Determining the Initial Response to Magnification Keys to Examination Efficiency

By: Stanley Woo - OD, MS, FAAO

One of the challenges in low vision rehabilitation is to manage examination time efficiently.  This includes not only the number of visits, but also the expectations for what can be accomplished at each visit.  In general, we counsel our patients that a low vision rehabilitation evaluation will take on average 3 to 4 visits.  The first visit identifies realizable goals and the patient’s response to magnification.  The second and third visit may involve critical training to reinforce key concepts in utilizing devices and techniques with their own materials.  The fourth visit for dispensing is an additional opportunity to verify the persistence of training and touch up techniques as needed.

This article will propose an efficient and rational basis for estimating magnification for reading.  Reading is one of the most important goals for people with central field loss (e.g. age-related macular degeneration, Stargardt’s, diabetic retinopathy, among others).  The goals may range from short-term tasks such as reading medicine labels and menus to more prolonged tasks such as reading books or newspapers.  The first step is to identify the desired target print size (see Table 1).  For instance, newspaper is typically 0.8M size (0.3logMAR), and large print Reader’s Digest print is about 1.6M (0.6logMAR). The case history helps to prioritize goals, and the potential to satisfy these goals must be established by the conclusion of the initial examination.

Table 1  Common print sizes and examples

M Print

Point

Reduced Snellen
(@ 40 cm)

Common size

10

80

20/500

½” letters

5.0

40

20/250

Newspaper headlines

4.0

32

20/200

Newspaper subheadline

2.0

16

20/100

Children’s books

1.6

12

20/80

Children’s books, Large Print Reader’s Digest

1.3

10

20/63

Magazine print

1.0

8

20/50

Text Books, Newspaper print

0.8

6

20/40

Paperback, Newspaper print

0.6

5

20/32

Newspaper stock table

0.5

4

20/25

Small Bible, footnotes

Once the target size print has been identified, the second step is to measure the patient’s near visual acuity.  Many continuous text test cards exist on the market, but look for both M notation and logMAR scales since they are standardized and repeatable.  In low vision rehabilitation (LVR), one should avoid reduced Snellen notation since the meaning is lost when the test distance is somewhere other than 40 cm.  For example, 20/40 reduced Snellen at 20 cm is actually the equivalent of 20/80 reduced Snellen.  The resulting ambiguity from writing 20/40 at 20 cm in the chart is confusing at best. 

Encouraging the patient to vary the distance at which they hold the test card is important since it gives the practitioner insight into how adaptable the patient might be.  A patient who holds the card at 15 cm may have substantial defocus, but is indicating that they are willing to tolerate defocus in exchange for an enlarged retinal image.  By contrast, a patient who is resistant to any attempts to hold the card closer may be in for a long visit since relative distance magnification is one of the easiest ways to meet their goals.  Thus, the key in the second step is to measure the test distance accurately with a tape measure and note the M size and logMAR print they were able to read. 

If there is a substantial disconnect between the patient’s habitual working distance and their spectacle add, it may be prudent to use a tentative add following trial frame refraction to re-measure near visual acuity more accurately.  The patient may have had a +2.50D add and no change in spectacle Rx.  However, since they held the material at 15 cm, a tentative add of 1/15cm or +6.75D would allow them to focus for this closer distance.  Re-measuring near visual acuity might reveal that they can now read smaller size print, which in turn serves as a more accurate starting point for the estimation of magnification.

To illustrate these two points, let’s assume that we have a patient a patient with AMD.  They wish to be able to read the newspaper, their mail and bills, and menus in a restaurant.  Initial near VA is measured as 2.0M print at 40 cm with their +2.50D add.  The first step is to identify the target size print consistent with their goal.  Newspaper size is 0.8M.  We note that the test distance is consistent with their spectacle add i.e. the patient wanted to hold things only at 40 cm and their +2.50D add focuses them for that distance.

The third step utilizes the logMAR scale that is present on continuous text reading charts.  I have duplicated the scale in Table 2 with sections labeled I. and II.  The 1st column indicates the size of the text in M notation.  However, the units in column 1 can be substituted to be cm, diopters, meters, or any other unit as will be explained later.  The 2nd column indicates the reduced Snellen equivalent in the event that the card is held at 40 cm.  The 3rd column indicates the size print in logMAR, and it is important to note that each step in print size corresponds to a 0.1 logMAR step.  Also, each text size on the near acuity card corresponds to a row on the table.  Note that the upper half (I) of the table is a duplicate of the bottom (II) with the following exceptions:  the first and second columns are a factor of 10 larger than the bottom half.  E.g. 5.0M corresponds to 20/250 instead of 0.5M corresponding to 20/25.       

