Équipement de dépistage manuel de la vision et environnement Copy
Manual Vision Screening Equipment and Environment
Manual vision screening requires the technician to perform a range of different tests, each with its own equipment and setup requirements. The following manual tests are performed in the occupational health setting which need specific testing environments. Other tests are conducted but the environment is unchanged for them and therefore will be covered under vision screening tests.
Far visual acuity – Snellen or a Tumbling E chart (other charts such as the Lea are also available and can be used, the methodology and set up is the same) Near visual acuity – Rosenbaum, Sloan letter or a Jaeger chart Colour vision – Ishihara test
Snellen Chart
A Snellen chart is an eye chart that is used to measure far visual acuity. Visual acuity (VA) commonly refers to the clarity of vision, but technically rates a worker’s ability to recognize small details with precision8. Snellen charts are named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862. The normal Snellen chart is printed with eleven lines of block letters from the European alphabet8. The first line consists of one very large capital letter, which is usually an E, H, or N.
Subsequent rows have increasing numbers of capital letters that decrease in size. Each line is numbered 1- 11 and measured in either or both meters and / or feet. The number indicated at the side of the row corresponds to the distance at which a normal eye can read that row. For example, the top row (marked 60) is read by a normal eye 60 metres away. The Snellen chart cannot be used on workers who are illiterate, or who are not European alphabet literate. Snellen charts are designed to be read at three or six metres. Snellen charts are made of a non-reflective material, with a matte finish, some with green and red colour bars and a grommet for hanging. The Snellen chart is a standard 22″ x 11″ in size. Figure 6.5 below is an example of a Snellen chart.
Numerator (top number) – the number of feet at which screening is done (the appropriate screening distance is noted at the top of the vision chart) Denominator (bottom number) – the smallest line on which the required number of optotypes (standardized letters or symbols for testing visual acuity) are correctly identified (line size is indicated on the chart as 10, 15, 20, 25, 30 or 32, 40, 50, 63, 80, 100, and 180)15.
For example
If one’s vision is “20/50” it means 7,16:
At 20 feet away, the smallest line that person can see is the 20/50 line A person with 20/20 vision could read that same line at 50 feet away
Similarly, if one’s vision is “6/15” it means7,16:
At 6 metres away, the smallest line that person can see is the 6/15 line A person with 6/6 vision could read that same line at 15 meters away
Tumbling E Chart
A tumbling E chart is an eye chart used to measure a worker’s visual acuity, useful for workers who are unable to read or cannot read the European alphabet for example Chinese and Arabs have their own writing and alphabet. The tumbling E chart contains rows of the letter “E” in various kinds of rotation. The worker is asked to state (usually by pointing) where the limbs of the “E” are pointing or are the legs of the table pointing “up, down, left or right.”
Like the normal Snellen chart, the first line consists of one very large capital letter “E”, followed by rows that have increasing numbers of capital letters E’s that decrease in size. Each line is numbered 1- 11 and measure in either or both meters and / or feet. The number indicated at the side of the row corresponds to the distance at which a normal eye can read that row. For example, the top row (marked 60) is read by a normal eye 60 metres away. The tumbling E chart is designed to be read at three or six metres. This chart is also made of a non-reflective material, with a matte finish, some with green and red colour bars and a grommet for handing. It is also a standard 22″ x 11″ in size. For both charts the visual acuity is quantified by how far the worker can “read or see”. Figure 6.6 below is an example of a tumbling E chart.
Figure 6.5 An Example of a Snellen Chart10 – NOT FOR CLINICAL USE
Figure 6 An Example of a Tumbling E Chart10 – NOT FOR CLINICAL USE
Preparation of the Far Visual Acuity Environment
Both the Snellen chart and the tumbling E chart require the same preparation therefore will be dealt with together in this section. Everything mentioned under the section “room preparation” should be in place for the area where far visual acuity is tested manually.
