Visual Acuity (EX. 1)

Exposure Duration

*Decreasing exposure duration will decrease the VA*. This can be offset by increasing the level of luminance to ensure the constancy of the number of absorbed quanta.

Larger pupil size

*Increasing pupil size, increases VA by letting more light in.* BUT a very large pupil (over 5 mm) will allow *spherical aberration* in which light rays striking the crystalline lens' periphery are refracted more strongly, causing the image to move forward toward the vitreous, producing myopia. Also, the larger pupil may also reveal irregular astigmatism, causing a blurred image.

Target and Eye movements

*Strict stability* of the retinal image is not required for optimum resolution. BUT if there is significant movement of the retinal image, the VA acuity will decrease. If a smooth pursuit movement will enable tracking of an object, the VA will only be slightly decreased.

Children

IMPORTANT: Present a block of letters so amblyopia is not missed!!!!!! For children sometimes 6 meter distances are not appropriate, then use 3 meters and adjust your VA result. 1. Birthday cake children acuity chart. 2. Tumbling E. 3. Landolt C. 4. Allen preschool vision test. 5. STYCAR visual acuity test (Screening Test for Young and Retardates) (eating items or toys) 7. Sheridan-Gardiner visual acuity test 8. HOTV 9. Lea symbols 10. Preferential looking 11. Preferential fixation 12. Candy bead tests (match candy with others at 40 cm) 13. Optokinetik drum- nystagmus until no more nystagmus 14. VEP visual evoked potential

Calculating VA

If a patient is a myope: the *inverse* of the distance at which the eye is able to focus the 20/20 line is going to be very close to the diopters of myopia.

Amblyope, crowding

If you know your patient is _______, do not present the whole chart because it will create a _______ effect. Show a single line or letter.

Procedure for Konig bars

A chart with pairs of Konig bars of different sizes with the width of the bar and the gap being the same. The smallest pair resolved as 2 bars gives a measure of subject's VA.

Recording VA at near

ALWAYS assume is at 40 cm, but if NOT, write the distance. Specify what chart is used Pinhole not done at near, only at distance and covering one eye.

Recording VA at distance

ALWAYS indicate eye and if with or without correction. Distance for near chart. Distance for CF (counting fingers) or having to walk toward the chart. (ex. CF @ 1) If other than Snellen was used, indicate!!!

Minimum recognizable acuity

Angular size of the smallest feature that one can recognize or identify. Snellen chart. Block letters five times stroke size. Normal defined at 20 ft, or 6 m. 20/20 = 5 min arc letter = 1 min arc stroke. MAR units (20/20)=1 , (20/40)=2

Near VA

Different near cards (hands, tumbling E, numbers, Figures, Spanish, English) *ILLUMINATE the chart* Place at *40 CM (~16 in.)*, if patient needs to bring it closer, then write your recording with that distance.

Patient with SRx

Do VA first WITHOUT correction and then WITH correction. At distance: present whole chart. Cover one of the eyes and say "read the smallest line of letters you can". Then switch eyes and cover the other eye.

1. squinting 2. patient 3. chart

Do not allow 1 (myopes and astigmatism show better VA), therefore look at the 2 NOT at the 3.

1 month

Fixation is central, steady and maintained, can follow a slow target, and convergence, preference of looking at face.

Luminance

Is a photometric measure of luminous intensity per unit area of light traveling in a given direction. Used as *indicator of brightness* and use foot-lambert units. **IF the relative luminance increases, the relative VA also increases** The standard luminance for VA chart is considered to be a minimum at 10 foot-lamberts. Increasing luminance from 100 to 1000 foot-lambert causes little increase in VA. Reducing luminance to 1 or even 5 foot-lambert causes VA loss. *Very high and very low luminance cause a reduction in VA* (this is evident clinically by a decrease in vision when it is very dark or very bright, like a blinding light)

Visual Acuity (conceptual)

Is the ability to distinguish details and shapes of objects. *First test performed* in all patients after the case history, even in follow up appointment. Each visit ALWAYS have to document VA. Tells us about the *preservation or degradation* of vision, how effective is a treatment given. Is important legally because implications such as licenses, pensions, insurances, lawsuits, claims, etc.

