Visual acuity is an often overlooked aspect of an emergency or outpatient department visit. Sometimes, this part of the physical exam happens intuitively and is not actively thought about during a patient’s examination. In reality, there are numerous instances where taking a moment to test visual acuity can severely change the diagnosis and workup for a patient regardless of the chief complaint. Whether the physician is evaluating a stroke, dizziness, headaches, red-eye, or a myriad of other conditions, the role of quantifying the visual acuity can be paramount. A full 8 point emergency department eye exam includes testing a patient’s visual acuity. One should always remember to ask the patient what their eyesight is before beginning testing. To better understand visual acuity, it is first important to define the term, know some basic ophthalmologic terms and procedures to document the visual acuity.
Defining visual acuity is as simple as stating it is the clarity or sharpness of one’s vision. Three terms that will readily be seen on an eye exam are Latin abbreviations, including OU, OS, and OD. The full form of these are:
- oculus uterque - both eyes (OU),
- oculus sinister - left eye (OS), and
- oculus dexter - right eye (OD).
The next bit of background information that is vital to a visual acuity exam is knowing how the numbers are recorded.
The numbers from a visual acuity exam all hinge on what a “normal” person can see at a distance of 20 feet. For example, take the term 20/20. What this number describes is that the patient being examined can see the same as what a normal person sees, or what normally would be seen, at 20 feet. Another example would be 20/40. A person with this visual acuity can see the letter at the 20/40 line at 20 feet, which a normal person could have recognized at a distance as far as 40 feet.
Visual acuity is a quantification of the sharpness of sight. It specifies a threshold. The acuity of vision is determined by the smallest retinal image, which can be appreciated. It is the measure of the tiniest object, which is clearly visible at a certain distance. To appreciate the form of an object, its several parts must be recognized. The angle created at the nodal point of the eye by the smallest resolvable object is known as the minimum angle of resolution (MAR). Visual acuity is of 4 types:
- Minimum visible acuity (minimum detectable) – This signifies the detection of an object or whether the object is present or not. This is quantified by the visual angle made at the nodal center of the eye by the tiniest object, which can be detected.
- Minimum resolvable acuity (minimum separable) – It is denoted by the smallest separation between 2 parts of an object (or 2 different objects), which can be resolved as 2 different parts by the visual system. A person of normal sight can differentiate 2 objects casting a visual angle of 1 minute (60 seconds or 0.017 degrees, 6/6 vision in Snellen's chart). This ability is determined the spacing of cones in the retina. However, some normal human beings may be able to resolve a stimulus as small as 30 seconds of arc (6/3 vision).
- Minimum recognizable acuity – It is denoted by the smallest feature that can be identified or recognized (like identification of a letter). Though 20/20 is considered as the gold standard minimum recognizable acuity for humans, the mean visual acuity in different age groups from 18- 80 years was better than 20/20. The sharpest mean visual acuity was noted in the 25-29 age group, and the mean ± standard deviation was -0.16 ± 0.06 in LogMAR (mean Snellen equivalent of 20/13.83).
- Minimum discriminable acuity – This is denoted by the smallest change in orientation, position, or size that can be appreciated. It is also known as hyperacuity. The smallest misalignment that we can realize is known as Vernier acuity. It is named after Pierre Vernier, who invented a scale that was used to navigate the ships. Humans can proficiently detect whether adjacent lines are aligned or not, and this leads to the widespread use of this scale.
Anatomy and Physiology
Minimum detectable visual acuity measures the ability to differentiate the intensity of the object and the background. In other words, it is a threshold of minimum detectable changes in contrast.
Minimum resolvable acuity and minimum recognizable acuity are limited by the spatial distance between the photoreceptors, aberrations, and pupil size (diffraction). When an object made of repeating black and white stripes is used to test visual acuity, there is a maximum spatial frequency beyond which the human visual system either sees a gray field or aliasing happens. In aliasing, the orientation or width of stripes is misperceived. At the maximum spatial frequency, in the retinal image of the object, the center of the white stripe falls on one cone photoreceptor, whereas the center of the black stripe falls on another adjacent cone photoreceptor. Normally, one cycle of this maximal spatial frequency casts a visual angle of 1 minute or 60 seconds of arc.
Minimum discriminable visual acuity is called hyperacuity, as it is much finer than what can be explained by the spatial separation between the photoreceptors. Multiple hypotheses on the mechanism, including changes at the photoreceptor level or beyond and optical properties, have been proposed to explain hyperacuity.
