Lecture 2
Why are trial frame refractions better for Low vision patients?
- Easier to observe & allow Px to adopt any particular eye & head movements that may aid vision *eccentric viewing, searching, nystagmus, squinting - Easier to make large changes / jumps in power - Aperture of trial lenses are larger than with phoropter *Avoids pinhole effect of moving eye to edge of lens behind phoropter - Can set the Vertex distance -. important for high Rx
With LV refraction, use the technique of Bracketing. What is the Goal of bracketing?
- Present Px with large dioptric changes (big steps) - Systematically reduce / narrow the dioptric change based on the Px's responses (ie. Improvement in VA) Goal: mid-point of final bracket centered around the Px's retina -> equality
Review of subjective refractions with trial frame
-Always use bracketing technique for Best sphere & Cyl -Use JND (from VA with ret result) as initial bracket size -Present option as: "better with the lens or without" instead of "which is better: lens 1 or lens 2" -Three steps: Best Sphere, Cyl Axis & Cyl Power -Use 1 line above VA threshold as "target" -Allow extra time for Px to make "decision" -Encourage EV if it improves VA
What is the equation for computing M size of sample text?
1 M = 1.5mm high
What M size is Newspaper, Magazines, or books?
1 M size.
What are some tips for low vision refractions?
1. Facilitate the Px's decision by limiting the choice -Present choice as: "is it better with the lens or without, or no difference" NOT as: "is it clearer with Lens 1 or Lens 2" 2. Allow extra time for Px to make decision 3. Can use an auto-refractor as a starting point for trial-frame refraction, but not a substitute
What are the steps for obtaining single letter near VA?
1. Have Px wear their habitual Near Rx 2. Assess: (i) OD; (ii) OS; (iii) OU -Monitor that Pt occludes untested eye 3. Have Px hold the Near VA card at their usual working distance (WD) & measure their WD -The WD does NOT have to be 40 cm. Encourage Px to hold Near VA card close to their eyes (Eg. <20 cm) 4. Have pt read/guess letters until they cannot go any further -Pt's threshold N VA is the last line in which the Pt got the majority of optotypes correct -Measure N VA w/ and w/out addition light -Monitor that the pt is not squinting or leaning forward 5. Assess/ Encourage Eccentric viewing
What are the steps for performing an MN Read?
1. Have Px wear their habitual Near Rx & hold MN Read card at their usual working distance (WD). Measure the WD 2. Assess: (i) OU 3. Have Px read each sentence aloud as quickly & accurately as possible until they cannot read any further. Encourage Px to guess -Record the time (to nearest 0.1 sec) it took Px to read each sentence -Mark on score sheet any words incorrectly read -Use optimal lighting conditions 4. Assess / Encourage Eccentric Viewing (EV)
What are the steps for testing the Pelli-Robson CS?
1. Leave over head room lights on [Chart is NOT back-lit] 2. Position Pt 1m in front of chart ~ at eye level with middle of the chart 3. Pt wears their DRx 4. Have Pt read/ guess letters until 2 letters are incorrectly identified within a triplet. A call of "O" for "C" & vice versa is considered correct 5. Encourage Pt to wait ~30sec before responding for each letter by threshold 6. Assess/ Encourage Eccentric Viewing (EV) 7. Record result as LogCS
What are the two contrast sensitivity charts?
1. MARS letter CS chart 2. Pelli-Robson Letter CS Test
What can you use to occlude the eyes in low vision?
1. Occlue eye with folded tissue or black lens. 2. For nystagmus Pt's: use fogging lens, do NOT occlude.
Be prepared to do "radical retinoscopy" What is this?
1. Performing ret at either a very close distance and/or off-axis position. Done to observe a better reflect. Corneal irregularities might fragment the ret reflex. (interpret movement of the brightest part of the reflex). -Guestimate new (ret) WD & make this your dioptric correction in the final Rx determination. -Beare of induced errors in RX from this technique. *Don't Fret your Ret!