The chart must be hung on a white background, do not frame or place behind plastic as this increases glare. When using a 6m chart measure from the chart a distance of 6m and mark this point clearly as this is where the worker being tested will stand When using a 3m chart measure from the chart a distance of 3m and mark this point clearly as this is where the worker being tested will stand Measure 1.6m from the floor to the bottom of the 6/6 or 20/20 line and hang the chart. It must be straight. The lighting / illumination of the Snellen chart is important. The chart must be illuminated by a 100-watt incandescent lamp / globe. The spotlight should be hung at 1.2m in front of and slightly above the patient.
If the testing area has normal fluorescent lighting, turn this lighting off and the spotlight on when testing. The area should be quiet so that the worker being tested can hear the instructions and be relatively private but not necessarily confidential. Ensure there are enough clean eye occluders (spray bottle caps, spoons, professional eye occluders). Do not use hands, paper, towels and tissues or anything that allows light through it like clear plastic. Sterilizing liquid for eye occluders including a clean and dirty tray (container) for used and unused eye occluders. Scrap paper and a pen are required for the technician to write the results on. A clean pointer with a sharp point is required so that the technician can point to the letters required.
Figure 6.7 Various examples of Eye Occluders14
General Care6
At the beginning of the day dust the charts with a soft cloth dampened in a mild solution of soap and water. Clean the pointer with disinfectant. Prepare the sterilizing fluid for the used occluders. Soap all the occluders as per the sterilizing agents’ instructions, rinse the occluders in hot water, dry so that they are clean to start with and place inside the clean container. Use dust cover to protect the occluders and prevent dust accumulation when not in use. After an occluder has been used ask the worker to place it in the used container ready for washing, do not reuse it until it has been cleaned and disinfected.
Near Vision Charts
There is a range of manual near vision charts that can be used. The Jaeger, Sloan, and Rosenbaum are some of the most common chart tests used in occupational health. They are all available for download on the internet or can be purchased through a good medical wholesaler. For any of the near vision tests the room should be set up and prepared as indicated earlier in the chapter under “room preparation”18.
Jaeger Chart
The Jaeger chart is the most common eye chart used in testing near vision acuity in the workplace. It is a card on which paragraphs of text are printed, with the text sizes increasing from 0.37mm to 2.5mm. The paragraph with the smallest print is No. 1 or J1 which is a 20/20 optotype. The paragraph with the next larger print is No. 2 or J2 which is 20/25 optotype, and so on, to the largest print. As you progress to larger print, the lettering size increases for lesser visual acuity. Persons with normal vision should be able to read the smallest print in good lighting, at a comfortable reading distance of 14 inches / 35.56cm.
To make it easier for the technician to identify what the worker is reading, it is suggested that a back-to-back print in high definition / quality copy is made. An occluder is also required for near visual acuity testing for all the charts. The examination must be conducted in a well-lit room with the source of light behind the worker to prevent shadows and reduce glare, see figure 6.7 below which depicts an example of a Jaeger chart.
The area should be quiet so that the worker being tested can hear the instructions and be relatively private but not necessarily confidential. Ensure there are enough clean eye occluders (spray bottle caps, spoons, professional eye occluders). Do not use hands, paper, towels and tissues or anything that allows light through it like clear plastic. Sterilizing liquid for eye occluders including a clean and dirty tray (container) for used and unused eye occluders. Scrap paper and a pen is required for the technician to write the results on.
General Care6
At the beginning of the day dust the charts with a soft cloth dampened in a mild solution of soap and water. Clean the pointer with disinfectant. Prepare the sterilizing fluid for the used occluders. Soap all the occluders as per the sterilizing agents’ instructions, rinse the occluders in hot water, dry so that they are clean to start with and place inside the clean container. Use dust cover to protect the occluders and prevent dust accumulation when not in use. After an occluder has been used ask the worker to place it in the used container ready for washing, do not reuse it until it has been cleaned and disinfected.
Sloan letter near vision chart
Sloan letter near vision charts are used in the aviation industry15. They have proportionally spaced lines in capital European letters and the line sizes range from 20/400 to 20/10 (6/120 to 6/3) equivalent. The letters are printed on two sides, and it is 17.8 cm x 22.9 cm. Either side of the chart can be used when screening or testing, it is advisable to use them interchangeably to avoid memorising by the person being tested. The charts are numbered “1” and “2” at the top to report which test was used.
