Test That Determines the Smallest Letters That Can Be Read on a Standardized Chart

March 05, 2022 Post a Comment

Definition/Introduction

Invented in 1862 past a Dutch ophthalmologist named Herman Snellen, the Snellen nautical chart remains the about widespread technique in clinical practice for measuring visual vigil.[1][2] The Snellen chart serves as a portable tool to quickly assess monocular and binocular visual acuity. Throughout history, there have been several charts utilized by eye care professionals. The near common in clinical practice is the Snellen chart, while research studies more frequently use logMAR charts, such as the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart.

The Snellen Chart uses a geometric calibration to measure visual acuity, with normal vision at a altitude being prepare at 20/20. The numerator represents the distance that the patient is standing from the chart (in feet), while the denominator represents the distance from which a person with perfect eyesight is withal able to read the smallest line that the patient can clearly visualize. For example, a patient continuing 20 feet away from the nautical chart who can clearly read until the line of font that a person with normal visual acuity can read from 40 anxiety abroad would be measured as 20/xl vision. A like assessment for testing almost vision can be washed using a pocket carte du jour held about fourteen inches from the patient's eyes. There are simply nine letters on the nautical chart, known equally optotypes: C, D, E, F, L, O, P, T, and Z. Finally, the sizing of letters is geometrically consequent, meaning that optotypes representing 20/40 are twice the size of those representing 20/20.

Other notable visual acuity charts include the post-obit:

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 chart. The Bailey-Lovie chart contains xiv rows, each with five 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 by 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 nautical chart is tested from a shorter distance (13 anxiety rather than 20 anxiety), has the same amount of letters in every row (five letters each), and has an equal spacing of both the letters and the rows on a logarithmic scale. Shamir et al. found the ETDRS to be significantly more accurate and reproducible than the Snellen chart, yet its adoption in clinical practice has been limited due to its price, longer testing time, and size of the chart.[3]

Jäger: Invented in 1854 by the Austrian ophthalmologist Jäger, this chart tests near vision. The card contains xi short paragraphs, which are numbered from 1 (smallest font) to eleven (largest font). The card is held roughly fourteen inches from the olfactory organ at a comfortable reading altitude for testing.

Tumbling Eastward: Dr. Snellen likewise developed this nautical chart, which he designed for children and those unable to read or unfamiliar with the Roman alphabet. This nautical chart displays a majuscule 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 E, this chart displays a C - which some say appears more similar to a broken ring - facing in various directions. The chart was adult past the Swiss ophthalmologist Landolt and is regarded equally a laboratory standard past the International Council of Ophthalmology.

LEA Symbols Chart: This chart was designed for visual vigil 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. determined that the LEA symbols chart performs better than the Snellen chart for visual acuity assessments in preschool children.[iv]

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 various sizes and orientations. Schulze-Bonsel et al. determined that the FrACT tin exist utilized to quantify the visual acuities previously characterized as "hand motility" and "counting fingers."[v]

Snellen Chart Procedure

Position the patient sitting 20 anxiety from the chart. Note that projecting the nautical chart on a mirror can exist a useful way to simulate the xx-foot testing distance in shorter clinical lanes.
Ensure that the room and Snellen chart are adequately illuminated.
The patient may habiliment his/her current corrective lenses to assess for corrected visual acuity.
In the absence of corrective lenses, a pinhole may be used, which often resolves the refractive error. This is done past assessing visual acuity while looking through a pinhole.
Test the eyes one at a time, first with the worse eye while covering the expert eye.
Ask the patient to brainstorm reading messages from the top.
The smallest line read represents the visual acuity.
Patients unable to read the large East on the nautical chart (Figure ane) when wearing the best cosmetic lens have a visual vigil measuring less than 20/200, which is termed "legally bullheaded" in the United States.
For patients unable to meet the center chart, check if they can count fingers (CF vision), see manus movements (HM vision), or perceive the low-cal from a flashlight (LP vision). If the patient fails to identify the light, record this as no calorie-free perception (NLP).
Echo the procedure to measure the opposite eye.[6][seven]

