Measurement of Ocular Deviation Flashcards
why are ocular deviations measured?
- diagnosis
- find maximum deviation
- consistency between examiners
- monitor progression
- to relate to the fusion range
- to calcular AC/A
what distance should ocular deviations be measured?
should be measured at 6m.
1/3rd of a metre for near.
and >6m for distance exo.
is Rx correction used?
deviations measured with and without correction.
with correction:
- myopic and exo = smaller deviation
- hyperopic and eso = smaller deviation
what are the objective tests for deviations?
- using total dissociation
i. e. the prism cover test - using corneal reflections
i.e Hirshberg
Krimsky
Prism reflection test
what are the subjective tests for deviations?
- subjective prism cover test
- maddox rod
- maddox wing
why can objective techniques not be relied upon?
the total dissocation techniques do not allow the practitioner to distinguish between a manifest and a latent deviation
how is a prims cover test (PCT) carried out?
- perform cover/uncover test
- place prism of estimated strength in front of deviated eye if manifest or any eye if latent deviation
- put base down in front of hyper-eye
- put base out for eso’s
- put base in for exo’s
- make sure px’s head is erect
- select a target - DV snellen chart and NV budgie stick letter
- perform alternating cover test and estimate angle
- increase the prism until no movement is seen
- increase prism until the opposite movement is seen
- reduce the prism again until no movement is seen
- record results in prism dioptres - where no movement was detected
why do you need to make sure patients head is erect?
because those with vertical deviations can reduce the angle of deviation by tilting their heads and reduce the symptoms.
this means you cannot find the full angle of deviation if head is tilted.
what deviations can the prism cover test measure?
what deviations can the prism cover test not measure?
can be used to measure latent, manifest, vertical and horizontal deviations.
the test cannot measure cyclo-deviations.
what do you require to do a prism cover test?
requires co-operation from px
accommodative target required - as accommodation can affect the size of deviation and therefore need to control it.
target needed - a letter on the line above that which thet weakest eye can see. and a spot light is used if less than 6/60.
what is the prism cover test / simultaneous cover test? and why is it used?
the test is used to measure the habitual angle and only works for manifest deviations and when measuring microtropia
- perform the cover/uncover test to estimate the angle of deviation (because we dont want to allow full dissociation)
- place the prism of estimated strength in front of deviated eye in manifest deviation
- use prisms to neutralise the deviation
what is a microtropia?
microtropia is where you have a very small deviation and have ARC (abnormal retinal correspondence) - where you fixate with an extra foveal point - and they often tend to have a little phoria which you can see with a cover test.
what are the advantages and disadvantages of prism cover test?
advantages:
- quick and easy to perform
- accurate measurement, down to 2prism D.
disadvantages:
- dependent upon ability to fixate accurately (children or if poor vision in one eye)
- cannot be used to measure cyclodeviations
- unreliable in the presence of poor visual acuity
- difficult to measure combined horizontal and vertical deviations
what do corneal reflections do?
they estimate the angle in manifest deviations only
what is a normal coreal reflection?
normally our corneal reflections are displaced slightly nasally because our macula lies slightly temporal to our visual axis
what is the angle kappa?
angle kappa is the angle formed between the pupillary axis and the visual axis.
what does a large angle kappa indicate?
large angle kappa gives the appearance of a psuedostrabismus
when is angle kappa:
positive?
zero?
negative?
angle kappa:
- positive when fovea is temporal and corneal reflection nasal to the posterior pole
- zero when the fovea coincids with the posterior pole
- negative when the fovea is nasal and corneal reflection temporal to the posterior pole
how do you do the Hirschberg test?
- px fixates on pen torch at 33cm
- corneal reflections noted in fixing eye and compared with the other eye
- displacement estimated
1mm displacement = 12degrees (20-22prism D) deviation approx.
when can the Hirschberg test be done?
sometimes its the only thing that can be done with difficult incooperative individuals i.e. children, or those with poor visual acuity in the deviated eye
how do you do the Krimsky test?
