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Identify patients who can benefit from 0.12 D correction
The theory of "just noticeable difference" has been used in low vision rehabilitation for years.
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In 1975, The Refraction Letter, a monthly Bausch + Lomb periodical, published a lively debate about the clinical legitimacy of 0.25 D vs. 0.50 D as endpoint criteria for a refraction and vision correction. Forty years later, with advancing refractive technologies – the most progress occurring in the past 10 years – new questions are now arising as to the legitimacy of 0.125 (1/8) D vs. 0.25 D as the endpoint criteria for contemporary refraction and vision correction.
The practical considerations involved in such a question obviously include the subjective issues of patients’ wants and needs for 1/8 D accuracy and, equally, the objective technological ability to measure (refract) and fabricate corrective devices at 1/8 D. Addressing these issues is neither an academic, rhetorical or theoretical exercise any longer. Today’s vision care patients seem to be demanding higher resolution vision correction, and new vision care measuring and correcting technologies have reached levels of precision capable of achieving such accuracy.
Subjective considerations
As with most refinements in vision correction, the overriding measure of satisfaction and awareness of an improvement is directly proportional to the patient’s visual needs and his or her personality profile. The obvious – as well as the subtler – nature of a patient’s actual and perceived visual wants and needs dictate the value and success of fine-tuning a refraction by 1/8 D. Thus, as is the case in all good clinical care, a careful and vigilant patient history and interview is the key to effective implementation of 1/8 D in vision care.
Demographic factors directly influence the value of 1/8 D refinement in vision correction as well. Patients younger than 50 years or so who generally have less incipient retinal and/or lens changes occurring are better candidates for refractive refinements greater than the traditional 0.25 D. Also, myopes, who are generally more discriminating about visual resolution, would prove to be more aware and appreciative of 1/8 D precision in their vision correction. The degree of myopia is not as much a mitigating factor as is the inclusion of an astigmatic element in the compound prescription. Needless to say, accuracy of the cylindrical correction (second order) trumps any refinement in the sphere.
But beyond these stated and, to an experienced clinician, somewhat obvious considerations, there is also the quantifiable measure of “just noticeable difference” or JND. Scientifically speaking, JND is a quantification of the patient’s ultimate best correctable visual acuity directly related to their subjective acceptance of any dioptric refinement in the refraction and their final vision correction.
There actually is a scientific basis to the classic refractive question, “Which is better, one or two?” The subjective refraction is based on a classic, scientific, mathematical, psychophysical principle, the Weber-Fechner Law of Difference Threshold, or more commonly referred to as just noticeable difference (notated and designated specifically in scientific literature as JND).
The Weber-Fechner Law (JND) is a mathematical (logarithmic) formula (I/I = k), which states that as stimulus levels (e.g., diopters or I) increase or decrease (I), the magnitude of change must increase proportionately (k) to remain noticeable (i.e., “Which is better...?”). Thus, if we divide the denominator of the patient’s best corrected visual acuity (BCVA) by the logarithmic factor of 100 (or simply move the decimal two places to the left), we have calculated the patient’s JND, or the maximum dioptric power at which the patient can discern a difference in his or her response to “Which is better, 1 or 2?” This is also the delimiting level at which the patient is able to discern a difference in his or her final vision correction with eyeglasses, contacts, IOLs or refractive surgery.
For example, when the patient’s BCVA is 20/20, his or her JND is 0.20 diopters (for practical purposes, 0.25 D in a phoropter). In fact, JND has been used in low vision rehabilitation for years (i.e., a BCVA of 20/200 will generally not discriminate well beyond +/–2.00 D). Conversely, when visual acuity is 20/12, the JND would be 0.12 (or 1/8 D in the phoropter). At BCVAs of 20/10, 20/12 and even 20/16 (practically speaking, the best Snellen acuities the human eye can obtain, notwithstanding theoretical 20/6 and 20/8 reports), by the Weber-Fechner Law, the empirical, quantifiable dioptric level for these threshold acuities at which a patient can appreciate a refinement in their refraction and in their vision correction is 1/8 D.
Thus, patients with certain personality characteristics, within certain demographic categories and recording BCVAs of 20/16 or better may be both subjectively and scientifically good candidates for refraction and vision correction at a 1/8 D level. Perhaps the contemporary eye care practitioner should be conducting a 1/8 D discrimination assessment for these appropriate patients to discern the patients’ optimal and desired endpoint refraction. The practitioner that answers these discriminating patients’ wants and needs may well be rewarded with added patient satisfaction in their final prescription.
