Vision Testing in Occupational Health - PDF

www.ohacademy.co.za Vision Testing in Occupational Health More than meets the eye! 20 June 2024 Dr Greg Kew Occupational Medicine Specialist Overview of presentation Introduction – Purpose of vision testing industry – What does the OH prof...

www.ohacademy.co.za Vision Testing in Occupational Health More than meets the eye! 20 June 2024 Dr Greg Kew Occupational Medicine Specialist Overview of presentation Introduction – Purpose of vision testing industry – What does the OH professional need to know? Part A: Basics of vision – anatomy & physiology (5 min) Part B: The core elements of vision testing (25 min) Part C: Standard Setting (briefly) (5 min) Part D: Vision testing & the Law (not today) The purpose of vision testing To assess the visual function of employees, to determine fitness for work (eg driving) Occasionally, it is part of a screening program for adverse effects of exposure to a hazard (eg methanol) Also as part of a normal clinical encounter (eg IOD involving a foreign body in the eye) Therefore … What must the OH Professional know?  It is important that the OH professional understands: – The fitness for work requirement (minimum standard in the OREP) for each visual function (acuity, colour, peripheral vision, etc) – The performance characteristics of the vision tests available (purpose, predictive value and limitations) … and is able to apply this knowledge appropriately, so as to correctly: – Assign the correct medical (vision) tests – Determine defendable minimum medical standards of fitness – Interpret the outcomes, and recommend appropriate action PART A: ANATOMY & PHYSIOLOGY OF VISION Key aspects of visual function to consider Consider what it takes to “see”.. Key aspects of visual function to consider Consider what it takes to “see”... Small fine details - far Key aspects of visual function to consider Consider what it takes to “see”... Small fine details (near) Key aspects of visual function to consider Consider what it takes to “see”... contrast Key aspects of visual function to consider Consider what it takes to “see”.. depth and dimensions Key aspects of visual function to consider Consider what it takes to “see”... colours Key aspects of visual function to consider Consider what it takes to “see”... things outside your immediate focus Key aspects of visual function to consider Consider what it takes to “see”... things in the dark (“night vision”) Key aspects of visual function to consider Consider what it takes to “see” complex images such as... motion Key aspects of visual function to consider Consider what it takes to “see” (recognise) complex images such as... animals Key aspects of visual function to consider Consider what it takes to “see” (recognise) complex images such as... faces Key aspects of visual function to consider Consider what it takes to “see” (recognise) complex images such as... emotions Consider the visual demands confronting a driver How is all this magic possible?  Visual information processing The journey comprises FOUR key steps involving visual information processing The eyes  Gather & focus light waves  Convert (“transduce”) light waves to “electrical” impulses (neural signals) The optic nerves  Transmit the signals to the brain The thalamus (& midbrain)  Does some “primary” processing  Redirects the signals to many parts of the brain (mostly to the visual cortex) The visual cortex  Interprets the various signals and forms a coherent meaningful image (colour, motion, form, depth, etc) Anatomy of the eye Light is gathered & focused onto the retina  The retina transduces the light to neural impulses Photoreceptor cells Single layer of melanocytes Visual spectrum: 400nm - 700nm Also amacrine cells & horizontal cells in this layer (retinal interneurons, modulate the information flow from photoreceptors to bipolar cells) The human retina The retina’s projections to the brain Optic nerves LGN (thalamus): Major relay centre for the visual pathway via the optic radiations. Optic tracts Hypothalamus (limbic system) Pretectal area in midbrain: pupillary reflexes, circadian rhythm accommodation reflex, smooth pursuit, regulating REM sleep Emotions, memory (“limbic system”) Sup Colliculus (tectal area in midbrain): Multiple layers. Topographic mapping of body relative to Visual attention outside world. (thalamus) “Dorsal stream” (“Where Pathway”) Motion, representation of object locations Optic radiations (left & right) “Ventral stream” (“What Pathway”) Form recognition & object representation; long term memory Bottom line… It is NOT the eyes that see, but the BRAIN! PART B: VISION TESTS Key aspects of visual function to consider What are the key components of visual function? Visual acuity (distance and near) Contrast sensitivity Colour vision Peripheral vision (visual fields) Depth perception (and its association with stereopsis) “Night vision” VISUAL ACUITY Visual Acuity Testing  The word “acuity” comes from the Latin acuitas, which means sharpness  Distance visual acuity / near visual acuity  Application: – Drivers and operators of hazardous self-propelled mobile equipment (land, sea, air) – Operators of equipment with hazardous moving parts – Employees required to see small objects or small print. (small print, circuit boards, etc.)  