Eschenbach Low Vision Training Program

Edited by:Thomas Porter, ODAsst. Professor & DirectorLow Vision ServiceSt. Louis University, Dept. of Ophthalmology

© 2015 Eschenbach Optik of America, Inc. (Ver. 2)

Module 2: Visual Acuity, Magnification, Illumination & Contrast Sensitivity

The Seven Steps to Dispensing Low Vision Aids®

1. Make sure the patient is under the current care of an eye doctor

2. Identify the patient’s visual goals

3. Determine the magnification required

4. Determine the impact of illumination

5. Select the appropriate vision aids

6. Train the patient in the use and care of the chosen vision aid

7. Schedule a follow-up visit

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Visual Acuity Measurement• In more conventional eye care settings,

accurate visual acuity measurement is necessary for medical-legal reasons

• In LV practices, measuring VA often provides the initial insight into the level of functional vision loss, the need for eccentric viewing, and even the type of training that may be required

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Visual Acuity Measurement• Measuring VA is traditionally broken down into

distance VA and near VA.

• Each has some unique characteristics, requirements and points to ponder in the LV setting

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Visual Acuity Measurement

Why test distance VA• To determine what the vision level ‘really is’• Best corrected VA can be used to predict

magnification requirements for both distance and near tasks

• Testing VA can be used to predict the need for and ability of the patient to access and use eccentric viewing points

• Refractive issues

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Low Vision Training Module #2

Distance Acuity Charts options

• Avoid the ‘standard’ projected eye charts

• Printed Charts• Illuminated Charts

Considerations• Snellen, Metric, or Log

• Illumination

• Filtering and Testing distance

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Measuring Near VA• OD, OS, OU – (watch and listen)

Types of Charts• Single Optotype• Single word• Continuous Text

How each may help you• Good single letter and word but poor CT

often indicates a need for eccentric training

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Low Vision Training Module #2

Predicting Magnification Requirements

• When you can’t make an image clearer, then you must make it larger – magnification

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Predicting Magnification Requirements

• By having a method of predicting magnification, you can streamline the exam and conserve chair time

• Remember all methods only predict the level

• In many cases the prediction is the ‘lowest level’ that will achieve a goal

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Predicting Magnification Requirements

Among the methods often used• Kestenbaum’s Rule• Enlargement Ratio based on acuity

demand• Eye chart ‘recommendations’• Trial and Error

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Methods often usedKestenbaum’s Rule

• Reciprocal of the distance VA = dioptric add that will allow near reading at the appropriate reading distance

• Example: 20/200 distance VAo Reciprocal would be 200/20 = +10.00Do Therefore with +10.00 over the distance

correction, the patient should be able to read about 20/50 type at 4 inches

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Methods often usedEnlargement Ratio based on acuity demand

• By knowing both the VA and the specific VA requirement of a task, a ratio is formed that will help predict the magnification requirement.

• Example: 20/200 distance VA and the goal is newsprint for which the demand is accepted as 20/50.

• A simplified version would be 200/50 or 4x larger

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Methods often used (a neat variation)Enlargement Ratio based on acuity demand:

Measure the VA in metric VA as well as the power of the bifocal that the patient is using

• Example: 4M VA (Snellen 20/200) using a +3.00D add. The goal is newsprint for which the demand is accepted as 1M (20/50 Snellen).

• Then simply multiply the enlargement ratio 4/1 or 4 x the add (in this case +3.00D giving you a +12.00 add) which should allow newsprint.

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Methods often usedEye chart estimations

• Based loosely on assumed E/RTrial and error

• Least desirable due to time wasted with inappropriate powers

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The method often used for distance tasks is the ratio formed by the distance VA divided by the ‘estimated’ task VA. An example would be the VA is 20/200 and the task requires 20/50, therefore 200/50 = 4x

Predicting Distance Magnification Requirements

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• The flaw is that there is no accepted list of visual demands based upon various tasks

• Due to field of view issues the lowest power that will achieve the goal is desirable.

Predicting Distance Magnification Requirements

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• In the normal (healthy) aging process, more light is required to do various tasks. This is due to several factors

• Gradual loss of optical transparency of all ocular media

• Loss of contrast sensitivity• Increased illumination is an important tool

to enhance performance

Illumination and Contrast Sensitivity

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• Sunlight is often the standard used as the benchmark in a LV practice

• Each artificial light source has strengths and weaknesseso Fluorescent/LED– less contrast, veiling

glare, cooler, color shifted

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• Each artificial light source has strengths and weaknesseso Incandescent – yellow spectral output,

directional issues (area not uniformly illuminated)

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• Each artificial light source has strengths and weaknesseso Halogen – Good color balance

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Contrast sensitivity is the ability for the eye to ‘visually’ separate an object from the background

Normal view (good contrast sensitivity)

Poor contrast sensitivity

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• Contrast loss is almost universal in ocular pathology and congenital eye conditions

• Even a small contrast loss has a big impact on visual performance

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Contrast Sensitivity• There are many methods used to

measure and document contrast losso Vistecho EDTRS

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Contrast Sensitivity

• There are many methods used to measure and document contrast losso Others

§ (Clinical pearl) Test each eye § separately then repeat with both eyes

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Contrast sensitivity can be enhanced three ways:

1) Improved lighting2) Enhanced figure ground relationship3) Absorptive filters

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Improved Lighting

More important than the wattage or type of light is the position of the light• angle (at or below eye level, off

to the side• distance (“inverse square law”)

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Enhancing figure ground relationship

Choose the color of an object’s background to be as far opposite in contrast than the object (or vice versa)

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Absorptive Filters

Selective colored filters attenuate the blue portion of the visible spectrum and improve contrast

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• Contrast loss may not be noticeable unless tested for specifically

• Some patients may even have normal VA as tested in the office (but still complain of “foggy vision”)

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Small contrast loss = big functional loss

• Contrast loss can affect not only the patient’s visual acuity, but also their field of view

• These two negative impacts on vision can have a significant impact on the ability of the patient to accomplish their normal daily activities

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Questions?

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Thank You!