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Adult Health
문제

A nurse is assessing a patient's visual acuity using the Snellen chart. Which finding would indicate normal vision?

해설
20/20 vision indicates normal visual acuity, meaning the patient sees at 20 feet what a person with normal vision sees at 20 feet. Other options (20/40, 20/60, 20/100) represent varying degrees of reduced vision.
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심화 해설

Understanding the Snellen Chart and Visual Acuity

The Snellen chart is the standard tool used in clinical settings to measure distance visual acuity. When you see a fraction like 20/20, the numerator (top number) represents the distance in feet at which the patient is standing from the chart, which is typically 20 feet. The denominator (bottom number) indicates the distance at which a person with "normal" vision can read that same line. Therefore, 20/20 vision means that at 20 feet, the patient can read the line that a person with normal visual acuity is expected to read at 20 feet.

Interpreting the Findings

A finding of 20/20 vision in both eyes is the clinical benchmark for normal distance visual acuity. The other options represent increasing degrees of visual impairment. For example, 20/40 vision indicates that what a person with normal vision can see clearly at 40 feet, this patient can only see at 20 feet. This level of impairment often meets the legal requirement for driving restrictions in many jurisdictions. As the denominator increases, as seen in 20/60 and 20/100 vision, the visual deficit becomes more profound, signifying that the patient must be much closer to an object to discern the detail that a normally sighted individual can perceive from a greater distance.

Clinical Significance and Pathophysiology

While the Snellen chart is excellent for measuring central visual acuity, it is important to understand that "normal vision" is a multifaceted concept. The provided research highlights that visual acuity is just one component of visual function. A study on binocular night vision goggles demonstrates a critical dissociation: individuals can achieve a high level of visual acuity, such as 20/25 or better, yet still have profoundly impaired stereoacuity, which is the ability to perceive depth from binocular cues . This underscores that a patient with 20/20 Snellen acuity could still have significant functional vision problems. Furthermore, the primary cause of preventable visual impairment in children globally is uncorrected refractive errors (UREs), which directly reduce Snellen acuity scores. A study from Jakarta found the prevalence of UREs surged to 40% post-pandemic, highlighting that a finding of less than 20/20 vision, such as 20/40 or worse, is a serious public health indicator that requires prompt referral and correction to prevent impacts on education and quality of life . The systematic review on school vision screening confirms the importance of accurately detecting these deficits, as visual impairment can significantly affect a child's academic performance and interpersonal growth . In a post-surgical context, monitoring changes in visual acuity, such as the stability of a 20/20 outcome in a fellow eye after unilateral refractive surgery, is crucial for assessing long-term success and managing conditions like anisometropia .

임상 시나리오

Clinical Practice Guide: Snellen Chart Assessment
Procedure for Accurate Testing
  • Position the patient exactly 20 feet from the well-illuminated Snellen chart.
  • Test each eye separately, asking the patient to cover one eye without applying pressure.
  • Instruct the patient to read the smallest line of letters possible, starting from the top.
  • Record the visual acuity as a fraction (e.g., 20/20) for each eye, noting any missed letters.
  • If the patient wears corrective lenses, test with and without them to assess corrected and uncorrected acuity.
Interpreting Results and Nursing Actions
  • 20/20: Normal distance vision. Document and continue routine screening.
  • 20/40 or worse: Indicates visual impairment. Notify the provider as this may affect driving eligibility and safety.
  • For any abnormal finding, ensure patient safety by orienting them to the environment and keeping pathways clear.
  • Refer patients with new or worsening deficits for a comprehensive ophthalmologic examination.
Key Clinical Considerations
  • The Snellen chart measures central visual acuity only; it does not assess peripheral vision, color vision, or depth perception.
  • Legal blindness is defined as 20/200 or worse in the better eye with best possible correction.
  • Always consider age-related changes and health conditions like cataracts or diabetic retinopathy that can affect acuity.

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