Visual Acuity Testing: apparatus  Electronic visual acuity tester (ie. Keystone Orthoscope; Bausch & Lomb Orthorator; AOC Site-Screener; Titmus Vision Tester; Keystone Telebinocular; OPTEC 2000 Vision Tester, Projector with screen).  Hand-held lens devices, with a set of standardised reading cards (ie. the “Bioptor”)  A wall-mounted reading card, with standardised images. (ie. a Snellen chart, LogMAR chart). Note that the Snellen chart was primarily designed for far vision, although it can be adapted for near vision. But which is the device of choice???  Advantages of Test Charts Inexpensive Easily done well Easily transported No electricity required Easily compared with commonly understood (and legally referred to!) standards  The Snellen Chart  Named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862  Commonly only the ten letters C, D, E, F, L, N, O, P, T, Z are used.  The “optotypes” are intended to be seen and read as letters, which have a particular block-like appearance  the height and width of the optotype is five times the thickness of the line. 1 2 3 4 5 1 2 3 4 5  Principles behind the Snellen notation The optotypes are designed so that, at a test distance of 6m (20ft), the symbols on the line subtend an angle of five minutes of arc* on the retina, and the thickness of the lines and of the spaces between the lines subtend one minute of arc. * There are 60 minutes of arc in one degree; 360 degrees in a circle. 360 x 60 = 21 600 minutes of arc in a full circle. This is what Snellen defined as “standard vision” – the ability to read these letters at 6m Line Standard vision is thereby designated 6/6 (or 20/20) Letter thickness = height = 1 minute of 5 Other acuities are expressed as ratios with a numerator (top arc minutes number) of 6. of arc Letter width = 5 minutes of arc Principles behind the Snellen notation Hence: 20/20 = 6/6 (feet versus meters) 6/6 score means the test subject sees an object at 6m, as clearly as a person with normal vision would see it at 6m 6/12 score means the test subject sees an object at 6m, just as a person with normal vision would see it at 12m 6/24 score means the test subject sees an object at 6m, just as a person with normal vision would see it at 24m The test subject sees an object at … just as a person with normal vision this number of meters… 6 /6 would see it at this number of meters The test subject sees an object at … just as a person with normal vision this number of meters… 6 /12 would see it at this number of meters The test subject sees an object at … just as a person with normal vision this number of meters… 6 /24 would see it at this number of meters But the “standard” Snellen Chart has problems  Does not meet international eye chart design guidelines  Some letters are easier than others to identify (“Snellen font”) International Vision Chart Standards  American National Standards Institute, Inc. (2010). American National Standard for Ophthalmics – Instruments – General-Purpose Clinical Visual Acuity Charts. ANSI Z80.21-2010. Revision of ANSI Z80.21-1992 (R2004). Approved May 27, 2010.  World Health Organization. (2003). Consultation on development of standards for characterization of vision loss and visual functioning. Retrieved from http://whqlibdoc.who.int/hq/2003/WHO_PBL_03.91.pdf  BS 4274-1:2003 "Test charts for clinical determination of distance visual acuity — Specification“ replaced the old BS 4274-1:1968(British Standards Institution) "Specification for test charts for determining distance visual acuity“.  International Council of Ophthalmology (ICO) – 1984  National Academy of Sciences – National Research Council (NAS-NRC) – 1980  The 6 standards for visual acuity charts 1. Optotypes should be of approximate equal legibility. “Optotype” is the name for the picture, symbol, letter, or number the test subject is to identify. Approximate equal legibility helps to prevent guessing. The Sloan font should be used because Sloan letters are approximately equally legible one from another. 2. Each line on an eye chart should have the same number of optotypes. Some 11 x 14 charts may have fewer than 5 optotypes on the top two lines to fit a light box. This is acceptable; you are concerned with lines 20/50 and below. 