Used to Test Vision, the Snellen Eye Chart Typically Has What Single Letter in the Top Row?

Definition/Introduction

Invented in 1862 by a Dutch ophthalmologist named Herman Snellen, the Snellen chart remains the virtually widespread technique in clinical practice for measuring visual acuity.[one][2] The Snellen chart serves every bit a portable tool to quickly appraise monocular and binocular visual vigil. Throughout history, there have been several charts utilized past eye care professionals. The most common in clinical exercise is the Snellen chart, while research studies more oftentimes use logMAR charts, such equally the Early Handling of Diabetic Retinopathy Study (ETDRS) chart.

The Snellen Nautical chart uses a geometric scale to mensurate visual vigil, with normal vision at a altitude beingness set at 20/20. The numerator represents the distance that the patient is standing from the nautical chart (in feet), while the denominator represents the altitude from which a person with perfect eyesight is all the same able to read the smallest line that the patient can clearly visualize. For case, a patient standing twenty feet abroad from the chart who can clearly read until the line of font that a person with normal visual vigil can read from 40 feet away would be measured every bit 20/40 vision. A similar assessment for testing about vision can be done using a pocket card held nearly 14 inches from the patient's eyes. In that location are only nine letters on the chart, known equally optotypes: C, D, E, F, L, O, P, T, and Z. Finally, the sizing of letters is geometrically consistent, meaning that optotypes representing 20/40 are twice the size of those representing 20/20.

Other notable visual vigil charts include the following:

• Bailey–Lovie :Recognizing the concerns of the Snellen chart, which are covered in the next section below, in 1976, the Australians Bailey and Lovie redesigned the Snellen nautical chart. The Bailey-Lovie chart contains xiv rows, each with 5 messages. The size of the characters decreases from the first to the last row. The accuracy of this chart is considered superior to the Snellen chart.

• ETDRS: Developed in 1982 past a researcher named Rick Ferris, the ETDRS is a modification of the Bailey-Lovie nautical chart to overcome the limitations of the Snellen chart. The ETDRS chart is tested from a shorter distance (xiii feet rather than 20 anxiety), has the same corporeality of messages in every row (five messages each), and has an equal spacing of both the messages and the rows on a logarithmic calibration. Shamir et al. institute the ETDRS to be significantly more authentic and reproducible than the Snellen nautical chart, yet its adoption in clinical exercise has been limited due to its cost, longer testing fourth dimension, and size of the chart.[3]

• Jäger: Invented in 1854 by the Austrian ophthalmologist Jäger, this chart tests near vision. The bill of fare contains 11 brusque paragraphs, which are numbered from 1 (smallest font) to 11 (largest font). The carte du jour is held roughly 14 inches from the nose at a comfortable reading distance for testing.

• Tumbling E: Dr. Snellen as well developed this chart, which he designed for children and those unable to read or unfamiliar with the Roman alphabet. This chart displays a uppercase alphabetic character E facing in various directions. While observing each character, the patient points in the direction that the E is facing.

• Landolt C: Comparable to the Tumbling Due east, this nautical chart displays a C - which some say appears more similar to a broken ring - facing in various directions. The chart was developed by the Swiss ophthalmologist Landolt and is regarded every bit a laboratory standard by the International Council of Ophthalmology.

• LEA Symbols Chart: This chart was designed for visual acuity screening in children to eliminate the bug of language barriers. The chart utilizes symbols and creates a play environment to entertain the pediatric patient during the exam. Vivekanand et al. adamant that the LEA symbols chart performs better than the Snellen chart for visual acuity assessments in preschool children.[4]

• Freiburg Visual Acuity Test (FrACT): The FrACT is an automated, computerized test that displays large Landolt C optotypes on a monitor screen. The monitor randomly presents Landolt C characters in diverse sizes and orientations. Schulze-Bonsel et al. determined that the FrACT tin can be utilized to quantify the visual acuities previously characterized equally "hand move" and "counting fingers."[5]

Snellen Nautical chart Procedure

1. Position the patient sitting xx anxiety from the nautical chart. Note that projecting the chart on a mirror can be a useful way to simulate the xx-foot testing distance in shorter clinical lanes.

2. Ensure that the room and Snellen nautical chart are adequately illuminated.

3. The patient may wear his/her current corrective lenses to assess for corrected visual acuity.

4. In the absence of corrective lenses, a pinhole may be used, which oftentimes resolves the refractive error. This is done past assessing visual vigil while looking through a pinhole.

5. Test the optics one at a time, beginning with the worse center while covering the good middle.

6. Ask the patient to begin reading messages from the top.

7. The smallest line read represents the visual vigil.

8. Patients unable to read the large Eastward on the chart (Effigy 1) when wearing the best corrective lens have a visual vigil measuring less than 20/200, which is termed "legally blind" in the United States.

