Visual Acuity_2023.pdf

Visual Acuity Dr. Ariette Acevedo Rodríguez, O.D. PPO1 Visual Acuity (VA) • The ability to distinguish details and shapes of objects • This is the 1st test to be performed in all patients after case history • No exceptions-medicolegal implications • This test will provide information about: • Pre...

Visual Acuity Dr. Ariette Acevedo Rodríguez, O.D. PPO1 Visual Acuity (VA) • The ability to distinguish details and shapes of objects • This is the 1st test to be performed in all patients after case history • No exceptions-medicolegal implications • This test will provide information about: • Preservation or degradation of vision • Effectiveness of a treatment given • Legal implications such as licenses, pensions, insurances, lawsuits, claims, professions, etc… Visual Acuity Anatomy • Retinal photoreceptors • Rods and Cones • Rods: used for vision under dark or dim conditions • At low levels of illumination (Scotopic vision) • Have low spatial acuity • Cones: responsible for color vision and high spatial acuity • Central fovea only has cones • Active at higher levels of illumination (Photopic vision) • 3 types of cones photopigments, sensitive for specific wavelengths: • S-Cones: short-wavelength: Blue • M-Cones: middle-wavelength: Green • L-Cones: long–wavelength: Red Macula • Macula lutea and area centralis • Has retinal ganglion cell complex (RGCC) • Superior and inferior temporal vascular arcades demarcate it • Yellow appearance • Due to xanthophyll carotenoids: lutein and zeaxanthin • Has a foveal avascular zone and fovea centralis depression • V-Shape Macula Rods and Cones • We have ~120 million rods and 6-7 million cones • Most cones are concentrated in the fovea • The blind spot, which corresponds to the optic nerve, contains no photoreceptors • Fovea centralis is completely rod-free • Highest capacity for visual acuity due to the amount of cones • The farther away from the fovea centralis the VA starts to decrease: • Paramacular area ranges from 20/200-20/400 Macula • The angular size of the foveola is 54 minutes of arc (54’ arc), which corresponds to almost 1 degree in diameter, subtended at the nodal point of the eye. • 1’ arc = 1/60 degree • The 20/200 (6/60) letter “E” subtends an angle of 50’ arc, fitting well into the fovea. • If the object is projected 1 degree away from the fovea, there is a 60% reduction in the maximum VA. • Eccentric viewing and fixation Visual Acuity Definition Visual Acuity • Visual acuity is defined as the resolving power of the eye or the ability to see two separate objects as separate. • It is often referred to as the minimum separable resolution: • The ability to see a gap • Visual Acuity is limited by several factors: • Diffraction • Aberrations • Photoreceptor density in the eye • Other factors that affect VA: • Refractive error • Illumination, contrast • Location of the retina being stimulated Visual Acuity • VA is the resolving power of the visual system, to see two separate objects as separate. • The eye can resolve two objects as separate if they are separated by an angular distance of 1 minute of arc. • A gap has a visual angle of 1’ arc • The “normal” resolving power of the eye is the ability to detect a gap with a width of 1’ arc. • Visual acuity is specified in terms of the angular size of the gap for the smallest-sized letter the patient can identify. Snellen Visual Acuity • Universal method of measuring VA • The width of the stroke equals the width of the gap • The “best” letter is “E” because it has 3 stroked and 2 gaps. • Each stroke has 1’ arc separation, the whole letter is 5’ arc. • “T” and “L”, although used, do not have a gap • Advantaged of Snellen VA: • Reduces the chance of guessing • Letters are a familiar task • People expect to read in a visual examination Snellen Fraction • The Snellen fraction expresses the angular size of optotypes by specifying the testing distance and the height of the letters. • The number used to indicate the height of the letters is the distance at which the letter height subtends 5’ arc. !"