3. Horizontal spacing between optotypes should be equal to the width of the optotypes on a line (red box). 4. Vertical spacing between lines should be the height of the optotypes in the next line down (blue box). 5. The size of optotypes should progress down the chart by 0.1 log units between rows. This makes it easier to convert to alternative testing distances. Typically, these charts are referred to as “proportionally spaced” in a catalogue description. 6. Optotypes should be black on a white background with luminance between 34.4 lux and 68.9 lux. This is to achieve adequate contrast between the letters & the background.  The LogMAR Chart The LogMAR chart scores visual acuity with reference to the Logarithm of the Minimum Angle of Resolution. Also known as or Bailey-Lovie chart or ETDRS chart (Early Treatment Diabetic Retinopathy Study) Developed at the National Vision Research Institute of Australia in 1976, & is more accurate than the Snellen chart The LogMAR Chart  Each line comprises the same number of test letters (effectively standardizing the test across letter size)  The Sloan font is used (Sloan letters are approximately equally legible one from another)  Letter size from line to line varies logarithmically, as does the spacing between lines.  Scoring: – Zero LogMAR indicates standard vision – Positive values indicates poor vision – Negative values indicates good vision. – This is less intuitive than other VA notations. LogMAR versus Snellen Font differences for the 3 chart types (LogMAR) Scoring conversion tables are available  “Illiterate Chart” Specified “LEA Symbols” to be used Interpretation along the same lines as the standard LogMAR chart  Visual Acuity Test Charts These tests of visual acuity do not meet international eye chart design guidelines Common errors in use of Test Charts The chart does not conform to the international standards Incorrect sizing of chart letters for a 6m (or 3m) distance. Poor quality chart. Poor quality lighting Incorrect distance between subject and chart. Permitting the subject to view the chart with both eyes.  Visual Acuity Testing: near vision  Standardised reading cards  Same principle as for LogMAR Chart  Tip: Use a 40cm cord Jaeger Chart  Errors of refraction impacting visual acuity (better known as hyperopia) Both hyperopia and presbyopia are vision conditions where there is an error of refraction with the light not reaching the correct part of the retina. Hyperopia is a condition that you can have from birth or get as a child while presbyopia is a condition that is a consequence of becoming older CONTRAST SENSITIVITY What is “Contrast Sensitivity”?  Contrast sensitivity is the ability to distinguish between finer and finer increments of light versus dark (contrast).  Especially important in situations of low light (night driving), fog or glare - reduced contrast between objects and their background.  Important - even with 20/20 visual acuity, one can have eye or health conditions that may diminish one’s contrast sensitivity and impair vision.  What is “Contrast Sensitivity”? With respect to the image on the right:  a person with normal visual acuity but poor contrast sensitivity might see the trees in the foreground clearly (high contrast),  but have trouble seeing the contours of the mountains against the sky in the background (low contrast).  Symptoms of impaired Contrast Sensitivity Problems with night driving, including inability to see traffic lights May require extra light to read Structures can become lost against the background Inability to see spots on clothes, work surfaces or dishes Causes of Impaired Contrast Sensitivity Low contrast sensitivity can be caused by cataracts, glaucoma or diabetic retinopathy. Changes in contrast sensitivity also can occur after and other types of refractive laser surgery (LASIK, PRK (photorefractive keratectomy)) (better or worse). In most cases, people with cataracts notice a significant improvement in both visual acuity and contrast sensitivity after cataract surgery. Importance of Contrast Sensitivity  Population data has been obtained for visual acuity and contrast sensitivity (Grimson et al., 2002; Haymes et al., 2002; West et al., 2002) and related to real world performance.  A strong relationship between high spatial frequency contrast sensitivity loss and visual acuity with self-reports on driving difficulty was shown in 288 drivers over the age of 55 with cataract compared to a control group of 96 drivers with no indication of cataract (McGwin et al., 2000).  A separate study from the United Kingdom showed that automobile crash involvement increased for drivers with below average low contrast visual acuity (Sladeet al., 2002).  