Assigned status Update Pending by Antonio Jaime Villarreal Gonzalez, MD on January 4, 2016.
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.    
PTK is considered a bridge between medical and surgical management of different corneal diseases. PTK can be used for therapeutic and / or refractive indications. 
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. 
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. 
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.  This helps in reducing irregular astigmatism and improving visual acuity.
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. 
Ocular coherence tomography
Ocular coherence tomography (OCT) is a noncontact, cross-sectional, high-resolution representation of cornel configuration, before and after PTK. The quantitative differences in the epithelial thickness after PTK can be monitored for epithelial hyperplasia and anterior stromal changes.  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. 
- 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)
- 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
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%).  
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. 
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. 
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.
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.  This gives a guideline for the depth of ablation.  After firing 70 to 80% of the target ablation, the patient must be examined on the slit-lamp.
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 do not affect the healing of the epithelium and may reduce the need for oral analgesics. Oral analgesics can be given as required. However, NSAIDs must be used with caution as topical nepafenac is reported for corneal melt after PRK. Once the defect heals and BCL is removed, topical corticosteroids such as Fluorometholone alcohol must be given four times daily, tapered by one drop per week, over a month. Topical lubricants are prescribed for a month, four to six times daily. 
Best corrected visual acuity (BCVA) improves and Uncorrected Visual Acuity (UCVA) may reduce after PTK, because of the induced refractive error.   Occasionally, PTK may be combined with PRK and vision may improve in those patients. The visual outcome may be unaffected when PTK is performed, to give symptomatic relief, as in bullous keratopathy. 
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.  
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.  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.  Systemic diseases such as diabetes mellitus and autoimmune disorders are contraindications for PTK. The decreased corneal sensation may affect epithelial healing, with an increased risk of stromal melt. Any viral activity within six months is a contraindication. 
- Recurrence of primary disease pathology
- Reactivation of herpes simplex virus
- Delayed healing of epithelium
- Corneal opacity and scarring
- Allograft transplant rejection
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