- Snellen chart
- Eye cover
Tests for visual acuity: It can be grouped into 3 types:
1. Detection acuity tests- Ability to detect the smallest stimulus e.g.
- Boeck candy beads
- Catford drum test
- Dot visual acuity test
- Schwarting's metronome
- STYCAR graded ball's test
2. Recognition acuity tests- Ability to recognize the stimulus.
- Bailey-Hall cereal test
- Beale Collin picture charts
- Landolt C test
- Lighthouse test
- Lipmann HOTV test
- Sheridan letter test
- Sjogren hand test
- Snellen charts
- Snellen E test
3. Resolution acuity tests
- Optokinetic drum.
- Preferential looking test
Vision Test in Different Ages
Vision Test in Infancy
- Cardiff acuity cards
- Catford drum test
- Fixation test
- Preferential looking test- Teller acuity cards test
- Reflex response
- Visual evoked responses
Vision Test in 1-2 years
- Boeck candy test
- Sheridan's ball test
- Worth's ivory ball test
Vision Test in 2-3 years
- Coin test
- Dot visual acuity test
- Miniature toys test
Vision Test in 3-5 years
- Landolt's C
- Lippman's HOTV test
- Sheridan letter test
- Tumbling E
Brückner Test helps in detecting strabismus. A direct ophthalmoscope is used to obtain a red reflex simultaneously in both eyes. A patient with strabismus shows an increased light reflex in the deviated eye. It also detects high anisometropia.
Relative Afferent Pupillary Defect (RAPD) detects asymmetric or unilateral disease of the optic nerve and/or retina. Such a pupil is also called Marcus Gunn pupil. The swinging flashlight test is done in a semi-dark room with the patient looking straight. A bright light source is shone on an eye (A) for 3 seconds. Then, the light is rapidly shifted to the other eye (B). If this eye's pupil dilates (instead of the normal pupillary constriction), the B eye is considered to have a relative afferent pupillary defect
Cover Test can detect the amblyopic eye in a pre-verbal child. Such children resist or cry when the better eye is covered or occluded. However, when the amblyopic eye is covered there is minimal resistance, if at all present.
To understand the applications of testing visual acuity, it is important to understand the different modalities used to test visual acuity. The straightforward and simplest of these methods is with a Snellen chart. Care should be taken to have the patient stand at an appropriate length from the chart in question and to test both eyes simultaneously as well as each eye individually. The visual acuity should be recorded as uncorrected (without glasses or contact lenses) and corrected (with glasses or contact lenses) visual acuity.
Methods of Recording and Reporting VA
The results of visual acuity tests are usually noted with "V," "VA," or "Va." Distance visual acuity is recorded as "UCVA" (uncorrected visual acuity) or "BCVA" (best-corrected visual acuity). Near vision may be identified by “NV.” Usually, distance visual acuity is taken first. By convention, initially, the visual acuity of the right eye is recorded, followed by the left eye, and then the binocular vision is recorded.
Küchler chart: This is believed to be the oldest known eye chart (around the 1830s to 1840s) invented by German Ophthalmologist Heinrich Georg Küchler. The original chart included images of birds, guns, farm equipment, frogs, and others. Dr. Küchler published another chart with 12 rows of letters later.
Snellen chart: This chart was described in 1862 by Herman Snellen, a Dutch Ophthalmologist. The standard Distance used for this chart is 20 feet or 6 meters, as at this distance, the rays are almost parallel, and the patient usually does not accommodate to see at this distance. Chart luminance should be 80-320 cd/m2. Usual luminance is 160 cd/m2. This chart is very commonly used. Snellen's chart has fewer letters in the upper part of the chart and the number of letters increases as the visual acuity tested becomes finer (lower part of the chart). The spacing between the letters in the same horizontal line varies in different lines. The letters use serifs. Snellen called the targets used in the chart optotypes.
The vertical height of a 6/6 letter casts 5 minutes of arc at the nodal point of the eye when seen from 6 meters. The vertical height of each arm of E at 6/6 line casts an angle of 1 minute of arc, or in other words, the thickness of each stroke of the letter is 1 minute of arc. The height and width of the articles are the same (each is 5 times the stroke width). Thus, the height: width ratio is 5:5.
Thus, the height of a letter at the 6/6 line is derived by the formula.
Tan (5min)= height of the letter in meter/6
Or, height of letter in meter= 6*tan 5 minute=6*0.0015 meter= 0.009 meter= 9 mm
Similarly, the height of a letter in any line= 6 meters* tan (5x MAR)
The procedure to calculate MAR in minutes is described later.