.....communication problems

It is expected that a 3 year old child have a VA of 20/20. Failure of this VA in the absence of a retractive error, amblyopia, or ocular disease may be due to......

2 years

Picture matching.

6 months

Reaches out accurately for toys.

Konig bars

In research, they can be used to determine the VA. Pair of black bars on a white background. For normal visual acuity, the width of each bar is 1 min of arc and the gap between the bars is 1 min of arc. Length is usually five times the width but it is not critical.

Illumination

Recommended luminance levels of 12-20 foot-candles for distance acuity charts. Commercially available chart projectors provide luminance levels in excess of this amount when used correctly. Consider the luminance of the chart projector and the overall room illumination. If the room is extremely bright it will cause miosis (constricted pupil), therefore acuity will be artificially high. Recommended room lighting in the vicinity of the projector chart to be kept low otherwise will reduce contrast of the letter on the chart. Room lighting in the vicinity of the projector screen should be no greater than 13% of the projector luminance.

Recording VA

Record if patient needs eccentric fixation. Record VA in terms of the smallest line of letters read. Has to read half or more of that line.

Minimum discriminable acuity (hyperacuity)

Refers to determining the relative location of two or more visible features with regard to each other. Angular size of the smallest change in feature than one can identify. ex. Vernier acuity, one vertical line is displace until observer can determine if upper line is to the right or left of lower line. Threshold of discrimination is 3 seconds. Clinical example: Aligning the mires when reading a patient's ocular pressure with Goldmann application tonometry. Here the threshold is 1 second of arc.

Percentage Acuity

Representation of VA according to percentages. The resolution of the gap of 1 min of arc in width represents VA of 100%. It is DA (distance acuity) times 100.

English acuity

Representation of VA according to the English system. The resolution of the gap of 1 min of arc in width represents VA of 20/20.

Decimal acuity

Representation of VA according to the decimal system of measurements. The resolution of the gap of 1 min of arc in width represents VA of 1.0. Good for plot charts and research. Normal VA is given a value of 1.0. It decreases with the increase of the gap (letter size). For an acuity half as good as 1.0, the letter should be twice as large.

Metric Acuity

Representation of VA according to the metric system. The resolution of the gap of 1 min of arc in width represents VA of 6/6.

Visual Activity (math definition)

Resolving power of the eye, or the ability of the eye to see two separate objects as separate. ABILITY TO SEE A GAP. Ability to resolve a spatial pattern separated by a visual angle of ONE MINUTE OF ARC

Clinical Pearls

Show the whole chart from the 20/50 (if using projector) IF patient cannot see the 20/50, then show the 20/400. IF patient cannot see the 20/400 and you have the low Vision chart, then use it. If you do not, then do all the other steps like walking towards chart, finger counting, etc. MEMORIZE CHARTS, need to observe patient at all times.

Small pupil size

Smaller pupil has a larger depth of focus. By decreasing the aperture of the pupil, a smaller blurred circle will be projected to the retina and will sharpen the retinal image. This smaller blurred circle results in a greater *depth of focus* with a small pupil. A pinhole device increases the depth of focus by decreasing the size of the blurred circle projected on the retina. If a pupil is too small, however, it will block too much light and decrease VA.

5 years

Snellen chart by matching or naming.

Contrast

The difference in color and light between parts of an image. **Increasing contrast increases resolution at a given luminance level** In the clinic it is used high contrast, that is, black letters on a white background. Reality: Objects and their surroundings are of varying contrasts. The relationship between VA and contrast allows a better understanding of our visual perception. VA will decrease if contrast is below 90%.

Retinal eccentricity

The further fixation is from the center of fovea, the lower the level of acuity will be. Foveal visual acuity refers to the one that only has cones. *At the fovea, the visual acuity will be maximal* The angular size of this rod-free area (the fovea) is 54 minutes of arc, almost 1 degree in diameter, subtended at the nodal point of the eye. *If light focus away from the fovea by 1 degree, there is a reduction of 60% of the maximum acuity.* The 20/200 subtends an angle of 50 min of arc, which fits well in the rod-free area.

1. pupil 2. lens 3. retina

The light enters the through 1, it passes through the 2 and is projected on the 3 at the back of the eye.

