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Tony.Ching.AAO (talk) 14:03, May 14, 2024 (PDT)

Visual Acuity Testing in Adults

Visual acuity (VA) is a fundamental measure of visual function that assesses the eye’s ability to distinguish fine details and the shape of objects in the direct line of vision. It serves as a cornerstone of the ophthalmic exam, informing diagnosis and treatment decisions. All adults, regardless of risk factors, must undergo a comprehensive medical eye evaluation with increasing frequency as patients age, aimed at preventing or minimizing vision loss through early detection of ocular diseases, visual dysfunction, and underlying systemic conditions. Visual acuity may also be performed periodically to monitor disease processes and progression in patients with a known eye condition1,2.

Contents Conceptual Framework Minimum Angle of Resolution Types of Visual Acuity Optotype Chart Types Snellen Chart ETDRS Chart Bailey-Lovie Chart Landolt C and Tumbling E Charts Visual Acuity Testing Protocol Distance Vision Setup and Environmental Conditions Correction Status Monocular Testing Low Vision Acuity Designation Near Vision Summary of Documentation and Reporting Factors Affecting Visual Acuity Functional Visual Loss Legal Context References

Conceptual Framework Minimum Angle of Resolution Visual acuity is defined as the reciprocal of the minimum angle of resolution (MAR) expressed in minutes of arc. The minimum angle of resolution is the smallest angle of separation at which two points can be distinguished as separate, best represented by the letter E on the Snellen chart. For an ideal human eye, under optimal conditions, the MAR is approximately 1 minute of arc. The photoreceptor mosaic of the fovea centralis constrains the theoretical limit of resolution. The fovea centralis is the central 1–2 mm of the macula and contains the highest density of cone photoreceptors in the retina, with center-to-center cone spacing of approximately 0.5 arcmin3. According to the Nyquist–Shannon sampling theorem, accurate reconstruction of a spatial signal requires a sampling frequency at least twice the highest frequency present in that signal4. Because each cone acts as an independent sampling unit, the minimum resolvable spatial frequency is determined by the spacing between adjacent cones: with a cone separation of ~0.5 arcmin, the Nyquist limit of the foveal mosaic is ~1 arcmin, meaning the eye cannot reliably resolve details finer than 1 arcmin without aliasing artifacts. This 1 arcmin Nyquist limit corresponds precisely to the angular subtense of each stroke of the letter E at the 20-foot line on the Snellen chart, explaining why 20/20 represents the theoretical norm for best-corrected visual acuity in a healthy eye under optimal conditions5,6. In practice, factors such as optical aberrations, pupil size, and neural processing set the functional limit close to, but rarely better than this theoretical ceiling.

Types of Visual Acuity Type Description Clinical Use Minimum (detectable) visible acuity Detection of a single point of light against a background Contrast sensitivity Minimum (separable) resolvable acuity Discrimination of two points as separate (MAR) Standard chart testing Minimum recognizable (optotype) acuity Recognition of a letter or symbol Snellen, ETDRS charts Minimum discriminable acuity Detection of a small positional change (Vernier) Hyperacuity; amblyopia research

Optotype Chart Types Snellen Chart The Snellen chart was created by Dutch ophthalmologist, Dr. Hermann Snellen, in 1862 and is the most widely used chart in clinical practice7. It portrays a row of letters with each element subtending 1 arcmin at the designated letter distance. The letters are sized on a 5x5 grid, generating an overall height that subtends an angle of 5 arcmin. This can be used for evaluating both near and distance vision. The Snellen chart provides a quick and adequate evaluation of visual acuity; however, several limitations remain. The chart has an unequal number of letters per line, increasing from one letter at the 20/200 line to eight letters at the 20/20 line. As lines include more letters, the spacing between letters decreases, creating a crowding effect. Lastly, some letters may be easier to read than others. This lack of standardization within a chart makes it unreliable for purposes of statistical analysis8.

ETDRS Chart The ETDRS chart was developed in 1982 to overcome the limitations of the Snellen chart and standardize VA measurements for clinical research. It is the gold standard for clinical research. Tested from 4 meters (13 feet) with each row containing 5 letters, each of equal spacing on a logarithmic scale. Scoring is represented as logMAR = 1.1- (0.02 x total letters correct)9. Bailey-Lovie Chart Developed in 1976 by Bailey and Lovie as an effort to redesign the Snellen chart, it became the precursor to the ETDRS chart. Containing a total of 14 rows with 5 letters each, featuring logarithmic scaling and equal letter counts per row. More commonly used in optometric and low-vision settings 10. Landolt C and Tumbling E Dr. Snellen developed the tumbling E chart for children as well as for adults with language barriers or cognitive impairments. This displays the letter E facing different directions, allowing a patient to identify which direction the E is facing. Similarly, the Landolt C displays a capital C or open ring facing various directions. Patients may be able to identify patterns as an alternative assessment of their visual acuity. The Landolt C, developed by Dr. Landolt, is regarded as the lab standard by the International Council of Ophthalmology11.

