Dr.Narayan Bardoloi, N03755
Introduction
Being essentially a cosmetic procedure the patients after laser procedure are very sensitive if the result is not perfect.. Aside from residual refractive error or overcorrection, theses patients frequently have some form of irregular astigmatism induced by small optical zones and or decentered ablation. These types of refractive errors are difficult to correct with standard treatments because of their irregular nature. These types of patients would benefit from either wave front guided or topo-guided ablation.
Purpose–
To evaluate the efficacy of topographically guided excimer laser photo-ablation to retreat unsuccessful myopic LASIK patients with irregular astigmatism.
Methods
At least three months after primary LASIK 07 eyes of 04 patients were submitted for LASIK and PRK enhancement. 3 patients with six eyes had bilateral irregular ablation following faulty Lasik surgery done by one surgeon. On enquiry with the particular surgeon it was gathered that after flap lifting the surgeon used to wash the stromal bed with BSS before firing laser. This resulted in irregular ablation as the areas covered by water were never ablated. The seventh eye had decentered ablation. All the patients had subjective complaints of ghosting, star bursts, halos and monocular double vision.
Pre operative measurements include UCVA, BSCVA, and Manifest and Cycloplegic refraction; slit lamp examination, detailed fundus examination with binocular indirect ophthalmoscope, corneal tomography with Oculyzer, pupil size and white to white corneal diameter.
Six repeatable and highly reproducible tomographic maps were obtained in each eye through oculyzer. The oculyzer is linked to the socialized T-CAT software to capture and transfer and transfer treatment data to the Allegretto wave Excimer Laser. The target asphericity for all the eyes was set to Q=0.46 which is believed to be theoretically optimum for the eye’s physiology. T-CUT software is based on the principle of fitting the best fit asphere and removing the excess tissue in order to turn the irregular cornea into a rotationally symmetric aspheric cornea. At the same time, one can adjust the aim asphericity (Q-value) of the cornea within a range of 0-0.6, as well as desired refraction in the terms of sphere and cylinder.
Five eyes were selected for Lasik while two because of paucity of residual stromal tissue underwent PRK without Mitomycin C. Because of the high irregularity one eye was taken only for asphericity postponing the refractive correction three months later.
In the Lasik procedures the previous flaps were lifted using sinsky’s hook and forcep and ablation done. The epithelium in the PRK patients were removed using 20% ethyl alcohol and chilled BSS applied at the end of the ablation. Bandaged Contact Lens was put in all cases of PRK.
Post operatively Flouromethasone eye drop 4 times a day, Vigamox eye drop 4 times a day and Moistane eye drop one hourly given to the operated eyes.
Results– Mean follow up was 27.8 months ± 8.2(SD).
The preop UVA was 0.81+/- 0.68 log MAR (0.2 to 2), which improved to 0.13 ± 0.14 log MAR (0.1 to 0.7) in 6 months= 0.008
The BSCVA improved from 0.07+-0.07 log MAR (0 to 0.2) preop to 0.05 +/-0.08logMAR (-0.1to 0.2). p= 0.01
The asphericity (Q value) improved from pre op +0.08+/- 1.03 to +0.04+/-1.05 (-1.37 to +1.46) p= 0.04, pair student t test
Subjective symptoms like glare, halos, starbursts, ghost images and monocular diplopia were absent in most of the cases.
Discussion
Opinion varies in regards to treatment modalities to be undertaken in case of irregular cornea. A strong group belief that wave front guided ablation is most suitable in these cases as they take care of the aberrations of both corneal surface and of internal structure, namely the lens of the eye. All elements of the optical system (i.e. tear film, anterior corneal surface, corneal stroma, posterior corneal surface, crystalline lens, vitreous and retina) are taken into consideration with the wave front system. As the derived information is converted into mathematical data (i.e., Zernike Polynomials), we can classify as well as quantify the aberrations.
In practice, there are limitations to this technology. The wave front measurements are taken at a certain point in time, and hence, are essentially static measurements. The eye, however, is a dynamic optical system. In certain lighting condition and with corresponding pupil diameters, all the aberrations may be correctable. This may not be true in other situations.(eg change in light intensity and accommodative states) .Furthermore, if we add in factors such as wound healing, ablation predictability, flap induced aberrations and changes in the tear .
- Because it based on the corneal surface, it is theoretically possible to factor in the asphericity (Q value)and maintain the natural aspheric shape of the cornea.
The major disadvantage of topography –guided treatments is that it ignores the rest of the refracting media because it concentrates mainly on the corneal contour. This may be responsible for occasional refractive surprises surgeons encounter.
In our study two of the eyes had to taken for second sitting as the amount of irregularity precludes refractive correction. We regularize the corneas in the first sitting and subsequently after three months refractive correction was under taken.
Similar studies on topo graphy linked Lasik and PRK enhancement show reduction in BSCVA, UCVA, Q value in irregular corneal resulting from keratoplasty, scar and decentered ablation. Knorz et al (1)found significant improvement on UCVA , a significant reduction of corrective cylinder, and a more regular corneal topography in most of the patients after topography guided LASIK with the exception of those with central island after previous photo-ablative refractive treatment. Kymionis et al(2) reported a general increase in UCVA and BSCVA , better re-centration of previously decentered ablations, without a significant change in spherical equivalent . Our result of topography-guided:LASIK and PRK in patients with irregular astigmatism showed also a significant improvement of UVA, a significant reduction of corrective cylinder , and a more regular corneal topography in all patients, without losing BSCVA
Alessio et al(3) performed topography –guided PRD in patients with decentered myopic ablation and irregular astigmatism after penetrating keratoplasty and showed a significant decrease in sphere and cylinder and a gaain in BSCVA in all patients with irregular astigmatism and 50% of the patients with decentered ablation.
The ideal Q value of -0.46, as suggested by Manns et al,(4) was targeted in all cases. But it was not achieved completely as we feared that going more towards negative Q value would necessitates more ablation.
Conclusion- The enhancements using topographically guided excimer laser photo-ablation resulted in stable and satisfactory outcome with significant reduction of refractive cylinder and increase of uncorrected visual acuity , without loss of BSCVA in patients with severe corneal irregularities.
Bibliography
1.Knorz, Topgraphically . Topo–guided Laser In situ Keratomileusis to treat corneal irregularities, Opthalmology 2000;107:1138-1143
2.Kyminonis GD, Panahopoulou SI, Aslanides IM, Plainis S et al. Topographically supported customized ablation for the management of decentered laser in situ keratomileusis : Am J Ophthalmol 2004;137:806-811
3.Allesio G, Boscia F, La Tegola et al. Corneal Interactive programmed topographic ablation customized photorefractive keratectomy for correction of post keratoplasty astigmatism. Ophthalmology 2001;108:2029-2037
4.Manns F, Ho A, Parel FM et al .Ablation profile for wave front –guided correction of myopia and primary spherical aberration. J Cataract Refract Surg 2002;28:766-774