#$ %&#$'()" • 𝑉𝐴 = *&#$'()" '$ +,&), -"$$". #/0$"(%# 1!2.) • i.e: 20/200 • Testing distance = 20ft. • The smallest letter that could be read subtends 5’ arc when viewed at 200ft. Snellen VA • The letters of the 20/20 subtends an angle of 5’ arc at the eye when viewed at 20ft (6m) away. • VA in Snellen notation is determined by the smallest line of letters of the chart that a person can correctly identify at a certain distance. • Recorded as VA = D’/D • D’: chart working distance (viewing distance) • D: the distance at which the letters of the line subtend 5’ arc • 5’ arc whole letter, each stroke is 1’ arc Snellen VA • For visual acuity of 20/20 (6/6) the whole letter subtends an angle of 5’ arc at the eye when viewed at 20ft (6m). • For patients: • 20/200: what a normal eye can see at 200 feet away, this eye can only see at 20ft. Minimum Angle of Resolution (MAR) • The reciprocal of the Snellen Notation equals the angle (arc minutes) that each stroke of the letter subtends at the person’s eye. • This can also be expressed as a logarithm (log10) of MAR • LogMAR is commonly used in research to specify visual acuity. • Ex: • 20/20 (6/6) MAR = 1 • LogMar = Log(1)= 0 • 20/200 (6/60) MAR= 10 • LogMAR (10)= 1 Specification of Visual Acuity • To determine how much a letter measures with a known testing distance (6m/20ft) and a visual angle of 5’ arc . • 𝑇𝑎𝑛 𝜃 = ! "#$%&'() • This is important for proper chart calibration according to the length of the exam room. Specification of Visual Acuity • • • • ! 𝑇𝑎𝑛 𝜃 = "#$%&'() X= linear width of gap Distance= exam room length 𝚹= 1/60 • 1 min of arc = 1/60 degree ! • 𝑇𝑎𝑛 𝜃 = "#$%&'() • * 𝑇𝑎𝑛(+,) = ! + • 0.000291 = ! +. • 𝑋 = 0.000291 6 • 𝑋 = 0.001746m • 𝑋 = 1.746𝑚𝑚 (linear width of gap) • 1.746 (5) = 8.73mm (linear width of whole letter) Calibration • Ideally each letter should have 3 strokes and 2 gaps. • Best letter is ”E” • Each stroke and gap subtends 1’ arc. • The linear width of each gap is: 1.746mm. • A letter that subtends 5’ arc at 6 meters measures 8.73mm. • 20/20 line • This is useful when calibrating the charts according to the projector's location and room size. Calibration • When calibrating the VA chart, measure the height of the 20/200 (6/60) letter E. • It should measure 87.00mm • If not calibrated move or adjust the projector until the correct size is achieved. • For distances shorter than 6m (20ft), the height if the 20/200 (6/60) letter ”E” is provided in the following table. VA Measurement Procedure Visual Acuity • Every patient needs to have VA measured at distance and near with and without correction. • This test should be performed at the beginning of the examination, following case history. • Instruct your patient that you are going to be measuring their visual acuity. • Equipment: • Occluder, pinhole occluder, distance chart (Snellen or equivalent), near chart (reduced Snellen or equivalent), transilluminator Equipment Procedure • When testing VA always start with the OD, then OS, then OU. 1. DVA sc OD, OS, OU 2. DVA cc OD, OS, OU 3. NVA sc OD, OS, OU 4. NVA cc OD, OS, OU • If the patient was not able to see the 20/30 line or better at distance with habitual correction, take pinhole VA. • Record as: testing distance/smallest line read • i.e.: 20/200, 10/50, 20/60 Distance • Illumination has to be dim • If completely dark will create contrast sensitivity (another test) • Present the whole chart (20/50-20/15) • If patient cannot read 20/50, then go to 20/400 • Cover OS (testing OD) • Ask the patient: “read the smallest line of letters you can” • After OD, move to test OS (cover OD) • Do not allow squinting, moving forward or lifting of the occluder • You have to observe your patient NOT the chart • If your patient has eccentric fixation/viewing it needs to be specified Memorize • It is recommended you memorize the classical Snellen chart 20/4020/15, at least. • Even though they may vary by a letter or two, most are very similar. • 20/40: FZBDE • 20/30: OFLCT • 20/25: APEOTF • 20/20: TZVECL • 20/15: OHPNTZ Record • Record in terms of the smallest line of letters read and distance of chart • To be correct the patient must read at least half or more of the line. • 20/20-2 or 20/20+2 • If you know the patient is amblyopic, do not present the whole chart • Crowding effect leads to lower VA • Show single line or isolated letters, but specify this method was used. Snellen Chart 20/400 20/200 20/80 20/100 What if my patient cannot see 20/400? • 1st option is to use a low vision (LV) chart • SOSH, Bailey Love or Feinbloom chart • Performed at 10 feet away from the patient • Record the distance/smallest line able to read • May record according to test distance (10ft) or convert to Snellen (20ft) • If LV chart is not available have your patient walk towards the chart until they can see the 20/400 letter • Record the distance at which they could see it for the first time • i.e.: 5ft: 5/400 • Avoid recording NOT ABLE TO PERFORM • A VA measurement has to be recorded What if my patient cannot see 20/400? • If the patient