Low Vision Math Made Easy for the Primary Care Optometrist

Transcription

1 Low Vision Math Made Easy for the Primary Care Optometrist Karl Citek, OD, PhD, FAAO I. Introduction Case Example Esther, 82 yowf, AMD OS>OD, referral from another ECP Habitual Rx: OD x030 OS sph Add OU DVA cc: OD 20/400, OS FC Goals: Watch TV, read newspaper What additional and/or different testing would you do? What low vision devices, if any, would you prescribe? What lifestyle/environmental changes would you recommend? II. Making Sense of Visual Acuities A. Distance 1. Finger Counting i. ICD-10 (WHO) definition: 1m (3ft) " 1/60 (20/1200) ii. But can vary depending on actual finger size, from about 20/800 to 20/1400 iii. Nonetheless, a huge difference from the largest chart presentation, 20/400 iv. Try at a greater distance, such as 2m (6ft) 2. Walk-up acuity: reduce viewing distance by bringing patient to chart, or chart to patient i. Works best with printed (e.g., Lighthouse ETDRS, Bailey-Lovie, Lea) and hand-held (e.g., Feinbloom) charts a. Use actual viewing distance and seen optotype size, e.g., 2/32 1. Most charts are calibrated for 4m or 6m, rather than 20ft 2. Later convert to 20ft Snellen equivalent, e.g., 2/32 = 20/320 b. AVOID direct, pre-calculated Snellen equivalent 1

2 1. 8ft is easily confused 2. Information, such 8ft, may be lost over time 3. Unless the chart is exactly 20ft away, this is NOT 8/160 ii. Use EXTREME caution with projected or computer-generated charts with non-20ft viewing distances. For the adventurous: a. Measure ACTUAL viewing distance, e.g., 14ft b. Calculate new denominator for a large optotype, e.g., 20/200 is really 14/140 c. Reduce viewing distance until patient can see optotype, e.g., 6ft d. Record VA based on new viewing distance and denominator, e.g., 6/140 e. Later convert to 20ft Snellen equivalent, e.g., 6/140 = 20/ What testing would you do with Esther? B. Near 1. Must record actual viewing distance i. Consider add power and/or uncorrected ametropia, e.g., uncorrected 5D myope should be tested at 20cm 2. Nearpoint card with M notation preferred i. Record near acuity, e.g., 33cm 3. For nearpoint cards listed only with reduced Snellen, MUST know calibration distance, usually 40cm, but sometimes 33cm or 30cm 4. What testing would you do with Esther? III. Improving the Subjective Refraction A. Just Noticeable Difference (JND) 1. Test lens powers based on denominator of 20ft Snellen equivalent 2. Refining sphere power i. Divide Snellen denominator by 2, then divide by 100 ii. For each +/- sphere lens power, round to nearest 0.25D iii. Example: 20/160 " 160/2 = 80 " 80/100 = 0.80 " +/-0.75D 3. Refining cylinder power 2

3 i. Divide Snellen denominator by 100 ii. For each +/- cylinder lens power, round to nearest 0.25D iii. Example: 20/160 " 160/100 = 1.6 " +/-1.50D 4. Refining cylinder axis i. With axis rotation, error in cylinder power in intended meridian is based on cosine function a. 10-deg rotation reduces intended power by about 3% b. 25-deg rotation reduces intended power by almost 18% c. 45-deg rotation reduces intended power by 50% ii. HOWEVER, with axis rotation, induced cylinder power in orthogonal meridian is based on sine function a. 3-deg rotation induces about 10.5% of unintended power b. 7.5-deg rotation induces 26% of unintended power c. 15-deg rotation induces over 50% of unintended power iii. Multiply percentage by nominal cylinder power to determine if axis rotation is potentially problematic a. For 1D cyl, 7.5-deg rotation induces about 0.25D unintended power b. For 4D cyl, 7.5-deg rotation induces about 1D unintended power B. Loose lenses (trial lenses, lens holder/frame, lens rack) vs. phoropter 1. Phoropter is cumbersome, slow, even useless when JND lens power >+/-0.25D 2. Qualifies as complex refraction 3. Use hand-held JCC lens of consistent with JND, e.g., +/-1.00D or +/-1.50D C. How would you refine your testing of Esther? IV. Improving the Low Vision Patient s Ability to See A. Increase Retinal Image Size, i.e., Magnification 1. Make the object larger, e.g., large print book 2. Reduce the viewing distance; remember walk-up acuity? 3. Make the image larger with optical or electro-optical devices B. Increase Luminance Contrast and Color Contrast 3