1 minute of arc

The normal eye can resolve two objects as separate if they are separated by an angular distance of..... Needed: Retinal focus, intact neural elements and brain faculty to interpret.

MAR

The reciprocoal of the Snellen notation equals the angle (in min of arc) subtended by the strokers of the letter at the person's eye. This angle is also used to specify visual acuity. Can also be given in logMAR.

Rods and cones

The retina has these two basic types of photoreceptors.

Snellen Acuity chart

The universal method for measuring VA. Snellen letter: with of the stroke equals the width of the gap. Most Snellen charts have letters that are 5 units high and 4 units wide. "best" letter is E because it has 3 strokes and 2 gaps. T and L are used but do not have a gap.

Blind spot

This is *caused by the optic nerve* which is void of any photoreceptors.

Tumbling E

This is presented to preschoolers or those who cannot read. Letter E is placed in any of 4 positions and subject reports which way the legs of the E point.

VA with PINHOLE

This test is done IF with the patient's correction at distance, he/she CANNNOT read more than *20/30* with the best correction BROUGHT. IF patient does not have correction, then do it without correction. During subjective refraction it is taken if patient cannot reach 20/30. Done at DISTANCE ONLY.

Cones

Three types of cones for basic color vision. Red, Green, and Blue. (based on their wavelengths, short, medium, long)

Minimum resolvable acuity

Type of VA. Ability to determine the presence of, or distinguish among, more than one identifying feature in a visible target. Separation of two features, finest high contrast detail visible, for width of light and dark bar of a grating, limit about 1 min arc for fovea. By varying the object size or spacing between object components, one could determine the minimum resolvable threshold. Applied clinically with the Snellel visual acuity and optotypes presented up, down, left, or right. Determined by photoreceptor sampling.

Minimum visibility

Type of VA. Detecting the presence or absence of a target. Detection of a feature. Accomplished by gradually increasing size of a target. This results in an increase in the target's contrast until it is rendered minimally visible. Actually limited by contrast sensitivity, depends on background illumination.

Developmental aspects of VA

VA sharpens as an infant ages. Age at which child will attain normal level of adult visual acuity depends of technique used to determine the VA. 1. Visual Evoked Potential (20/20 at 6 to 12 months) 2. Preferential Looking Tests (20/20 at 18 to 24 months) 3. Optokinetic Drum (for nystagmus) (20/20 at 20 to 30 months) On AVERAGE you will retain your ability to see 20/20 until age 62. After this age, only half retain their ability to see 20/20.

Refractive error

VA will decrease if the optics of the eye are defocused. ***The amount of visual acuity drop is proportional to the amount of defocusing. (.25 D of defocusing per line of VA)

Snellen notation

Visual acuity in Snell notation is: the standard viewing distance over the distance in which the letter subtends 5 min of arc) D'= usually 20 ft. or 6 m D= distance at which each letter of this line subtends 5 minutes of arc (each stroke of the letter subtending 1 minute)

Limits of VA

Visual acuity is limited by diffraction, aberrations and photoreceptor density in the eye.

Interaction effects

When objects are too close together, the level of visual acuity decreases.

Rods

used for vision under dark or dim conditions

Extraocular muscles

will move the eyeball in the orbits and allow the image to be focused on the central retina or fovea.

Visual Acuity Notations

1. Decimal acuity 2. English acuity 3. Metric acuity 4. Percentage acuity

Standard testing conditions for VA

1. Letter size 2. Testing distance 3. Chart 4. Background illumination 5. Contrast

Three types of VA:

1. Minimum visibility 2. Minimum resolvable acuity 3. Minimum discriminable acuity

Other factors of variation of VA

1. Size of retinal mosaic 2. Optical aberrations of the eye 3. Retinal image Magnification 4. Optical media clarity 5. Ability to interpret blurred images

Charts

1. Snellen Chart 2. Landolt ring (Landolt C) 3. Tumbling E 4. Konig bars 5. Pediatric charts (LEA, HOV, ALLEN, TELLER)

Patient cannot read 20/400 at distace

1. You can use SOSH (low vision charts) at 10 feet away. 2. If possible patient walks to the chart 3. Counting fingers (3 feet, 2 feet, 1 foot) 4. Hand motion 5. Light projection 6. Light perception - if not seen then you record NLP (no light perception)

VA is affected by:

1. refractive errors 2. illumination 3. contrast 4. location of the retina being stimulated 5. pupil size 6. exposure duration 7. target and eye movement

1 min of arc is.....

