Evaluation of Visual Impairment

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Evaluation of Visual Impairment
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Various Perimetry Tests

• Central field only
• Confrontation test
• Tangent screen
• Damato campimeter
• Central and Peripheral field
• Automated bowl perimeter
• Humphrey visual field analyzer
• Manual Goldmann perimeter
• Macular field
• Scanning laser ophthalmoscope

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Confrontation Test
Tangent Screen
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Damato Campimeter
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Humphrey Visual Field Analyzer
Goldmann Manual Bowl Perimeter
Examiner Patient
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Documentation of Visual Field Deficits

• Visual field is plotted on a field diagram
• Visual field 60 superiorly, 75 inferiorly, 100
  temporally, 60 nasally
• Diagrams come in different formats
• Isopters
• Absolute scale
• Gray scale

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Isopter Diagram
isopter
Seen area

• Isopter boundary between a region where a target
  is visible and where it is not visible
• Defines the sensitivity of the field much like a
  topography map

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Absolute Scale Diagram

• Uses symbols to describe sensitivity of field
• One mark indicates loss
• Another mark indicates decreased sensitivity
• Each perimeter will have its own legend

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Gray Scale Diagram

• Sensitivity of field is described using different
  shades of gray
• Light shadinghigh threshold
• Black shadingno response to target

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• Threshold tests
• Will have several isopters or symbols
• Depicting varying sensitivity of the field
• Screening tests
• Will have one isopter or one set of symbols
• Depict only areas of intact and impaired retinal
  function

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Classification of Visual Field Deficits

• By location in field
• Central (called a scotoma) nasal
• Para-central temporal
• Peripheral superior
• Hemianopsia inferior
• Quadrantanopsia right or left
• By Density
• Absolute
• Relative or threshold
• Also referred to as depression in the field

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Learning Objectives

• Understands visual function testing
• Demonstrates correct test procedure
• Identifies factors affecting test performance
• Knows acceptable and unacceptable test
  modifications
• Modifies test appropriately for selected patient
• Interprets test results accurately

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Low Vision Examination

• Typically completed by low vision ophthalmologist
  or optometrist
• Completed not to determine whats gone but what
  usable vision remains
• Very important distinction
• Patients often come in with defeated attitude
• Evaluation can change that by emphasizing ability
  to use remaining vision

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Primary Components of Exam

• Acuity
• Contrast sensitivity function
• Visual field integrity
• Color vision
• Functional anomalies

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Acuity

• Ability to see small detail at a specified
  distance
• Acuity is a fraction representing distance over
  letter size
• Two types of acuities are measured
• Distance (no accommodation)
• Reading (requires accommodation)

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Acuity Measurement

• In U.S. most commonly expressed as Snellen
  equivalent fraction
• 20/20 is considered normal vision
• Ratio of test distance to the distance at which
  smallest optotype subtends 1 minute of visual arc
  (or angle)

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• 20/20 optotype subtends 1 minute of arc (minimum
  angle of resolution-MAR) at 20 feet

E
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• Larger letter subtends a larger number of minutes
  and has a larger MAR
• 20/200 optotype subtends 10 minute of arc at 20
  feet

E
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Metric Measurement

• Result of international push for uniformity
• Uses M units
• 1M equivalent to size of optotype subtending 5
  min of arc at 1 meter
• 1.454 mm
• Sloan letters
• 10 Sans serif letters of same size
• CDHKNORSVZ

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• San Serif
• BLOCK LETTERING
• Serif
• EXTRA STUFF

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Standard Acuity Charts

• Design to obtain most accurate measurement in
  normal to near normal acuity range
• More lines to measure discrete differences
• As acuity declines, fewer lines
• Stops at 20/200 (the big E)

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For Acuity Below 20/200

• Switch to subjective measurement
• Count fingers
• Notation CF at X feet
• Hand movement
• Notation HM or HMO
• Light perception only
• Notation LPO
• No light perception
• Notation NLP

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Low Vision Test Charts

• Provide more discrete assessment of acuity in low
  vision range
• Accomplished by measuring acuity at intermediate
  distance of 1 meter
• Instead of standard 10 or 20 feet
• Extends measurable range to 20/1200
• Best charts use logarithmic progression
• all variables controlled but size

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ETDRS Chart

• Same of optotypes per line
• Spacing between letters and rows are proportional
  to size of letter
• 1 log unit between each level
• Enables letter by letter measurement

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Low Vision Charts for OTs
LeaNumbers chart
Colenbrander chart
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Test Protocol

• Hold chart 1 meter and at eye level from the
  patient
• Be sure the surface of the chart is evenly and
  adequately illuminated
• The patient should be wearing their eyeglasses
  with distance correction
• Instruct the patient to start at the TOP of the
  chart and read the letters down row by row
• Acuity is recorded from the last line, on which
  the patient could read the majority of letters
  accurately