4 C. Increase Useful Lighting / Decrease Harmful Glare 1. Type of light source i. Lamp/bulb style ii. Shading 2. Distance of light source i. Applying the Inverse Square Law 3. Position of light source i. Height and location with respect to object of interest ii. Reflectance of object of interest, e.g., matte paper vs. ipad glass screen D. What environmental/lifestyle changes would you recommend for Esther? V. Increasing Retinal Image Size A. Distance viewing " Telescopes 1. Expected acuity improvement i. Multiply initial acuity by telescope power ii. Example: 20/160 with 4x, expect 20/40 i. Image is too dim: Galilean telescopes transmit about 75-80% of visible light, but Keplerian telescopes can transmit as little as 50% ii. Image is not clear: some devices can have poor optics that create images with blur and/or chromatic aberration iii. Image field of view is small and/or patient has a central scotoma and cannot use device properly with eccentric viewing B. Near viewing " Magnifiers (Hand, Stand, High-Plus Spectacles) 1. Expected acuity improvement i. Decrease viewing distance based on ratio of desired acuity to initial acuity ii. Increase add power accordingly iii. Example 4

5 a. Initial acuity: 4M 40cm (viewed with 2.50D add!) b. Desired acuity: 1M (20/50), e.g., newsprint c. Predicted distance: 40cm x (1M/4M) =10cm d. Minimum predicted add: 100/10cm = 10D iv. What if you have only distance VA? a. Kestenbaum s rule suggests that the near add power to view 1M (20/50) can be predicted by taking the inverse of distance VA b. ASSUMES that the patient has no central scotomas, media opacity, etc. c. Gives a very rough, first-order approximation based on entering, pre-exam information, BUT... d. ALWAYS measure near acuity without low vision aids first i. Testing is based on high-contrast charts, but patient tries to view medium- to lowcontrast objects ii. Testing is conducted with good quality, high level lighting, but patient tries to view with suboptimal lighting iii. Some devices may be of poor quality, with aberrations and/or image distortion, especially in the periphery iv. Some devices may be difficult for patients to manipulate: e.g., to use with the proper viewing distance, to hold stable, to use with eccentric viewing C. Near viewing " Electro-Optical Devices (ipad, tablet, e-reader, etc.) 1. Expected acuity improvement i. Depends on magnification provided by device, most have a manual or on-screen indicator ii. Divide initial acuity by magnification, e.g., 4M (20/200) / 5x = 0.8M (20/40) i. Device is set to high magnification but has small screen, i.e., field of view ii. Device contrast and/or brightness setting is inappropriate; if available, increase brightness, change contrast to enhanced or reverse D. Intermediate viewing " Telemicroscopes 5

6 1. Combines afocal telescope power and near add power based on desired viewing distance: fixed-focus telescope with add (either cap over objective or incorporated into objective power) or focusable i. Determine best near add first ii. Determine desired intermediate viewing distance and appropriate add iii. Calculate required telescope power by dividing near add power by intermediate add power iv. Reduce telescope power which increases field of view if intermediate acuity demand is less than that at near v. Example a. Near add: 10D b. Desired intermediate distance: 50cm " 2D add c. Required telescope power: 10D/2D = 5x i. Brightness and/or contrast provided by telescope is reduced ii. Reduced field of view may make viewing difficult VI. Verifying Low Vision Device Power A. Telescopes 1. Image-to-object size comparison 2. Vergence amplification via lensometry B. Optical magnifiers 1. Lensometry for low powers and/or small devices 2. Image vergence neutralization for stand magnifiers C. CCTV and e-devices 1. Image-to-object size comparison VII. Conclusion 6