depth of the corneal dissection as close as possible to Descemet cornea. e area graed depends on the extent of the disease pro- membrane without perforation and if this is not achieved the cess to be addressed. visual outcome may be compromised.  Indications  Indications { Opacication of the supercial one-third of the corneal { Disease involving the anterior 95% of corneal thickness stroma not caused by potentially recurrent disease. with normal endothelium and the absence of breaks or 282 Keratoplasty outcomes and lower rejection rates are achieved, but intraopera- tive complication rates are higher than DSAEK.  Indications include endothelial disease such as Fuchs endo- thelial corneal dystrophy and a localised tectonic gra for patching of a corneal perforation.  Advantages { Relatively little refractive change and a structurally more intact globe. { Faster visual rehabilitation than penetrating keratoplasty. { Suturing is minimized.  Disadvantages { Signicant learning curve. { Specialized equipment is required. { e visual outcome is oen suboptimal regardless of the gra clarity. Reasons include: (a) dierences in gra thick- ness, (b) gra irregularities, (c) high order aberrations, (d) Fig. 8.8 Deep anterior lamellar keratoplasty for chemical in- donor–recipient interface brosis. jury. A superior conjunctival limbal autograft has also been { Endothelial rejection can still occur (Fig. 8.10). performed (arrows indicate the edge of conjunctival graft). TIP Endothelial keratoplasty (DSAEK) results in more rapid visual improvement and less risk of rejection than penetrating scars in Descemet membrane (e.g. keratoconus without a keratoplasty. history of acute hydrops, supercial trauma – Fig. 8.8). { Chronic inammatory disease such as atopic keratocon- junctivitis that carries an increased risk of gra rejection.  Advantages Limbal stem cell grafting { No risk of endothelial rejection, although epithelial/sub- Stem cells are undierentiated cells that give rise to the dierenti- epithelial/stromal rejection may occur. ated cells that form tissues and organs. Tissue-specic stem cells { Less astigmatism and a structurally stronger globe com- are important for maintaining homeostasis and for tissue repair pared with penetrating keratoplasty. aer injury. Corneal epithelium is constantly self-renewing and { Increased availability of gra material since endothelial does so throughout life. Progeny of corneal stem cells divide and quality is irrelevant. dierentiate to form basal corneal epithelial cells (transient ampli-  Disadvantages fying cells) that dierentiate into wing cells (post-mitotic cells) { Dicult and time-consuming with a high risk of and, nally, supercial squamous cells (terminally dierentiated perforation. cells). { Interface haze may limit the best nal visual acuity. Various techniques have been described to replenish the { Postoperative management is similar to penetrating limbal stem cell population in patients with severe ocular sur- keratoplasty except that lower intensity topical steroids face disease and associated stem cell deficiency. These include are needed and sutures can usually be removed aer 6 transplantation of a limbal area of limited size from a healthy months. fellow eye, complete limbal transplantation of a donor annulus and exvivo expansion by culture of either host or donor stem cells with subsequent transplantation. In patients with uni- Endothelial keratoplasty lateral ocular involvement in Stevens–Johnson syndrome and Endothelial keratoplasty involves removal of diseased endothe- ocular cicatricial pemphigoid, donor tissue should not be taken lium along with Descemet membrane (DM) through a corneo- from the unaffected eye (autologous). scleral or corneal incision. Folded donor tissue is introduced  Signs of limbal stem cell deciency (LSCD) include: through the same small (2.8–5.0 mm) incision. Descemet strip- { Conjunctivalization of cornea with goblet cells (conrmed ping (automated) endothelial keratoplasty (DSAEK) uses an auto- with confocal microscopy, impression cytology with acid mated microkeratome to prepare donor tissue and is currently the Schi stain or monoclonal antibody against cytokeratin 19). most commonly performed technique. A small amount of poste- { Supercial and deep corneal vascularization. rior stromal thickness is transplanted along with DM and endothe- { Fibrovascular pannus. lium (Fig. 8.9). In Descemet membrane endothelial keratoplasty { Persistent epithelial defects. (DMEK) only the DM and endothelium are transplanted. { Scarring. Because the gra is only 5–10 μm thick it is dicult to handle.  