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Examination of Visual Acuity



Ovid: Field Guide to the Neurologic Examination

Authors: Lewis, Steven L.
Title: Field Guide to the Neurologic Examination, 1st Edition
> Table of Contents > Section 2
– Neurologic Examination > Cranial Nerve Examination > Chapter 12
– Examination of Visual Acuity

Chapter 12
Examination of Visual Acuity
PURPOSE
The main purpose of the examination of visual acuity
during the neurologic examination is to determine how well the optic
nerves are functioning.
WHEN TO TEST VISUAL ACUITY
Visual acuity should be assessed during the neurologic
examination if the patient has any visual symptoms or if the patient is
thought to have a condition that can ultimately affect vision. In the
absence of these situations, it is not imperative to check visual
acuity in all patients as part of a routine neurologic examination.
NEUROANATOMY OF VISUAL ACUITY
Visual acuity of each eye depends on the integrity of
the eye (anterior compartment and posterior compartment, including the
retina) and the optic nerve. Lesions posterior to the optic chiasm
affect visual fields (see Chapter 13, Visual Field Examination) but do not cause abnormal visual acuity within the intact fields.
EQUIPMENT NEEDED TO TEST VISUAL ACUITY
A pocket-sized eye chart card used to determine reading vision (also called a near card).
HOW TO EXAMINE VISUAL ACUITY
  • In a brightly lit room, have the patient
    hold the visual acuity card in one hand, while using the other hand to
    keep one eye closed. If the patient needs visual correction for
    reading, have the patient use this.
  • Have the patient hold the card
    approximately 14 in. away. (Holding the card at a specific, exact,
    distance is not imperative from the standpoint of the neurologic
    examination. Have the patient hold the card at a reasonably comfortable
    distance, which usually ends up being approximately 14 in. away anyway.)
  • Point to the 20/20 line on the card and
    ask the patient to read the numbers. If the patient is unable to read
    the 20/20 line, point to the next higher line until the patient is able
    to read most of the numbers correctly. Report this line as the visual
    acuity.
  • Repeat with the other eye.
NORMAL FINDINGS
With the appropriate refractive correction for reading,
most patients with normal optic nerve function should have close to
20/20 vision in each eye.
ABNORMAL FINDINGS
  • The abnormal finding on visual acuity
    testing is diminished visual acuity. Depending on the clinical
    situation, a visual acuity of 20/25 or worse can

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    be considered abnormal, as long as the acuity is tested with the patient wearing the appropriate refractive correction.

  • Visual acuity can be affected by
    dysfunction of the eye or the optic nerve. The finding of abnormal
    visual acuity despite appropriate refractive correction suggests optic
    nerve dysfunction, because most patients with normal optic nerve
    function can be corrected with the appropriate lenses to nearly 20/20
    for reading. Other ophthalmologic (e.g., retinal) causes of diminished
    visual acuity may need to be excluded through ophthalmologic
    consultation, however.
ADDITIONAL POINTS
  • Patients sometimes need coaxing to read the smallest line that represents their true acuity.
  • It often helps to shine a bright light on
    the card to improve brightness and contrast, allowing demonstration of
    the best visual acuity.
  • If the patient normally uses reading
    glasses but does not have them available, the patient’s near vision can
    be tested through a pinhole. A pinhole acts like a lens that corrects
    the refractive error for near vision of most patients (who have normal
    optic nerve function) to close to 20/20, albeit within a small field of
    vision. A pinhole can be created on the spot by taking a small piece of
    cardboard and making a hole in it with a safety pin. The patient should
    be asked to hold this up to each eye and to read the card through the
    hole. Try this on yourself (especially if you need correction for
    reading) and see how well it works.

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