Table 2  Near VA card scales

I.  M Size/D/cm

Snellen

logMAR

40

20/2000

2.0

32

20/1600

1.9

25

20/1250

1.8

20

20/1000

1.7

16

20/800

1.6

13

20/630

1.5

10

20/500

1.4

8.0

20/400

1.3

6.0

20/320

1.2

5.0

20/250

1.1

4.0

20/200

1.0

II.  M Size/D/cm

Snellen

logMAR

4.0

20/200

1.0

3.2

20/160

0.9

2.5

20/125

0.8

2.0

20/100

0.7

1.6

20/80

0.6

1.3

20/63

0.5

1.0

20/50

0.4

0.8

20/40

0.3

0.6

20/32

0.2

0.5

20/25

0.1

0.4

20/20

0.0

Back to step 3.  You will recall that the patient was able to read 2.0M print size at 40 cm.  However, they wish to be able to read the smaller 0.8M print size which corresponds to newspaper.  Looking at the first column, we start at 2.0M print and then count the number of rows/steps downwards (i.e. towards smaller print) that it takes to go from 2.0M print to the patient’s goal of 0.8M print.  You should count a total of 4 steps.  This number of steps is critical since it also represents the number of rows/steps that we have to count upward in add power or downward in distance.

In our example, the patient read 2.0M print at 40 cm with a +2.50D add.  If we look at column 1 again in Table 3, we notice that the 1st column is now labeled in diopters (D).  Remember that we determined that the target size print was 4 steps smaller than the current print size the patient could read.  Consequently, we need to make the add power 4 steps stronger as well in order to be able to see it clearly.  Counting from +2.5D upwards because the add has to be stronger to read the smaller print.  As indicated by the “****” counting 4 steps from +2.50D leads us to +6.00D.  Thus, we can conclude that to read 0.8M print this patient would need a tentative add of +6D.  There is no need for any guessing as to what power is necessary, thus speeding up the examination process and removing uncertainty.  Upon completing the refraction, the a +6D lens can be inserted into a trial lens and the patient encouraged to read 0.8M print.  If successful, the patient’s response to magnification is deemed positive.  Demonstrating that the patient was able to read newspaper size print to the family or friends accompanying is an excellent way to encourage support.    

Table 3  Counting steps to determine add power (excerpt from Table 2)

D

Snellen

logMAR

+10   ***

20/500

1.4

8.0

20/400

1.3

+ 6.00 ****

20/320

1.2

5.0

20/250

1.1

4.0

20/200

1.0

3.2

20/160

0.9

+ 2.5 ****

20/125

0.8

2.0

20/100

0.7

To complete the example, we return to the goal of reading menus.  We can choose 0.5M print as a target to assure that all types of print will be accessible.  Counting from 2.0M to 0.5M is 6 steps.  Counting 6 steps stronger from the +2.50D add leads us to +10D.  Thus, we would select an illuminated hand-held magnifier that was at least +10D.  One could be selected from your diagnostic kit to confirm a positive and consistent response from the patient for the desired task.

In summary, we have demonstrated that the continuous text near charts with M notation and logMAR scales may provide a quick and easy estimation for magnification.  By being able to predict what power increments are necessary to read the target size print, the examination may proceed more efficiently while still demonstrating to the patient that their goals of reading are realizable.

Table 4.  Steps to identify initial magnification

  1. Identify target print size based on patient goals.
  2. Measure an accurate continuous text near VA.
  3. Count the number of steps from the print size in #2 to the target in #1.
  4. Count upwards the same number of steps in D to determine the initial add.
  5. Trial the initial add and measure the patient’s response to magnification.

References

  1. Bailey IL.  Visual acuity measurement in low vision.  Optometric Monthly.  April 1976;116-122.
  2. Westheimer G.  Scaling of visual acuity measurement.  Arch Ophthalmol.  1979;97:327-330.
  3. Strong G, Woo GC.  A distance visual acuity chart incorporating some new design features.  Arch Ophthalmol  1985;103:44-6.