In each of these charts, the letters (black) and background (white) have high contrast. For a printed chart, the contrast is usually higher than a projector-based chart. The projector based chart should be used in a dark room.
The size of a 6/36 letter is 60% of the 6/60 letter. On the other hand, a 6/6 letter’s size is 66% of a 6/9 letter. Thus, the change of size from one line to another line is not uniform. Another issue with Snellen’s chart is that finer visual acuity has more letters, which, due to the crowding phenomenon, makes it difficult to read smaller letters, especially in patients with amblyopia. The letters used may not have similar legibility. Reportedly, errors were more common with certain letters (S, F, C, B) than others (A, L, T, Z).
There are 7 lines in Snellen's chart. The usually denoted visual acuity in each line is as follows (from uppermost to the lowermost line):
- 6/60 (contains one letter)
- 6/36 (two letters)
- 6/24 (three letters)
- 6/18 (four letters)
- 6/12 (five letters)
- 6/9 (six letters)
- 6/6 (seven letters)
As the lower rows have more letters, the chart has an A-shaped appearance when seen from far.
John Green's chart: This chart (1868) had used letters without serif and had up to 11 letters in each row. The spacing between letters was proportionate, and there was a geometric progression of the letter size in different rows.
LogMAR chart: To overcome the limitations of the Snellen chart, the LogMAR chart was introduced by Bailey and Lovie (1976) from Australia. Such charts may provide higher sensitivity and reliability of the measurement of visual acuity.
LogMAR is the abbreviation of log (base 10) of the MAR (the angle created at the nodal point of the eye/ visual angle expressed in minutes). For statistical analysis, logMAR visual acuity is the best option.
The features of the Bailey-Lovie chart are
- The letters (optotypes) have equal legibility/equal difficulty in recognition. This chart uses 10 letters (without serif, see illustration) from the British Standard test charts for checking visual acuity as advocated by the British Standard Institution in 1968.
- An equal number (5) of letters in each line.
- The height of a letter is 5 times the stroke width. The width of a letter is 4 times the stroke width. Thus, the ratio of height to width for each letter is 5:4.
- Distance between each letter in a single line is equal to the width of letters (uniform inter-letter spacing). This creates a similar contour interaction in each line and possibly cast a similar crowding effect in each line.
- The distance between 2 rows is the height of the letters in the lower row (uniform inter-line or inter-row spacing)
- The ratio of the size of each letter in one line to the adjacent line is constant (geometric progression or progression in logarithmic steps). Weber Fechner's law states that the perceived intensity of a stimulus is proportional to the logarithm of the intensity of the stimulus. The fixed difference between adjacent lines is 0.1 Log unit (the lower line has less value than the upper line). This translates to a size change by around 1.25 times or the 10th root of 10 (upper line-letters being larger).
- Each letter has a LogMAR value of 0.02 in each line.
- The Snellen notation (eg., 6/60, 6/48, 6/38, and more) and LogMAR values (like 1, 0.9, 0.8, and more) are given on either side of each row when seen from a 6-meter distance.
- For nonstandard testing distance, the distance may be changed in a logarithmic scale at 0.1 log unit steps. The possible distances are taken in geometric progression (multiplication by 0.8) and include 6, 4.8, 3.8, 3 meters, and so on. Notably, these distances can be remembered by dividing the denominator of the Snellen notations of different rows (like 6/60, 6/48, 6/38, 30, and more) of this chart by 10. A correction factor is added to the LogMAR value noted in the chart, depending on the distance used. For 4.8 meters distance, the correction factor is 0.1; for 3.8 meters, it is 0.2; for 3 meters, it is 0.3, and so on. The correct LogMAR value of VA at a distance is the sum of the LogMAR value of the lowest row seen plus the correction factor for the distance. As an example, if a person sees the row of 6/30 (LogMAR 0.7) at 4.8 meters (correction factor 0.1), the corrected visual acuity in LogMAR is 0.7+0.1=0.8. For a distance of 7.5 meters, the correction factor is -0.1 (minus point one). Nonstandard testing distances may be needed in
- nonstandard design of examination room
- patients with low VA
- validation of VA scores
- The chart size is 80 cm (height) x 70 cm (width)
- Appearance like V when seen from a distance
Disadvantages of the chart include:
- A longer time is taken
- Larger chart
Most used 2 LogMAR charts are:
- Bailey-Lovie chart and
- ETDRS (early treatment of diabetic retinopathy) chart - This chart was described by Ferris and colleagues (the National Eye Institute, USA). The ETDRS chart uses a luminance of 160 cd/m2. Each optotype (Sloan letters) has the same height and width (the ratio is 5:5). Thus, these letters are wider than the letters of the Bailey-Lovie chart. So, it was designed to be read at a distance of 4 meters instead of 6 meters, like Snellen’s or Bailey-Lovie chart. Small rooms may also be used to check visual acuity using this chart. The Snellen fraction at 4 meters may be converted to conventional American notation by multiplying 5 to both the numerator and the denominator. Thus 4/5 is equivalent to 20/25. At 4 meters, the visual acuity is maximum, and the dispersion of visual acuity is minimum. Refraction for infinity can be calculated by deducting 0.25 diopter from the refraction at 4 meters. Nonstandard distances used with the ETDRS chart are taken in a geometrical progression (3.2 m, 2.5 m, 2 m, 1.6 m, and more). The height and width of the chart are 60.3 cm and 63.5 cm, respectively. The reading of a single letter is allowed once only.
