Low Vision Assessment Components Job Aid 1 Eye Dominance Often called eye dominance, eyedness, or seeing through the eye, is the tendency to prefer visual input a particular eye. It is similar to the laterality of right or left handedness; however, the side of the dominant eye and the dominant hand do not always match. Approximately two-thirds of the population is right-eye dominant. In a small portion of the population neither eye is dominant. Dominance seems to change depending upon direction of gaze due to image size changes on the retinas. A person's dominant eye is determined by subjective alignment of two objects presented at a stereo disparity distance. There are a number of ways to do this: In the Dolman method (preferred method), also known as the hole-in-the-card test, the subject is given a card with a small hole in the middle, instructed to hold it with both hands, and then instructed to view a distant object through the hole with both eyes open. The observer then alternates closing the eyes or slowly draws the opening back to the head to determine which eye is viewing the object (the dominant eye). The Miles test. The observer extends both arms, brings both hands together to create a small opening, then with both eyes open views a distant object through the opening. The observer then alternates closing the eyes or slowly draws opening back to the head to determine which eye is viewing the object (the dominant eye) Alternate Tests: The Porta test. The observer extends one arm, and then with both eyes open aligns the thumb or index finger with a distant object. The observer then alternates closing the eyes or slowly draws the thumb/finger back to the head to determine which eye is viewing the object (the dominant eye) The observer extends one arm, forms a small, circular opening with the thumb and index finger, then with both eyes open views a distant object through the opening. The observer then alternates closing the eyes or slowly draws the opening back to the head to determine which eye is viewing the object (the dominant eye). Extra-Ocular Movements (EOMs) Eye movements such as fixation, scanning and tracking are required for a person to evaluate their surrounding environment, read text across a page and quickly visualize objects that enter their field of vision unexpectedly. EOM evaluation tests the ability of the eye muscles to move the eyes in different directions. You will ask the patient to perform three different tests at both near (between18 and 24 inches) and far (between 2 and 3 feet) distances. Tests include: Fixation Ask patient to fixate on a target (penlight, highlighter, your finger) in all positions of gaze Smooth Pursuit Ask patient to follow a target as you move it across their full range of horizontal and vertical eye movements Saccades Hold two objects at equal distance away from the patient and ask them to shift their gaze between objects
Low Vision Assessment Components Job Aid 2 Near Visual Acuity The reason for testing near visual acuity is to determine how a person copes with near tasks such as reading, writing, eating, and ADLs. The results of this test will tell you how well a person can see detail at a short distance (within arm s distance from the body). Decisions regarding print size or low vision devices can be determined from your findings. These test results are not the only determinates for structuring your intervention; you must also consider reading speed, fatigability from reading, availability of glasses, and low vision devices when deciding on the best type of reading material. Test near visual acuity using the Low Vision Lea-numbers chart. Begin testing at the standard distance of 1 meter. If you test at other distances, document the viewing distance used. Determine the row the patient can read clearly and accurately first using both eyes (OU) and then use the eye piece to test the right (OD) and left (OS) eyes individually. Reading Acuity For patients with low vision, the emphasis is not on optics, but on functional performance. Accurate measurement of reading performance is important, since it is the basis for the prescription of magnification. Reading tests require a much larger retinal area than do letter recognition tests. Furthermore, reading (not letter recognition) is the function most patients with visual complaints want to regain. Reading performance has two aspects. The first is the visual function aspect (how the eye functions); the second is the functional vision aspect (how the person functions). Under the visual function aspect we are primarily interested in reading acuity the smallest print that can be read at a given distance. Related to functional vision, we are interested in other aspects, such as reading fluency, reading endurance, and reading comprehension. The latter aspects are rarely considered in a routine eye exam, but are very important in the context of vision rehabilitation, where specific training regimens have been developed to help patients with low vision regain reading skills. Cards with standardized sentences with a fixed number of spaces and characters are preferred for testing reading acuity, such as the MNRead Acuity Chart. Make sure chart illumination is even, with no shadows or glare Viewing distance is 40cm or 16in You can use the findings from the near visual acuity test (letter size M) to determine which line you will start with on the reading acuity test, or ask the patient to read the sentences aloud starting from the top. Continue until he/she makes mistakes. Using the score sheet, mark words the patient misses or reads incorrectly. Continue down the card until they cannot read any words in a sentence. Encourage patient to guess even when he/she cannot read the words. Count the number of sentences the patient reads, including the larger print sentences that you may have skipped, and count the number of errors made (words missed or read incorrectly). Calculation to determine logmar: Acuity = 1.4 (# of sentences X 0.1) + (# of errors X 0.01)