is unable to walk and there is no LV chart, then: • Counting fingers (CF)** • Present fingers at 3ft, 2ft, 1ft • Record as CF @ Xft. • Hand Motion (HM) • Performed at 1ft • Light Projection (LPj) • Present light from transilluminator from different angles • Light Perception (LP) • No Light Perception (NLP) Clinical Pearls • Show your patient the whole chart from the 20/50-20/20 line • If the patient cannot see the 20/50 line, then show the 20/400. • If the patient cannot see the 20/400, use a LV chart • If there is no LV chart, then: • Ask the patient to walk, if the patient cannot walk then: • CF, HM, LPj, LP • Memorize the charts!! • Observe your patient all the time VA with Pinhole • Usually done when a patient at distance, with habitual correction cannot see better than 20/30. • This means pinhole is done at distance, with correction if available • If no Srx then perform without correction • If during subjective refraction the patient cannot see 20/20, also perform PH • If no improvement is seen with a PH, report NIPH Near VA • Performed using a near chart • Typically, Reduced Snellen, but it is recommended you have several near charts • Tumbling E, Figures, Paragraphs • Place the chart at 40cm (~16in.) • Full room illumination with overhead lamp Illuminating the chart • If a patient needs to bring the material closer, record the distance • For myopes the inverse of the distance will be the amount of myopia Near VA Chart • Types of notation for near VA: • Reduced Snellen Acuity Card: testing distance 40cm (16in.) • Early Treatment Diabetic Retinopathy Study (ETDRS) • Standardized VA testing • Jaeger Acuity Chart • 20 letter sizes classified from J1 to J20 • Point system • Each point is 0.35mm • M notation • Based on meter unit Recording Visual Acuity • Will be recorded as: .)$%#'/ "#$%&'() 01&22)$% 2#') 34 2)%%)5$ 5)&6 • Always indicate the eye, distance or near, and with (cc) or without (sc) correction • Always indicate testing distance • 40cm, 6m, 20ft, ect… • For CF 1ft-3ft • Always indicate the chart used "#$%&'() 4531 (7&5% • If the patient walks/gets closer to the chart record as: • If LV chart was used, performed at 10ft, then record: • If PH was used, must be indicated *, ! 8,, Recording Visual Acuity • Examples: • DVA sc OD 20/20 OS 20/40 OU 20/20 (Tumble E) • DVA cc OD 20/20 OS 20/40 cPH 20/20 (Snellen) • If a patient has glasses: • DVA sc OD5/400 OS 20/20 • DVA cc OD 20/40 cPH 20/25 OS 20/20 • If patient cannot walk: • DVA cc OD CF@3ft OS HM • With LV Chart: • DVA cc 10/700 (20/1400) NIPH (SOSH) Recording Visual Acuity • Near VA Recording • It is always assumed it is performed at 40cm, but it is important to record testing distance • In some patients it can vary • Examples: • • • • NVA sc OD 20/20 OS 20/20 NVA sc OD 20/20 OS 20/20 @ 10cm NVA cc OD 20/200 OS 20/100 (Allen Cards) NVA cc OD 20/200 c eccentric fixation OS 20/20 • Pinhole is only done at distance VA Notations • Decimal Acuity: • The resolution of the gap of 1’ arc in width represents a VA of 1.0 • English Acuity • The resolution of the gap of 1’ arc in width represents a VA of 20/20 • Metric Acuity • The resolution of the gap of 1’ arc in width represents a VA of 6/6 • Percentage Acuity • The resolution of the gap of 1’ arc in width represents a VA of 100% VA Notation • Snellen Acuity into Metric Acuity • First convert the Snellen fraction into decimal acuity then convert the decimal acuity into metric acuity • Or solve for X • 20/20= 1.0= 6/6 • 20/200= 0.10 = 6/60 • 20/200 = 6/X • X= 60 VA Notation • Decimal Acuity (DA): • Good for plot charts and research, seldomly used clinically • Division of the fraction • Example: • 20/40 = 0.50 • If converting DA to Snellen • 0.50 = 20/X • 0.5X = 20 • X = 40 (20/40) • The DA decreased with the increase of the gap (letter size) • If DA is 0.5, the letter should be 8.726/0.5 = 17.45mm • If DA is 2.0, the letter size would be 8.726/2.0 = 4.363mm • For percentage acuity: • Multiply DA by 100 Snellen Acuity and MAR Snellen Acuity (Fraction) Snellen Acuity (Decimal) 20/20 20/25 20/30 20/40 20/50 20/100 20/200 1.0 0.80 0.67 0.50 0.40 0.20 0.10 Minimum Angle of Resolution 1 1.25 1.50 2 2.5 5 10 Pediatric VA Evaluation Allen Cards (D&N) LEA Cards HOTV Birthday Cake Kindergarten Chart Developmental Aspects • Visual milestones: • Soon after birth the newborn can fix and follow a light source, face or a large/colorful toy. • By 1 month old they should have central, steady and maintained fixation. They can follow a slow target, converge and prefer looking at a face. • By 3-month-old