Emerging Technologies to Assess the Eye’s Optical Quality: Focus on Double-Pass Techniques

Subjective assessment of vision is the bread-and-butter of Ophthalmology and Optometry, and its formal origins can be traced back to Benito Daza de Valdes’s publication of Uso de los Antoios para Todo Genero de Vistas in 1623. In this first systematic work on visual testing, Daza de Valdes put together sight testing tables, and discussed the importance of using convex lenses after cataract surgery. To this day, visual acuity, contrast sensitivity, and glare testing remain essential components of basic vision assessment for any eye care professional. However, one major shortcoming for these tests is that they are subjective. Objective measurements of visual function can also be an important part of the eye exam, but progress in objective vision assessment has lagged due to a relative lack of appropriate technologies. An important component of overall visual function is the eye’s optical quality. Of course, even a basic slit lamp exam yields important information about optical quality. However, all eye care professionals have been faced with patients reporting visual acuity (or contrast sensitivity, glare, etc.) at levels much lower than would be expected from an eye exam.

Objective assessment of optical quality can serve multiple purposes. For one, not all patients can reliably communicate the quality of their vision effectively. This group of patients not only includes infants and children, but also elderly individuals with cognitive disorders as well as many others with altered mental status. As such, if the overall optical quality of an eye could be assessed objectively, then the eye care professional could intervene appropriately without relying completely on the ability of the patient to communicate. In addition, specific objective markers of vision quality would allow patients to be followed up using dependable, unchanging metrics over time. This in turn would allow adjustment of therapies to optimize care.

Several technological breakthroughs have allowed Ophthalmologists to assess optical quality more objectively. Perhaps the most promising such technologies have been digital corneal topography and wavefront aberrometry. Although wavefront technology has been used by astronomers for decades in order to reduce higher order aberrations induced by the earth’s atmosphere, it has been introduced into clinical eye care relatively recently. Wavefront aberrometry measures aberrations over the entire eye, in that it takes into account not only spherocylindrical refractive error, but also spherical aberration, trefoil, coma, secondary astigmatism, and several other “higher order” aberrations described by Zernike polynomials. Such higher order aberrations are thought to contribute to over 20% of the total number of aberrations in a normal eye, and may contribute to a much greater percentage of aberrations in eyes with diseases such as keratoconus or those that have undergone previous surgery. New customized refractive ablation systems take into account many of these higher order aberrations, and there are some data to support the view that such customized ablation systems perform exceedingly well for many patients.

However, the currently available measurement of Zernike polynomials may not be the ultimate method of describing the optical quality of the eye. There is evidence to suggest that repeating higher order aberrations measurements leads to measurement variability per patient, even in the short run. Also, for customized ablation, only aberrations in the cornea are treated. As refractive surgery techniques advance, it will be more and more important to differentiate aberrations induced by the cornea from those induced by the crystalline lens.

A variety of aberrometry techniques are available in the clinic, including those that use Hartmann-Shack, Tscherning and ray tracing systems. A relatively new development in measuring the eye’s optical function uses a “double pass” technique that is somewhat different than traditional aberrometry techniques. The OQAS (Optical Quality Analysis System) from Visiometrics is currently the only available "next generation" aberromter. The double pass instrument images and analyzes the size and shape of a singular light spot that enters the eye and is reflected from the retina. As such, the light passes twice through the media of the eye. The double pass technique may become particularly useful in measuring the aberrations in eyes with highly aberrated corneas, post surgical eyes, or eyes that have irregular astigmatism. Traditional aberrometers provide excellent information on “virgin” corneas, but there is some evidence to suggest that the data generated by the double pass technique has higher correlation with specific subjective complaints following refractive surgery. One reason may be that double pass systems capture the effect of scatter while abberometers fail to do so. Because double pass systems take into account the effects of scatter, in the future they could be used to grade visual disability due to cataract.

Additional research comparing traditional aberrometry and double pass instruments will help further characterize the shortcomings and benefits of each system. However, it is clear that emerging technologies continue to improve assessment of the eye’s optical quality more objectively.