Measurement of ocular deviations Flashcards
list the reasons why we measure ocular deviations
- diagnosis
- find maximum deviation
- consistency between examiners
- monitor progression
- to relate to fusion range
- to calculate AC/A
why is diagnosis an important point of measuring ocular deviations
as some deviations are greater at distance than near or greater at near than distance & some deviations are equal at both distances
why is monitoring progression important when measuring ocular deviations
incase the deviation gets larger
eg a large eso at distance can be dangerous, and cannot respond to exercises & large exo at near can be normal and fixed by exercises
why is it important to calculate AC/A when measuring ocular deviations
if have higher than 1D or accommodation at 4 prism dioptres, we have eso at near & if we converge less than 1D = exo at near (for every 1D or accommodation, we converge by 4prism dioptres)
what distance should all measurements of ocular deviations be taken at
- 6 meters
- at 1/3 metre
- at > 6 meters plus for distance exo
why is it important to state whether the px was wearing refractive correction when measuring their ocular deviation
to know the effect of the refractive error on the deviation
what is the objective technique of measuring ocular deviation, using total dissociation
prism cover test
what is the objective techniques of measuring ocular deviation, using corneal reflection
- hirschberg
- krimsky
- prism reflection test
what are the subjective techniques or measuring ocular deviation
- subjective prism cover test
- maddox rod
- maddox wing
what is a disadvantage of a measuring technique which tests the eyes in total dissociation
do not allow the practitioner to distinguish between a manifest and latent deviation
(but thats irrelevant as we would have already done a cover test to know if they have latent or manifest deviation)
what does the prism cover test (PCT) require from the patient
co-operation
what type of target is required for the prism cover test
accommodative (which requires precise fixation & controls accommodation)
why is it important to have a target which controls accommodation during the prism cover test
as accommodation can effect the size of the deviation
what type of target is used with the prism cover test if px has VAs better than 6/60
letter on the line above that which the weakest eye can see e.g. R 6/18, L 6/12 use 6/24 letter
what type of target is used with the prism cover test if px has VAs worse/less than 6/60
spot light
why is it important to use a target which is a letter on the snellen chart on the line above that which the weakest eye can see in the prism cover test
so each eye can separately resolve that letter, when the other eye is covered
which type of deviations is the prism cover test used to measure
- latent
- manifest
- vertical
- horizontal
which deviation is the prism cover test not able to measure and under which exception can it work
cyclo deviation - unless they have an iris freckle
which type of technique must be done first in the prism cover test and why
cover/uncover test to determine if the px has an eso or an exo deviation and the size
once the cover/uncover test is done to determine the deviation and the size of it, what is done next
place prism of estimated strength before:
- deviated eye in manifest deviation
- either eye is latent
in the prism cover test, which direction must the base of the prism be placed for a hyper deviated eye
base down
in the prism cover test, which direction must the base of the prism be placed for a eso deviated eye
base out
in the prism cover test, which direction must the base of the prism be placed for a exo deviated eye
base in
in the prism cover test, which direction must the apex of the prism be placed
in the direction that the eye is deviated
why is it important for the patients head to be erect during the prism cover test
for a cycle or vertical deviated eye, the px may find that my tilting their head, they can reduce the symptoms/angle of deviation
in the prism cover test, once the estimated prism is placed before the eye, what must be performed next and why
alternating cover test
to see the effect that the prism has on the angle of deviation/estimate the angle
in the prism cover test, once the alternating cover test is performed and the angle of deviation is estimated, what must you do with the prism
- increase prism until no movement is seen
- increase prism until opposite movement is seen
- then reduce prism again until no movement is seen
in the prism cover test, how must you record the results
in prism dioptres, the point at which no movement was detected
eg distance: 10 prism dioptres exo, 6 prism dioptres L/R with rx
near: 6 prism dioptres eso, 2 prism dioptres L/R without rx
(prism dioptres should be the prism triangle sign instead)
during the prism cover test, why is it important to always make sure the eyes are dissociated when doing the alternating cover test and changing the prism on one of the eyes whilst keeping the other eye covered
