The objective scatter index (OSI) is a tool for surgeons to properly distinguish between the need for a clear lens surgery or a cataract surgery, a decision that can have a medico-legal impact.1 (See “Other OSI Clinical Applications,” below.)
Here, we discuss what the OSI measures, the data it provides, and how it is acquired.
What It Measures
The OSI measures the intraocular light scatter from an infrared laser diode during the diode’s interaction with heterogeneous ocular media (e.g., a small opacity or a diffuse opalescence of the lens), leading to severe retinal image degradation (e.g., glare).2 This is important, as, sometimes, a patient’s visual symptoms do not correlate with their visual acuity or with the clinical exam of their crystalline lens, and vice versa.
As an example, sometimes, a deterioration of the quality of vision is attributed to early lenticular changes without necessarily degrading visual acuity. This concept, dysfunctional lens syndrome, was first presented by Dan Durrie, MD, at the annual meeting of the American Academy of Ophthalmology in Chicago, USA, in 2016.
The Data It Provides
The more scattered the light during its passage through the eye, the higher the OSI. An OSI less than 1 (green) is usually indicative of no ocular scatter and good ocular transparency. An OSI between 1 and 3 is indicative of a loss of transparency and early cataract (yellow). An OSI between 3 and 7 is indicative of a mature cataract (orange). And finally, an OSI of more than 7 (red) is suggestive of severe alteration of ocular transparency. (Figure 1).3
How It Is Acquired
The Optical Quality Analysis System (OQAS) (HD Analyzer II, Visiometrics SL, Keeler USA) uses a double-pass aberrometry approach, shining an infrared laser diode light on the retina, and analyzes both the retinal image obtained and its reflection.4 A caveat: Since the double-pass retinal images are affected by both ocular aberrations and intraocular scattering, it is important to correct second-degree aberrations, such as spherocylindrical aberrations, when measuring OSI, and to make sure to acquire the test with the patient’s best-corrected visual acuity (BCVA) and in a good environment to minimize external alterations.3
The first pass shines a 2 mm-in-diameter infrared light through the patient’s pupil, focalizing the light on the fovea. The retinal image obtained is called the Point Spread Function (PSF).
The second pass collects the reflected light after its passage through the patient’s pupil. The obtained image by the device’s digital camera is similar to the image focalized on the fovea when the patient is fixating on the instrument’s light target.
During the second pass, there is scattering of light through the vitreous cavity, the lens, the anterior chamber, the cornea, and the tear film. (Figure 2) This is how through the analysis of the retinal PSF, the surgeon can estimate the amount of ocular scatter and predict its effect on BCVA and its impact on contrast sensitivity.
The amount of ocular scatter is then interpreted by the device as the index described above. The infrared light can also be projected inside the eye at different distances, so
you can study the effect of accommodation on the
measurements.4
The measurements given by the OQAS device have proven to be reproducible and repeatable, making this index a reliable tool to use in everyday practice.5
Happy Patient, Happy Practice
By using the OSI, surgeons can protect both their patients and their practices by distinguishing between the need for a clear lens surgery or a cataract surgery. Additionally, the OSI can be utilized for other clinical applications, such as in refractive surgery. A happy patient equals a happy practice. CP
OTHER OSI CLINICAL APPLICATIONS
OSI can objectively help clinicians in these clinical scenarios as well:
• Preoperative assessment of age-related cataract. Measured in optimal conditions, it is directly correlated to the Lens Opacities Classification System (LOCS) III and to the patient’s symptoms.
• Predicting phacodynamics. The higher the OSI, the higher the ultrasound energy and time needed during cataract surgery.8 This is applicable in cases of nuclear cataracts because OSI is dependent on light scatter measured through the patient’s pupil. Any cortical peripheral lens changes would not affect similar measurements by the digital camera.8
• Determining laser capsulotomy. In early posterior capsular opacification (PCO), it can sometimes be difficult to know whether the visual disturbances felt by the patient are caused by the PCO itself or by something else. The OSI can help clinicians decide whether the patient should undergo a laser capsulotomy. Also, the surgeon can compare the OSI before and after capsulotomy. (Figure 3)1
• Assessing the implication of flap microstriae. In cases in which visual disturbances are described by the patient and flap microstriae are identified on slit lamp exam, OSI can help the surgeon assess the implication of those microstriae on the visual disturbances felt by the patient to determine whether they are optically significant.1
References
1. Principles of the analysis of ocular optical quality with OQAS. https://www.gatinel.com/wp-content/uploads/2008/05/OQAS-Gatinel.pdf.
2. Monferrer-Adsuara C, Mata-Moret L, Castro-Navarro V, et al. An objective scatter index cutoff point as a powerful objective criterion for preoperative nuclear cataract decision-making based on ROC analysis. J Cataract Refract Surg. 2019;45(10):1452-1457.
3. Artal P, Benito A, Pérez GM, et al. An objective scatter index based on double-pass retinal images of a point source to classify cataracts. PLoS One. 2011;6(2):e16823.
4. Gatinel D. A quoi sert l’instrument OQAS? [What is the OQAS instrument used for?] Accessed May 29, 2024. https://www.gatinel.com/2011/04/a-quoi-sert-linstrument-oqas/
5. Saad A, Saab M, Gatinel D. Repeatability of measurements with a double-pass system. J Cataract Refract Surg. 2010;36(1):28-33.
6. Garcin T, Grivet D, Thuret G, Gain P. Using Optical Quality Analysis System for predicting surgical parameters in age-related cataract patients. PLoS One. 2020;15(10):e0240350.
7. Cabot F, Saad A, Mcalinden C, Haddad NM, Grise-Dulac A, Gatinel D. Objective assessment of crystalline lens opacity level by measuring ocular light scattering with a double-pass system. Am J Ophthalmol. 2013;155(4):629-635, 635.e1-2
8. Garcin T, Grivet D, Thuret G, Gain P. Using Optical Quality Analysis System for predicting surgical parameters in age-related cataract patients. PLoS One. 2020;15(10):e0240350.