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Acceptable Modifications to the Test Procedure
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Acceptable Modifications to the Test Procedure

• Answers
• Forced choice-is it this or this?
• Matching
• Yes/no
• Display
• Single optotype at a time
• One line a day
• Patient can turn the head but NOT move closer

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Reading Acuity

• Ability to read text
• Tests acuity at near distances to 20/400
• Reading is the primary visual task completed at
  near distances
• Text cards are more accurate than single letter
• Requires accommodation
• Variety of test cards are available
• Lighthouse Childrens Continuous Text card
• Warren text reading card
• Mnread Acuity test chart

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Warren Text Card
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Test Procedure for Reading Acuity

• Patient should have reading glasses on
• Be sure surface of card is evenly and adequately
  illuminated
• Hold card 16 inches/40cm from patient
• Instruct patient to start with top line and read
  down as far as possible
• Acuity is recorded at the last line of text the
  patient is able to accurately read

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Mnread Acuity Test Chart

• Combination reading acuity and performance test
• Very versatile test-always used
• Measures reading acuity to 20/400
• Grade 2-3 level

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Front of card
Back of card
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Measures Three Reading Components

• Reading acuity
• Smallest print size patient can read without
  significant errors
• Critical print size
• Smallest print size patient can read with maximum
  speed
• Maximum reading speed
• Reading speed patient can obtain when not limited
  by print size

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MNread Test Procedure

• Patient wears reading glasses, sits 16 inches (40
  cm) from chart
• Instructed to read each sentence out loud
  beginning with top sentence
• Patient is to stop after each sentence and not
  proceed to next sentence until so instructed
• Examiner times patient as sentence is read and
  notes any errors
• Patient continues reading until he/she cant make
  out the print
• Encourage patient to guess even when he/she
  believes a word is unreadable

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Scoring Mnread

• Locate print size in logmar units on X axis
• Size progresses from right to left, largest to
  smallest
• Locate time in seconds on Y axis on left side
• Mark intersection of two axis
• Repeat for all lines read
• Connect lines to shown progression on graph

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Reading Speed
Time in seconds to complete sentence
Largest print
Smallest print
Print Size
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Physicians Shorthand
V
OD 20/200 OS 20/100
CC or CC
OD right eye OS left eye
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Important Reasons For Knowing Acuity

• Determine the level of visual impairment for
  billing services
• ICD-9 codes are assigned to each level of
  impairment and in many states serve as the
  primary diagnosis for billing
• Snellen or metric acuity can be used to determine
  minimum diopters needed to read standard 1M size
  print

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Contrast Sensitivity Function

• a.k.a. low contrast acuity
• Measures ability to see an image as it degrades
  in contrast from its background
• Provides critical information about edges and
  borders and variations in luminance
• Important to measure because most environment
  features are low contrast
• Decreases with any deterioration of macular
  function

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Peli Robson Test

• Provides logarithmic score of CSF
• Sloan letters in groups of 3 ranging from 100 to
  1 contrast
• Scored letter by letter
• Impairment
• Disability

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LeaNumbers Low Contrast Chart

• Designed for clinic
• Durable and portable
• Children and adults
• Symbols and numbers
• Measures CSF between 25 - 1.2
• Performance on test is interpreted more
  subjectively

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Test Procedure

• Patient wears reading glasses and sits 40 cm from
  chart
• Chart should be well illuminated
• Patient is instructed to read first symbol on
  each line
• If patient does not see a symbol at first,
  encourage patient to focus on the symbol for a
  while and see if it appears

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Visual Field Evaluation
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Perimetry Testing

• All testing involves three components
• Sustained fixation on a central target
• Presentation of a second target of a specified
  size/luminosity in a designated area of the field
• Acknowledgment of the second target without
  breaking fixation on the central target

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Static vs Kinetic Test Strategy

• Static
• Target appears in a specified location without
  moving to that location
• Kinetic
• Target is moved into a specified area of the
  until it is detected and acknowledged

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Testing Defines the Visual Sensitivity of the
Field

• Every point within the retinal visual field has a
  specific visual threshold
• Defined as the weakest test stimulus just visible
  in that location under the specific test
  conditions
• Indicates the sensitivity of the photoreceptors
  to stimuli
• Threshold levels vary within the field
• Hill of vision concept
• Lowest at the fovea
• Highest at the periphery

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Circles represent isopters in the field diagram
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Threshold vs. Screening Test Strategy

• Threshold
• target is presented in specific location in field
  and increased in intensity until it becomes
  visible
• Very accurate, discrete measurement
• Screening
• Target is presented one time at supra-threshold
  value