Problems of Concern

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

Chart Pattern

Every line of the chart contains a dissimilar number of characters, i.due east., viii characters on the 20/20 line and only 1 letter on the 20/200 line.[eight] This presents confusion when a patient's medical record shows, for case, a gain of 3 letters of acuity, which could signal a small comeback on the 20/twenty line or a gain of an entire acuity line in the 20/lxx acuity. Furthermore, the progression between lines is variable, with different spacing from line to line. As such, a two-line improvement in vigil could represent a gain of 33% from 20/thirty to 20/20 or a gain of l% from 20/100 to xx/50. The lack of standardization in line progression, besides as the spread of the measurements, makes it difficult to make statistical comparisons on acuity data.[nine] Additionally, the distance between rows and characters varies considerably, creating a crowding phenomenon of adjacent contour interactions, reducing acuity.[x] Finally, some letters (i.due east., Fifty, T, A) are reportedly easier to run across than other messages (i.due east., B, S, C).[11]

Language Barriers

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

Cerebral and Attentiveness Limitations

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

Clinical Significance

Interpreting the Snellen chart

An of import component of the cranial nerve 2 (optic nerve) exam is testing the visual acuity. Visual vigil assessment has been used clinically for hundreds of years as a useful tool to judge the clarity of a patient'due south vision. When interpreting visual acuity using the Snellen chart, it is important to note that 20/20 vision is not synonymous with perfect vision, but rather, indicates a measure of a patient's clarity of eyesight from a altitude. Overall visual ability encompasses a diversity of other vision skills, including peripheral vision, eye coordination, depth perception, power to focus on objects, and colour vision. Furthermore, visual vigil assessment using the Snellen nautical chart does not requite a patient'south prescription. The prescription is a separate measurement that must be evaluated by an eye professional in an office setting.

Hyperopia (farsightedness) describes the condition where patients have clear eyesight from a altitude merely cannot focus well on objects that are near. Another blazon of farsightedness is presbyopia, which typically develops afterwards age 40 due to a loss of the elasticity in the eye'due south natural lens. On the other hand, myopia (nearsightedness) describes the condition where patients can see objects that are near well merely have trouble seeing objects at a distance. A comprehensive eye examination past a licensed eye professional person is needed in order to diagnose the condition compromising a patient'due south vision and determine whether refractive therapy is needed or more extensive workup is required for vision-threatening diseases, such as coloboma or carotid-cavernous fistula.[12]

Understanding Abbreviations in Clinical Do

The post-obit abbreviations are commonly used in the reporting of visual acuity measurements.

OD: Right eye
Bone: Left eye
OU: Both eyes
VA: Visual vigil
DVA: Distance visual acuity
NVA: Near visual vigil
CC: With correction
SC: Without correction
PH: Pinhole
CF: Counting fingers
HM: Mitt motion
LP: Light perception
NLP: No light perception
+/-: Letters are seen on the adjacent line (+) or letters missed (-), i.e., 20/l+1 means the patient read all messages on the 20/50 line and discerned ane letter on the twenty/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.[xiii][fourteen] While the potential applications of this software are phenomenal for patients to rail their eye illness comeback and ultimately reduce the price of care, no Snellen visual acuity apps have been identified that could accurately measure visual acuity within one line. Further validation is warranted to explore the future applications of this endeavor.

Nursing, Centrolineal Wellness, and Interprofessional Team Interventions

An interprofessional care team of nurses, allied health professionals, and physicians must maintain a collaborative and communicative environs to reach positive patient outcomes. Regarding the Snellen chart, the interprofessional team oftentimes includes primary intendance physicians, hospitalists, ophthalmologists, optometrists, technicians, and nurses. Standardized testing procedures, as outlined above, also as consistent reporting of visual acuity measurements, provide greater reliability of Snellen chart test results. An example of this tin can be seen in the instance of a hospitalist testing the visual acuity of a temporal arteritis patient in dim lighting and reporting the acuity as 20/seventy. In dissimilarity, the next twenty-four hours, the ophthalmologist uses the Snellen nautical chart in good lighting and measures the patient'southward vigil to be 20/50. Despite the progression of vision loss overnight, the Snellen chart results misleadingly propose an improvement in visual acuity because of the lack of standardization in testing procedures.