Low Vision Assessment Components Job Aid 3 Contrast Sensitivity Contrast sensitivity measures two variables size and contrast, while acuity measures only size. Contrast sensitivity is the visual ability to see objects that are not outlined clearly or do not stand out from their background. The ability to see a shade of gray on a white background or to see white on a light gray background declines with age. A person with low contrast sensitivity may have vision difficulties such as the following: Trouble seeing traffic lights or cars at night Not being able to see spots on clothes, counters, or dishes Missing facial gestures Not seeing whether a flame is burning on a stove Needing a great deal of light to read Experiencing tired eyes while watching television Contrast sensitivity testing can provide a true "real world" vision of a patient s functional vision. The ability to differentiate between light and dark is as much an important sense as 20/20, allowing us to be able to carry out everyday tasks safely. Lea-numbers Low Contrast Flip Chart with 10M Optotypes Contrast sensitivity is measured by determining the contrast level in the flip chart the patient perceives. For consistency and reliability perform the test at a fixed distance from the light source. Place the flip chart 3 meters or 9.8 feet from the patient. The patient reads the first number on the line; if able to identify the first number flip to the next page. Continue in this manner until an error is made with the first letter; when they can not read the first letter, have them continue reading the numbers aloud across the page. When the patient is unable to identify 3 or more numbers, STOP the test. Document how many numbers the patient identified correctly starting with the first page of the chart, not including the page with 3 or more errors. For example: Patient read the 25% card (5 numbers) and the 10% card (5 numbers), on the 5% card read 3 numbers and on the 2.5% card read 2 numbers. Therefore the lowest contrast perceived is 5% and according to the contrast sensitivity formula, this patient s contrast sensitivity is: 100/5 = 20. The higher the number, the more detail a patient can see. A contrast sensitivity of 1 tells us a patient can only see stark contrast of pure black and white; a contrast sensitivity of 170 or 0.6% tells us a patient has no difficulty seeing faint objects or distinguishing detail.
Low Vision Assessment Components Job Aid 4 Writing Assessment Color Discrimination Writing is a complex task requiring the integration of refined skills. Visual skills enable the writer to locate where writing belongs on the page and scan for accuracy. Visual perceptual skills allow the writer to visualize the necessary spacing between letters, words, and paragraphs. Writing often becomes inefficient for the older person with low vision. For example, older people with macular degeneration lose central vision, thus losing the ability to see the tip of the pen and cannot see letters and numbers as they are being formed. Sentences and paragraphs often look crowded with poor spacing, and letters are written on top of each other. Rows of words tend to drift up or down the page. Writing numbers in columns to perform calculations is difficult and often results in calculating an incorrect amount, making the task of balancing a check book frustrating. People with poor peripheral vision as with glaucoma or retinitis pigmentosa have initial difficulty locating the tip of the pen or edge of the page. It becomes difficult to line up writing on a page because of the inability to see the entire page at once. The following items are needed for the writing assessment: various sizes of blocks (large and small), lined paper, blank paper, a sample bill, blank check and register, Bucket Sentence for visual scanning. Stack 3 large blocks first on a wide surface and then on a narrower surface (gross hand / eye coordination). Progress to small blocks if able. Have the patient to write their name in a specific area, i.e. top left hand corner, of lined paper. Create a grocery list of 5 items they commonly use. On a blank sheet of paper, have the patient write a short paragraph describing where he/she was raised. Assess cognitive component of writing. Have the patient write a check for one of his/her bills and enter it in the ledger. Test the ability to track and visually scan across a page. Have the patient read the Bucket Sentence and circle each n they see in the sentence. The aging eye receives significantly less light than younger eyes, making it difficult for older adults to distinguish between colors and intensities of color. Decreased color discrimination results mostly from cell loss in the fovea. As the eye ages, it requires greater intensities of color for the visual system to perceive stimuli. In older adults, all pastels colors may appear gray and dull, and dark colors may all appear similar. Using the packet of color cards consisting of five basic colors (red, orange, yellow, green, blue) with five grades of saturation: Choose the color that is most prevalent in the current surroundings. Select the lightest hue (least color saturation) within the designated color. See if the patient can identify the color, if not continue presenting cards in that color until he/she is able to identify color. Select the next most prominent color and repeat the process.
Low Vision Assessment Components Job Aid 5 Maximum Reading Speed Reading speed is an objective measure of reading performance. Research has shown that patients (with either normal or low vision) often require letters that are two or three times larger than their acuity limits to achieve their maximum reading speeds. The MNRead acuity charts can be used to measure reading speed at different print sizes, and therefore can be used to determine the print size that supports the patient's maximum reading speed. You will need a stopwatch and piece of blank paper for this test. Patient reads each line aloud as quickly and accurately as possible Time each section within 1 second Reading speed is measured in words per minute. The MNRead acuity chart sentences each have the same length: 10 standard length words so reading speed = 600 / (time in seconds) More precise reading speed measurement when words are missed or read incorrectly: Reading speed = 60 x (10 - errors) / (time in seconds) On the print size chart, plot the number of seconds for each logmar size and to determine where the reading speed (WPM) drops off. The WPM on the graph should be roughly the same WPM you arrived at in the calculation.