Algorithm for surgical interventions for LSCD However, the gras are able to attach to the host tissue with less { All associated abnormalities (lids, conjunctiva, intraocular unevenness, which leads to more rapid visual recovery. Visual pressure) should be treated rst.  CHAPTER Corneal and Refractive Surgery 8 283 A B C D Fig. 8.9 DSAEK. (A) Scleral tunnel incision (arrowheads) with an air bubble in the anterior chamber to facilitate visualization of Descemet membrane; (B) the far edge of Descemet membrane is stripped using a reverse Sinskey hook (arrow); (C) the donor graft is inserted through the scleral tunnel using an endoglide system; (D) once the graft has unfolded and centred it can be gently stroked into position, making sure that there is no uid in the donor/ recipient interface. (Courtesy of M Leyland.) { In total bilateral stem cell deciency, use of allogras from a living related donor or a cadaver donor is the only option.  Indications { Congenital: for example, aniridia. { Traumatic: chemical and thermal burns. { Chronic inammatory disease: Stevens–Johnson syn- drome, ocular cicatricial pemphigoid. { Ocular surface malignancy. { Contact lens-related pathology.  Advantages { Regeneration of corneal surface epithelium. { Visual improvement and improved comfort.  Disadvantages { Autologous gra: conjunctivalization, lamentary kerati- tis, scarring of donor eye. Fig. 8.10 Endothelial graft undergoing rejection. (Courtesy of { Allogenic gra: infection (especially in immune- C Barry.) compromised patients), rejection. { When the visual axis is involved, sequential sector con- junctival epitheliectomy should be considered with a sec- tor limbal transplant. TIP An important sign of limbal stem cell deciency is { In total unilateral stem cell deciency, auto limbal trans- colonization of the cornea with goblet cells, detected with impression cytology. plantation is the procedure of choice. 284 Refractive Procedures or lens, the principal refracting components. Myopia, hyper- metropia (hyperopia) and astigmatism can all be addressed, though correction of presbyopia is yet to be achieved on a con- sistently satisfactory basis.  e patient population consists of people with stable refrac- tive error who desire an improvement in visual acuity or func- tion aer surgical correction.  A thorough preoperative assessment is mandatory. Not everyone who requests this treatment is suitable. People who require a high level of quality vision, e.g, certain professions (pilot, air-trac controller, truck-driver) and specic hobbies (contact sports, marksman, amateur astronomer) must under- stand the limitations of the surgery before proceeding.  Informed consent is vitally important.  Contraindications include: people with unacceptably high expectations, dry eye syndrome, non-stable refractive error, Fig. 8.11 Keratoprosthesis. (Courtesy of R Bates.) some systemic diseases (SLE, rheumatoid arthritis, immu- nocompromise) and those with ocular disease (chronic blepharitis, previous herpes simplex, glaucoma, macular degeneration). KERATOPROSTHESES  e advantages and potential risks should be discussed and Keratoprostheses (Fig. 8.11) are articial corneal implants used in the common adverse side eects such as under- and over- patients unsuitable for keratoplasty. e modern osteoodontokerato- correction, dry eye symptoms and eventual presbyopia should prosthesis consists of the patient’s own tooth root and alveolar bone be emphasized. Care should be taken to discuss which adverse supporting a central optical cylinder and is usually covered with a buc- events are likely to be transient and which may be permanent. cal mucous membrane gra. Surgery is dicult and time-consuming  Alternatives, including new and emerging technology for and is performed in two stages, 2–4 months apart. In experienced treatment, should be discussed. hands the retention rate is about 85% at 18 months. Approximately  Patient satisfaction depends on patient expectations and the 80% of patients achieve visual acuity between counting ngers and surgical outcome. Most are satised aer the surgery, but some 6/12 and occasionally even better. A poor outcome is usually associ- may be unhappy because of induced ocular side eects and ated with pre-existing optic nerve or retinal dysfunction. High patient visual aberrations, even though the intended correction has motivation is a key factor to consider before proceeding to surgery. been achieved.  