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
