Phototherapeutic Keratectomy
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Excimer laser phototherapeutic keratectomy (PTK) has been found to be an effective treatment for a variety of superficial corneal disorders. Corneal surface irregularity, epithelial instability, and superficial opacity may all benefit from the procedure. [1] [2] [3] [4]
PTK is considered a bridge between medical and surgical management of different corneal diseases. PTK can be used for therapeutic and / or refractive indications. [5]
History
Since late 1980s and early 1990s the excimer laser has been used to reshape the anterior corneal curvature in a procedure known as photorefractive keratectomy (PRK) or keratomileusis in situ (LASIK). Excimer laser phototherapeutic keratectomy (PTK) was approved by the U.S. Food and Drug Administration (FDA) in 1995 to treat a variety of anterior corneal pathologies and nowadays is a technique very used for many surgeons around the world.
Preoperative Evaluation
The key to a successful outcome depends on proper case selection. Systemic diseases such as uncontrolled diabetes, collagen vascular diseases (rheumatoid arthritis, systemic lupus erythematous) may have delayed epithelial healing, representing a problem for this therapeutic approach. [6]
Visual improvement after PTK may due to reduction on scar density or removal of leukomas, as well as a reduction of irregular astigmatism.
The main diagnosis and planning of PTK is based on clinical judgment, on slit lamp examination, and the amount of refractive error. Imaging techniques are used to plan and manage the postoperative outcome. [5]
Corneal topography
Corneal topography helps in the planning and following up of patients after a PTK procedure. It also helps in correcting the pre-existing refractive error by the topography-guided laser treatment. [7] This helps in reducing irregular astigmatism and improving visual acuity.
Ultrasound biomicroscopy
The ultrasound biomicroscopy measurement overestimates the corneal pathology, and the ablation required is inversely proportional to the depth of the pathology and does not help in the planning of the procedure. [8]
Ocular coherence tomography
Ocular coherence tomography (OCT) provides a noncontact, cross-sectional, high-resolution representation of corneal configuration, before and after PTK. In general, PTK should be performed to remove less than 1/3 of the corneal thickness, and to leave behind at least 250 microns of residual stroma; these values can be quantified using OCT. The quantitative differences in the epithelial thickness after PTK can be monitored for epithelial hyperplasia and anterior stromal changes. [9] OCT is used to measure the depth of opacities in the cornea and it helps in the planning of either a lamellar or a penetrating procedure depending on the depth of the opacities. [5]
Therapeutic Indications
- Spheroidal degeneration
- Salzmann nodular degeneration
- Calcium band keratopathy
- Recurrent corneal erosion syndrome
- Bullous keratopathy
- Anterior corneal dystrophies (Cogan, Meesman, Reis-Bückler, etc)
- Superficial scars (leukomas)
- Keratitis
- PTK combined with or before and after other surgeries
- PTK combined with amniotic membrane graft
- PTK after pterygium surgery
- PTK before and after keratoplasty
- PTK before cataract surgery
Technique
The procedure is generally done under topical anesthesia, unless some additional surgical procedure is combined with PTK (local anesthesia) or when the procedure is done in pediatric patients (general anesthesia). The topical anesthetic agents used are Xylocaine 4% or Proparacaine HCl (0.5%). [10] [11]
After applying a speculum, the epithelium is debrided manually with a hockey stick knife or with the excimer laser (transepithelial). After debriding the epithelium, ablation is performed either by asking the patient to look into the green fixation light or with the laser centered manually. In an eccentric or peripheral lesion, the surgeon needs to manually rotate the patient’s head or eye for ablation or use the joystick to place the laser beam in the desire location. [12]
PTK is performed with an excimer laser (193 nm) and works on the principle of photoablation, by breaking the bonds between molecules. One pulse of laser removes approximately 0.25 microns of corneal tissue, making it more precise, while keeping the tissue removal depth under better control; also, the treated surface was smooth and regular, improving the visual outcome. [13]
