Geetha Sravani, PhD Candidate
Eye length (EL) is one of the major factors that determine the optical image quality and is also an important measure to track myopia onset and progression. In this regard, measurement of peripheral EL is equally important as the measurement of central or axial EL, as the periphery of the eye is thought to play a significant role in the onset and progression of myopia.
Over the years there have been significant advances made in procedures used to measure EL. Here, the authors reviewed the available techniques for EL measurements, including the peripheral EL measurements and the limitations of the existing techniques/measurements.
Although there have been no instruments designed explicitly for measuring peripheral EL, techniques and methods such as ultrasonography, interferometry, optical low coherence interferometry and optical coherence tomography developed to measure the EL can also be used to measure peripheral EL. Each of these methods has its own advantages and disadvantages. For example, while ultrasonography is an efficient and inexpensive technique, it is limited in application as it is invasive, has low resolution and requires a high level of expertise. Other techniques, such as optical low coherence interferometry are non-invasive and have high penetration depth. However, they also suffer from limitations: they are challenging to use with opaque ocular media and may also suffer from poor signal-to-noise ratio. Optical low coherence reflectometry, however, has a high signal-to-noise ratio.
Specifically, with respect to measurements of peripheral EL, commercially available instruments (such as IOL Master 500, Lenstar LS 900, spectral domain optical coherence tomography, ACMaster) were modified to measure peripheral EL at eccentricities varied from 2° to 40° in humans and from 2° to 52° ± 6° in animals. It was reported that Lenstar LS 900 produced high accuracy and precision (0.02 ± 0.02mm). Peripheral eccentricities measured with this instrument are 30° and 35° in vertical and horizontal meridians, respectively. One of the issues that is debated with peripheral EL measurement techniques is the influence of eye or head turn on the measurements; it was considered that there would be differences in peripheral measurements due to change in the eyeball shape when eyes were rotated versus head was rotated. Other considerations for peripheral EL measurements include the type of off-axis targets, accommodation, pupil diameter, wavelength of light used for measurement, etc.
To conclude, the authors reported that while there was a good agreement between most optical biometers, the Lenstar LS 900 was considered to offer better results in terms of modification for peripheral EL measurements. There is a further need for instrumentation/techniques that can accurately measure peripheral EL, which are usable and affordable.
Peripheral eye length measurement techniques: a review
Ingrid Ornella Koumbo Mekountchou, Fabian Conrad, Padmaja Sankaridurg, Klaus Ehrmann
Along with the rising myopia epidemic is the increasing interest in any ocular parameter that might inform understanding of myopia progression. The relationship between eye length and myopia has long been established, but the recent interest in the central and peripheral retina, eye shape, retinal contour and refractive error development is attracting more clinical and research interest in peripheral eye length measurements. Therefore, peripheral eye length measurements are an important step in the ongoing research involving the peripheral retina. Since the first measurement of peripheral eye length reported in 1991, many techniques and methods have been developed that vary in many aspects. These techniques involve custom-built or modified commercially available instruments, with the use of off-axis targets and other considerations such as eye or head turn of the subject. The wide range of methods and instruments used for peripheral eye length measurements make it difficult to compare results and may account for some of the variations in the reported results. Specifications of the different methods are presented along with their advantages and disadvantages. Although researchers acknowledge a good agreement between the modified commercially available optical biometers for peripheral eye length measurement, the Lenstar LS 900 appears to offer better results. Nevertheless, the introduction to the market of an instrument specially designed for peripheral eye length might overcome the issues noted with other methods and could allow for more insights in future research involving the peripheral retina. Moreover, future studies may be able to track peripheral eye length changes and its relationship to the progression of myopia and find out if those changes are responsible for or correlated with important eye conditions.
Koumbo Mekountchou, I. O., Conrad, F., Sankaridurg, P., & Ehrmann, K. (2020). Peripheral eye length measurement techniques: a review. Clinical and Experimental Optometry, 103(2), 138-147.