The standardized letters/shapes used to test visual acuity are called optotypes.
Various optotypes used for testing visual acuity include
- Landolt’s C chart- The chance of randomly guessing the correct response is 25% (1 of 4 configurations, opening up, down, right, or left). All the configurations have similar legibility.
- Illiterate E chart (by Hugh Taylor)- The chance of guessing is 25% (1 of 4 configurations, opening up, down, right, or left).
- When all letters are used, the chance of randomly guessing the correct letter is quite low (1/26 or around 4%).
- Snellen’s chart uses a serif font (see illustration).
- ETDRS chart used Sloan letters. There are 10 Sloan letters. Thus, the guess rate is 1/10, or 10%. The Sloan letters are named after Louise Sloan. The Sloan letters include C, D, H, K, N, O, R, S, V, and Z. The characteristics of Sloan letter include equal difficulty in recognition of each letter when compared with Landolt's C.
- Bailey and Lovie chart used the British set of 10 letters, and the guess rate is 10%. These letters are D, E, F, H, N, P, R, U, V, and Z.
- Other optotypes especially used for children include
- Allen’s Pictures and
The charts can be
- Shown on computer or LCD (liquid crystal display) screen, or
Procedure for Checking Visual Acuity
The visual acuity is denoted by the smallest optotype seen by a person. For the Snellen chart, the patient is 6 meters away from the chart. The normal room illumination is used. For projector-based vision charts, a darkened or dim room is preferred. The usual contrast of the visual acuity charts is at least 80%. If the person reads the top letter of Snellen's chart only, it is written as 6/60, which means that the top letter, which should be read at 60 m, is being read at 6 m. It can be written as 0.1 on the decimal scale and 20/200 as per the American standard. 6/60 equals +1.00 on the logMAR scale. Similarly, if the second line that should be read at 36 m is being read at 6 m should be written as 6/36. It can be written as 0.16 on the decimal scale and 20/120 as per American standard. 6/36 equals +0.78 on the logMAR scale. This progression can continue up to the seventh line, written as 6/6 on Snellen’s chart or 1.0 on the decimal scale. This is equivalent to +0.00 on logMAR, and the patient reads the smallest letter at 6 m.
If the patient cannot read 6/60 at 6 m, then the distance is gradually reduced. If he sees the top letter at 5 meters, then it is written as 5/60. If he sees it at 4 meters, it is written as 4/60, and similarly 3/60, 2/60, and 1/60. If he cannot see the largest letter at 1-meter, then he is asked to count fingers close to his face. Hand movement (HM) is tested in a patient who cannot count fingers close to the face. For HM visual acuity, the fellow eye is closed. The light is shone from behind the patient over a hand, which is kept at 60 cm away in front of the eye. Suppose the patient is asked to tell whether the eye is not moving, moving up-down, or moving right-left. If the patient gives a correct response (vertical or horizontal movement of the examiner's hand with direction) at least 4 of the 5 times he/she is tested, then HM visual acuity is said to be present. Projection of rays (PR) is checked by closing the other eye and asking the patient if he/she can correctly detect the direction from which the light from an indirect ophthalmoscope is projected over the eye (superior, inferior, nasal, or temporal). The responses of PR are recorded as + (present) or - (absent) in these quadrants. Light Perception (PL) is checked when the patient cannot appreciate hand movements. The light of an indirect ophthalmoscope (maximum illumination) is put over the eye from the front (90 cm away), and he/she is asked if the presence of light is appreciated or not. Testing this with the other eye before checking the abnormal eye may help the patient cooperate and better comprehend the test. PL vision may not be quantifiable for statistical analysis.