Review of the steps for trial frame refraction
1. Properly adjust trial frame on Pt (pd, height, temple, vertex, tilt) 2. Perform Retinoscopy (static and/or radical) 3. Occlude L eye & measure VA in OD 4. Perform a subjective fraction (in trial frame) 5. Measure VA at end of refraction & record result (both VA & refraction) 6. Repeat procedure for OS 7. If you determine a new Rx, show pt their vision through their current (habitual Rx) compared to the Rx you found.
When will we need to perform color vision?
1. Px's complain of difficulty with color discrimination -Blue - yellow defects are most common in acquired vision loss -Pseudochromatic plates do NOT identify blue - yellow defects! 2. Also required for certain certifications: - Driving -Various vocations
Why is contrast sensitivity measurement important?
1. Research has shown that CS has been strongly associated with: -Reading, Mobility & Driving performance -Face recognition -ADL's 2. Indicated when it is unclear, which eye is better functionally and / or explain why a Px: -Prefers the eye that is not of the best VA -Is responding poorly to rehabilitative devices despite good VA
Continuous text is the second method for assessing near VA What are the three ways to measure continuous text?
1. Threshold Print size (TPS) (ie. Near Acuity): Smallest print size that Pt can read the majority of words correctly 2. Critical Print Size (CPS): Smallest print size Pt can read with maximum speed 3. Maximum reading rate (wpm): Pt's reading speed when not limited by print size
The third step is to determine the Rx Cyl power, how do we do this?
1. Use either ±0.75D or ±1.00D JCC (If no cyl found in your ret, elicit cyl) 2. Hold JCC handle 450 to the Rx cyl axis 3. Use bracketing technique to determine cyl power -Use big changes in cyl power (eg. ± 1.00 DC) -Reduce bracket size every time you hit reversal until the cyl power bracket can no longer be reduced (ie. ±0.25 DC) -Maintain spherical equivalence by adding +0.25 DS for every -0.50 D of cyl 4. Cyl power determined when you reach equality
The first step is to determine the best sphere, what are the steps to this?
1. Use ret VA as initial JND/bracket size. Adjust JND by re-measuring VA every time you hit reversal 2. Initially assess best there in both the "plus" and "minus" power direction 3. Once you hit reversal but the VA remains the same, test for equality. Once equality is achieved -> best sphere determination is complete. *Present options as: "better with lens or without"
What are the steps for testing MARS CS?
1. leave overhead lights on 2. Pt holds chart either at 40 or 50cm 3. Pt wears near Rx 4. Have pt read/guess letters until 2 consecutive letters are incorrectly identified. Note: *NO credit given for "O" called a "C" & vice versa* 5. Assess/ encourage eccentric viewing (EV) 6. Record result as Log CS
what are some important tips to remover when performing MARS CS?
1. ↓ chart memorization by having Px read lines backwards 2. Monitor that Px occludes untested eye properly 3. Monitor that Px is not squinting, or leaning forward
The second step is to determine the Cyl Axis. How do we do this?
1.Use either ±0.75D or ±1.00D JCC *If no cyl found in your ret, elicit cyl 2.Hold JCC handle coincident with the ret cyl axis *Use bracketing technique to determine cyl axis *Make big changes initially in axis direction (eg. in 150 steps) *Reduce bracket size with subsequent testing Eg. 15 degrees -> 10 degrees -> 5 -> 1 3. Cyl axis determined when you reach equality
IfPxreads20/100at30cm,whatMnotation would they see at 50 cm?
3.3 M
What are some details about the MARS letter CS chart?
Chart is hand-held & consists of: - 8 rows each with 6 letters - Contrast of each successive letter (LR), ↓'es by 0.04 log units - Each correctly identified letter is assigned a value of 0.04 log CS - Px reads letters until they make 2 consecutive errors - Chart held at 50cm or 40cm - Px wears their habitual Rx
What are some characteristics of the Pelli-Robson Letter CS chart?
Chart is set at 1 m & consists of: - 8 rows each with 2 sets of triplets - *Letters within a triplet have the same contrast* - Each correctly identified letter is assigned a value of 0.05 log CS except for the 1st triplet - *A call of "O" for "C" & vice versa is considered correct* - Px's read letters until they incorrectly identify 2 out of 3 letters wrong within a triplet (*the two within the triplet do not have to be consecutive*) - Must allow Px at least 30 sec per letter by threshold
How is Near VA recorded?