Indications { Bilateral blindness from severe, but inactive anterior segment disease with no realistic chance of success from conven- TIP People who require a high level of quality vision because tional keratoplasty, e.g. Stevens–Johnson syndrome, ocular of their occupation or hobbies need to understand the cicatricial pemphigoid, chemical burns and trachoma. limitations of refractive surgery and make an informed decision { Vision of counting ngers or less in the better eye. before proceeding to surgical correction. { Intact optic nerve and retinal function, without marked glaucomatous optic neuropathy. Correction of myopia  Complications  Surface ablation procedures (see below) can correct low– { Glaucoma in up to 40%. Management is inevitably extremely moderate degrees of myopia. challenging and requires aggressive IOP-lowering medica-  Laser in situ keratomileusis (LASIK – see below) can correct tion and oen the insertion of a drainage shunt. moderate to high myopia depending on initial corneal thick- { Membrane formation behind the prosthesis (25%); usually ness, but for very high refractive errors one of the intraocular managed with YAG-laser membranectomy. procedures below is necessary. { Tilting or extrusion.  Refractive lenticule extraction (SMILE – see below) is a small { Retinal detachment and endophthalmitis (about 3% per incisional laser technique for the correction of myopia and patient year). myopic astigmatism.  Clear lens exchange gives very good visual results but carries a small risk of the complications of cataract surgery (see Ch. 10), REFRACTIVE PROCEDURES particularly retinal detachment in individuals with high myopia. Introduction  Iris clip (‘lobster claw’) implant is attached to the iris (Fig.  Refractive surgery encompasses a range of procedures aimed 8.12A). Complications include subluxation or dislocation due at changing the refraction of the eye by altering the cornea to dislodgement of one or both attachments, an oval pupil,  CHAPTER Corneal and Refractive Surgery 8 285 A B Fig. 8.12 Phakic intraocular implants for correction of myopia. (A) Anterior chamber iris claw implant with anterior iris attachment at 3 and 9 o’clock; (B) emplacement of a posterior cham- ber phakic implant between the iris and anterior lens surface. (Courtesy of J Krachmer, M Mannis and E Holland from Cornea, Mosby 2005 – g. B.) endothelial cell loss, cataract, pupillary-block glaucoma and retinal detachment.  Phakic posterior chamber implant (implantable contact lens, ICL) is inserted behind the iris and in front of the lens (Fig. 8.12B) and supported in the ciliary sulcus. e lens is com- posed of material derived from collagen (Collamer) with a power of −3 D to −20.50 D. Visual results are usually good, but complications include uveitis, pupillary block, endothelial cell loss, cataract formation and retinal detachment.  Radial keratotomy (Fig. 8.13) is now predominantly of his- torical interest. Correction of hypermetropia (hyperopia)  Surface ablation procedures can correct low degrees of hypermetropia.  LASIK can correct up to 4 D.  