These charts usually come with a 40 cm cord attached to ensure the proper screening distance. Use the cord to measure 40cm between the chart and temple near the worker’s right eye. Ensure the cord remains tight to maintain distance throughout the test. An occluder is also required for near visual acuity testing for all the charts. To test intermediate vision the chart must be held at 81.3cm from the person being tested. The examination must be conducted in a well-lit room with the source of light behind the worker to prevent shadows and reduce glare, see figure 6.8 below which depicts an example of a Sloan letter chart.
The area should be quiet so that the worker being tested can hear the instructions and be relatively private but not necessarily confidential. Ensure there are enough clean eye occluders (spray bottle caps, spoons, professional eye occluders). Do not use hands, paper, towels and tissues or anything that allows light through it like clear plastic. Sterilizing liquid for eye occluders including a clean and dirty tray (container) for used and unused eye occluders. Scrap paper and a pen are required for the technician to write the results on.
General Care6
At the beginning of the day dust the charts with a soft cloth dampened in a mild solution of soap and water. Clean the pointer with disinfectant. Prepare the sterilizing fluid for the used occluders. Soap all the occluders as per the sterilizing agents’ instructions, rinse the occluders in hot water, dry so that they are clean to start with and place inside the clean container. Use dust cover to protect the occluders and prevent dust accumulation when not in use. After an occluder has been used ask the worker to place it in the used container ready for washing, do not reuse it until it has been cleaned and disinfected.
Rosenbaum near vision chart
A Rosenbaum card can be used for illiterate workers when testing near visual acuity. The chart is held 136cm from the person being tested. It is usually pocket size, plastic, glare-proof and provides clear, legible, and precise numbering. It measures 2/800 testing at 136cms. The examination must be conducted in a well-lit room with the source of light behind the worker to prevent shadows and reduce glare. An occluder is also required for near visual acuity testing for all the charts. See figure 6.9 below which depicts an example of a Rosenbaum chart.
The area should be quiet so that the worker being tested can hear the instructions and be relatively private but not necessarily confidential. Ensure there are enough clean eye occluders (spray bottle caps, spoons, professional eye occluders). Do not use hands, paper, towels and tissues or anything that allows light through it like clear plastic. Sterilizing liquid for eye occluders including a clean and dirty tray (container) for used and unused eye occluders. crap paper and a pen is required for the technician to write the results on.
General Care6
At the beginning of the day dust the charts with a soft cloth dampened in a mild solution of soap and water. Clean the pointer with disinfectant. Prepare the sterilizing fluid for the used occluders. Soap all the occluders as per the sterilizing agents’ instructions, rinse the occluders in hot water, dry so that they are clean to start with and place inside the clean container. Use dust cover to protect the occluders and prevent dust accumulation when not in use. After an occluder has been used ask the worker to place it in the used container ready for washing, do not reuse it until it has been cleaned and disinfected.
Figure 6.8 An example of a Jaeger Eye Chart19 – NOT FOR CLINICAL USE
Figure 6.9 An example of a Rosenbaum Eye Chart20 – NOT FOR CLINICAL USE
Figure 6.10: Two Examples of Sloan Letter Charts21 – NOT FOR CLINICAL USE.
Ishihara Chart
The Ishihara Colour Test is a test to determine if a worker has colour blindness. It is named after Dr Shinobu Ishihara who first published the test in 1917 as a professor at the University of Tokyo22. The Ishihara plates are widely used as a test for colour vision. Originally designed for the purpose of detecting congenital red-green colour vision deficiency (CVD), the test also has some value in demonstrating acquired colour vision defects.
