What is a visual acuity test? Show A visual acuity test is an eye exam that checks how well you see the details of a letter or symbol from a specific distance. Visual acuity refers to your ability to discern the shapes and details of the things you see. It’s just one factor in your overall vision. Others include color vision, peripheral vision, and depth perception. There are several different types of visual acuity tests, most of which are very simple. Depending on the type of test and where it’s conducted, the exam can be performed by:
No risks are associated with visual acuity tests, and you don’t need any special preparation. You may need an eye exam if you feel you’re experiencing a vision problem or your vision has changed. A visual acuity test is one part of a comprehensive eye exam. Children frequently take visual acuity tests. Early testing and detection of vision problems can prevent issues from getting worse. Optometrists, driver’s license bureaus, and many other organizations use this test to check your ability to see. Two commonly used tests are Snellen and random E. SnellenThe Snellen test uses a chart of letters or symbols. You’ve probably seen the chart in a school nurse’s office or eye doctor’s office. The letters are different sizes and arranged in rows and columns. Viewed from 14 to 20 feet away, this chart helps determine how well you can see letters and shapes. During the test, you’ll sit or stand a specific distance away from the chart and cover one eye. You’ll read out loud the letters you see with your uncovered eye. You’ll repeat this process with your other eye. Typically, your doctor will ask you to read smaller and smaller letters until you can no longer accurately distinguish letters. Random EIn the random E test, you’ll identify the direction the letter “E” is facing. Looking at the letter on a chart or projection, you’ll point in the direction the letter is facing: up, down, left, or right. These tests tend to be more sophisticated when performed at an eye clinic than in a nurse’s office. At an eye doctor’s office, the chart might be projected or shown as a mirror reflection. You’ll look at the chart through a variety of different lenses. Your doctor will switch out the lenses until you can see the chart clearly. This helps determine your ideal eyeglass or contact lens prescription, if you need vision correction. Visual acuity is expressed as a fraction, such as 20/20. Having 20/20 vision means that your visual acuity at 20 feet away from an object is normal. If you have 20/40 vision, for example, that means you need to be 20 feet away to see an object that people can normally see from 40 feet away. If your visual acuity is not 20/20, you may need corrective eyeglasses, contact lenses, or surgery. You might also have an eye condition, such as an eye infection or injury, that needs to be treated. You and your doctor will discuss your test results as well as any treatment or correction that might be necessary. Community Eye Health. 2019; 32(105): 15–16. Published online 2019 May 13. PMCID: PMC6688402 Ophthalmic nurses and other allied health personnel can detect myopia using a Snellen chart and a pinhole occluder. Myopia is a refractive error that occurs when the eye is longer than normal or has a cornea which is too steep (p. 4). People with myopia, also known as short- or near-sightedness, can see near objects clearly, but objects further away appear blurred and out of focus. Normal vision can be restored by prescribing the correct spectacles or contact lenses. Ideally, refractive errors in children (myopia, hypermetropia and/or astigmatism) should be detected in eye screening programmes in schools or in the community. In the absence of such programmes, adults and children will present at the clinic with a range of conditions. It is therefore important to be aware of myopia and to look out for it when performing an eye examination. Myopia can be inherited, and there is also evidence that it is more likely to occur in communities where children spend less time outdoors and more time doing near work. It can be corrected using spectacles or contact lenses of the correct power, expressed in dioptres (D). People who require ≤ −0.50 D of optical correction are considered as having myopia. “There is evidence that myopia is more likely to occur in communities where children spend less time outdoors and more time doing near work.” High myopia affects around 10% of people with myopia, and is defined by the World Health Organization as requiring ≤ −5 D of correction. People with high myopia are at greater risk of macular degeneration, retinal detachment, glaucoma and cataract. Of these, cataract is the only reversible condition: sight can be restored in a single, quick surgical procedure. Retinal detachment can cause sudden visual loss and requires urgent surgical treatment to re-attach the retina; failure to do so can result in a complete loss of vision in that eye. Macular degeneration and glaucoma cause progressive visual loss that cannot be reversed, so early detection is essential as medication or surgery can, at best, slow down or halt their progression. This article describes how to detect myopia and when to refer someone to the appropriate health professional for a comprehensive eye examination, refraction and spectacle prescription. Indicators of myopia in children include:
People with myopia may complain of:
Ask about the person's previous eye history, e.g. spectacles or lazy eye, and about family history, including myopia (due to the familial link), glaucoma, diabetes mellitus and hypertension. Floaters and flashes of light are associated with retinal detachment and requires urgent referral to an ophthalmologist. Double vision is associated with a range of conditions, including strabismus (lazy eye) and neurological disorders, and requires referral to a medical professional or ophthalmologist. Ask about general health, medications, whether the person drives, and their hobbies. End with the open–ended question: “Is there anything else that you feel I should know about your eyes or your health?” If you have been thorough in your questioning, the answer should be no, which can be recorded in the notes as NOC (no other comments). Visual acuity testing must form part of every eye examination. ‘No refractive error’ at this distance is deemed to be 6/6 or 20/20, even though there may be even smaller lines on the chart, such as 6/5 or 6/4. Steps
Pinhole testing is mainly used for adults and older children. Children under 7–8 years old would struggle to see with a single pinhole. Occluders with multiple pinholes may work better, but if these are unavailable, refer all children with VA of < 6/6 for refraction. A pinhole occluder (an opaque disc with one or more small holes) is used to determine whether reduced vision is caused by refractive error. If this is the case, the pinhole will cause an improvement in visual acuity. If the pinhole worsens vision, this can indicate macular disease, central lens opacities or other causes of reduced vision. If there is no change in visual acuity, this might be caused by amblyopia. Children and adults suspected of having these conditions must be referred. A pinhole occluder is an opaque disc with one or more small holes. In people with myopia, visual acuity increases with the use of a pinhole. TANZANIA Steps
If the person can read more letters with the pinhole than without, they are likely to have a refractive error, such as myopia. All patients (adults and children) whose acuity improves with a pinhole, and/or who present with symptoms consistent with a refractive error, should be referred for a full refraction and an eye health examination. Refer patients with signs or symptoms of eye disease for a comprehensive eye examination (including a slit lamp examination, if possible) if you are unable to carry one out yourself. Articles from Community Eye Health are provided here courtesy of International Centre for Eye Health |