9. For patients unable to see the middle chart, check if they can count fingers (CF vision), see hand movements (HM vision), or perceive the low-cal from a flashlight (LP vision). If the patient fails to identify the light, tape this every bit no lite perception (NLP).

10. Repeat the procedure to measure the reverse eye.[6][7]

Issues of Concern

While the Snellen chart is prized every bit a quick, low cost, and portable tool, in that location are several drawbacks to its use. These disadvantages, outlined below, gave nascency to the improvements seen in newer charts, such as the ETDRS.

Chart Design

Every line of the chart contains a different number of characters, i.e., 8 characters on the 20/twenty line and simply ane letter on the 20/200 line.[8] This presents confusion when a patient's medical record shows, for example, a proceeds of 3 letters of acuity, which could indicate a minor improvement on the twenty/20 line or a gain of an unabridged vigil line in the 20/70 vigil. Furthermore, the progression between lines is variable, with different spacing from line to line. Every bit such, a two-line improvement in acuity could stand for a gain of 33% from 20/xxx to 20/twenty or a gain of 50% from twenty/100 to 20/50. The lack of standardization in line progression, as well every bit the spread of the measurements, makes it hard to make statistical comparisons on acuity data.[nine] Additionally, the distance between rows and characters varies considerably, creating a crowding miracle of next profile interactions, reducing acuity.[ten] Finally, some letters (i.eastward., 50, T, A) are reportedly easier to see than other messages (i.e., B, Southward, C).[11]

Language Barriers

The use of the Snellen chart is limited to those patients familiar with the Roman alphabet and complimentary of any other language, literacy, or verbal communication difficulties.

Cognitive and Attentiveness Limitations

Administering the Snellen nautical chart examination requires that the patient focus his/her attention for the duration of the test and cooperate by following the instructions given. For those patients who are unable to cooperate due a physical or mental inability or for pediatric patients, this test may evidence challenging to administer.

Clinical Significance

Interpreting the Snellen chart

An important component of the cranial nerve two (optic nerve) exam is testing the visual acuity. Visual acuity assessment has been used clinically for hundreds of years every bit a useful tool to gauge the clarity of a patient's vision. When interpreting visual acuity using the Snellen nautical chart, information technology is important to note that xx/20 vision is not synonymous with perfect vision, but rather, indicates a measure out of a patient'due south clarity of eyesight from a distance. Overall visual ability encompasses a diversity of other vision skills, including peripheral vision, eye coordination, depth perception, ability to focus on objects, and colour vision. Furthermore, visual acuity cess using the Snellen chart does not give a patient's prescription. The prescription is a separate measurement that must exist evaluated by an centre professional in an function setting.

Hyperopia (farsightedness) describes the condition where patients have clear eyesight from a distance but cannot focus well on objects that are near. Another type of farsightedness is presbyopia, which typically develops later historic period forty due to a loss of the elasticity in the centre's natural lens. On the other hand, myopia (nearsightedness) describes the condition where patients can see objects that are near well simply have trouble seeing objects at a distance. A comprehensive middle exam by a licensed eye professional person is needed in order to diagnose the condition compromising a patient's vision and determine whether refractive therapy is needed or more all-encompassing workup is required for vision-threatening diseases, such as coloboma or carotid-clangorous fistula.[12]

Understanding Abbreviations in Clinical Practice

The following abbreviations are commonly used in the reporting of visual acuity measurements.

• OD: Right heart

• OS: Left eye

• OU: Both eyes

• VA: Visual acuity

• DVA: Distance visual acuity

• NVA: Near visual acuity

• CC: With correction

• SC: Without correction

• PH: Pinhole

• CF: Counting fingers

• HM: Mitt motion

• LP: Light perception

• NLP: No lite perception

• +/-: Letters are seen on the next line (+) or messages missed (-), i.e., twenty/50+one ways the patient read all letters on the 20/50 line and discerned 1 letter of the alphabet on the xx/40 line.

Future Directions

Mobile smartphone apps, such as the SightBook mobile app, have been developed to allow patients to remotely monitor their visual acuity.[13][xiv] While the potential applications of this software are phenomenal for patients to track their middle disease improvement and ultimately reduce the cost of care, no Snellen visual acuity apps have been identified that could accurately mensurate visual acuity inside one line. Further validation is warranted to explore the future applications of this attempt.