- px fixes on a light at 33cm and position of fixing eye is noted
- prism strength is increased until the corneal reflexes in the deviated eye are equal to the position in the fixing eye initially
- can be used with prism before deviating eye (prism reflection test) i.e. in children as otherwise they wont let you put a prism in front of their fixing eye
using prism bars to make the corneal reflections more symmetrical
what are the advantages and disadvantages of using corneal reflections?
advantages:
- uncooperative px’s e.g. children, learning difficulties etc.
- easy to do
- px blind or eyes with very poor vision - meaning its the only thing that can be done
disadvantages:
- not suitable for latent deviations and can only be done for manifest deviations
- will not detect microtropia
- acommodation is not controlled as using a spot torch
what are you trying to find in a subjective cover test?
what target is used and why?
trying to find the prism strength that neutralised the movement of images as the cover is moved from one eye to the other. - increase the prism until no movement seen
- if exo = target being looked at moves in the same direction as cover
- if eso = if target is moving in the opposite direction to the cover
the target is the letter on the line above that the weakest eye can see - and spot light is < 6/60. a letter is used as it controls accommodation.
what are the advatages and disadvantages of a subjective cover test?
advantages:
- useful for small vertical deviations
disadvantages:
- good subjective observations required from patiet
- inaccurate in presence of ARC
- cant use on young children
how do you do the subjective cover test?
- perform the cover/uncover test to estimate the angle of deviation
- place prism of estimate strength in front of strabismic eye if manifest deviation or either eye if latent deviation
- px has to note movement of target
if the target moves:
- in the same direction as occluder - increase base-in prism
- in the opposite direction as occluder - increase base-out prism
- record the prism and power which gives no movement
what is the maddox rod test?
- series of high powered cylinders which blur a spot of light into a streak at 90degrees to the direction of striations
- other eye views spot light
- normally red - but can be clear or blue
- maddox rod lenses are added to the trail frame
how do you do the maddox rod test?
- carry it out in a darkened room as you dont want any additional light sources to blur the streak or produce extra streaks
- place the maddox rod in either eye for latent - not used for manifest deviations
- prism is placed in front of the other eye apex in the direction you want the spot to move:
- prism increased until spot and image/streak are coindident
- prism at this point is equal to the angle of deviation
what is the maddox double rod test?
it is a subjective test when you are looking at cyclodeviations.
used when the px reports that the line is tilted or if torsion is suspected
how do you do the maddox double rod test?
- two maddox rods are inserted into the trial frame and different colours can be used ideally
- one colour before each eye
- cylinder axis vertical produces horizontal streaks
- px or practitioner rotates the rods so that they appear straight or parallel
- amount of cyclodeviation is measured on trial frame in degrees
- used for distance and near viewing in primary position
what are the advantages and disadvantages of maddox rod?
advantages:
- detects and measured torsion in cyclodeviations
- can be used to detect presence of ARC
disadvantages:
- accommodation is not controlled
- not accurate in the presence of ARC
- not good for manifest deviations
- slight head tilt may stimulate vertical deviations or cyclodeviation
how do you do the maddox wing test?
in this test, one eye will see the arrow and one eye will see the number scale. the number tells you the size of the deviation and the odd/even tells you whether its eso/exo.
- px is asked what number the white arrow is pointing to and what number the red arrow is pointing to
- have to be careful as if the arrow falls between numbers i.e. between 9 and 11, then px may report 10
therefore: - even numbers = exo-deviations
- odd numbers = eso-deviations
- if the red horizontal arrow is not parallel with the white horizontal grading scale - torsion is indicated
- the px is asked to adjust it themselves until it is parallel
- if the red line is tilted upwards = incyclo torsion
what are the advantages and disadvantages of maddox wing?
advantages:
- easy to use
- measures latent and manifest deviations, horizontal, vertical and cyclodeviations
disadvantages:
- can be done for near only
- set distance of 33cm
- septa bend easily not fully preventing peripheral fusion
- accomodation effects the results - instruct the patient to keep the numbers clear: accommodation increases exos and reduces esos
- PD is fixed