Conductive keratoplasty (CK) involves the application of Fig. 8.13 Radial keratotomy. (Courtesy of C Barry.) radiofrequency energy to the corneal stroma in order to cor- rect low–moderate hypermetropia and hypermetropic astig- matism. Burns are placed in one or two rings in the corneal (Fig. 8.14A) in the axis of the correcting ‘plus’ cylinder (the periphery using a micro-needle high-frequency probe. e steep meridian). e resultant attening of the steep meridian resultant thermally induced stromal shrinkage is accompanied coupled with a smaller steepening of the at meridian at 90° to by an increase in central corneal curvature. Complications the incisions reduces astigmatism. e desired result can be con- are infrequent, but the disadvantages include early over- trolled by varying the length and depth of the incisions and their correction, signicant regression and induced astigmatism. distance from the optical centre of the cornea. Arcuate keratot- CK may also be helpful for presbyopia (see below). Laser ther- omy may be combined with compression sutures placed in the mal keratoplasty with a holmium laser is no longer used. perpendicular meridian, when treating large degrees of astigma-  Other modalities include clear lens extraction and phakic lens tism such as can occur following penetrating keratoplasty. implants as described above for myopia. Intraocular surgical  PRK and LASEK can correct up to 3 D. procedures are the only options for high degrees of refractive  LASIK can correct up to 5 D. error.  Lens surgery involves using a ‘toric’ intraocular implant incorporat- ing an astigmatic correction (Fig. 8.14B). Postoperative rotation of the Correction of astigmatism implant away from the desired axis occurs in a minority of cases.  Limbal relaxing incisions/arcuate keratotomy involves mak-  Conductive keratoplasty (see ‘Correction of hypermetropia’ ing paired arcuate incisions on opposite sides of the cornea above). 286 Refractive Procedures A B Fig. 8.14 Correction of astigmatism. (A) Arcuate keratotomies; (B) toric intraocular implant in situ – markings incorporated in the lens (arrows) facilitate correct orientation. (Courtesy of C Barry – g. A.) Correction of presbyopia  Corneal multifocality: presbyLASIK. Several dierent  Lens extraction, either to treat cataract or for purely refractive approaches are under development utilizing a laser procedure purposes. Acronyms used include clear lens exchange (CLE), to alter the shape of the cornea such that a bifocal or transi- refractive lens exchange (RLE) and presbyopic lens exchange tional eect is induced. Laser manufacturers have developed (PreLEx). Much research eort is being applied to the devel- specic treatment algorithms based on the near-addition target opment of eective accommodating and multifocal prosthetic (or planned according to the subject’s age) to create a multifo- lenses. cal corneal prole. However, the eective change in refraction { Implantation of a bifocal, trifocal diractive, multifocal comes from the controlled change in the corneal asphericity refractive, extended depth of focus or ‘accommodating’/ from the centre to the periphery. e benet of presbyLASIK pseudo-accommodative intraocular lens implant can opti- over monofocal ablation is not yet clear. cally restore reading vision, but glasses may still have to be  Scleral expansion surgery. Results have been inconsistent and used for some tasks. If appropriately screened, most recipi- unpredictable and this technique has not achieved sustained ents of multifocal IOLs are happy with the visual outcome. popularity. However, dissatisfaction may occur as a consequence of  Intracorneal inlays provide substantial benet in presbyopia, nocturnal glare, halo and reduced contrast sensitivity. In though in the past the biocompatibility of some materials has patients with intractable visual symptoms IOL exchange been relatively poor and complications such as corneal haze surgery should be undertaken. In some jurisdictions, and extrusion can necessitate removal. implantation of a multifocal IOL is a contraindication to { Types: (a) refractive corneal inlays (Fig. 8.15A and B), (b) the holding of a private or commercial pilot’s licence, or to corneal reshaping inlays and (c) small aperture inlays (Fig. military service. 