In Canada, it has been acknowledged that reduced contrast sensitivity can affect driving ability in spite of the driver having “adequate” visual acuity; additionally, they recommended that further research is needed to understand what level of reduced contrast sensitivity represents an unacceptable driving risk (Canadian Ophthalmological Society, 2000).  Although high contrast acuity is undoubtedly important for some everyday tasks, natural scenes are predominantly composed of low contrast information (Brady & Field, 2000).  Contrast sensitivity has been found to be a better predictor of target detection and recognition than standard visual acuity measures for pilot’s attempting to detect ground-to-air targets in field studies (Ginsburg et al., 1983) and in simulators (Ginsburg et al., 1982), for detection and discrimination of faces (Beard & Ginsburg, 1991), for military tank detection in outdoor scenes (Rohaly et al., 1997) and for simulated aircraft on a runway (Ahumada & Beard, 1997a,b).  Thus, measuring the ability to see low contrast images may be worth considering when determining vision standards and tests for individuals needing to see small objects at low contrast levels (Kleven & Hyvärined, 1999).  Tests for impaired Contrast Sensitivity  A contrast sensitivity test measures the ability to distinguish between finer and finer increments of light versus dark (contrast).  Different tests and charts are available to test and score contrast sensitivity. The Pelli-Robson Contrast Sensitivity Test chart is recommended.  The chart contains letters of equal size, arranged in groups of 3 letters divided by a gap. Contrast is reduced from one group of letters to the next and graded in Log units. COLOUR VISION  Functional anatomy of the eye The 7 million cone cells play a key role in colour vision (dominate the fovea centralis inside the macula) The 120 million rod cells play a key role in light / dark detection & movement (minimal role in colour) (dominate the periphery of the retina) Note the “blind spot” where the optic nerve exits the eye  “Colour Vision” and the retinal cone cells  Three classes of photopigment that have Red or “L” cones – respond to long wavelength light maximum sensitivity in the long-wavelength (red), middle-wavelength (green) and short- wavelength (blue) parts of the visual spectrum. (note: not the primary colours!)  Normal colour vision = “normal trichromacy”. Green or “M” cones – respond to medium wavelength light  Colour vision impairments arise from cone photopigment abnormalities; – Only 1 photopigment present (monochromatism) Blue or “S” cones – respond to short wavelength light – Only 2 photopigments present (dichromatism). – All 3 photopigments are present, but with abnormal spectral sensitivity (anomalous trichromatism). Monochromats Single-cone photopigment-type cells, or no functioning cone cells at all. Visual acuity (sharpness) is poor Colour discrimination is not possible, and people are truly colour blind, seeing everything in shades of grey and white. Very rare for both inherited and acquired defects. Dichromats and Anomalous Trichromats Dichromats  Deficient in one of the three photopigments.  3 types: – Protanopia (red affected) Red - green colour deficiency is – Deuteranopia (green affected) a common chromosomal defect affecting 2 photopigments – Tritanopia (blue affected) Anomalous Trichromats  Have all 3 photopigments, but have an altered spectral sensitivity of one of the three colour receptor mechanisms -> wide spectrum of severity What do people with colour blindness see? (green affected) Deuteranopia Tritanopia (blue affected) Protanopia (red affected) Classification & prevalence Protan defects: Deutan defects: Tritan defects red-sensitive cones green – sensitive cones blue-sensitive cones affected affected affected Dichromatism Protanopia Deuteranopia Tritanopia. (missing 1 photopigment) (Inherited - 1% prev in (Inherited - 1% prev in men) Rare for inherited. men) More common for acquired. Anomalous Protanomalous Deuteranomalous Tritanomalous trichromatism trichromatism trichromatism trichromatism (Inherited - 1% prev in (Inherited - 5% prev in men) men)  Inherited red-green vision impairment  Genes that specify the red- and green-sensitive photopigments are located on the X chromosome and abnormalities are inherited as an X-linked recessive trait. Therefore, a much higher prevalence of red-green colour deficiency in men (8%) than in women (0.4%).  