Clinics, Refraction Flashcards
What are the 3 levels that make up binocular vision?
-simultaneous perception
-fusion
-stereopsis
what are the advantages of binocular vision?
-increases FOV
-compensates for physiological blind spot
-binocular summation
-stereopsis
where is the physiological blindspot?
the optic disk
when does BV start developing in babies
not present at birth, starts developing mainly during first 2/3 years and stops after 8/9 years
what is the definition of binocular vision?
Condition in which both eyes contribute
towards producing a percept which may or
may not be fused into a single impression
what makes up the near vision triad?
-conversion
-accommodation
-miosis = pupil constriction
what is near point of accommodation?
the distance at which the eyes see when fully accommodated
how does near point of accommodation change with age?
it increases (i.e. the point the object becomes clear)
name 5 causes of accommodative dysfunction
- Systemic and ocular medication
- Ocular trauma
- Inflammatory disease
- Metabolic disorders e.g. diabetes
- Down’s syndrome and Cerebral palsy (reduced amplitude)
- Idiopathic
name 5 symptoms of accommodative dysfunction
- Headache
- Asthenopia (eyestrain)
- Near vision blur
- Difficulty in reading
- Difficulty in changing focus
why is it important to know dynamic ret?
it may be the only way to assess accommodation in children
what are the two methods of dynamic ret?
-(monocular estimation method) MEM
-Nott
what shows lag and what shows lead in dynamic ret?
-lag = with movement so add positive lens
-lead= against movement so negative lens
when reading, why is it normal for us to use less than required AoA? What does this mean?
as we dont focus exactly on the target but instead somewhere behind it due to depth of focus. this means the accommodative response is less than the accommodative stimulus meaning there is accommodative lag
what is normal accommodative lag?
between +0.25DS- +0.75DS
where do eyes focus in relation to the near target in accommodative lag and lead?
-in lag they focus behind the target as response is smaller than the stimulus
-in lead they focus in front of the target as the response is greater than the stimulus
what could it mean if dynamic ret results are >+1.00DS? (accommodative lag)
- near esophoria because convergence system tries to compensate for the insufficient accommodation
- accommodative dysfunction (paresis, fatigue)
- under-corrected hyperope or over-corrected myope
- Higher + diopters (e.g., +2.00DS) shows presbyopia signs
what could it mean if dynamic ret results are < +0.25DS? (accommodative lead)
- Near exophoria
- Spasm of accommodation
- Under-corrected myope
- Latent hyperopia
how can you modify how you do dynamic ret for children?
to maintain appropriate fixation and accommodation, you can ask children to read some of the letters out aloud or to name/describe details in the picture
what are the most common errors when assessing accommodation with dynamic ret?
- Not realising that a small lag of accommodation is normal.
- Taking too long to make a judgement as to whether the reflex is moving ‘with’ or ‘against.’
- Nott method: inaccurate measurement of the distance of the target to the patient and the retinoscope distance from the patient that gives reversal.
- MEM method: leaving the lens in place for too long. The lens can alter the accommodation of the eye.
when might you do Nott dynamic ret instead of MEM?
if you dont have trial lenses to hand
when does presbyopia start becoming clinically significant? what can cause it to occur earlier?
-clinically significant in the fifth decade of life
-occurs earlier in people living in hot climates, people with short arms, short working distances & hyperopes
what happens to the comfortable near point with age?
it increases because accommodation reduces but the percentage of accommodation stays the same as before aging
when determining working distance, what do reading and computer screen distances range from?
-reading : 33-40cm
-computer screens: 50-60cm
what are the common errors when assessing reading addition?
- Assessing the near ADD without the distance correction in place
- Not insisting on the patient to read the lower line they can identify clearly
- Presenting the positive slide of the flippers when refining final add instead of the negative
- Not considering the hobbies of the patient i.e., knitting, sewing, crafting , modelling
- Not considering the occupation of the patient i.e. a contrabass player may benefit from an intermediate add alone for the distance he reads his music
when determining the range of vision:
-how do you find the closest point of near vision?
-how do you find the furthest point of near vision?
-closest point = 1/(near add + AoA)
-furthest point = 1/(near add)
what are the closest and furthest points of near vision in the range of clear vision
-closest point is where px uses total accommodative amplitude through near add
-farthest point is where px relaxes their accom through near add
give 3 tips when assessing near reading addition
-stimulate the workstation environment
-keep referring back to h+s checking the needs of your patient
-check the range of your near add
in general between what ages do people become presbyopic?
between ages 40-45 years old
what factors make the age of becoming presbyopic differ?