8.15C and D). { ‘Monovision’ consists of the targeting of IOL-induced { Inlay position. e inlay can be placed either in a stromal refractive outcomes so that one eye (usually the domi- pocket or under a ap. Advantages of a stromal pocket nant) is optimized for clear uncorrected distance vision include stable centration, minimal damage to corneal and the other for near or intermediate vision, in order to nerves and corneal striae. e advantage of placing the facilitate both good distance and near vision when the eyes inlay under a ap is that the procedure can be combined are used together. Suppression of the blurred image and with excimer laser ablation to correct refractive error. neuro-adaptation by the patient is required for this to be { Complications involve: (a) lens decentration, (b) dry eyes, (c) successful. keratolysis, (d) vascularization, (e) opacication and (f) epithelial  Conductive keratoplasty (see ‘Correction of hypermetropia’ ingrowth. ese complications are less likely to occur with mod- above). ere is some evidence that CK can impart a degree of ern implants as they are thin, have a small diameter and have high multifocal functionality to the cornea. permeability with improved biocompatibility characteristics.  Laser-induced monovision refers to the use of laser refrac- { Late side eects include: (a) haloes, glare and disturbance tive surgery to optimize one eye for distance and the fel- of night vision, (b) loss of contrast sensitivity, (c) diculty low for near or intermediate vision (see above under ‘Lens reading in dim light, (d) tear instability and (e) variable extraction’). vision.  CHAPTER Corneal and Refractive Surgery 8 287 A B C D Fig. 8.15 Intracorneal inlays for presbyopia correction. (A) and (B) Refractive inlay; (C) and (D) small aperture inlay – utilizes the pinhole eect. (Courtesy of C Barry.) hypermetropia up to 3-4 D, astigmatism up to 5 D and myopia Laser refractive procedures up to 6–8 D depending on initial corneal thickness. For refractive To settle any contact lens-induced corneal distortion prior to errors outside these parameters an alternative technique should be denitive keratometry, so contact lenses should probably be dis- considered. To decrease the risk of subsequent ectasia, a residual continued for 2 weeks and hard/rigid gas permeable lenses for at corneal base at least 250 μm thickness must remain aer ablation. least 3 weeks (some surgeons suggest 1 week for each year of wear e amount of tissue removed, and therefore the amount of refrac- to date). tive error correctable, is limited by the original corneal thickness. It follows that high refractive errors can be addressed only by an Laser in situ keratomileusis intraocular procedure. In addition to treatment of a wider range of Laser (or laser-assisted) in situ keratomileusis (LASIK) is a very refractive errors, advantages over surface ablation include greater common refractive procedure. Refraction should be stable for at postoperative comfort, faster visual rehabilitation, more rapid sta- least one year before LASIK surgery is undertaken. e excimer bilization of refraction and milder stromal haze. e major disad- laser, which can ablate tissue to a precise depth with negligible dis- vantage is the potential for serious ap-related complications. ruption of surrounding areas, is used to reshape corneal stroma  Preoperative evaluation includes: exposed by the creation of a supercial ap. e ap remains { Complete ocular examination. attached by a hinge to facilitate accurate and secure reposition- { Tear lm osmolarity to exclude dry eye syndrome. ing. Myopia is corrected by central ablative attening and hyper- { Corneal pachymetry to determine residual stromal bed metropia by ablation of the mid-periphery so that the centre thickness and to exclude mild keratoconus. becomes steeper. LASIK can generally be used to treat higher { Corneal topography to exclude irregular astigmatism, ker- refractive errors than surface ablation techniques (see below): atoconus, and forme fruste keratoconus. 288 Refractive Procedures { Corneal OCT to produce an epithelial map of the cornea. bed and undersurface of the ap. Ethanol or mitomycin C { Corneal and whole eye wavefront aberrometry. can be applied, but this can result in adverse eects.  