Visual Acuity - Clinical Skills 1

Visual Acuity

Angular measurement relating testing difference to the minimal object size resolvable at that distance

-Spatial resolving capacity of the visual system

What is Visual Acuity based on?

Central Vision

Fovea

Central of Cones

Where should we measure Resolvable Acuity?

Curved Surface at the Retina

What type of measurements do we use to measure the Resolvable Activity?

Minutes and Seconds

Why do we not use Degrees or Arcs?

They are too large

Why do we use angular measurements to measure the Resolvable Activity?

Retina is a curved, NOT flat

List the reasons why we test Visual Acuity (4)

1. Determines Normalcy

2. Determines Refractive Decisions

3. Monitors Ocular Health

4. Vision Standards

In absence of Ocular Disease, how should patients see?

20/20

What Visual Acuity is better than 20/20?

20/15

Up to what age should our eyesight be better than 20/20?

50 years of age

List the reasons as to why patients may not see as well as 20/20 (3)

1. Uncorrected Refractive Error

2. Ocular Disease

3. Amblyopia

Amblyopia

Developmental disorder resulting in reduced vision

What does Amblyopia cause? (5)

1. Uncorrected (high) Refractive Error

2. Uncorrected Asymmetric Refractive Error

3. Deprivation (Ptosis/Cataract at early age)

4. Strabismic

5. Toxic/Nutritional

What is Strabismus?

Eye turn

What can Visual Acuity be used to help estimate?

Spherical Equivalent Refractive Error

-This supports your objective/subjective refraction

What can Visual Acuity be used to help monitor?

Ocular Disease

What is the main factor that decreases Visual Acuity?

Ocular Pathologies

Do certain professions have Visual Acuity requirements?

YES

-List may include Pilots, Military, Police

Do driving acuity requirements vary for each state?

YES

Who must you report to when a patient falls below the acuity requirement to drive?

DMV

List the two types of theories of the resolution of the eye:

1. Optical Limitations

2. Neural Limitations

What determines the Resolution?

Pupil

Diffraction

Pupil is too small

Refraction

Pupil is too large

What type of limitation is Rayleigh Criterion?

Optical Limitation

Airy Disc

Central bright circular region of pattern produced by light diffracted when passing through the Pupil

Pupil

Small circular aperture in the eye

What is the pattern of an Airy Disc?

Concentric rings

Describe what has to occur when 2 Airy Discs are considered resolvable:

The center of one lies at the edge of another

What is Resolution approximated at?

1 minute of arc with optimal pupil size of 2.5 mm

Rayleigh Criterion

What type of limitation is the Receptor Theory?

Neural Limitation

Resolution is dependent on...

Packing density of retinal receptors & neural interactions

What has the best resolution?

Fovea

Why does the Fovea have the best resolution?

High density of cones

Distance between the centers of Cones separation within the Fovea

2 µm

What distance of the separation of centers of Cones permits the separation of 2 points when they fall on the center of 2 receptors?

4 µm

What resolutions do both Optical & Neural systems provide?

Around 1 minute

List the types of Visual Acuity (4)

1. Detection Acuity

2. Resolution Acuity

3. Localization Acuity

4. Identification Acuity

What are the two types of Detection Acuity?

1. Minimum Visible Acuity

2. Minimum Distinguishable Acuity

Minimum Visible Acuity (Detection Acuity) is associated with...

Brightness

Describe Minimum Visible Acuity (Detection Acuity)

Determined by the brightness of object relative to its background illumination

-Patient would NOT need to identify the object

-Image is NOT relative to the angle it subtends

What does visibility rely on?

Intensity, NOT size

What is an example Minimum Visible Acuity (Detection)?

Star

Minimum Distinguishable (Detection Acuity) is associated with...

Contrast

Contrast Discrimination

Distinguish whether light falling on photoreceptors is greater than falling on its neighbors

Describe Minimum Distinguishable Acuity (Detection Acuity)

Ability to detect an object against a plain background

-Threshold is 0.5 seconds of arc

-IE: dark line on white background

What does the width of the retinal image depend on?

Diffraction

Resolution Acuity

Ability to distinguish smallest space between two similar objects

-30 seconds of arc is threshold

-IE: dots and lines

What are the clinical applications to the Resolution Acuity?

Preferential Looking and Retinometer

What happens are shapes are moved closer together?

It's harder to resolve the two shapes or gaps & it DECREASES angular size

Minimum Angle of Resolution (MAR)

The angular size of the gap between the smallest letters a patient can read

Localization Acuity

Ability to discriminate the alignment of 2 objects

-Minimum spatially discriminable

-Misalignment Acuity

-Threshold is 2-6 seconds of arc

What does the angular displacement of 2 objects determine?

Acuity

Clinical Applications of Localization Acuity (4)

1. Stereoacuity

2. Keratometry

3. Lensometers

4. Applanation Tonometers

Identification Acuity

Minimum Legible Acuity - Recognition Resolution

-Patient must recognize or identify optotypes

-Threshold is 30 seconds to 1 minute of arc

Examples of Identification Acuity charts:

Snellen Acuity Chart and SOSH Chart

What is the most commonly used clinical chart?

Identification Acuity - Snellen Acuity Chart

Limitations of Snellen Acuity Chart

1. Fewer targets at the top of the chart

2. Some optotypes are more recognizable than others