-ethnic group
-length of arms
-different working distances
-wether they are hyperopic or myopic to begin with
(those with long arms/working distances and myopes will see prebyopic change later, hyperopes, short arms and working distances later)
how does aoa change with age?
it reduces
why does average reading addition continue to increase after age 6o even though between 55-60 years objective tests indicate accommodation is 0?
due to the increases in add needed by some older subjects with reduced VA who use a reduced working distance to provide some magnification to offset the VA loss.
how can you avoid patients being unhappy with their glasses due to reading add?
determine the range of clear near vision required by the patient and prescribe glasses that fulfill those requirements so use a trial frame instead of a phoropter to more accurately determine working distances
what does determination of reading add begin with?
a tentative add
from studies (check clinical key page 95) what should tentative add be based on?
the patients age (1/2 aoa was second most reliable method)
what could unequal estimates of tentative add mean?
-the distance refractive correction has not been adequately balanced and needs checking
what is tentative add of patients aged 45, 50 and 55?
+1.00
+1.50
+2.00
most reading adds are equal for both eyes true or false
true, prescribing of unequal additions between the eyes is the exception and is rarely satisfactory
in some cases, what can a reading add that is low for a patient’s age and WD mean?
that the distance refraction has been over-plussed/under-minused.
why should you keep reading add as weak as possible?
to keep range of clear vision as long as possible
what is usually the max reading add you give to a px?
+3.00
what are the most common errors in reading add determination?
- Estimating the tentative addition of patients over 60 years of age based on their age and not their working distance.
- Not determining the patient’s near vision needs and subsequently prescribing an addition that gives an inadequate range of clear near vision for those needs.
how do you use the mallet unit to determine if the patient has an eso or exo deviation?
-eso deviation: when the bottom line moves towards the right eye and the top moves towards the left eye, it is an uncrossed
-exo deviation: If the bottom line moves towards the left and the top line moves towards the right, then its crossed
how can you use the mallet unit to determine of the Px has marked supression?
if they cannot see both markers simulatenously. this means you cannot test fixation disparity
what are the most common errors when using the mallet unit to measure fixation disparity?
-Decentration errors due to poorly fitting trial frame/phoropter or badly centred lenses
-not taking lowest possible prism as the measure
-not checking for potential prism adaptation
in fixation disparity, what does potential prism adaptation mean you must do?
leave the lowest prism power that neutralises the fixation disparity in place for a period of time (several minutes)
what is prism adaptation?
where in patients with normal binocular vision, two to three minutes after the introduction of a prism, the slip that was initially corrected by the prism reappears.
before doing motility testing, what should you do?
- get the Px to take off their glasses if they have them
- establish whether the Px has an abnormal head posture
- if they do not have an abnormal head posture just tell them to keep their head straight for the test
- if they do have an abnormal head posture then do motility in habitual head posture and then in normal head posture
- make sure corneal reflections are present and symmetrical
What is maddox double rod for?
measure the angle of a cyclodeviation
what’s the main difference between congenital and acquired cyclodeviations?
congenital are usually asymptomatic whereas acquired are symptomatic and usually also involve vertical elements like vertical diplopia
what could cause a cyclo-deviation?
a problem with the oblique muscles but could also (unlikley) be superior and inferior rectus
for excyclo-deviation, how do the eyes rotate, what does the patient see and which of the extraocular muscles is underacting?
-eyes rotate outward
-patient sees intorted image
-superior oblique
for excyclo-deviation how do the eyes rotate, what does the patient see and which of the extraocular muscles is underacting?
-eyes rotate inwards
-patient sees extorted image
-inferior oblique
why does double maddox rod work?
the patient is dissociated so can measure both cyclophoria and cyclotropia
what is laevoversion
when the eyes are moving in the direction of the left
what is dextroverison?
when the eyes are moving in the direction of the right
what is leavoelevation and leavodepression?
when the eyes are moving in the direction of top left and bottom left
what is dextroelevation and dextrodepression?
when the eyes are moving in the direction of top right and bottom right
when testing ocular motility, what should you do if the patient reports diplopia?
follow it up by asking the patient where the lights are displaced in relation to each other, horizontal? vertical? diagonal?
if the left eye underacts on dextroversions, what extraocular muscle is defective?
left medial rectus
what is accommodative fatigue?
where some Px only have the ability to achieve a normal AoA for a short period of time
what is jump convergence testing?
the patient’s ability to make rapid changes in vergence either between distance (6m) or near (15cm) or between two different near targets (6cm and 15cm, or 15cm and 50 cm).
how would poor jump convergence look compared to NPC?