Technique { Diuse lamellar keratitis (‘sands of the Sahara’ – Fig. 8.16E) { A suction ring centred on the cornea is applied to the globe may develop 1–7 days following LASIK. It is characterized which raises the intraocular pressure substantially. by granular deposits at the ap interface. Treatment is with { e ring stabilizes the eye and provides the guide track for intensive topical antibiotic and steroid. a mechanical microkeratome, which is advanced across { Bacterial keratitis (Fig. 8.16F) is rare. e organism must the cornea to create a thin ap. In recent years, the creation be cultured and the sensitivity determined so that an of the ap is undertaken using a femtosecond laser, which appropriate topical antibiotic can be prescribed. allows a precise and uniform ap thickness of 100–120 μm { Epithelial hyperplasia can result in regression and is diag- to be made. nosed using an epithelial OCT map. { e ap is reected and the bed reshaped by the laser, fol- { Persistent epithelial defects. lowed by ap repositioning (Fig. 8.16A). { Pressure-induced stromal keratitis (PISK) is a late-onset { Wavefront-guided LASIK is a variation of LASIK, where visible uid cle which occurs in the interface as a result the excimer laser ablates a precise pattern based on mea- of elevated IOP, usually secondary to prolonged topical surements from a wavefront aberrometer. steroid use. is may present as an under-correction or { ‘Optomised’ LASIK treatment refers to an ablation algo- regression. rithm, patented by some lasers in standard mode to pro- { Corneal ectasia (see Ch. 7). is is a devastating complica- duce larger optical zones and better corneal asphericity, in tion which is fortunately uncommon (0.2–0.6%). Ectasia is order to reduce unwanted visual aberrations and postop- more likely to occur when there is: (a) abnormal preopera- erative glare. tive topography (e.g. forme fruste keratoconus or pellucid { LASIK Plus is a procedure where cross-linking is under- marginal degeneration), (b) low preoperative central cor- taken aer the laser treatment. It might be considered neal thickness, (c) low post-ablation residual stromal bed in patients at risk of developing postoperative ectasia or thickness, (d) greater percentage corneal tissue altered, (e) refractive regression (refractive error greater than −7 D). high myopia, (f) young age. Careful pre-procedure screen- A riboavin solution is applied to the stromal bed while ing should be performed to detect any predisposition. the ap is open, followed by irrigation of the stromal bed, replacement of the ap and application of ultraviolet light.  Intraoperative complications include ‘buttonholing’ (pene- TIP LASIK may cause transient symptoms of dry eye and glare secondary to subepithelial haze. tration) or amputation of the ap, incomplete or irregular ap creation and, rarely, penetration into the anterior chamber. is is signicantly less likely to occur when the ap is created Surface ablation procedures with a femtosecond laser. Like LASIK, photorefractive keratectomy (PRK) employs excimer  Early postoperative complications laser ablation to reshape the cornea. PRK is able to correct myopia { Flap decentration. up to 6 D, astigmatism up to around 3 D and low–moderate hyper- { Wrinkling (Fig. 8.16B), distortion or dislocation of the ap. metropia. e main disadvantages compared with LASIK are the { Interface debris. lower degrees of refractive error correctable and slower epithelial  Late postoperative complications healing with unpredictable postoperative discomfort. However, as { Refractive stability can take up to 3 months depending on a ap is not created there is a lower risk of serious complications the amount of treatment performed. Under-correction than with LASIK, including corneal ectasia and late ap disloca- (primary treatment) and over-correction (retreatment) tion and it may be the procedure of choice for patients at higher may occur. than average occupational or leisure-related risk of eye injury. It is { Dry eye and tear lm instability are the commonest side also suitable for patients rendered ineligible for LASIK due to low eects, occurring in up to 70%, but usually improve spon- corneal thickness. Other indications for surface ablation rather taneously in time. than LASIK include borderline dry eye syndrome, epithelial base- { Transient light sensitivity syndrome. is usually occurs ment membrane disease, prior corneal transplantation or radial between 2 and 8 weeks aer surgery and is more likely to keratotomy and large pupil size. occur aer hyperopic LASIK (9%) than myopic LASIK  Technique (5%) or SMILE (1%). Symptoms settle on topical steroids. { e corneal epithelium is removed prior to ablation. { Subepithelial haze (Fig. 8.16C) leading to symptoms of Methods used may include a sponge, an automated brush glare, haloes and ‘star bursts,’ which are especially noticed (Amoils epithelial scrubber) and alcohol. at night. is usually settles within 3–6 months. { Ablation of the Bowman layer and anterior stroma (Fig. { Epithelial ingrowth under the ap (1–2%) (Fig. 8.16D). 