Visual Acuity Assessment

VISUAL ACUITY ASSESSMENT

CHAPTER ONE

RELATED TASKS

• define visual acuity
• list types of visual acuity
• explain minimum detectable resolution va
• explain minimum resolvable va
• explain minimum separable va
• demonstrate stereo acuity
• outline visual acuity notation
• explain visual acuity test charts

explain recording of distant va

j. explain common errors occur during taking va

• k. explain factors affecting the measurement of va
• l. demonstrate how va charts are made
• m. demonstrate distant visual acuity measurement

n. describe photo transduction.

INTRODUCTION TO VISUAL ACUITY

Visual acuity is defined as the spatial resolving capacity of visual system.

Refers to the sharpness of vision or patient’s ability to recognize a minimum size target.

• It provides information on;
• refractive status of the eye
• indication of macular function

indication of neural intergrity.

Visual acuity compares the sharpness of both eyes, if its similar or different.

TESTS OF VISUAL RESOLUTION

• The ability of the eye to distinguish fine details ( visual performance).
• They include;
• Minimum detectable resolution.
• Minimum separable va.
• Minimum resolvable va.

Stereoacuity.

MINIMUM DETECTABLE RESOLUTION

Minimum detectable resolution refers to the threshold of an individual’s visual system to detect the presence of a spot or line stimulus against its background.

Does not require the discrimation of target details but requires the individual to perceive the presence or absence of an aspect of the stimuli presented.

MINIMUM RESOLVABLE VA

• Minimum resolvable visual performance involves the resolution of details.

Clinical evaluation of the va is based upon this type of visual performance .

it involves the measurement of the smallest symbols, shapes, letters that can be correctly by the px.

it is separated into 2 forms;

Form sense( landolt C , tumbling E)

True minimum legible ( uses complex patterns such as letters or numerals). referred to as letter optotypes.

MINIMUM SEPARABLE / VERNIER ACUITY

This involved the individual’s ability to detect that a group of points or lines are separate and distinct eg break in a line.

The px has the task of determining the minimum separation between line targets that allows them to distinguish the lines from each other.

STEREOACUITY

Stereoacuity involves the use of both eyes. It represents the ability of the px to resolve slight differences in distance of target objects when looking at special stereoscopes.

NOTATIONS OF VISUAL ACUITY

• There are various different ways in which they can be specified, namely;
• snellen fraction
• decimal notation
• minimum angle of resolution
• logarithm of minimum angle of resolution
• visual acuity rating

visual efficiency

1. Snellen Fraction

Based on the snellen principle that, for 2 objects to be distiguished as separate, they must be separated by a minimum angle of resolution of one minute of arc at the nodal point of the eye.

in the construction of letter E on letter chart , the thickness of the limbs and the space btn them each needs to subtend an angle of 1 minute of arc.

Visual Acuity Assessment

The snellen fraction is an expression of the angular size of optotypes by specifying the test distance and the height of the letters.

• the snellen fraction is denoted as;
• visual acuity =numerator/denominator
• where , numerator =test distance
• denominator =distance at which letter subtends 5’of arc.

NB; 6meters is assumed to be an optical infinity.

Letter size and testing distance

• The range of letter sizes on most charts from top to bottom is as;
• 6/60,6/36,6/24,6/18,6/12,6/9,6/7.5,6/6,6/5.

The 6/60 letter is 10 times the 6/6 letter.

If cannot be seen the move the chart closer in 3m , 1m or perfome the finger counting , hand movement, light projection.

Visual Acuity Assessment

In US the test distance if expressed in feets while other many countries in meters , that is 20/20=6/6.

2. Decimal notation.

• this reduces the snellen fraction to a decimal quantity.
• that is 20/20( 6/6) =1.0 decimal notation
• 20/200(6/60)=0.1 decimal notation

this system does not specify the testing distance.

3. Minimum Angle of Resolution ( MAR)

this is expressed n minutes of an arc , it is equal to the reciprocal of the decimal acuity or snellen fraction.

that is; 20/40( 6/12)= 2MAR

4.Logarithm of minimum angle of resolution ( LogMar)

• its is merely the logarithm of MAR.

that is 20/200=6/60=10MAR→logMAR=log 10= 1.0

5. Visual acuity rating(VAR)

• this is derivered from the LogMar values

VAR =100 -50 (LogMAR).

Visual acuity test charts and designs.

VISUAL ACUITY CHART DESIGNS.

SNELLEN CHART

the original snellen design comprised a single large letter at the top of the chart and smaller letters below.

the number of optotypes increase as size gets smaller.

2. BAILEY – LOVIE DESIGN/LOGMAR CHARTS

bailey lovie design recognized some of inherent flaws in the snellen design and developed a set of principles that make the va same at each size level.

• has the following characteristics;
• a logarithimic size progression ( constant ratio from one letter to next.)
• same number of letters at each size

the spacing btn letters and rows are proportional to the letter size.

CHART FORMATS

• There are may be various types of chart formats that va charts are present in , includes;
• Printed charts
• Projector charts

Charts on display screens

DISTANCE VISUAL ACUITY MEASUREMENT.

PROCEDURE

• Should be conducted under adequate illlumination conditions.
• give proper instructions to the patient , “how well they can see”
• use the occluder ( cover) to cover the px LE and record VA of the RE
• proceed to occlude the RE and record the VA of the LE.

remove the occluder and recorf the VA of both eyes.

don’t correct the mistakes that the patient may make.if the patient can no longer see, urge him to try a few more letters by point that are easy to avoid nemorization of letters , start with the poor eye

RECORDING.

If the patient reads an entire line correctly and stops there, then you record the line as his VA e,g 6/6 if the patient correctly reads more than half the number of letters on a line , you score him as that line minus the number of letters that could not be read or were miscalled.

PINHOLE VISUAL ACUITY

The pinhole consists of either a single small hole or multiple small holes.

the pinhole allows narrow bundle of light rays to enter the eye and prevents light scatter to provide an adequate image without the need for additional optical aids.

Common errors when taking visual acuities.

• allowing the px to decide their own acuity
• permitting the patienbt to look around the occluder or through fingers
• using dirty chart or one of low illumination.

placing the chart at an incorrect working distance.

not recording the result immediate and guessing the wrong result at the end of the examination.

Factors affecting the measurement of visual acuity

• refractive error
• illumination and pupil size
• spacing between letters and lines (crowding phenomenon)
• choice of letters / optotypes
• design of the letters
• contrast
• time

chart luminance

PHOTO TRANSDUCTION PROCESS