A woman will only have colour deficiency if both of her X chromosomes carry similar abnormal genes (recessive)  The different types of red-green colour deficiencies do not occur with the same frequency - deuteranomalous trichromatism is the most common type. Gender related prevalence of colour vision impairment  Inherited blue vision impairment  Abnormalities of blue photopigment are inherited as an autosomal dominant trait.  Therefore men and women are affected equally.  The prevalence of inherited tritanopia is rare - estimated to be about 1 in 10 000 and that of tritanomalous trichromatism to be about 1 in 500 (0.2%). Acquired colour vision impairment  It is estimated the 5% of the population have an acquired defect as severe as the 8% with an inherited defect  A highly-varied and unpredictable group of defects (difference in colour perception between eyes, or confined to one eye and/or localised in one part of the visual field)  Increasing age brings increased vulnerability Causes of acquired impaired colour vision  Medical conditions: diabetic complications to eyes, glaucoma, retinitis pigmentosa and age-related macular degeneration, multiple sclerosis and liver conditions.  Medications (antibiotics, barbiturates, anti-tuberculosis drugs, high blood pressure medications, Viagra, quinine)  Chemicals (carbon disulphide, carbon monoxide)  Substance abuse (cannabis, alcohol) Test Selection What aspects of colour vision are you testing for? Basic “primary colours”? Complex colour discrimination, including contrasts, hues and saturation Something in between? Special applications (coloured wires, lab work involving titration to a colour change) Test environment  Lighting levels in the test area must be adequate: – Artificial “natural daylight” fluorescent illumination – The light source should provide a minimum of 200 lux at the surface of the test for young subjects, and higher for older subjects.  Light source should be at an angle of 45° to the plate surface  Tungsten lighting is unsuitable Colour Vision: apparatus Simple colour plates Useful for determining colour discrimination from basic (primary & secondary colours), but no hues, shades or contrasts Colour Vision: apparatus Coloured wires  Anyone required to work with electrical wiring  Preferably 2 identical sets, and the subject is asked to pair up the wires of similar colour.  Colour Vision: apparatus Ishihara colour test plates Subtle hues and contrasts are tested Best for screening for red-green colour deficiency. Appropriate for laboratory workers, quality assurance in clothing & textiles, etc. The City University Test (2nd & 3rd editions) Grading the severity of red-green colour deficiency. Possible grading of protan and deutan defects if the red-green test is failed. Identifying tritan defects.  Farnsworth-Munsell HueColour Vision Test The aim of the test is to order the shown colour plates in the correct order—any misplacement can point to a colour vision deficiency  Colour Vision: apparatus Lantern Tests  For testing seafarers  Previously the Holmes-Wright type B lantern test,  Now replaced by the Fletcher CAM lantern test. Note: The SA Maritime Safety Authority (SAMSA), in its Maritime Medical Standards Code, now prescribes using the Ishihara Chart or equivalent – and use of Lantern Tests “as appropriate”)  The Farnsworth Lantern Test (FALANT)  In the FALANT test, the subject is are presented with a combination of two coloured lights which may be any combination of RED, GREEN and/or WHITE.  The combination may be two lights of the same colour and may be presented more than once during the test.  Used for aircraft pilots Note: Its seems that many Civil Aviation Authorities accept the use of the Ishihara Chart or equivalent – and use of Lantern Tests “as appropriate”)  Chromatic correction for colour blindness  EnChroma developed optical lens technology that selectively filters out wavelengths of light at the precise point where this confusion or excessive overlap of colour sensitivity occurs.  This re-establishes a more accurate ratio of the colours of light entering the eye so that when someone with red-green colour blindness wears a pair of EnChroma glasses, they perceive a more accurate range of colours—and the experience can be life changing. https://enchroma.com/blogs/beyond-color/how-enchroma-glasses-work PERIPHERAL VISION (VISUAL FIELDS)  Visual field testing Taking a point directly ahead as zero degrees, the normal human visual field extends to:  Approximately 60 degrees nasally away from the midline (past the nose, toward the opposite side) in each eye  To 95 degrees temporally (towards the ears, or outwards),  Yielding a horizontal span of about 190 degrees.  