PTK provides excellent outcomes in astigmatism, corneal opacity

Paris-Patients with irregular astigmatism not only have axial corneal curvature asymmetry, but corneal opacity also might complicate the picture. Damien Gatinel, MD, PhD, described how phototherapeutic keratectomy (PTK) provides excellent visual outcomes in this patient population.

Irregular astigmatism is not only characterized by principal corneal meridians that are not orthogonal, unaligned mires, and axial asymmetry, but opacity can also be present, according to Dr. Gatinel, assistant professor and head of the Anterior Segment and Refractive Surgery Department at the Rothschild Ophthalmology Foundation, Paris. Higher-order aberrations, notably coma, can predominate the wavefront aberrations.

Dr. Gatinel pointed out that he finds double-pass aberrometry (OQAS instrument, Visometrics, Spain) to be helpful in his practice to quantify the level of optical scatter, i.e., related to the spatial extent of the light intensity point spread function, across the retina, and quantified by a metric referred to as the optical scatter index (OSI).

“In my practice, an OSI of less than 1.5 is usually within the normal range,” he noted. He demonstrated a case of severe haze with an OSI that was about seven times higher than normal, in which the best-corrected visual acuity (BCVA) was 20/40.

In such cases, Dr. Gatinel recommended using the classic rigid gas-permeable contact lens test. After the contact lens is put on the cornea and neutralizes the irregular astigmatism, if the point spread function measured with double-pass asymmetry remains similar to that before the lens was applied, then opacity is likely playing a major role.

“The principal aim then would be to reduce the opacity,” he said.

“The positive diagnosis of irregular astigmatism includes visual symptoms such as glare, halos, and monocular diplopia; and the results of corneal topography, corneal tomography, aberrometry (ocular wavefront aberration), and double-pass aberrometry to detect light scatter,” Dr. Gatinel summarized.

He emphasized the importance of the stability and the instability of the astigmatism. With acquired irregular astigmatism, after refractive surgery, for example, the possible presence of ectasia within the optical zone must be considered. With stable irregular astigmatism, treatment can be considered, but re-treating undiagnosed post LASIK ectasia should be avoided, he pointed out.

Dr. Gatinel considered four important guidelines with irregular astigmatism.  He advises waiting for the astigmatism to stabilize, eliminating the ectasia after refractive surgery, and evaluating the respective effects of both scatter and higher-order aberrations. In addition, customized ablation is possible in some cases, but careful patient selection is important.

PTK with mitomycin C should be the preferred treatment option for most patients with irregular astigmatism, according to Dr. Gatinel.

“I usually like to perform transepithelial PTK whenever the irregularities are at a higher level than Bowman’s layer or the epithelium,” he explained.

However, in some specific cases, he performs a topography-guided ablation in cases of congenital astigmatism or stable forme fruste keratoconus. He described a patient with severe adenovirus keratitis with a marked reduction in the BCVA. The topography showed irregular astigmatism with high optical scatter, and Dr. Gatinel chose to perform transepithelial PTK.

No mechanical debridement was performed; the procedure begins with laser treatment. After applying balanced saline solution (BSS) to regularize the corneal surface, the blue fluorescence from the epithelium could be seen when firing the excimer laser. Where the stromal layer is reached, there is a sudden disruption of the blue fluorescence. The depth of the ablation is measured, and in this case it was 33 µm, which corresponded to the minimal epithelial thickness. He then cleared the epithelium over 5 mm centrally using further PTK laser ablation. One month postoperatively, the patient had less opacity but still had substantial scatter. However, the improvement continued out to 1 year postoperatively, at which time the scatter was greatly decreased, Dr. Gatinel reported.

“In my practice, this is an excellent technique to reduce opacities without inducing too much secondary irregular astigmatism; the corneal epithelial layer is the best masking agent. In addition, a hyperopic shift can be prevented,” he said and emphasized the importance of performing a minimal ablation to achieve the desired effects.

In the case of another patient with irregular astigmatism but no opacity, he performed a topography-guided ablation (PRK) using the Nidek EC 5000 laser (CATz mode). In this case, he removed the epithelium and performed the laser treatment but did not treat a myopic –0.75-D refractive sphere to avoid a hyperopic shift. The patient achieved 20/20 uncorrected visual acuity 6 months after treatment.

“PTK is an efficient technique for treating superficial corneal opacities and irregularities. Choosing an adequate strategy results in satisfactory outcomes. Minimally invasive ablations should always be tried first, and the smoothing effect of the epithelial layer can be used to regularize the corneal surface with transepithelial PTK,” Dr. Gatinel concluded.

Dr. Gatinel did not indicate any financial interest in this topic.

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