they should have binocular vision and eye coordination. Eyes should follow a morning light or face and offer a responsive smile. • By 6-month-old they should be able to reach accurately for toys. • By 9-month-old they should have the ability to look for hidden toys. • At 2 years old they should be able to picture match. • At 3 years old they should be able to letter (single) match • At 5 years old they should be able to match or name the Snellen chart optotypes. Developmental Milestones • Axial length: • At birth: 16-17mm • 1 year old: 20-21mm • Adolescence-Adulthood: 23-25mm • Expected VA: • 1 month old: 20/800 (6/240) to 20/200 (6/60) • 2-month-old: 20/150 (6/45) • 4-month-old: 20/60 (6/18) • 6-month-old: 20/20 (6/6) Visual Acuity in Children • Preferential fixation • Preferential looking (Teller Cards) • 20/20 by 18-24 months • Optokinetic drum • 20/20 by 20 -30 months • Visual Evoked Potential (VEP) • 20/20 by 6-12 months Visual Acuity Charts and Acuity Tasks Detection Recognition Resolution Location Target Detection • Target detection: the perception of the presence or absence of an aspect of the stimuli, not the discrimination of target detail. • The Landot C and Illiterate (Tumbling E) are examples of target detection, more specifically detection of the location of the gap. Tumbling E • Presented to children or those who cannot read. • Letter E is placed in 4-8 different position and the subject reports which way it is facing. Landolt C (Broken Wheels) • Landolt Ring is a ring with a gap. • The width of the stroke and the gap are both 1’ arc. • Same as Tumbling E • Ask the patient, where the gap is located • Chart starts with 20/200 (6/60) and ends with 20/20 (6/6) VA Limitations • On average, people will retain their ability to see 20/20 until the age of 62. • After this age, less than 50% will retain their ability to see 20/20. • Cataracts • Glaucoma • Age-Related Macular Degeneration (ARMD) VA Limitations • Visual acuity is limited by: • Diffraction, aberrations and photoreceptor density. • May also be affected by: • • • • • • • Refractive errors Location of the retina being stimulated Illumination Contrast Pupil Size Exposure Duration Target and Eye Movement VA Limitations: Refractive Error • VA will decrease if the optics of the eye are defocused. • For example: • If the amount of defocus at distance is 1D, the visual acuity may decrease from 20/20 to 20/50. • In theory for every -0.25D of RE, a line of VA is decreased. • For hyperopes, due to the ability to accommodate and compensate refractive error, VA is affected to a lesser degree. VA Limitations: Retinal Eccentricity • The further away from fixation (fovea), the lower the acuity level. • The highest visual acuity corresponds to the fovea. • The angular size of the fovea is 54’ arc (almost 1 degree) at the nodal point of the eye. • The 20/200 (6/60) letter “E” subtends an angle of 50’ arc, fitting well in the fovea. • If light focuses away from the fovea by 1 degree, there is a reduction of 60% of maximum acuity. • Strabismus, macular holes, macular pathologies will all affect the ability to see 20/20. VA Limitations: Luminance • Luminance is the photometric measure of luminous intensity per unit area of light travelling in a given direction. • This is used as an indicator of brightness and is measured in foot-lambert units. • The standard luminance for a VA chart is considered to be minimum, at 10 foot-lamberts. • If the relative luminance increases, the relative visual acuity also increases. • But be careful, increasing luminance from 100 to 1000 foot-lamberts causes little increase in VA, while reducing luminance to 1 or even 5-foot lamberts causes VA loss. Variation with Luminance and Contrast • In clinic the charts used, have a high contrast: black letter on a white background. • Contrast is the difference in color and light between parts of an image. • Objects and their surroundings are of varying contrast. • The relationship between VA and contrast allows for a better understanding of our visual perception. • Increasing contrast, increases resolution at a given luminance level. • Contrast sensitivity is another method of VA measurement Testing Condition: Illumination • It is recommended a room lighting in the vicinity of the projector chart to be kept low (not off), otherwise will reduce the contrast of the letters on the chart • Room lighting in the vicinity of the projector screen should be no greater than 13% of the projector luminance. Pupil Size • Most patients have pupil sizes between 2-5mm at ordinary