Myopia Profile


Model eyes and the GRAS Module of the OCULUS Myopia Master – Q&A with David Kern

Posted on October 17th 2022 by David Kern

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Our new Q&A format is designed to explore a particular clinical topic, intervention, product or research paper with an expert. Here, we explore the Gullstrand Refractive Analysis System (GRAS) Module in the OCULUS Myopia Master with ophthalmic optics expert David Kern, from Germany. We also provide you some additional tips and links to further reading on putting this into practice.

Kate-profile-thumbnail.png What is a model eye and how does it relate to myopia management? Q&A with David Kern (1920 × 1500 px).png A model eye represents the optical components of an average human eye like curvatures of the corneal and crystalline lens surfaces, refractive indices and distances between the surfaces and the retina. There are several eye models, a well-established and most popular one is the Gullstrand eye. Myopic eyes represent a deviation from a perfect emmetropic eye. The aim of myopia management is to use a risk analysis to assess eye growth in adulthood, which is directly related to severe eye diseases. Appropriate interventions attempt to inhibit axial length growth as much as possible in order to reduce the risk. In doing so, it is necessary to know the individual components of the eye that create myopia, but also to be able to choose an effective treatment method.

You may remember the concept of a 'model eye' from back in your student days. Our new article Model eyes in myopia management explains more on how understanding the ocular power components of the eye can be applied to clinical diagnosis and individualized management decisions in childhood progressive myopia.

Q&A with David Kern (1920 × 1500 px) (1).png Could you explain the GRAS Module in the OCULUS Myopia Master?Q&A with David Kern (1920 × 1500 px).png The Gullstrand Refractive Analysis System (GRAS), available in the myopia software of the Myopia Master, compares individual measured parameters with the Gullstrand Eye and simulates the corresponding refractive changes in the spectacle plane. In a GRAS examination, the Gullstrand parameters are replaced by the individual measurements of axial length, corneal radius and total refraction of the respective patient. The crystalline lens parameters can be calculated via ray tracing in the Gaussian space. The outcome is not interchangeable with the output of an IOL formula, because of the differences in geometry between IOLs and the natural crystalline lens. Each of the measured parameters, their interrelations and the calculated crystalline lens power are inserted into the original Gullstrand eye model. Paraxial ray tracing simulation identifies the refractive change in diopters in the spectacle lens for each component, respectively. The Gullstrand model is a great reference for adults, but not for children (<22 years). For this reason, a correction model for children was established using 7,628 eyes to compensate the influence of a natural eye growth. The GRAS module in the OCULUS Myopia Master software is correcting each parameter automatically depending on age.

An interview with Max Aricochi, an Optometrist practicing in Austria, explains how he uses the GRAS Module for clinical communication, and to distinguish between refractive and axial myopia. Read more in this Q&A Interview with Max on the OCULUS Myopia Master.

Kate-profile-thumbnail.png How could we best use the GRAS Module information in practice?Q&A with David Kern (1920 × 1500 px).png The demonstration of the deviation to the Gullstrand eye supports the detection of the origin of the refractive error within a quick look. The principle question can be answered if the cause is based on axial length or refractive deviations due to higher corneal or crystalline lens power. The advantage of the age-adapted GRAS module allows clinicians to detect pre-myopia, but also constellations like hyperopic refraction with axial elongation, moderate myopia with dramatic long eyes, crystalline lens myopia by reason of accommodation and more. This information improves the work-flow in clinical practice and is elementary to prescribe further investigations but also the choice of treatment. Depending on the origin of ametropia, the eye must be investigated regularly with fundus imaging and/or retinal OCT, binocular vision, accommodation, corneal topography and biomechanics or crystalline lens imaging.

In David's response above he identifies several less typical presentations where understanding the contributing structure to the refraction is extremely helpful. See this in action in this case study, where measurement of corneal topography and axial length helped to determine the cause of what appeared to be rapid myopia progression: Is it really fast progressing myopia, or something else?

Q&A with David Kern (1920 × 1500 px) (1).png What information could we miss by not measuring the cornea in myopic children?Q&A with David Kern (1920 × 1500 px).png It is easy to understand that the result of the total refractive error of an eye depends on the refractive elements, the crystalline lens power and corneal power, but also the location of the retina, the axial length. All parameters are required for a full refractive analysis and GRAS wouldn't work if one of these is missing. The corneal power is a very stable parameter and the properties should be stable after the second year of live. Thus, a significant corneal power change should activate alarm signals to every eye care professional, with exception of intentionally changes like corneal surgery or specialty contact lens. Progression of corneal power indicates a development of corneal ectasia and the earlier the diagnosis, the higher the treatment success. In case of an Ortho K lens fit, the GRAS module can nicely show the refractive power change pre to post treatment in the cornea power and the total refraction, when crystalline lens power and axial length remain stable.

It is crucially important to recognize that corneal changes can contribute to the appearance of myopia progression, which can occur without changes in axial length. As described in our article Measuring the whole eye in myopia, corneal changes and progression of astigmatism are NOT typical in progressive myopia. These can instead indicate risk or presence of corneal ectasia, as mentioned above.

Understanding corneal curvature can also help to identify myopes with a lower refractive power but longer axial lengths. Read more in this case study Are you measuring the cornea in myopia management?

Kate-profile-thumbnail.png How could we explain a refraction-analysis of the eye to parents and patients?Q&A with David Kern (1920 × 1500 px).png Short-sightedness or far-sightedness is a common term for parents and children. Nevertheless, it helps to show the light rays in the schematic eye to explain the reason for the patient's visual defect better. GRAS supports eye care practitioners with an individual demonstration of the patient's eye and light paths. But also, the origin of ametropia can be easily explained with the bar diagram. Eye care professionals can present themselves very professionally by a simple patient education and communicating the patient a high-quality impression of the analysis. This leads to significantly greater acceptance of rewarding the performance of physicians accordingly.

The screenshot below (sourced here) shows the output from the Gullstrand Refractive Analysis System (GRAS) Module in the OCULUS Myopia Master, which models the power of ocular components to chart a child's axial length, corneal power, crystalline lens power and refraction as contributions to their overall refractive error. In the image below, the right eye power components are charted in purple and the left eye components in blue.

The patient's ocular power components are indicated as either more myopic (left side of central vertical line) or more hyperopic (right side of line) compared to their age-matched peers. In this example, it is evident that axial length is a fairly close match to the level of refractive myopia observed. The corneal power is quite close to the emmetropic value for the model eye, and the crystalline lens power trends towards being slightly more hyperopic than expected for that patient's age. This indicates axial length as the primary contributor to this patient's myopia.


Further reading

Meet the Authors:

About David Kern

David Kern is application and product manager at OCULUS Optikgeräte GmbH with over 12 years of experience in the field of optics. After three years of training in ophthalmic optics, he studied ophthalmic optics and psychophysics with a Master of Science degree. His experience includes theses in the field of perception and visual performance as well as progressive corrective lenses. David Kern has been with OCULUS since 2017 and is product manager for the Myopia Master® and the Pentacam®. Myopia management, contact lens fitting using topography and refractive diagnostics are among his main areas of expertise.

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