The Ishihara test makes numbers out of dots that are a different colour than the dots surrounding them. Someone who is colour vision deficiency (CVD) sees all of these dots as the same colour, whereas someone with normal vision can distinguish the different colours. The Ishihara plates come in 3 series, 14- test plates which are less accurate than the Ishihara 24- or 38-plate series. The concise edition’s 14 test plates differ from the other Ishihara test plates in that they are printed on a white background instead of individual white cards interleaved onto a black surround and the sizing is different. For basic colour vision screening identifying the primary colours, red, blue, and yellow plus white and black is sufficient. In some occupations such as train drivers, aviation workers and electricians a more detailed colour vision assessment is required hence the use of the Ishihara plates. Sit approximately 75cm from the person being tested and hold the plates at eye level. Ensure natural reading light with no glare on the plates, as this can alter the colour of the pictures. Do not photocopy frame or laminate the plates as this changes the contrast, shading and colour of the plates.
Worker requirements and room hygiene
Some measures need to be taken to prevent the spread of infectious germs from the worker and technician and vice versa. Set up a hand washing or sanitising station near the vision screener. Technicians must thoroughly wash their hands before testing and after each assessment. During COVID-19, have disposable surgical masks and gloves available for the technician and ensure the worker has a mask on when visiting the clinic and throughout the vision screening assessment24. It is recommended that feet remain covered by shoes. Allowing technicians to wear open-toed shoes or sandals is not advised. Keep in mind that these recommendations are not intended to contradict or replace policies required for infection control at the clinic or company where vision screening is being performed.
Standard Healthcare Precautions in Healthcare Settings
According to the World Health Organisation, standard precautions are meant to reduce the risk of transmission of bloodborne and other pathogens from both recognized and unrecognized sources26. They are the basic level of infection control precautions that are to be used in all healthcare facilities.
Consulting / examination rooms used for vision screening, common waiting areas, and other areas where workers may have potentially contaminated surfaces or objects that are frequently touched by staff and workers (doorknobs, sinks, toilets, other surfaces, and items in close proximity to workers) should be cleaned routinely with registered disinfectants, following the manufacturer’s instructions for amount, dilution, and contact time. Housekeeping surfaces such as floors and walls do not need to be disinfected unless visibly soiled with blood or body fluids. They may be routinely cleaned with a detergent only or a detergent/disinfectant product. Most disinfectants are not effective in the presence of dirt and organic matter; therefore, cleaning must occur first before disinfection. Wet a cloth with the disinfectant, wipe away dirt and organic material, then with a clean cloth apply the disinfectant to the item and allow it to air dry for the time specified by the product manufacturer.
Hand hygiene is a major component of standard precautions and is the most effective method to prevent the transmission of pathogens found in a health care setting. The use of personal protective equipment should be guided by a risk assessment and the extent of contact anticipated with blood and body fluids, or pathogens. In addition to practices carried out by health workers when providing care, all individuals (including workers and visitors) should comply with infection control practices in healthcare settings. Respiratory hygiene/cough etiquette is now also considered part of standard precautions.
Hand hygiene technique27
Hand washing (40–60 sec): wet hands and apply soap; rub all surfaces; rinse hands and dry thoroughly with a single-use towel; use the towel to turn off the faucet. Hand rubbing (20–30 sec): apply enough 70% or more alcohol sanitiser product to cover all areas of the hands; rub hands until dry.
Summary of when to wash hands in an occupational health clinic setting:
Before and after any direct contact with and between workers, whether or not gloves are worn Immediately after gloves are removed Before handling an invasive device After touching blood, body fluids, secretions, excretions, non-intact skin, and contaminated items, even if gloves are worn During worker care, when moving from a contaminated to a clean body site of the worker After contact with inanimate objects in the immediate vicinity of the worker
Standard Precautions include
Hand hygiene
Use of personal protective equipment (e.g., gloves, masks, eyewear)
Respiratory hygiene / cough etiquette
Sharps safety (engineering and work practice controls)
Safe injection practices
Sterile instruments and devices
Clean and disinfected environmental surfaces
Universal infection control measures must be conducted on all the equipment that comes into contact with the worker, it is important to follow the manufacturer’s instructions:
The automated vision screener chair and workstation must be wiped down with a disinfectant cloth between each worker The worker response eye occluder must also be washed and sterilized after use. The vision testing room should be cleaned at least once daily with a disinfectant solution under normal circumstances but with COVID-19 after each worker
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