8.17) is performed, generally taking 30–60 seconds. In Treatment is not always required, but when the vision is modern systems, sophisticated tracking mechanisms aected, repeat surgery should be considered. Aer liing adjust laser targeting with eye movement and will pause the ap the epithelial cells are scraped from the stromal the procedure if the eye is signicantly decentred.  CHAPTER Corneal and Refractive Surgery 8 289 A B C D E F Fig. 8.16 Laser in situ keratomileusis (LASIK). (A) A ap is elevated either with a microker- atome or a femtosecond laser, the stromal bed is precisely reshaped with an excimer laser and the ap is repositioned; (B) retroillumination showing wrinkling of the ap; (C) sub- epithelial haze; (D) epithelial ingrowth; (E) diuse lamellar keratitis (‘sands of the Sahara’); (F) bacterial keratitis, with an arrow showing the edge of the ap. (Courtesy of M Leyland – g. D; S Tuft – g. E; R Bates – g. F.) 290 Refractive Procedures A B Fig. 8.17 Photorefractive keratectomy (PRK). (A) Laser ablation of Bowman layer and anterior stroma after removal of the epithelium. This results in reproling of the anterior curvature of the cornea and therefore changes its refractive power; (B) active laser ablation of the anterior surface of the cornea. (Courtesy of C Barry – g. B.) { e epithelium usually heals within 48–72 hours. A ban- is then applied and the epithelium repositioned. Epi-LASIK dage contact lens is generally used to minimize discomfort. employs a mechanical device, an epikeratome, to elevate a Subepithelial haze invariably develops within 2 weeks and hinged sheet of epithelium with an oscillating blunt plastic commonly persists for several weeks to months. Dimin- blade and alcohol application is not usually required. Some ished nal visual acuity is rare but there may be decreased recent reports suggest that healing occurs more rapidly if the contrast and nocturnal glare. Intraoperative application epithelium is simply detached entirely without replacement of mitomycin C (mitomycin-LASEK or M-LASEK) may (ap-o Epi-LASIK). In Trans-PRK, epithelial ablation is per- reduce haze. formed with the laser prior to the refractive ablation, reducing  Complications include slowly healing epithelial defects, cor- operative time and possibly conferring other benets. neal haze with blurring and haloes, poor night vision and regression of refractive correction. Uncommon problems Small incision lenticule extraction (SMILE) include decentred ablation, scarring, abnormal epithelial heal-  Small incision lenticule extraction – SMILE is a technique ing, irregular astigmatism, reduced corneal sensation, sterile that uses a femtosecond laser (Carl Zeiss VisuMax) to cut a inltrates, infection and acute corneal necrosis. lens-shaped piece of corneal tissue (a lenticule) within the  Variations of PRK. A range of procedural variations with intact cornea. is is then removed using a minimally invasive correspondingly varying terminology have been described. 4 mm incision. When the lenticle is removed via a LASIK-style LASEK (laser epithelial keratomileusis or laser-assisted sub- ap the procedure is termed refractive lenticular extraction epithelial keratectomy), Epi-LASIK (epipolis or epithelial (ReLex). LASIK; epipolis is a Greek word meaning supercial), modi-  Results are similar to those obtained with LASIK. ed PRK, advanced surface (laser) ablation (ASA or ASLA)  Advantages and Trans-PRK (trans-epithelial PRK) are variations of PRK { Low risk of ap-related complications. that utilize a variety of techniques to try to reduce discomfort { Biomechanical advantage. and post-laser haze and to speed visual recovery. ASA and { Faster recovery of dry eye symptoms. modied PRK are sometimes used generally to refer to all sur- { Quicker corneal reinnervation. face ablative procedures. In LASEK the epithelium is detached { Surface disturbance is minimal in comparison to surface and peeled back aer pre-treatment with dilute alcohol. Laser ablation procedures.