Nursing, Allied Health, and Interprofessional Squad Monitoring

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

Review Questions

Figure

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

References

1.: Tsui E, Patel P. Calculated Decisions: Visual vigil testing (Snellen chart). Emerg Med Pract. 2020 Apr 01;22(4):CD1-CD2. [PubMed: 32259420]
2.: Zapparoli K, Klein F, Moreira H. [Snellen visual vigil evaluation]. Arq Bras Oftalmol. 2009 November-Dec;72(6):783-eight. [PubMed: 20098899]
3.: Shamir RR, Friedman Y, Joskowicz L, Mimouni M, Blumenthal EZ. Comparing of Snellen and Early Handling 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 S, Bhat SS. Is LEA symbol meliorate compared to Snellen chart for visual acuity assessment in preschool children? Rom J Ophthalmol. 2019 Jan-Mar;63(i):35-37. [PMC free commodity: PMC6531776] [PubMed: 31198896]
v.: Schulze-Bonsel K, Feltgen N, Burau H, Hansen L, Bach G. Visual acuities "manus motion" and "counting fingers" can exist quantified with the freiburg visual acuity exam. Invest Ophthalmol Vis Sci. 2006 Mar;47(iii):1236-forty. [PubMed: 16505064]
vi.: van Gijn J, Gijselhart JP. [Snellen and his optotypes]. Ned Tijdschr Geneeskd. 2012;156(16):A4416. [PubMed: 22510417]
7.: Sue S. Test altitude vision using a Snellen chart. Customs Eye Health. 2007 Sep;20(63):52. [PMC free article: PMC2040251] [PubMed: 17971914]
8.: Bailey IL, Lovie JE. New pattern principles for visual acuity letter charts. Am J Optom Physiol Opt. 1976 November;53(11):740-5. [PubMed: 998716]
9.: Wadhwa RR, Azzam D. StatPearls [Net]. StatPearls Publishing; Treasure Isle (FL): Jul 21, 2021. Variance. [PubMed: 31869153]
x.: Kaiser PK. Prospective evaluation of visual vigil assessment: a comparison of snellen versus ETDRS charts in clinical practice (An AOS Thesis). Trans Am Ophthalmol Soc. 2009 Dec;107:311-24. [PMC free article: PMC2814576] [PubMed: 20126505]
11.: Mathew JA, Shah SA, Simon JW. Varying difficulty of Snellen letters and common errors in amblyopic and fellow optics. Arch Ophthalmol. 2011 Feb;129(2):184-seven. [PubMed: 21320964]
12.: Azzam D, Bordoni B. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): April xiv, 2021. Embryology, Optic Fissure. [PubMed: 32119320]
thirteen.: Phung L, Gregori NZ, Ortiz A, Shi West, Schiffman JC. REPRODUCIBILITY AND Comparing OF VISUAL Acuity OBTAINED WITH SIGHTBOOK MOBILE APPLICATION TO Almost Card AND SNELLEN CHART. Retina. 2016 May;36(v):1009-20. [PubMed: 26509223]
14.: Perera C, Chakrabarti R, Islam FM, Crowston J. The Eye Telephone Written report: reliability and accuracy of assessing Snellen visual acuity using smartphone applied science. Heart (Lond). 2015 Jul;29(7):888-94. [PMC free commodity: PMC4506341] [PubMed: 25931170]
xv.: Kim YJ, Radloff JC, Stokes CK, Lysaght CR. Interprofessional didactics for health science students' attitudes and readiness to work interprofessionally: a prospective accomplice study. Braz J Phys Ther. 2019 Jul - Aug;23(iv):337-345. [PMC free article: PMC6630190] [PubMed: 30245041]

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