If the surface is rough, a masking agent such as hydroxypropyl methylcellulose (HPMC) 0.7 to 2% is spread to smooth the cornea. This masks the deeper tissue and only the protruding irregularities of the cornea are exposed for laser ablation. Masking fluids need to be applied intermittently during the procedure, although applying fluid too many times will delay the procedure and may further increase the irregularities of the surface after the procedure.[14]
A few visual and auditory signals during the procedure may help in guiding the extent of the treatment. A snapping sound is heard when the tissue is ablated, and when the masking fluid gets ablated, a soft sound is heard. A bubbling sound indicates too much of methylcellulose. The ablation of the epithelium shows blue fluorescence in the dark room, while stromal ablation does not produce blue fluorescence. [15] This gives a guideline for the depth of ablation. [4] After firing 70 to 80% of the target ablation, the patient must be examined on the slit-lamp.[10]
Post procedure management
Poor epithelial healing would result in haze formation or infection. Either BCL such as the Silicone hydrogel lens or patching of the eye is required to facilitate the healing. Prophylactic antibiotic eye drops such as newer generation fluoroquinolone is instilled till the epithelium heals, and the BCL is removed. If the eye is patched, then antibiotic ointment is instilled twice a day with the patch, till the epithelium heals. Topical NSAIDs such as Diclofenac sodium, reduce postoperative pain and may reduce the need for oral analgesics.[16] Oral analgesics can be given as required. However, NSAIDs must be used with caution as they have been reported for corneal melt after PRK.[17] Once the defect heals and BCL is removed, topical corticosteroids such as fluorometholone or prednisolone acetate must be given four times daily, tapered by one drop per week, over a month. Preservative free artificial tears for should be used four to six times daily. [5]
Clinical outcome
Best corrected visual acuity (BCVA) improves and Uncorrected Visual Acuity (UCVA) may reduce after PTK, because of the induced refractive error. [3] [4] Occasionally, PTK may be combined with PRK and vision may improve in those patients.[18] The visual outcome may be unaffected when PTK is performed, to give symptomatic relief, as in bullous keratopathy. [19]
A significant amount of tissue removal will induce refractive error, the type and amount depends on the site and depth of ablation; central ablation will flatten the cornea and induce a hyperopic shift, like a photorefractive keratectomy, similarly peripheral ablation will result in removal of tissue in the periphery, like hyperopic ablation, and will induce myopia. [3] [4][12][20]
Contraindications
Phototherapeutic keratectomy should be limited to anterior corneal pathology only, as deeper scars need deeper ablations and may result in haze formation or a hyperopic shift. [13] The best candidates for PTK are patients with opacities in the anterior 10 – 20% of the cornea, without significant irregularity and thinning. For patients who have both superficial and deep stromal lesions, only superficial lesions are easily treated. [21] Systemic diseases such as diabetes mellitus and autoimmune disorders are contraindications for PTK.[6] The decreased corneal sensation may affect epithelial healing, with an increased risk of stromal melt. Any viral activity within six months is a contraindication. [5]
PTK Complications
- Recurrence of primary disease pathology
- Infections
- Reactivation of herpes simplex virus
- Delayed healing of epithelium
- Corneal opacity and scarring
- Allograft transplant rejection
References
- ↑ Sher NA, Bowers RA, Zabel RW, Frantz JM, Eiferman RA, Brown DC, et al. Clinical use of the 193-nm excimer laser in the treatment of corneal scars. Arch Ophthalmol. 1991;109(4):491-8.
- ↑ Stark WJ, Chamon W, Kamp MT, Enger CL, Rencs EV, Gottsh JD. Clinical follow-up of 193-nm ArF excimer laser photokeratectomy. Ophthalmology. 1992;99(5):805-12.
- ↑ 3.0 3.1 3.2 Campos M, Nielsen S, Szerenyi K, Garbus JJ, McDonnell PJ. Clinical follow-up of phototherapeutic keratectomy for treatment of corneal opacities. Am J Ophthalmol. 1993;115(4):433-40.