Objective measurement considerations
Identification of objective factors in potential patients who could benefit from 1/8 D refinement in their refraction and vision correction would be merely academic were it not for evolving practical, clinical and technological means of accurately measuring certain critical, contributory objective parameters. Forty years ago, such clinical technology and instrumentation was not available. Today it is.
The objective measures that invalidate or corroborate the subjective considerations of patient candidacy for 1/8 D enhancement of their vision correction include, but are not limited to, certain optical, retinal and neural factors in the visual system.
Objective factors that would preclude any realistic expectations of a patient appreciating a 1/8 D change in their refraction and final prescription cover a wide range of clinical considerations, some not measureable as little as 10 years ago. Limiting clinical factors that have been measureable since the days of trial frames and manual phoropter refractions include large pupils (greater than 6 mm) and/or higher degrees (greater than 2 D to 3 D) of second order cylinder (regular or irregular astigmatism).
But measuring patient discrimination levels at 1/8 D is more demanding on the patient’s subjective assessment (“Which is better...?”) than the manual phoropter provides. Such refined discrimination requires a seamless and instantaneous transition, without momentary occlusion or pause, between dioptric lens options. With the advent of electronic digital refractors, this uninterrupted, instant transition is now possible in certain current models.
But other critical measurements of objective factors in 1/8 D prescribing have become available in the past 10 years as well. The highly precise measurement of higher order (third and fourth) aberrations (at the retinal plane) with modern wavefront aberrometry can now identify and accurately measure a patient’s irregular astigmatism and spherical aberrations. At the “clinical awareness threshold” ( 0.43 RMS), these higher order aberrations (HOAs) begin to decrease the quality of vision and undermine the likelihood of 1/8 D sensitivity. Using the Thibos formula (diopters = 4 3 RMS error/area of pupil), an RMS value of 0.43 is equivalent (for a 6 mm pupil) to about 0.47 D. Thus, 1/8 D falls well within the threshold of dioptric sensitivity (JND) for subjectively qualified patients with fewer HOAs (less than 0.43 RMS).
Finally, neural interpretation, the ultimate assessment of sight, dictates patients’ acceptance, rejection or sensitivity to all optical components (sphere and cylinder) and retinal imaging (RMS) measurements. This neural component of the human visual system continues to be elusive and immeasurable. New research and technology in this field is showing some progress and promise. However, relative to this discussion of refractive patients discriminating at 1/8 D sensitivity levels, the science must continue to rely on the patient’s subjective JND (or “Which is better...?”) for neural interpretations.
Correcting considerations
The final test in the veracity of 1/8 D refracting being a legitimate, valuable and accepted level of vision correction becomes the practical, watershed consideration of the question: Can 1/8 D refractive refined measurements, which are now technologically achievable, be converted into a correcting device, that is, can they be effectively and efficiently produced, fabricated and mass manufactured? And, equally, can these vision correction devices demonstrate their effectiveness and subjective vision benefits through scientifically verifiable, evidence-based testing?
The advent of digital lens surfacing in the fabrication of spectacle lenses has advanced to a point that now allows for correction as refined as 0.10 D, theoretically, to 0.01 D. Practically speaking, 0.10 D is equivalent to about 1/8 D. As to whether such levels of correction accuracy can be manifested in a final frame and lens assembly is somewhat questionable, with only anecdotal evidence and reports. Thus, between subjective considerations and limiting mechanical issues, an astute practitioner and enthusiastic patient can resolve the potential of 1/8 D vision correction.
New horizon
The question of refining vision refraction and correction to a 1/8 D level has been answered and verified through a review of scientific principles including the Weber-Fechner Law of Difference Thresholds or JND; scientific testing; and evidence-based, subjective clinical results. The increasing understanding of optical, retinal and neural vision components and the development of advanced refracting technologies and correcting lens materials and designs makes the 1/8 D refraction and correction a reality.
The clinical and technical ability to identify patients that can discriminate, recognize and benefit from refraction and correction at the 1/8 D level is a new horizon in modern vision care. Practitioners and patients may benefit from an understanding, on a case-by-case basis, of patients’ wants, needs and JND.
- References:
- Bruce AS, et al. Optom Vis Sci. 2014;doi:10.1097/OPX.0000000000000377.
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- Dell’Osso, LF. Improving visual acuity of the nystagmic child. The Refraction Letter. 1975;18:3.
- Diopsys Web site. http://www.diopsys.com.
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- For more information:
- Louis J. Catania, OD, FAAO, is in private practice at Nicolitz Eye Consultants in Jacksonville, Fla., and a Primary Care Optometry News Editorial Board member.
Disclosure: Catania is an advisor to Marco Ophthalmic.