Nursing, Centrolineal Health, and Interprofessional Team Interventions

An interprofessional care team of nurses, centrolineal health professionals, and physicians must maintain a collaborative and communicative surround to achieve positive patient outcomes. Regarding the Snellen chart, the interprofessional team often includes principal care physicians, hospitalists, ophthalmologists, optometrists, technicians, and nurses. Standardized testing procedures, equally outlined above, equally well as consistent reporting of visual acuity measurements, provide greater reliability of Snellen nautical chart test results. An example of this can exist seen in the example of a hospitalist testing the visual acuity of a temporal arteritis patient in dim lighting and reporting the acuity as 20/70. In contrast, the next day, the ophthalmologist uses the Snellen chart in good lighting and measures the patient'southward vigil to exist xx/fifty. Despite the progression of vision loss overnight, the Snellen chart results misleadingly suggest an improvement in visual acuity considering of the lack of standardization in testing procedures.

Nursing, Allied Health, and Interprofessional Squad Monitoring

Monitoring the attitudes, education, and readiness of interprofessional team members to use and translate the Snellen chart tin can exist vital for enhancing team dynamics. Kim et al. demonstrated that a single interprofessional education session for health science students, with Snellen chart training, improved the interprofessional attitudes, knowledge of the roles of other professions, and the perceived-readiness to work together.[15] [Level 3]

Review Questions

Figure

A Snellen eye nautical chart for visual acuity testing. Contributed by Daniel Azzam

References

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Zapparoli M, Klein F, Moreira H. [Snellen visual vigil evaluation]. Arq Bras Oftalmol. 2009 Nov-December;72(six):783-8. [PubMed: 20098899]

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Shamir RR, Friedman Y, Joskowicz L, Mimouni M, Blumenthal EZ. Comparison of Snellen and Early on Treatment Diabetic Retinopathy Study charts using a computer simulation. Int J Ophthalmol. 2016;9(1):119-23. [PMC free article: PMC4768517] [PubMed: 26949621]

four.

Vivekanand U, Gonsalves Southward, Bhat SS. Is LEA symbol better compared to Snellen chart for visual vigil assessment in preschool children? Rom J Ophthalmol. 2019 Jan-Mar;63(1):35-37. [PMC free article: PMC6531776] [PubMed: 31198896]

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Schulze-Bonsel Yard, Feltgen N, Burau H, Hansen 50, Bach M. Visual acuities "paw motion" and "counting fingers" can be quantified with the freiburg visual acuity test. Invest Ophthalmol Vis Sci. 2006 Mar;47(3):1236-40. [PubMed: 16505064]

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van Gijn J, Gijselhart JP. [Snellen and his optotypes]. Ned Tijdschr Geneeskd. 2012;156(16):A4416. [PubMed: 22510417]

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Sue Southward. Test distance vision using a Snellen nautical chart. Community Eye Health. 2007 Sep;xx(63):52. [PMC free article: PMC2040251] [PubMed: 17971914]

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Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt. 1976 November;53(eleven):740-v. [PubMed: 998716]

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Wadhwa RR, Azzam D. StatPearls [Cyberspace]. StatPearls Publishing; Treasure Island (FL): Jul 21, 2021. Variance. [PubMed: 31869153]

x.

Kaiser PK. Prospective evaluation of visual acuity assessment: a comparison of snellen versus ETDRS charts in clinical practice (An AOS Thesis). Trans Am Ophthalmol Soc. 2009 Dec;107:311-24. [PMC gratuitous article: PMC2814576] [PubMed: 20126505]

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Mathew JA, Shah SA, Simon JW. Varying difficulty of Snellen messages and mutual errors in amblyopic and fellow eyes. Arch Ophthalmol. 2011 Feb;129(two):184-seven. [PubMed: 21320964]

12.

Azzam D, Bordoni B. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): April 14, 2021. Embryology, Optic Fissure. [PubMed: 32119320]

xiii.

Phung L, Gregori NZ, Ortiz A, Shi Westward, Schiffman JC. REPRODUCIBILITY AND Comparing OF VISUAL Acuity OBTAINED WITH SIGHTBOOK MOBILE Application TO Near Card AND SNELLEN CHART. Retina. 2016 May;36(5):1009-twenty. [PubMed: 26509223]

14.

Perera C, Chakrabarti R, Islam FM, Crowston J. The Middle Phone Written report: reliability and accurateness of assessing Snellen visual acuity using smartphone technology. Eye (Lond). 2015 Jul;29(7):888-94. [PMC free article: PMC4506341] [PubMed: 25931170]

15.

Kim YJ, Radloff JC, Stokes CK, Lysaght CR. Interprofessional education for health scientific discipline students' attitudes and readiness to work interprofessionally: a prospective cohort study. Braz J Phys Ther. 2019 Jul - Aug;23(iv):337-345. [PMC gratuitous article: PMC6630190] [PubMed: 30245041]

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Source: https://www.ncbi.nlm.nih.gov/books/NBK558961/

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