Several notation systems are used to express distance visual acuity and may allow conversions between systems. The Snellen fraction is expressed as distance tested/distance at which the letter subtends 5 arcmin (e.g., 20/20 in feet or 6/6 in meters). The logarithm of the minimum angle of resolution (logMAR) is the preferred notation in clinical research and represented by the following formula12: logMAR = log10(MAR) Snellen 20/20= log10(20/20)= logMAR 0 Snellen (ft) Snellen (m) logMAR 20/10 6/3 −0.3 20/20 6/6 0.0 20/40 6/12 +0.3 20/100 6/30 +0.7 20/200 6/60 +1.0

Visual Acuity Testing Protocol13 Distance Vision Setup and Environmental Conditions Testing Distance: 20 feet standard for Snellen (14 feet for ETDRS) Illumination: controlled room lighting with backlit or front-lit chart with standardized luminance of 80-160 cd/m2 Positioning: chart should be positioned at eye level or slightly below Correction Status Test with current correction unless directed otherwise. Add notations as below: SC (sine correctione): without correction CC (cum correctione) with correction PH - with pinholes Pinhole vision may not improve for all patients, but if visual acuity improved with pinhole occluder, the deficit is likely an optical (refractive) etiology. If no improvement is noted, document as “NI.” Monocular Testing Each eye is tested separately with right eye (OD) first (convention) then left (OS) Occlude the fellow eye with an opaque occluder and ask patient to read left to right, line by line, until they miss more than half of a line. Prefer showing multiple lines during test instead of isolating one line or one optotype. Ensure patients are using the distance portion of their glasses. If fewer than half of the line's optotypes are missed: record as line minus missed (e.g., 20/20-2) and if more than half are missed: record the last full line read plus correct optotypes on the next line (e.g., 20/25+2). Use Numbers, Tumbling E’s, or LEA Symbols for low literacy or non-English speakers. After optimization of environment and addition of pinhole, if no letters are legible proceed to test at shorter distance explained below Low Vision Acuity Designation When a patient is unable to identify any letters on the chart, proceed with further testing in the order below. Allow patients to move their head or use eccentric fixation for best vision. If side/peripheral vision helps, note as Ecc or EF (eccentric fixation) or head tilt left, right, up or down, etc. 1. Count fingers (CF)- patient correctly counts fingers held at specified distance (e.g., CF at 3 feet) 2. Hand motion (HM)- patient detects direction of hand movement at specified distance 3. Light perception (LP)- patient detects presence/absence of light; if patient can identify direction of light (superior, inferior, nasal, temporal) record as LP w/ P (light perception with projection) 4. No light perception (NLP)- no perception of any light Binocular testing Test both eyes vision together in cases such as latent nystagmus or when filling out specific forms such as the Department of Motor Vehicles (DMV) form. Near Vision Near vision is tested at standard reading distance, typically 14-16 inches. Common systems include the Snellen-equivalent near cards. The same principles of optimized lighting and monocular testing apply. Ensure patients are using the bifocal portion of glasses, if applicable. Near acuity is important in evaluation of presbyopia, low vision rehabilitation, and macular disease monitoring.

Summary of Documentation and Reporting 1. Distance at which the chart was read (standard 20 ft / 6 m, or specify if modified) 2. Correction status (sc, cc, ph) 3. Occlusion (monocular, right or left, binocular) 4. Notation (Snellen, logMAR, etc.) 5. Number of letters read on the best line (important for ETDRS) 6. Testing conditions if non-standard (reduced distance, dim lighting) Example: SC VA OD: 20/80+2, OS: 20/40−1. CC VA OD: 20/25 +2, OS: 20/20. Pinhole OD: 20/20. Testing at 20 ft, Snellen chart.