-more prevalent
-more closely associated with symptoms i.e., convergence insufficiency
when may you assess jump convergence?
if symptoms suggest a convergence insufficiency even if NPC is normal
how is vergence facility different to jump convergence?
-result is quantitative instead of qualitative
-distance of the test target does not change
what is accommodative facility?
where you test the ability to assess the ability to change amount of accommodation exerted by testing inertia. Patients complaining of adjusting from distance to near vision or from near to distance should get this tested
who is accommodative facility not appropriate for?
people with presbyopia as their accommodation has naturally declined so you usually measure it on people under 30
what initial observations should you look for before assessing BV?
- Mobility / navigation
- One eye covered / closed
- Facial structural characteristics
- Abnormal (compensatory) head
posture - Ocular alignment (gross estimate)
- Nystagmus
- Existing Rx
- Albinism
what can proptsosis and pseudoproptosis be a sign of?
-graves disease
-TED
which can cause BV problems - ocular motility check is very important here especially if the onset is new
what kind of deviations is abnormal head posture found in and why?
incomitant deviations as the AHP replaces the lack of movement in paretic eye and so helps fuse the images and reduce diplopia
what are the three types of AHPs and what do they each aim to correct?
-head/ face turn = horizontal deviation correction
-head/face tilt = vertical/ cyclo deviation correction
-chin up/ down = vertical deviation correction
what issues can cause patients to look like they have an AHP?
-using head posture to fix nystagmus by finding the null point
-to maximise their visual fields if they have a hemianopia
-if they have musculoskeletal problems that mean they have ahp
what are the 4 components of vergence?
-Tonic vergence is where the eyes are in resting state where there is no image to focus and fuse.
-Proximal vergence is where the brain makes a decision on how close the target we want to look at is and so adjust the eyes accordingly so the image falls on the retina. At this point, image is still not on the fovea
-Accommodative vergence is where the brain judges how much accommodation is needed to make this image on the retina clear at this point some people may already have the image on the fovea - others may now have the image closer to the fovea but not yet exactly there causing some remaining retinal disparity which is a phoria
-Fusional vergence - brain uses this to correct the remaining retinal disparity so image falls on fovea
what are you testing in fusional reserves?
whether the system has enough capacity to make the fusional vergence adjustment by seeing how much fusional reserve the patient has as the eyes become too tired when using all fusional vergence all the time. You see how much prism it takes to dissociate the eyes
how much of you fusional vergence do you use for comfortable vision?
between 1/3 to 2/3
when checking horizontal fusional reserves, what are the two aspects you check and how do you correct them?
-positive fusional reserve = ability to converge = base out prism
-negative FR = ability to diverge = base in prism
what should you be watching when assessing fusional reserves and why
watch the patients eyes as they may not report the image to have gone double even though the eye has turned out (or in) again
when does blur occur when testing fusional reserves?
when the Px does not have enough accommodation to make themselves converge or diverge enough to meet the demand
in fusional reserves, when may a a patient be unlikely to report blur?
-if they are presbyopes being tested at near
-when assessing negative FR at distance (if they do report blur, there may be under corrected hyperopia or overcorrected myopia at distance)
what is fixation disparity?
-When the amount of misalignment is too small to be seen but can still cause symptoms in the patient - asthenopia.
-Patients do not see double even though they have a small misalignment because the object falls in Panum’s area.
-Some fixation disparity can be normal
give 3 asthenopic symptoms
-eye strain
-headaches
-blur
-watery eyes
what is an eso fixation disparity in terms of where the image falls on the retina?
where the image falls on the nasal side of the fovea so on the temporal field.
what is an exo fixation disparity in terms of where the image falls on the retina?
the image falls on the temporal side of the retina so the nasal field
what is the mallet unit used for?
to measure the amount of prism needed to correct fixation disparity
in what conditions does fixation disparity occur?