Clinically, Near VA is recorded in terms of print size and not Snellen VA Print size: height of printed text. -Clinically, one measures the smallest resolvable print size that can be seen = resolution limit *But.. must still record viewing distance to give a meaningful (VA) measure
What kind of illumination should be used?
Directional light source positioned at the side or behind the shoulder of the Px - Do NOT have light shine directly in Px's eye -Ensure additional light provides uniform, glare-free lighting over the near acuity task - Have Px read sample text. While Px is reading text (letters), introduce the additional lighting & ask Px whether it (reading) is "better with or without the light"
T or F: we do logMAR grading for near
False because you have to have set distances to do logMAR
T or F we can use snellen for near
False! If you use snellen for low vision at near, you will get the answer wrong on the exam.
T or F: low vision tests typically have binocularity.
False. Often absent due to large difference in VA between the two eyes
What is the cutoff in which a pt probably functions monocularly?
If the difference in VA between OD & OS is> 0.3 log MAR (3 lines) Px probably functions monocularly Assess, if indicated, using the 4 Worth Dot Test Driving - check for suppression - gross "color vision" test for driving performance
What are some important tips to keep in mind with the Pelli-Robson CS
Important Tips 1. ↓ chart memorization by having Px read lines backwards 2. Monitor that Px occludes untested eye properly 3. Monitor that Px is not squinting, or leaning forward
What notation is used for Near VA
In LV, use M Notation to specify print size: - The distance (in meters) that lower case print size subtends an angle of 5' of arc - Record measurement as: *Working distance (m) / print size (M units)
What would a patient's JND be if their VA is 10/150?
JND = 150/100 = ± 1.50 D
What would a patient's JND be if their VA is 20/320-2?
JND = 160/100 = ± 1.75 D
What is the equation for Just Noticeable difference (JND) used for initial "bracket size" for the sphere?
JND = Denominator of 10ft VA/100
What would a patient's JND be if their VA is 10/80?
JND=80/100 = ±1.00D
What are modified color vision testing for Low Vision patient's?
Jumbo D-15 or Worth 4-Dot
How do we score MARS CS?
MARS CS (Log CS) = (Total # of Letters Correctly Identified) * 0.04
How do you record the N VA int he chart?
N VA with ETDRS N Chart with +3.00D Add & additional illum: OD: 0.33 / 2 M-2 OS: 0.2 / 1.2 M+1 Even though the ETDRS Near chart is based on LogMAR principles, do NOT score the N VA using LogMAR principles. Use snellen principles for scoring Near VA
How do you score Pelli-Robson CS score?
Pelli-Robson CS (Log CS) = (Total # of Letters Correctly Identified - 3) * 0.05
What does this near acuity mean? 0.20 / 1M
Pt can read 1M print size at 20 cm *The WD does NOT have to be 40 cm! (this is why we don't use snellen)
What does this near acuity mean? 0.65 / 3 M
Pt can read 3M print size at 65 cm *The WD does NOT have to be 40 cm! (this is why we don't use snellen)
What is the best way to determine a starting point for a Low Vision Pt's Rx?
Retinoscopy. -Do retinoscopy using ret bars and not behind the phoropter -Can put WD lens in place or subtract the WD out later
A practitioner refers a LV Px to you and states that the Px's N VA is "20/70 at 26 cm". What is the Pt's 'true' Near VA in M Notation?
Step 1: compute the M print size associated with 20/70 Near snellen VA. = 0.26meters /1.4 M Step 2: compute reduced snellen VA from M notation = 107.69 Thus: 20/70 at 26cm ≡ 0.26/1.4M ≡ 20/100 at 40cm
Is a larger M size correlate with better or worse VA?
The larger the M size, the larger the print size & hence the worse the Px's N VA
T or F: 1 M = 20/50 Reduced Snellen
True, but only True at 40 cm ONLY!
What should be your initial "Bracket size"?
Use the pt's just noticeable difference (JND) as your initial "bracket size" for the sphere
Do we perform refractions in the phoropter or with our trial frames?
With our trial frames! Never use a phoropter for Low Vision Patient.