Finger counting at 6 meters is usually considered equivalent to 6/60 as the size of the 6/60 letter is roughly similar to a finger. Similarly, finger counting at 5 meters is around 5/60, and so on. This can be utilized to check visual acuity at the bedside if a vision chart is not available.
The distance of near vision testing may vary according to the work/occupation of the patient. The near distance may vary and include 25, 30, 33, 35, or 40 cm depending on the nature of the work of the patient. Usually, 33 cm is taken. Various charts for testing near vision include
- Snellen’s near vision chart
- Times New Roman near vision chart (N notation chart)- N48, N20, N16, N12, N8, N6
- Jaeger’s near vision chart
Representation of Visual Acuity
Snellen fractions (test distance 6 meter or 20 feet)
British system- 6/60, 6/36, 6/24, 6/18, 6/12, 6/9, and 6/6
American system- 20/200, 20/120, 20/80, and others
Decimal- The value comes from dividing the Snellen fraction’s numerator by denominator. Thus, in decimal 6/60, visual acuity is 6 divided by 60 or 0.1. Likewise, 6/12 visual acuity, when expressed in decimal, comes to 0.5.
Minimum angle of resolution (MAR, the angle created at the nodal point of the eye in minutes)- This is derived from dividing the denominator of the Snellen fraction by the numerator. Thus, the MAR in the case of a patient with a 6/60 vision would be 60/6 or 10 minutes. MAR of 6/36 would be 36/6 or 6 minutes.
LogMAR- This is the logarithm of MAR (base is 10). Thus, for 6/60 vision, MAR is 10 minutes. The LogMAR representation of 6/60 visual acuity would be Log10=1. Similarly, for 6/6 vision, MAR is 1 minute, and LogMAR is 0 (=Log 1).
Spatial frequency- This is expressed as cycles per degree (cpd). This is calculated by decimal acuity multiplied by 30. Thus 6/60 (0.1 decimal) is equivalent to a spatial frequency of 3 cpd. 6/6 (1 decimal) is 30 cpd.
M-unit- Louis Sloan introduced this notation for the size of the optotype. According to Snellen, VA= distance at with the patient recognizes the letter/ distance at which a normal person can recognize the letter. Sloan used the metric system and converted the previous formula to VA (in decimal)=test distance in meters or m/letter size in M unit or M= m/M. Thus, M=test distance in meters/VA in decimal.
According to the SI system, a patient with a visual acuity of 20/20 will be able to recognize a letter of 1M size at 1 meter. Similarly, a normal eye will be able to see a letter with a 20M size from 20 meters.
Various Parameters Which Affect Visual Acuity
- Ocular diseases
- Illumination of the chart
- Pupil size
- Type of optotype used
- Refractive error
- Retinal eccentricity
- Duration of exposure
- Crowding- interaction from adjacent contours
- Light exposure before the vision testing
- Patient cooperation
While at first glance, visual acuity may not seem important, there is a large clinical impact this exam can have. For instance, a finding of bitemporal hemianopsia could mean a patient has an optic chiasm lesion, mass, or possible aneurysm. Visual loss may be a feature of ocular diseases or a clue to the diagnosis of a disease of other systems, including the central nervous system.
Sometimes knowing where a patient cannot see in their visual fields can be as elucidating as knowing what they can see. A thorough examination of a patient’s visual fields by confrontation can be the clue that is necessary to make the diagnosis on a patient in the emergency department.
Visual acuity should be used in conjunction with other ophthalmologic tests such as intraocular pressure, visual field testing, refraction, a thorough slit lamp exam, and examination of the retina. All of these tests are within the scope of practice of an emergency medicine physician.
For preverbal child tests, other than vision charts are used. These include:
- Optokinetic nystagmus: In this test, nystagmus is elicited by passing a series of black and white stripes in front of the infant.
- Preferential looking test- This test is based on the fact that when an infant is presented with 2 visual stimuli, one striped and the other plain, the infant looks at the striped pattern for a greater amount of time. Teller acuity cards II and Cardiff acuity test utilize this method.
- Visual evoked response/potential (VER/VEP): This refers to the EEG response recorded from the occipital lobe in response to visual stimuli. A sweep-VEP is performed by showing the preverbal child a pattern of grids or bars. When the stripes are prominent enough for the child to see (discriminate), a response is noted. For smaller stripes, no impulse is generated.