levels for VA tests. • Increasing the pupil size in general increases visual acuity by allowing more light into the eye. • But a very large pupil (>5mm) will result in spherical aberrations in which the light rays striking the crystalline lens’ periphery are refracted more strongly, causing the image to move forward toward the vitreous, producing myopia. • In addition, the larger pupil may also reveal irregular astigmatism, causing a blurred image. Pupil Size • A small pupil (less than 2mm) has a larger depth of focus. • By decreasing the aperture of the pupil, a smaller blurred circle will be projected on to the retina, producing a sharper retinal image. • A smaller blurred circle equals to a greater depth of focus, leading to better vision. • A pinhole device increases the depth of focus by decreasing the size of the blurred circle projected on to the retina. • But if it is too small an aperture, it will block too much light and decrease visual acuity and visual field. Testing Distance • As long as the letter size of the 20/20 letter subtends 5’ arc at the spectacle plane, an emmetrope or hyperope will have no problems clearing up the letters even if the distance is less then 6m. • The problem will be with myopes, they will show artificially better VA if the distance is closer than 5m. • For charts less than 4m distance, myopes will be underminused and hyperopes will be overplused. • Therefore, if testing distance is less than 20 feet, use a mirror systems should be considered. Other Factors Affecting VA • Exposure Duration: • Decreasing target exposure duration will decrease VA • This can be offset by increasing the luminance to ensure the constancy of the number of absorbed quanta. • Interaction Effects • When objects are too close together, the VA decreases Target and Eye Movement • Strict stability of the retinal image is not required for optimum resolution (micro-drifts are necessary) • But significant movement of the retinal image, such as during saccadic movements, will produce a decrease in VA. • If a smooth pursuit movement enables tracking of an object, the VA will slightly decrease. Visual Acuity • Visual acuity is a highly complex function that consists of: • • • • Minimum visible: detection of a target Minimum recognizable acuity: angular size of the smallest feature Minimum resolvable Minimum discriminable or separable: smallest change in any feature Target Recognition Tasks • Recognition or naming of a target, such as with Snellen letter. • Target recognition tasks is the most used in clinical exams • Test objects are large enough that detection is not a limiting factor. • Careful letter choice and chart design to ensure that letter recognition tasks are uniform for different letter sizes and chart working distance. Target Resolution • Target resolution thresholds: the smallest angular size at which subjects can discriminate the separation between critical elements of a stimulus pattern • A pair of dots, a grating, a checkerboard Target Localization • Target localization: discriminating differences in the spatial position of segments of a test object, like a break or discontinuity in contour. • Vernier Acuity (a type of hyperacuity) the discontinuity is specified in terms of angular size Target Localization • Konig bars are used in research • Consist of a pair of bars on a background • For normal VA, the width of each bar is 1’ arc and the gap between the bars is 1’ arc • This concept is applied in Goldmann Applanation Tonometry • Procedure: • Present a pair of Konig bars of different sizes with the width of the bar and the gap beign the same. • The smallest pair resolved as 2 bars gives a VA measure Visual Efficiency • To quantify for visual loss for legal and compensatory purposes. • Visual efficiency is 1 or 100% if VA is 20/20 and 20% of visual efficiency if VA is 20/200. Reminders on PH • Pinhole Visual Acuity Purpose • To determine if a decrease in vision is correctable by lenses • PH taken when entering VA is worse than 20/30 at distance (while performing refraction) • If the cause of VA decrease is uncorrected refractive error, the VA is expected to improve through the pinhole. • If the cause is not optically based, there will be no improvement, and possible a decrease (NIPH) • Always observe your patient while taking VA • Determination of the correct VA is a legal aspect, be sure to do it correctly. Practice • Transform into Snellen Notation: • • • • • • Decimal acuity of 0.25 Decimal acuity of 0.50 DVA sc OD 2/200 DVA sc OD 10/500 MAR 1.50 MAR of 20/25