-both eyes have to be viewing the same target
-when the eyes are not dissociated
how do you make fixation disparity test work?
by triggering partial dissociation which is achieved by red/green filters or polaroid filters
in fixation disparity how does one of the bars seem to deviate?
as the bars on the mallet unit fall in panums area but since their images are not common to both eyes, they cannot be aligned as there’s no other image to align to
what patients is fixation disparity test not suitable for?
-those with va 6/12 or better in each eye
-those without binocular vision
-near vision no more than N10
why is it good to check fixation disparity on Px with certain BV symptoms?
- Establish presence of fixation disparity
- Determine how much prism is required to align the eyes
- Determine whether prism is a suitable remedy for this patient
do you test fixation disparity with or without refractive correction?
with
in fixation disparity what does it mean is only one bar is visible?
then one of the eyes is supressing so you need to ask the px which bar it is so you can decide which eye is supressing
what other options are there before prescribing patients with prism?
- Exercises
- Manipulation of Rx (think AC/A ratio)
- Behavioural alterations (e.g. regular breaks
from close work)
why may prism not be for all patients
as some will have prism adaptation where they adapt to the change in vergence driven by the prism so that it has no effect
why does prism adaptation happen?
due to physiological fixation disparity where the fast system of vergence responds and then the slow system responds
what are the two systems of vergence?
-Fast - system that responds when you quickly look from distance to near and back so drives repositioning of eye in response to prism. this is in horizontal vergences only
-Slow - when you look from distance to near and then focus on the near for a few minutes so resets the tonic position so fast system can respond to further vergence changes
how quick does the fast vergence system respond?
approx. 1∆/sec (latency 180-200ms)
how long does the fast vergence system take to complete?
12s
how long does slow vergence system take to respond?
4-5 minutes to complete
what triggers the slow vergence system?
motor response of fast fusion system
what happens in pathological cases of fixation disparity?
- Slow system does not respond correctly, just
the fast system - Prism adaptation does not occur
- If prism is indicated and other management
options are not available, then prescribe the
least amount of prism
what are the 4 methods of measuring AC/A ratio?
clinical
heterophoria
gradient
fixation disparity (textbook only)
when may you become suspicious of a high/low AC/A ratio?
cover test
what is the clinical method of determining AC/A ratio?
-you compare results if distance and near cover test
-if deviation is equal in the distance and near then AC/A ratio is normal
-if the deviation is 10 prism D more eso at near than distance then AC/A ratio is high
-if deviation is 10 prism D more exo at near than distance then AC/A ratio is low
what are the numerical methods of determining AC/A ratio?
heterophoria method
gradient method
how do you measure AC/A ratio using the heterophoria method?
- make sure px refractive error is fully corrected and they are wearing their distance correction
- Measure deviation using maddox rod at distance (knowing the distance) (no accommodation)
- Measure deviation using maddox rod at near (know the distance e.g. 33cm = patient needs to exert 3D of accommodation to see clearly) (accommodation)
this allows you to work out the accommodative effort - Measure the patients PD (HAS TO BE WRITTEN IN CM NOT MM OTHERWISE AC/A RATIO WILL BE WRONG)
then do the calculation
what is the formula for AC/A ratio using the heterophoria method?
AC/A = PD (cm) + (Phn- Phd)/D where:
- D = accommodative demand = 1/d
-eso deviations are +ve
-exo deviations are -ve
in heterophoria method of measuring AC/A ratio, what does it mean if AC/A ratio = PD in cm?
the distance and near phoria/tropia are the same so the amount of convergence induced is the amount required
what are the limitations to the heterophoria method?
the target stimulates both accommodative and proximal convergence so the AC/A ratio gets overestimated however this is still useful as it mimics real life use of vision
how do you work out the AC/A ratio using the gradient method?
- measure the deviation present through a range of spherical lenses in front of both eyes through the maddox wing starting from +3.00D (no accommodation) and then going down in 1D steps to -3.00D and giving you a range of responses for different amounts of accommodation exerted.
- You can then plot this graphically and work out the AC/A ratio by looking at the gradient of the plot
AC/A = ( Ph2-Ph1 )/( D2-D1 ) - phoria/tropia (deviation) is plotted on the y axis and accommodation D with each spherical adjustment is measured on the x axis
when using the gradient method for AC/A ratio, why would you not do it at distance with minus lenses instead?
as that would be less effective at stimulating accommodation
what are normal results for clinical measurement of ac/a ratio
> 10 more eso at near = high
10 more exo at near = low
what are normal results for heterophoria method of measuring ac/a ratio?