Factors Affecting Visual Acuity Visual acuity is influenced by a multitude of optical neural, retinal, cognitive, and environmental factors. Optical factors are among the most common and include refractive error, media opacities, and pupil size. Neural etiologies span the entire visual pathways including macular disease, amblyopia, glaucomatous neuropathy, optic neuritis, and compressive lesions, each capable of reducing visual acuity through distinct mechanisms. The retinal aspect includes conditions such as diabetic macular edema, vascular occlusions, retinal detachment, and neovascular AMD that impair and disrupt retinal architecture. Beyond the structural diseases, cognitive and behavioral factors can affect measured VA. This includes patient cooperation, fatigue, motivation and non-organic visual loss. Standardized testing conditions (testing distance, ambient lighting, chart illumination) help ensure an accurate VA that reflects true visual function14.

Functional Visual Loss Non-organic visual loss (NOVL), also known as functional visual loss, is defined as reported reduction in visual acuity without detectable organic cause. Prevalence reported at 5-12% in ophthalmology clinics and most commonly diagnosed in female patients15. NOVL is suspected when VA is inconsistent across testing with an otherwise normal comprehensive ophthalmology exam. Confirmatory tests have been developed to identify functional visual loss including the fogging test, vectograph charts, OKN drum test, and visual evoked potential16.

Legal Context US Legal Blindness: A visual acuity of 20/200 or less in the better-seeing eye with best conventional correction (meaning with regular glasses or contact lenses). OR a visual field (the total area an individual can see without moving the eyes from side to side) of 20 degrees or less (also called tunnel vision) in the better-seeing eye17. Driving Test: Most states require a minimum of 20/40 best corrected VA for unrestricted driving. Restrictions may be placed if corrective lenses are needed to reach 20/40. Driving with monocular vision is legal with no restriction. Vision worse than 20/100-20/200 fail to meet licensing standards18.

References 1. Dictionary of Eye Terminology. Accessed May 21, 2026. https://store.aao.org/dictionary-of-eye-terminology.html 2. Feder RS, Olsen TW, Prum BE, et al. Comprehensive Adult Medical Eye Evaluation Preferred Practice Pattern(®) Guidelines. Ophthalmology. 2016;123(1):P209-236. doi:10.1016/j.ophtha.2015.10.047 3. Rehman I, Mahabadi N, Motlagh M, Ali T. Anatomy, Head and Neck, Eye Fovea. In: StatPearls. StatPearls Publishing; 2026. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK482301/ 4. Curcio CA, Sloan KR, Kalina RE, Hendrickson AE. Human photoreceptor topography. J Comp Neurol. 1990;292(4):497-523. doi:10.1002/cne.902920402 5. Riggs LA. Visual acuity. In: Graham CH ed. Vision and visual perception. New York: Wiley, 1965:321–349. 6. Westheimer G. Visual Acuity. Annu Rev Psychol. 1965;16(Volume 16, 1965):359-380. doi:10.1146/annurev.ps.16.020165.002043 7. Azzam D, Ronquillo Y. Snellen Chart. In: StatPearls. StatPearls Publishing; 2026. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK558961/ 8. 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;107:311-324. 9. Ferris FL, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol. 1982;94(1):91-96. 10. Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt. 1976;53(11):740-745. doi:10.1097/00006324-197611000-00006 11. Colenbrander A. Visual Standards aspects and ranges of vision loss with emphasis on population surveys. Published online January 1, 2002. 12. Holladay JT. Proper method for calculating average visual acuity. J Refract Surg. 1997;13(4):388-391. doi:10.3928/1081-597X-19970701-16 13. Kniestedt C, Stamper RL. Visual acuity and its measurement. Ophthalmol Clin N Am. 2003;16(2):155-170, v. doi:10.1016/s0896-1549(03)00013-0 14. Levenson JH, Kozarsky A. Visual Acuity Change. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Butterworths; 1990. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK215/ 15. Kathol RG, Cox TA, Corbett JJ, Thompson HS, Clancy J. Functional visual loss: I. A true psychiatric disorder? Psychol Med. 1983;13(2):307-314. doi:10.1017/s0033291700050923 16. Manion GN, Stokkermans TJ. Nonorganic Vision Loss. In: StatPearls. StatPearls Publishing; 2026. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK599519/ 17. Disability Evaluation Under Social Security | Disability | SSA. Accessed May 21, 2026. https://www.ssa.gov/disability/professionals/bluebook/ 18. Vision and Driving. American Academy of Ophthalmology. May 14, 2024. Accessed May 21, 2026. https://www.aao.org/eye-health/tips-prevention/vision-driving Karine.Bojikian (talk) 11:23, May 31, 2026 (PDT)