to not allow them to fuse as any point as it results in underestimating the angle
what is the point in adding a prism to the deviated eye in the prism cover test
so that the object of regard falls on the fovea of both eyes, so when you occlude the other eye, no movement is seen
in XOP, when is reversal seen during the prism cover test
when either eye moves out, to take up fixation (eyes made eso)
what is the reason for carrying out a simultaneous PCT
to measure habitual angle
which types of cases in a simultaneous PCT carried out on
- manifest deviations ONLY
- microtropia (very small deviations, but abnormal retinal correspondence, so they fixate with an extra foveal point, and tend to have a little phoria)
what is the first thing to be performed during the simultaneous PCT
cover/uncover test to estimate angle of deviation and don’t allot for full dissociation
following the cover/uncover test in simultaneous PCT, what is carried out next
place prism of estimated strength before:
- deviated eye in manifest deviation
- use prisms to neutralise deviation
- in the end take prism and cover away together so the eyes do NOT dissociate
what are the advantages of a prism cover test
- quick and easy to perform (on horizontal, vertical deviations)
- accurate measurements, down to 2 prism dioptres
what are the disadvantages of a prism cover test
- depend upon ability to fixate accurately
- can not be used to measure cyclo deviations
- unreliable in the presence of poor visual acuity
- difficult to measure combined horizontal and vertical deviations
what are corneal reflections only used to measure
estimate angle in manifest deviations
when looking at a corneal reflection, what are the looking at the deviation between
centre of the pupil and the corneal reflection
what is angle kappa, in a corneal reflection
the angle formed between the pupillary axis and the visual axis
which direction is a corneal reflection seen in most cases
nasal, (macula sits temporal on the eye)
what is the angle kappa in a normal position (slightly nasal) of a corneal reflection
+ve angle kappa or 3 degrees, fovea lies temporally to posterior pole
what is the angle kappa in a central corneal position of a corneal reflection
angle kappa = 0, fovea coincides with the posterior pole
what is the angle kappa in a corneal reflection which lies temporally
-ve angle kappa, fovea lies nasally to posterior pole
most look like they have a manifest pseudo deviation
in the hirschberg test, what does the patient fixate on and at what distance
pen at 33cm
what is noted during the hirschberg test
corneal reflections noted in fixing eye and compared with other eye
how are the measurements estimated from the hirschberg test
displacement estimated 1mm = 12 degrees = to 20-22 prism dioptre deviation approx
(22 for children as they have steeper corneas & 20 prism dioptres for adults)
in the krimsky test, what does the patient fixate on and at what distance
light at 33cm
in the krimsky test, what is noted when the patient is looking at a light at 33cm
the position of the corneal reflex in FIXING eye
once the corneal reflex position of the fixing eye is noted, during the krimsky test, what is done with the prism
prism strength is increased until corneal reflexes in deviated eye is equal to position in fixing eye initially
in the krimsky test, which eye is the prism put before and why
prism put before the fixing eye so it is easier to see the corneal reflection of the deviating eye clearly
in the krimsky test, in which case will the prism be placed before the deviating eye
on young children called prism reflection test
but it is less accurate as its more difficult to view the corneal reflex
what are the advantages of measuring deviations via corneal reflections
- uncooperative patients eg infants, learning difficulties
- blind or eyes with very poor vision/amblyopic eye (as can only do on manifest deviations)
what are the disadvantages of measuring deviations via corneal reflections
- not suitable for latent deviations
- will not detect microtropias (small angle manifest deviations)
- accommodation not controlled = less accurate
which subjective test is the only reliable subjective test for manifest deviations
synoptophore
list the steps of how the subjective cover test is carried out
- find prism strengths to neutralise movement of images as cover is moved from one eye to the other
(reliance placed upon patients subjective responses) - target - letter on line above that which weakest eye can see
(spot light if less than 6/60) - letter target controls accommodation
what is the advantage of a subjective cover test
useful for small vertical deviations
what are the disadvantages of a subjective cover test
- good subjective observations required from patient (cannot use on children)
- inaccurate in presence of abnormal retinal correspondence
list the steps of how a subjective prism cover test is carried out
- perform cover/uncover test to estimate angle of deviation
- place prism of estimated strength before:
strabismic eye in manifest deviation
either eye if latent - patient notes movement of target