Generally between 3 and 7
<3 considered to be low
>8 considered to be high
what are normal results for the gradient method of measuring ac/a ratio?
- Average of ~4 found for the gradient method
- > 5 taken as high
how does ac/a ratio change with age?
it generally does not
-in presbyopia, it may decline slightly but otherwise will not dramatically be affected by presbyopia
-this is because this is because AC/A ratio is driven by how much accommodation is needed to see the stimulus rather than the amount of accommodation that is physically exerted to see the stimulus.
what is the hirschberg krimsky test?
Reflex tests to assess a deviation based on the corneal reflection
what patients are kirschberg and krimsky tests good for?
patients who cannot cooperate with subjective tests like young patients.
what are you looking for in hirschberg and krimsky tests?
symmetry in the reflections of the pupil
why is it normal that the reflexes in the Hirschberg and krimsky tests unlikely to be at the centre of the pupil
because of variations in angle kappa which is where the pupillary axis is usually more nasal than the visual axis. The angle between them is the angle kappa.
what makes angle kappa positive?
when the corneal light reflex is nasal to the centre of the pupil
what are normal variations in angle kappa?
-small positive angle kappa
-large positive angle kappa - where the light looks more nasal (looks like exotropia)
-large negative angle kappa - where the light looks more temporal (looks like esotropia but isnt)
what is an ectopic macula?
where the fovea is displaced which affects the visual axis and hence asymmetry may not mean deviation in this case. Otherwise asymmetry = deviation and symmetry = normal
how can you measure the amount of prism needed judging by corneal reflex?
what do you need to be careful of?
1mm = around 7 degrees tropia = ~22 prism diopters
and you can work out the mm of displacement as in adults cornea = 11-13mm diameters
be careful of megalo or micro cornea as that can cause corneal diameter to be different, and diameter also varies with age
what is krimsky test?
where after hirschberg you observe the position of the corneal reflex in front of the strabismic eye with prism and keep adding prism until the reflections become symmetrical again
who is bruckner for?
those not suitable for ret or cycloplegia and you want to know if they have uncorrected Rx and mainly anisometropia e.g. young children
what patients can you not do bruckner on?
if theyve had mydriatics or a cyclo deviation
what does bruckner test?
corneal light reflexes (hirschberg) and asymmetry of pupils giving rough indication of presence of anisometropia
what is the prism reflex test?
an objective screening test used to demonstrate the presence of fusional vergence
what patients can you not do prism reflex test on?
patients with no bvs
what is prism reflex test for?
to check fusional vergence in young children as a screening test e.g. is there’s little to no fusion on cover test
how does prism reflex test work?
allows you to demonstrate binocularity as a positive result as by using motor fusion to check for functional sensory fusion as motor fusion only occurs if there’s sensory fusion
in prism cover test, what happens to the other eye when the prism gets put in front of the opposite eye in a patient with normal BV. why does this happen?
it will move out but then move back in again to take up fixation because
1. intro of prism causes diplopia
2. eye with prism turns inwards to bring image to fovea
3. opposite eye turns out due to herrings law but this causes diplopia
4. it then flicks back in as a corrective vergence movement to restore BVS
what are all the tests that can be used to assess stereopsis?
-free fusion
-auto stereograms (lang I and II)
-anaglyphs (different colour filters in front of each eye) (TNO test)
-vectographs (Writ, titmus and stereo fly)
-transparent plates (frisby test)
-synoptophopre (used by orthoptists)
what is the free fusion test?
deliberately altering the vergence of the eyes to superimpose and fuse the two images so if the images produce retinal disparity and stereopsis is possible
what are random dot stereograms?
Where there’s an image consisting of random dots black and white like a QR code and a slightly different image is presented to each eye causing retinal disparity and so the Px views the these and reports what the PX can see
what are lang 1 and lang 2?
stereopsis tests set up with lenticular sheets, great to use for young children as they may try to reach for the target or you can look at their eyes and see where they look
what mistake may you make in lang 1 and lang II?
wobble the test as it may give a false positive result as the Px may see some movement which would be artifactual - not due to them having stereopsis
what minutes of arc does lang 1 test and what does Lang 2 test?
