Visual Acuity Flashcards
When are VAs measured?
first at every examination with the exception of chemical burns
How do VAs guide an examination?
determine if something is wrong optically or pathologically
Poor attention could result in…
decreased measured VA
What are the four testing methods?
minimum visible/detection, minimum separable/resolution, vernier, and recognition
What is detection acuity?
whether or not an object is present, similar to visual field test, not often used
When might you use detection acuity?
if child is thought to be blind or vision development is delayed, can child see 1mm candy at different distances
What is the main problem with detection acuity?
it is not standardized
What is resolution acuity?
can you see the stimulus as compared to the background
What is the stimuli in a resolution acuity?
square wave, sine wave, checkerboard
What is vernier acuity?
the smallest area of misalignment that can be detected between 2 stimuli, used experimentally not clinically
What is recognition acuity?
the most common, snellen, landolt C etc
What is landolt C?
a forced choice recognition acuity test useful for preschool or nonverbal patients, takes a bit of time
What is the takeaway from the visual acuity development studies in 1939, 1962, and 1978?
newer studies have more sophisticated tests and record better acuities in the the corresponding ages
What are the infant VAs from VEP?
2 months 20/150, 4 months 20/80, 6 months 20/30
What is a limit of VA testing in children?
cognitive abilities are not the same and take time to develop, boredom/loss of attention is a problem too
What are structural limits of VA testing in children?
cortical immaturities, foveal cone immaturities, and foveal pit
What is cortical immaturity?
incomplete myelination of optic pathways
What are foveal cone immaturities?
short and stubby cones until 4 yrs, less densely packed/no tight junctions until 3-4 yrs
What is foveal pit immaturity?
variability in morphology even age matched, by 17 months the pits are more adult like (1.5 years)
How do you test VAs?
age appropriate target, monocularly w/ no peeking if possible, binocularly if unable
What objective tests can be used to assess VAs?
retinoscopy, cover test, health
Infant VA testing
occlude with sticky patch or mom/dad’s hand, infants like to look at faces, should be concerned with asymmetric responses to oculsion
What is the Heidi smile test?
screening for development, eye contact at 8 weeks, social smile at 12 weeks, Heidi smile at 3 months (recognizing faces)
What is CSM?
central, steady, maintained a monocular penlight test for gross assessment of fixation and acuity
What does central mean?
light is centered on the pupil
What does steady mean?
the eye is steady, no nystagmus
What does maintained mean?
eye is fixating on target or picks up fixation
What does OD: CSM OS: CSUM mean?
poor acuity OS
What does CUSUM OD, OS mean?
nystagmus with poor acuity OD/OS
What is fixate and follow?
grosser measurement of fixation monocularly with penlight, document F&F OD, OS
What does vertical prims test?
determines if there is a fixation preference, suspect amblyopia if 2+ lines difference, strabismus too
What is the vertical prism procedure?
have a patient fixate on a near object, place 10 pd BD or BU in front of 1 eye, if child sees double the eyes will move up and down between the images and there is no fixation preference
What is forced preferential looking?
resolution VA, grey vs spatial frequency gratings, infants will look at something instead of nothing
How do spatial frequency gratings work?
one black and one white stripe= 1 cycle, thinner stripes= higher spatial frequency and better VA
What is the spatial frequency grating notation?
cycles per cm that can be converted to cycles/degree looking at a table or performing at 55 cm, cycles/cm=cycles/degree, closer=easier, farther=harder
What is TAC?
Teller Acuity Cards, forced preferential looking, ranges from 38 cycles/cm to 0.23 cycles/cm which is 20/2400 to 20/10
What is 30 or 32 cpd?
by convention 20/20 but cpd is not really equal to Snellen
What is the TAC set up?
test distance can be 38/55/84, correct 70-75%
What are preferential looking problems?
time consuming, boredom/attention, does not equate to snellen, objectivity of examiner, cost
What are preferential looking attention grabbers?
fan child, hide face, tap on card, puppet, noise maker
What are VA FPL norms?
1-2 months VA 1.3 cpd (20/470 @55 cm), 6 months VA 5 cpd (20/100 @55 cm), around 3-5 years have adult levels
When do kids reach adult level VAs?
3-5 years old
What are Lea paddles/gratings?
FPL test with different spatial frequency gratings
What is the Lea paddle technique?
@57 cm hold gray paddle over striped paddle, separate paddles at equal speeds and distances, see if child’s gaze follows stripes
What are advantages of lea paddles?
cost, portable
What are disadvantages of lea paddles?
not snellen equivalent, time consuming, observer bias boredom, look at your face instead
Why use a distance of 57 cm for lea paddles?
at 57.2 cm, cycles/cm=cycles/degree
What is OKN?
optokinetic nystagmus, involuntary eye movement induced by speed of motion of the visual field, induced to hold images stable on retina with head/world movemnt
How does OKN present?
smooth pursuit in direction of target and saccade back (jerk nystagmus)
What is the OKN procedure?
@40 cm, patient fixates stimulus, slow eye movement in direction of rotation and rapid movement backwards
What must a patient do to elicit an OKN response?
must pay attention to the stripes and accommodate to the stripes
Is OKN truly foveal?
no, deep central scotoma= reduction of gain by ONLY 10-30%, test not particularly affected by blur and can get a positive response even with high refractive error
T/F absence of OKN = blindness
false, could be lesions in OKN in cortex, cerebellum, brainstem, cortical dysplasia, cortical blindness
Why is it difficult to determine VA from OKN?
may be useful in determining abnormal binocularity in infants based on asymmetric responses, generally if response to horizontal and vertical you can anticipate 20/400
OKN in infants
binocular OKN is robust in infants regardless of direction, monocular 1-2 month old infants only show OKN with temporal to nasal motion due to cortical immaturities, symmetrical at 3 (or 5-6) months
Asymmetric OKN responses seen in children and adults with abnormal binocularity result from
strabismus, anisometropia, unilateral congenital cataract
What should you do when a child shows no visual response?
child should blink to startle and light, flick fingers at child’s eye or shine bright light
