2016-4-25 11:18:41 Eye World. link. https://www.eyeworld.org/article-measuring-visual-acuity-behind-clouds
Date published online: April 2016
“Using the RAM is inexpensive, reliable, easy for both patients
and technicians, portable, quick, accurate ... more importantly, it prevents surgical delays, unexpected poor outcomes and unnecessary surgeries.”
–Sunil Brijendra Jain, DOMS
Measuring visual acuity behind clouds
by Matt Young and Gloria Gamat EyeWorld Contributing Writers
The potential of a retinal acuity meter as a predictor of postoperative visual acuity in cataract patients
Preoperative measurement of retinal visual acuity in cataract patients with clouded ocular media or co-existing macular pathology has always challenged ophthalmologists
due to the unpredictability of postoperative visual outcomes.
In the presence of media opacity or macular edema, standard visual acuity measurement through the use of the Early Treatment Diabetic Retinopathy Study (ETDRS) chart has been found to be insufficient.
For this reason, a more accurate measurement of photoreceptor layer visual acuity in cases of media opacity or macular edema was needed.
Through the years, different tools to predict potential postoperative visual acuity for this patient population became available, including the potential acuity meter (PAM), laser interferometer, illuminated near card, potential acuity pinhole, and more recently, retinal acuity meter (RAM).
Overview of PAM and RAM
In 2014, Hatef and colleagues1 investigated the relationship between visual acuity measured by the ETDRS chart or the PAM and retinal architecture and function.
Clinical data demonstrated that visual acuity measured by PAM is a more accurate indicator of retinal function compared to that measured by the ETDRS chart in eyes with macular edema.
The use of PAM in evaluating retinal function in eyes with media opacities has been well supported by numerous studies since the early 1980s. Those studies have established that the PAM can distinguish patients with retinal and neural diseases in cataractous eyes from those without, and can accurately measure visual acuity in high myopes with moderate cataracts and poor vision.
Furthermore, the PAM can determine how much of the vision loss was attributable to cataracts. It has been found to be a useful qualitative predictor of retinal function in cases of mild to moderate corneal edema.
Meanwhile, RAM technology has been clinically tested for more than 10 years.
In a prospective observational study, Milia and colleagues2 evaluated the predictability of the RAM for postoperative visual acuity in dry AMD patients undergoing phacoemulsification with IOL implantation.
The retinal acuity meter prediction, according to the investigators, was considered acceptable if within 1 logMAR line of the final visual acuity measured at 1 month postoperatively.
“The accuracy of prediction was within acceptable limits for most of the patients,” said Maria Milia, MD, IASO Children’s Hospital, Athens, Greece.
The investigators noted, however, that predictability was most valuable in cases of nuclear and cortical cataracts and was not affected by factors such as stage of AMD, age, and sex.
“Subgroup analysis revealed that successful retinal acuity meter prediction was obtained in 24 patients [100%] with cortical cataracts, 37 patients [82%] with nuclear cataracts, and only 10 patients [43%] with posterior subcapsular cataract (PSC); this difference was statistically significant, indicating that accurate predictability was most likely in cortical and nuclear cataracts while PSC may compromise efficacy of the retinal acuity meter,” the investigators reported.
“Potential acuity measurement after cataract surgery in patients with macular pathology is a useful tool for both patients and surgeon; based on the retinal acuity meter preoperative measurements, patients with AMD can be offered cataract surgery with predictable outcomes while others can avoid unnecessary intervention,” they concluded.
Value of RAM in developing countries
In countries with increasing incidence rates of cataracts, the use of a RAM in predicting postoperative visual outcomes can not only help patients arrive at an informed decision but can help surgeons screen patients that would benefit most from the treatment procedure.
At the 2015 American Academy of Ophthalmology (AAO) annual meeting, ophthalmologists from India reported the potential of the RAM as a valuable predictor of visual outcomes in cataract surgery patients.
The team, headed by Sunil Brijendra Jain, DOMS, Indira Gandhi Eye Hospital and Research Centre, Lucknow, Uttar Pradesh, India, conducted a retrospective single center study involving 43 eyes of 40 patients to evaluate preoperative RAM acuity, cataract grade, presence of associated eye disease (i.e., corneal degeneration, adherent leucoma, nebular corneal opacity), and postoperative BCVA.
“Our study results ... indicate that RAM is clearly a valuable predictor of postoperative visual acuity in cataract patients,” Dr. Jain reported at the 2015 AAO annual meeting. Preoperative RAM acuity was 6/12 to 6/6 (logMAR 0.301 to 0) in 81.40% of eyes, and 6/36 to 6/18 (logMAR 0.778 to 0.477) in 18.60%. Postoperative BCVA was 6/12 to 6/6 (logMAR 0.301 to 0) in 86.05% of eyes; 6/36 to 6/18 (logMAR 0.778 to 0.477) in 6.98%; and 1/60 to 5/60 (logMAR 1.778 to 1.079) in 6.98%.
Compared to earlier models of the technology (i.e., interferometer and PAM), Dr. Jain said, the RAM offers advantages that include an ergonomic molded faceplate with a built- in cradle for the retractor, an improved disc rotating mechanism, ease of use, portability, high resolution digital photographic lettering, and clearly visible indices to identify the level of acuity being tested.
“Using the RAM is inexpensive, reliable, easy for both patients and technicians, portable, quick, accurate, and provides repeatable results and realistic expectations; more importantly, it prevents surgical delays, unexpected poor outcomes and unnecessary surgeries,” Dr. Jain said.
“RAM testing may be considered to be a part of preoperative cataract surgery protocol for cases of retinal/macular disease in which we do not expect BCVA to be 6/6 or optimum,” he concluded.
1. Hatef E, et al. Longitudinal comparison of visual acuity as measured by the ETDRS chart and by the potential acuity meter in eyes with macular edema, and its relationship with retinal thickness and sensitivity. Eye (Lond). 2014;28:1239–1245.
2. Milia M, et al. Predictability of postoperative visual acuity in patients with dry age- related macular degeneration using the retinal acuity meter. J Cataract Refract Surg. 2012;38:2198–2199.
Editors’ note: Drs. Jain and Milia have no financial interests related to their comments. Contact information