Lang I up to 400”
Lang II up to 200”
what is the TNO test?
where the patient uses red green filter glasses - similar to lang 1 but uses anaglyphs
in the TNO test on the 4th plate what does it mean if the patient only sees 2 circles instead of 3?
then there is suppression and whichever image is not present is the suppressed eye
in TNO test, what do the 1st three plates test?
whether or not the patient has stereopsis so are used as screening
in the last three plates in the tno test, what do they test?
the depth of stereopsis that the patient has
what minutes of arc does the TNO test test?
Up to 15” of arc on older versions, 60” on more recent
give examples of vectograph tests
writ, titmus and stereofly tests
what is a vectograph test?
involves presenting images with disparity and having the patient look at them binocularly with polarising lenses so if stereopsis is present, the image will look 3D (often used in 3d cinemas)
in the vectograph test,
-what minute of arc does the stereo fly test do?
-what minute of arc do the circles do
what minute of arc do the animals do?
800-3000’’
400-100’’
what is the frisby test?
where there are perspex plates which have 4 boxes of patterns and one of the 4 boxes looks different as a part of the image seen is printed on the other side of the plate creating retinal disparity. Thickness of plates affects retinal disparity so thicker plates = higher amount of retinal disparity and vice versa. By varying the distance you can increase retinal disparities testing. This test also needs to be held still as it can cause artefact images
what tests can you do for suppression and/ or ARC?
- Mallet unit
- Worth 4 dot test
- Bagolini lenses
- 4 pd test
how can you use the mallet unit to test for suppression?
by seeing whether the patient can see one or two of the red lines. Absence of one line indicates suppression and whichever eye is seeing the missing line is the one that is suppressed
how can you use the mallet unit to test for ARC? Why should you not rely on this though?
Can tell you if the patient has an ARC if the px who is known to have a deviation reports they can still see two lines. Not the best test for this though as some Px with arc do have foveal suppression so someone with ARC can still report one line.
what will a patient with a dominant eye see in the worth 4 dot test?
the white spot on the bottom may appear reddish or greenish
what are the most common errors with supression tests?
-doing the test without having the patients most accurate refractive correction in place
-asking the patients leading questions like can you see the 4 dots instead of what can you see
how can you use the worth 4 dot test to tell if the patient has ARC?
if they are a patient with strabismus who reports a normal result
in the worth 4 dot test how do you know when a patient has suppression?
If the patient is suppressing their RE, they will only report what is seen by the RE (three green spots) and if the patient is suppressing their LE then they will only report what is seen by the LE (2 red spots).
in the 4 dot test, how many spots does a patient with diplopia see?
5
what questions would you ask the patient when doing the worth 4 dot test?
how many lights can you see?
what colour are they
where are they
are they there all the time (if not they can have mild suppression)
what are bagolini lenses?
Clear lenses with fine striations on them set at oblique angles of 45 and 135 and 180 and 90 degrees in each eye and when you hold them up, they create a fine white line in the field of view - allows you to test suppression and arc.
what are the conditions for the bagolini test?
-correct RX in place
-room lights on
-need to be binocular
in the bagolini test why do you need the room lights on?
as lights off may cause the Px to dissociate and so would not mean the conditions are binocular
how do you know whether the patient has foveal suppression or foveal suppression + unharmonious arc in the bagolini test
In foveal suppression the patient will see two lines. One is continuous and one is discontinuous. If the lines intersect through the spots they just have foveal suppression but in rare cases, the lines may not even cross at the spot and in this case, the patient has inharmonious ARC
in the bagolini test, what will a patient with diplopia see?
two spots instead of 1
in uncrossed, the lines cross below the spot (left eye image moves left) and in crossed the lines cross above the spot (right eye image moves right)
what is the 4 PD test?
Objective test of central suppression scotoma. Needs a small power prism for a foveal suppression scotoma so you are only moving the image within the suppression scotoma and it’s not going outside of the scotoma. Means the image can be deviated within the suppression scotoma to determine whether there is any vergence movements to correct diplopia that would be caused without a suppression scotoma
what are the most common errors made when testing stereopsis?
-not having appropriate lighting (good without glare_)
-allowing head tilt
-not testing at correct distance (most are set for 40cm)
-not allowing enough time for the patient to see the figures
-not using the best corrected Rx
-allowing child to view stimuli before wearing the polarising lenses in polaroid tests
-not repeating the test when the result is abnormal
