Vision Screening

• Vision screening


  The purpose is to detect those people who have defective vision but who do not present with symptoms resulting in them seeking attention
• Important factors to consider when conducting vision screening

  • Effectiveness (Specificity & Sensitivity)
  • Cost Factors
  • Personnel
  • Target condition
  • Criteria for visual requirements
• Specificity


  Indicates how well a test can classify subjects who truly do not have the outcome of interest. Proportion of the true negatives identified
• Sensitivity


  Indicates how well a test can classify subjects who truly have the outcome of interest. Proportion of true positives correctly identified.
• Specificity = TN/(TN + FP)
• Sensitivity = TP/(TP + FN)
• Positive Predictive Value (PPV)

  Probability that a positive result indicates the presence a condition of interest. PPV = TP/TP + FP
• Negative Predictive Value (NPV)

  Probability that a negative test result indicates the absence of the condition of interest NPV =TN/TN + FP
• Specificity = 80%, Sensitivity = 90%
• Visual screening


  Aims to detect those people whose visual ability is below the standard required for a specific task
• Advantages of industrial vision screenings

  • Selection of personnel
  • Identify employees with visual disabilities
  • Improved employee-employer relationship
  • Increased productivity
  • Fewer accidents & reduced insurance costs
  • Reduced absenteeism
  • Compensation claims can be settled more easily
• Steps employer must take for a vision screening program

  1. Establish visual standards for various tasks

  2. Select appropriate screening method and tests

  3. Seek advice from optometrist
• Methods of determining appropriate visual standard for a particular occupation

  • Use of predetermined visual standards
  • Establish relationship between visual ability and job performance/competence
• Visual task analysis


  1. Analyse the visual factors required for the task

  2. Decide on vision criteria for job competence

  3. Choose a method of job grading competence

  4. Screen the vision of competent and incompetent employees and compare
• Factors to consider in visual task analysis

  • Distance & size of critical detail
  • Need for color discrimination
  • Depth perception
  • Head and eye posture
  • Field of vision
  • Eye movements
  • Contrast and illumination
• Dynamic visual acuity


  More strongly associated with accidents than static visual acuity
• Visual requirements for driving

  • Visual acuity (dynamic vs static)
  • Contrast sensitivity
  • Visual fields
  • Monocularity
  • Color vision
• Minimal vision standards for driving licenses range from 20/30 to 20/60; Horizontal and vertical fields range from 70 to 140 degrees
• Nonvisual factors for driving

  • Attentiveness
  • Mental state
  • Judgement
  • Reaction time
  • General health
  • Sensory or motor disorders
  • Roadworthy cars
• Factors considered for aviation visual requirements

  • Radial keratotomy and diurnal fluctuations
  • Glare sensitivity
  • Monocular pilots
  • Professional flight crew requirements
• Overhead panel viewing solutions for pilots

  1. Look-over bifocals to reduce head tilt

  2. Flip-down spectacles with additional near power

  3. Vocational multifocals with near segment in upper part and lower part of the lens .Special multifocal lens with small near viewing segment 4 . Progressive power lenses. 5. Soft and hard contact lenses
• Overhead Panels


  Possible solutions to difficulty in viewing overhead panels
• Solutions for viewing overhead panels

  1. Lift the bifocals (or look-overs) so that the head does not have to be tilted so far back

  2. Provide flip-down spectacles with an additional lens power to focus the overhead panel clearly when viewing through the upper distance part of the bifocal lens

  3. Use vocational multifocals with a near segment in the upper part of the lenses as well as in the lower part

  4. Construct a special multifocal lens to provide a small near viewing segment
• Bifocal Segment Height


  The segment can be set low so that it is used only for charts, manuals and reading, to allow looking over the top of the near segment
• The Need for Trifocals

  Considers residual accommodation
• Look overs


  Provide a variable focus depending on which part of the lens is used for viewing
• Soft Contact lenses


  Both hard and soft contact acuity lenses are acceptable. Hard lenses tend to induce more discomfort and can be displaced by propeller wash or strong wind.
• Sunglasses


  • Glare is often a cause of significant discomfort when flying above clouds or when flying into the sun
  • Narrow frames are suitable, thick temples are not suitable: impair the peripheral visual field
  • Lenses should not be too dark, and should transmit at least 15% of incident light: Neutral Density Filter lenses
  • ND filters: greyish tint that does not distort colour perception or adversely affect red signal detection and recognition
  • Polycarbonate Lenses are preferred because of their impact-resistance and ability to absorb ultra-violet and infrared rays
  • Polarising sunglasses should not be used when flying: produce a distorted and degraded visual field that poses a threat to air safety
• PHOTOCHROMIC LENSES


  Have disadvantages that render them unsuitable for use by pilots: their transition times are relatively slow, bleaching time from maximum to minimum density can be as long as 30 minutes or more, even when fully bleached, they still absorb slightly more light than untinted lenses
• Colour Vision


  Defective colour vision may be safe for aviation duties, e.g. mild deuteranomals ('red-green' colour blind)
• OCULAR PATHOLOGIES


  • Cataracts
  • Multiple Sclerosis (MS): Optic Neuritis
  • Glaucoma
  • Macula Diseases
  • Retinal Detachments
  • Retinal Injuries
• STRABISMUS


  VA (corrected or uncorrected) of worse than 6/12 is unacceptable, and a binocular VA worse than 6/9 is also unacceptable. Lack normal stereopsis.
• METHODS OF VISUAL SCREENING

  • MODIFIED CLINICAL TECHNIQUE
  • INSTRUMENT SCREENERS
  • SOFTWARE SCREENING PROGRAMS
  • NEW YORK STATE OPTOMETRIC ASSOCIATION(NYSOA) screening battery
  • PHOTOSCREENERS
• MODIFIED CLINICAL TECHNIQUES


  • Advantages: Flexibility, Type and magnitude of refractive error can be assessed, Ocular pathology may be detected, Presence of squint will be detected, Very few false referrals, Binocular status determined

  Disadvantages: Professional personnel needed, Expensive method of screening, Time consuming
• INSTRUMENT VISION SCREENERS


  • Advantages: Operated by lay technicians, Rapid screening, Always available for use, Low maintenance costs, Cheap method of screening

  Disadvantages: Lack of flexibility of tests and testing distance, Does not detect ocular pathology unless inferred by reduction in acuity, Cannot detect squints, Awareness of target may induce proximal accommodation & convergence – affects VA measurement and induce relative esophoria at distance, Can be unnecessary referral by lay operator of people with amblyopia or squint
• COMPUTERIZED: VISUAL EFFICIENCY RATING (VERA)

  Increased specificity when overlays of symptoms, classroom behaviors, and reading skills are included, no new clinical skills of school nurses, Provides evidence of visual skills dysfunction, Detects refractive errors and basic binocular vision dysfunctions