In the vertical plane, the range extends from approximately 60 degrees above and 75 below the horizontal meridian Vision Testing: Peripheral Vision The “Confrontation Test” This is conducted with the subject at a position such that the eyes are level with the examiner’s eyes. (1): The examiner’s arms are outstretched, such that the hands are at points approximately 70 deg lateral to the subject’s eyes. The subject is instructed to keep his eyes fixed on the examiner’s nose, and to indicate when he sees the examiner’s fingers move. (2 & 3): The examiner twitches his fingers at various points (quadrants) about the subject’s peripheral field of view, watching for the subject’s responses. Vision Testing: Peripheral Vision The Novissphere (1): This is conducted using a small plastic dome designed for peripheral vision testing, and an otoscope with a paediatric earpiece as a light source. (1,2,3): The examiner places the device lightly over the eye being tested. The subject is instructed to keep his eyes fixed on the aperture of the device, and to indicate when he sees the light. The examiner moves the light to various points (quadrants) over the device, watching for the subject’s responses. DEPTH PERCEPTION Depth Perception The ability to perceive spatial relationships, especially distances between objects, in three dimensions Depth perception – Important concepts  Depth perception arises from a variety of “depth cues”.  The important message from this is that one does not necessarily require two eyes (binocular vision) to be able to perceive “depth”. Cues to depth perception Oculomotor – based on sensing the position of the eyes & muscle tension Monocular – 2 categories: – Pictorial cues – Movement produced cues Binocular – based on binocular disparity (“stereopsis”) Oculomotor cues Convergence  Being aware of an inward movement of the eyes when we focus on nearby objects Accommodation  Getting feedback from changing the focus of the lens  Monocular Cues – “Pictorial” Occlusion One object partially covers another Relative height Objects that are higher in the field of vision are more distant  Monocular Cues – “Pictorial” Relative size  When objects are equal size, the closer one will take up more of your visual field Familiar size  Distance information is based on our knowledge of object size Perspective convergence  Parallel lines appear to come together in the distance Monocular Cues – “Pictorial” Atmospheric perspective  Distant objects are fuzzy & have a blue tint Monocular Cues – “Pictorial” Texture gradient  Equally spaced elements are more closely packed as distance increases Monocular Cues – “Pictorial” Shadows  Help to create a sense of distance  Motion Produced cues Motion parallax  Close objects glide rapidly past, but objects in the distance appear to move slowly Deletion & accretion  Objects are covered and uncovered as we move relative to them Range of effectiveness of different depth cues  Binocular depth cues – “stereopsis” “Stereopsis” comes from the difference between the images in the two eyes. Also known as “binocular disparity”.  Binocular depth cues – “stereopsis” Depth perception is provided by binocular disparity  A stereoscope creates “artificial” depth by using 2 pictures from slightly different viewpoints  3D movies use the same principle and viewers wear glasses to see the effect Illusions caused by manipulations of size constancy Depth perception Depth deception Illusions caused by manipulations of size constancy How do we test for depth perception? Near vision depth perception Horizontal Lang two-pencil test (Thread a needle)    How do we test for depth perception? Distance vision depth perception Exclude recent loss of vision in one eye. The problem with electronic equipment is that it specifically relies on stereopsis (binocular disparity) as a proxy for depth perception!! On-the-job testing. NIGHT VISION  “Night” vision  What is meant by “night vision”?  Key visual functions to consider: – Ability to identify objects in low light – Light-dark adaptation.  The rod cells of the retina play a key role A rod cell is sensitive enough to respond to a single photon of light  Rods (greyscale) dominate the periphery of the and is about 100 times more retina, and cones (colour) dominate the fovea sensitive to a single photon than centralis. cones. Also, multiple rod cells converge on a single interneuron, collecting and amplifying the signals. Conditions that are a problem with night work Impaired vision in low lighting (“night blindness”) Impaired light-dark adaptation Intra-ocular light scatter “Night” blindness (“Nyctalopia”) Problems to consider in screening: – Impairment of contrast sensitivity function (CSF) which is induced by intraocular light scatter Refractive surgery (photorefractive keratectomy, radial keratotomy Cataracts – True impairment of “night vision” (rods problem) Retinitis pigmentosa (cited as “most common”, but