Expanding Therapeutic Horizons
The limiting factor currently in treating highly aberrated corneas is the ability to obtain good wavefront data. Without a good wavefront, a topography-guided treatment may be the only option. But by definition, this fails to address aberrations on the posterior surface of the cornea, which commonly occur in concert with anterior surface injuries, or elsewhere in the eye. Registration of topography-guided or complex conventional treatments is another challenge. A precise correction is of limited value if it cannot be delivered by the treatment laser to the exact x-y and rotational coordinates on the cornea. My preference, therefore, is always to take the whole-eye optics into account and perform a wavefront-guided correction whenever possible. In complex eyes, this can be accomplished only with a high dynamic range wavefront sensor (see figure). Although several such prototypes or custom systems have been tested, including one in my clinic, developing a robust new aberrometer that can perform at a very high dynamic range has been challenging. What we demand from such a device is the accuracy and reliability of Hartmann-Shack technology, but at higher resolution than is currently possible today. It must prove to have low interscan and intrascan variability and high levels of precision in both normal and aberrated eyes. We want the capture rate and the number of eligible scans to be higher than with current technology – but without sacrificing the robustness of the rejection algorithms. Finally, a new aberrometer must also satisfy workflow concerns such as efficiency, ease-of-use and ergonomics. The first clinical units of a new aberrometry platform developed to meet all these parameters, the iDesign aberrometer (Abbott Medical Optics), are scheduled to arrive in the UK and Europe later this year. This device also integrates a new, spot-based corneal topographer. Essentially, similar spots can be used to reconstruct the corneal shape as well as the whole-eye wavefront, providing great insight for diagnosis of complex problems. Although we have other systems that integrate topography with wavefront (Topcon) or OPD-Scan (Nidek) for diagnostic purposes, only the Nidek is linked to a treatment laser and this OPD system employs a different technology and does not have the spatial resolution and Fourier wavefront reconstruction of the iDesign system. We anticipate a real-world dynamic range of about 8 D from the iDesign aberrometer, along with resolution of 210 μm. Much remains to be proven with this new platform, but even if laboratory projections aren’t fully met in clinical practice, it should be able to capture most eyes, including those with high cylinder, fairly significant distortions from corneal transplants or injuries, and diffractive lens implants.
I anticipate that a major indication for high dynamic range aberrometry will be in analysing and treating patients with loss of corneal tissue due to contact lens infections. 15 per cent of contact lens-related infectious keratitis results in reduced quality of vision, which can be devastating for the patient. There is often a small divot that is too small to justify a lamellar corneal transplant, but can be highly aberrating and impossible to capture and accurately reconstruct with conventional scanners and wavefront technology. Of course, imaging is not the only challenge in highly aberrated eyes. Some of these patients have lost such significant tissue from infection or injury that there may not be enough left to recreate the correct corneal curvature. Nevertheless, having the wavefront data opens the door to creative, staged solutions that greatly expand the therapeutic horizon for these patients. Based on my past experience with earlier platforms, what I expect surgeons to notice most immediately from a high dynamic range system is the remarkable clarity of the Hartmann-Shack spots. Even before the system constructs a wavefront map, one can subjectively see obvious problems just from the spot distortions. My guess is that we will also find the high resolution very helpful in tear film assessment, an entirely new application for wavefront sensing. From both a diagnostic and treatment perspective, I am very enthusiastic about high dynamic range technology. Dr Stevens is a consultant ophthalmic surgeon at Moorfields Eye Hospital in London. He is also a consultant to Abbott Medical Optics.
Tags: visual outcomes
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