- ↑ 4.0 4.1 4.2 4.3 Maloney RK, Thompson V, Ghiselli G, Durrie D, Waring GO, O'Connell M. A prospective multicenter trial of excimer laser phototherapeutic keratectomy for corneal vision loss. The Summit Phototherapeutic Keratectomy Study Group. Am J Ophthalmol. 1996;122(2):149-60.
- ↑ 5.0 5.1 5.2 5.3 5.4 Rathi VM, Vyas SP, Sangwan VS. Phototherapeutic keratectomy. Indian J Ophthalmol. 2012;60(1):5-14.
- ↑ 6.0 6.1 Ayres BD, Rapuano CJ. Excimer laser phototherapeutic keratectomy. Ocul Surf. 2006;4(4):196-206.
- ↑ Camellin M, Arba Mosquera S. Simultaneous aspheric wavefront-guided transepithelial photorefractive keratectomy and phototherapeutic keratectomy to correct aberrations and refractive errors after corneal surgery. J Cataract Refract Surg. 2010;36(7):1173-80.
- ↑ Rapuano CJ. Excimer laser phototherapeutic keratectomy in eyes with anterior corneal dystrophies: short-term clinical outcomes with and without an antihyperopia treatment and poor effectiveness of ultrasound biomicroscopic evaluation. Cornea. 2005;24(1):20-31.
- ↑ Wirbelauer C, Scholz C, Häberle H, Laqua H, Pham DT. Corneal optical coherence tomography before and after phototherapeutic keratectomy for recurrent epithelial erosions(2). J Cataract Refract Surg. 2002;28(9):1629-35.
- ↑ 10.0 10.1 Rathi VM, Vyas SP, Vaddavalli PK, Sangwan VS, Murthy SI. Phototherapeutic keratectomy in pediatric patients in India. Cornea. 2010;29(10):1109-12.
- ↑ Vyas S, Rathi V. Combined phototherapeutic keratectomy and amniotic membrane grafts for symptomatic bullous keratopathy. Cornea. 2009;28(9):1028-31.
- ↑ 12.0 12.1 Amm M, Duncker GI. Refractive changes after phototherapeutic keratectomy. J Cataract Refract Surg. 1997;23(6):839-44.
- ↑ 13.0 13.1 Fagerholm P. Phototherapeutic keratectomy: 12 years of experience. Acta Ophthalmol Scand. 2003;81(1):19-32.
- ↑ Kornmehl EW, Steinert RF, Puliafito CA. A comparative study of masking fluids for excimer laser phototherapeutic keratectomy. Arch Ophthalmol. 1991;109(6):860-3.
- ↑ Hersh PS, Burnstein Y, Carr J, Etwaru G, Mayers M. Excimer laser phototherapeutic keratectomy. Surgical strategies and clinical outcomes. Ophthalmology. 1996;103(8):1210-22.
- ↑ Förster W, Ratkay I, Krueger R, Busse H Topical diclofenac sodium after excimer laser phototherapeutic keratectomy. J Refract Surg. 1997;13(3):311-3.
- ↑ Feiz V, Oberg TJ, Kurz CJ, Mamalis N, Moshirfar M. Nepafenac-associated bilateral corneal melt after photorefractive keratectomy. Cornea. 2009;28(8):948-50.
- ↑ Zaidman GW, Hong A. Visual and refractive results of combined PTK/PRK in patients with corneal surface disease and refractive errors. J Cataract Refract Surg. 2006;32(6):958-61.
- ↑ Thomann U, Meier-Gibbons F, Schipper I. Phototherapeutic keratectomy for bullous keratopathy. Br J Ophthalmol. 1995;79(4):335-8.
- ↑ Dogru M, Katakami C, Yamanaka A. Refractive changes after excimer laser phototherapeutic keratectomy. J Cataract Refract Surg. 2001;27(5):686-92.
- ↑ Paparo LG, Rapuano CJ, Raber IM, Grewal S, Cohen EJ, Laibson PR. Phototherapeutic keratectomy for Schnyder's crystalline corneal dystrophy. Cornea. 2000;19(3):343-7.
