Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard mini Atlas Series Lasik_Aragawal, Jacob_2009

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A

B

C

Figures 3.8A to C: Decented ablation (Courtesy: Ming Wang).

(A) Demonstrates curvature and elevation maps for a patient

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with a decentered ablation. The elevation map shows a decentration of the optical zone. Note the inferior decentration of the treatment in this patient who previously underwent a myopic LASIK treatment. The key observation on curvature maps is the dioptric difference between the superior and inferior keratometric readings. The key observation on elevation maps is the misalignment of the center of ablation from the optical center; (B) Curvature (left) and elevation (right) maps for a keratoconic cornea are noticeably different. On the axial map, keratoconus appears as an area of inferior steepening. On the surface height map, the elevation appears superior to the area of thinning; (C) The elevation map prior to hyperopic LASIK and S/P hyperopic LASIK, with the difference map showing the induced change.

Significantly decentered excimer ablations (Figure 3.8) result in loss of best-corrected visual acuity due to irregular astigmatism, and cause symptoms such as glare, night vision difficulty, ghosting, and diplopia. Possible causes of decentration include poor fixation due to poor patient instruction, anxiety, over-sedation, blurry vision due to high refractive error or the exposed stromal bed causing difficulty seeing the laser’s target. It can also be due to improper stabilization of the patient’s eye with a Thornton ring during ablation. In order to prevent decentration, careful preoperative and intraoperative instructions are key, especially with regard to the fixation target, keeping

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A patient with a decentered ablation generally presents with the following clinical signs and symptoms:

1.A decentration of the ablation zone on corneal topography.

2.Increased higher order aberrations as measured using wavefront aberrometry, predominantly coma.

3.The appearance of a tail on point spread function.

4.Reduced best-corrected visual acuity that improves only with gas permeable lenses.

5.A cylinder measurement on autorefraction and wavefront that differs from manifest refraction, and

6.A history of reduced vision immediately following surgery that fails to improve with time.

Relieving patients of symptoms associated with decentration may be complex. The most frequently used method involves gas permeable lenses, which reshape the anterior cornea optically, restoring visual quality. These fittings often require reverse geometry lenses or aspheric lenses to be successful. This is time-consuming, and most patients do not want to venture down the road that motivated them to pursue refractive surgery initially.

Surgical options for treatment of decentered ablations are limited. For mild degrees of decentration following PRK, a small (3 to 4 mm) diameter ablation at the edge

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Wavefront-driven custom treatment may be used to correct decentrations, assuming the technology currently available is able to detect the irregularities reliably. Hartman-Schack aberrometers may fail when attempting to measure eyes with considerable irregularity, due to limitations of the lenslet array. While decentrations may increase higher order aberrations, attempting to correct the aberrations may not fully correct the topographical errors. These systems assume a normal prolate cornea in treatment planning, and the refractive error corrections may be less accurate. Thus, these treatments may be less effective than topographically-directed treatments.

Retreatment using conventional enhancement techniques rarely fully corrects the problem, and typically increases the effective decentration. This occurs because the neural axes (visual axis and line of sight) and the optical axis (geometrical) are not aligned in cases of decentration. Image placement on the fovea requires the eye to rotate, making full correction of the optical problem unlikely when all measurement and planning occurs on the visual axis. Conventional technology is not able to decouple these axes, and treats solely on the visual axis information.

The advancement of Scheimpflug imaging to create three-dimensional models of corneal shape may be the

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Figure 3.9A

Figure 3.9B

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Figure 3.9C

Figures 3.9A to C: Mitomycin C application (Courtesy: Francesco Carones). (A) The microsponge soaked with 0.02% MMC solution, positioned over the corneal stroma, immediately after scraping; (B) A very severe haze (grade 4) that was evident in a patient who was treated for –12.00 D correction by myopic PRK; (C) The slit-lamp examination of the eye seven years after treatment showed a transparent cornea, with no haze traces

Laser in situ keratomileusis (LASIK) is the most common refractive surgery procedure performed worldwide. This procedure involves creation of a disk-shaped corneal lamellar flap, usually hinged to the cornea nasally or superiorly. The lamellar flap can be made with either a mechanical microkeratome, or with a femtosecond laser.

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Although LASIK is generally safe and effective, surgical complications can occur. The technology improvements made the flap-creation process safer and safer, however, the most significant surgical complications involve problems with the lamellar flap. Flap complications have been reported in up to 8% of LASIK procedures when using 1st generation mechanical microkeratomes, whereas this figure dramatically reduced to less than 0.2% when using last generation mechanical microkeratomes or femtosecond lasers. Button-hole flaps, partial flaps, dissected flaps, excessive thin flaps have been become very rare, but still these complications may be associated with loss of best spectacle-corrected visual acuity, contrast sensitivity loss, and visual symptoms. The etiology of visual loss and side effects associated with these types of flap abnormalities is most often the high order aberrations and irregular astigmatism secondary to scarring and epithelial ingrowth.

Treatment of severe haze involves the use of pharmaceuticals applied topically. Corticosteroid produced some controversial results, and are frequently ineffective. The second category of drugs employed at this aim are antimetabilites, of which mitomycin C (MMC), 5 fluorouracil, and thio-pepa are those experimented the