- record prism power and direction that gave no movement
in the subjective prism cover test, what direction does the target move if you need to increase base in prism
same direction as occluder
if the target moves in the same direction of the occluder as the cover was removed in the subjective cover test, what deviation does that patient have
exo deviation
in the subjective prism cover test, what direction does the target move if you need to increase base out prism
opposite direction as occluder
what is the maddox rod
a series of high powered cylinders which blur a spot of light into a streak at 90 degrees to the directions of the grooves
with the maddox rod, which eye views the spot
the eye which does not have the lens of high powered cylinders placed infront of it
what colour is the lens of high powered cyls for the maddox rod
traditionally red (for distance), but sometimes clear or blue
when can you use a green lens of high powered cyls for the maddox rod test
green at near, to control accommodation
what does the displacement of the streak and spot indicate in the maddox rod
the degree of the angle of deviation
what will an orthoptic patient see in the maddox rod test
spot and streak on top
which type of deviation is maddox rod used for and not used for
latent
not for manifest
before which eye is the maddox rod lens placed before
either eye if latent
in which surroundings must the maddox rod test be carried out and why
darkened room as additional light sources produce additional streaks
in the maddox rod test, which eye is the prism placed before
before the eye which does not have the lens of high powered cyls
at which direction should the apex of the prism be faced in the maddox rod test
apex to face the direction we want the spot to move
- increased until sport and image coincident
- prism at this point is equal to the angle of deviation
what is seen by the patient if they have a right eso deviation in the maddox rod test
high powered coyly placed before right eye,
streak of light seen temporally to spot
when will you do a maddox double rod test
if px reports that the line is tilted or if torsion is suspected
state how the maddox double rod test is carried out
two maddox rods are inserted into trial frame, different colours rods can be used
- one before each eye, cylinder axis vertical produces horizontal streaks
- patient or practitioner rotates rods so that they appear straight or parallel
- amount of cyclodeviation is measured on trial frame in degrees e.g. is trial frame axis moved by 10 degrees to make horizontal line, cyclo deviation = 10 degrees
- used for distance and near viewing in primary position
in the maddox rod test, if the lens is before the right eye, when will there be an exo deviation
when px reports streak to be on the left of the spot
in the maddox rod test, if the lens is before the right eye, when will there be a left hyper deviation
when px reports the streak to be above the spot
in the maddox rod test, if the lens is before the right eye, if the patient reports the streak to be below the spot, what deviation to they have
right hyper deviation
in the maddox rod test, if the lens is before the right eye, if the patient reports the streak to be to the right to the spot, what deviation do they have
eso deviation
what are the advantages of the maddox rod
- detects and measured torsion in cyclo deviations
- can be used to detect presence of abnormal retinal correspondence
what are the disadvantages of the maddox rod
- accommodation not controlled (as no accommodative target)
- not accurate in the presence of abnormal retinal correspondence
- not good for manifest deviations
- slight head tilt may simulate vertical deviations
what will a patient report with the maddox rod if their accommodation is not under control
the spot and streak will be aligned, then not aligned, then aligned etc
what is the patient asked during the maddox wing test
what number the white arrow is pointing to and what number the red arrow is pointing to
what may the patient report if an arrow falls between 9 and 11 in the maddox wing
10
what do even numbers suggest with the maddox wing
exo deviation
what numbers do patients report when they have an eso deviation with the maddox wing
odd numbers
what will each eye see when they look through the two separate eye pieces of the mad wing
one eye will see arrow & other eye will see number scale, when eyes are dissociated i.e. they take up the fusion free position
when is torsion indicated with the maddox wing
if the red horizontal arrow is not parallel with the white horizontal grading scale
patient should be asked to adjust it themselves until it is parallel
what are the advantages of the maddox wing
- easy to use
- measures latent and manifest deviations, horizontal, vertical and cyclo deviations
what are the deviations of the maddox wing
- near only
- set distance of 33cm
- septa bend easily not fully preventing peripheral fusion (doesn’t give complete dissociation)
- accommodation effects results, instruct px to keep numbers clear
increases exos
reduces esos - PD fixed