1/60 of a degree.

Preferential Looking Tests

20/20 at 18-24 months

Visual Evoked potential

20/20 at 6 to 12 months

Advantages of Snellen

26 letters to choose. Reduces chances of guessing We are familiar with letters and people expect to read in a visual examination

Letter Size of E

3 strokes and 2 gaps. Each one is 1 min of arc. Each component (or each min of arch) measures 1.745 mm. E measures 8.7 mm. Useful when calibrating the charts according to the projector's location. (move the projector until image projected is the corrected size calculated.

Testing distance

As long as the letter size of the 20/20 letter subtends an angle of 5 min of arc at the spectacle plane, an *emmetrope or hyperobe* will have no problems clearing up the letters even if the distance is less than 6 m. The problem will be with *myopes* that will show artificially higher VA if the distance is closer than 5 m. For charts at less than 4 m in distance, myopes will be underminus, and hyperopes will be overplussed. IF testing distance is less than 20 ft, then use of mirror systems should be considered.

Binocular fixation preference

Behavioral evidence of decreased vision in one of the eyes can be seen in babies. Cover eye and watch babies behavior. What eye it prefers to look at toy with.

3 months

Binocular vision and eye coordination, eyes follow a moving light or face, responsive smile.

Very soon after birth

Can fix and follow a light source, face or large colorful toy.

Fovea

Center of the retina.

Foveal region

Cones are highly concentrated in this region, but this density decreases rapidly outside of the fovea.

Contrast

Contrast of the letters can be a problem when using polarized projector slides. To have *maximum contrast*, the line normal to the screen has to bisect the angle between the projector beam and the patient's line of sight. For polarized projection systems, use a mirror to determine the correct angle of the screen. The mirror (and angle of the screen) should be adjusted until the patient that is sitting in the chair can see the projector.

Contrast sensitivity

Contrast sensitivity is another method of VA measure. Is the ratio of the difference between max and min luminance of a test stimulus divided by the sum of the max and min luminance. VA will decrease if contrast is below 90%.

Landolt Ring (Landolt C)

is a ring with a gap. The width of the stroke and the gap are both 1 min of arc. The subject is asked to tell where the gap is located. (harder to guess than in Konig bars) Typical Landolt ring chart starts with 20/200 or 6/60 and ends with 20/20 or 6/6

3 years

Letter matching of single letters (ex. Sheridan Gardiner)

Log (MAR)

Log of Minimum Angle of Resolution Should be used when precise, quantitative assessment of the visual acuity is needed, such as all research studies in which VA is a dependent variable. The incremental size of letters is determined according to the base 10 logarithm of the critical detail in minutes of arc of the letters. The critical detail in min of arc of the smallest recognizable letters is considered to be the patient's MAR.

9 months

Looks for hidden toys.

Pupil Size

Most patients have pupil sizes between 2 to 5 mm at ordinary illumination levels for VA tests. *Increasing pupil size, increases VA by letting more light in.* BUT a very large pupil (over 5 mm) will allow *spherical aberration* in which light rays striking the crystalline lens' periphery are refracted more strongly, causing the image to move forward toward the vitreous, producing myopia. Also, the larger pupil may also reveal irregular astigmatism, causing a blurred image.

NIPH

No improvement with pinhole. Then problem is not refractive and vision cannot be fixed with a prescription.

Spatial resolution limit

is derived from the fact that one degree of a scene is projected across 288 nm of the retina by the eye's lens

Snellen Fraction-Snellen Acuity (SA)

Testing distance/designation of the smallest line read. (or distance at which the smallest letter subtends an angle of 5'arc) Could be metric or english unit. Almost always VA done at 6 m or 20 feet, therefore this is the numerator. To convert DA into SA: Multiplly the Snellen fraction numerator by the reciprocal of the decimal acuity.

Optokinetic Drum

for nystagmus. 20/20 at 20 to 30 months.