6. Measuring ocular deviations Flashcards
Give 3 reasons why you should measure the deviation
To diagnose
Decide when to manage
Monitor progression
How should the measurements be taken?
At distance, near and greater than 6m if deviation increases
With and without rx
With and without CHP if they have one
What are the objective methods of measuring deviation?
Using total dissociation= objective prism cover test (PCT)
Using corneal reflections: hirschberg and krimsky
When would you use objective methods?
Unreliable patient, very young, had a stroke
What are the subjective methods of measuring deviation?
Subjective prism cover test
Maddox rod
Maddox wing
Which method is the best and why?
Prism cover test as it reveals the maximum angle of the deviation i.e shows the px at their worst
PCT measures total deviation? What does this mean?
It measures tropia and phoria added up
How reliable is PCT?
Only as reliable as the clinician and required px co-operation
What target do you use for PCT?
Accommodative target so line above worst VA
What target should you use if px’s VA is worse than 6/60?
spotlight
What do you do first in PCT?
a cover/uncover test and alternating cover test to know what type of deviation it is
What eye do you place the estimated prism in front of if phoria? Tropia?
If tropia then the eye with the tropia
If phoria then either eye
What prism base do you select to correct?
Base opposite deviation
RSOT?
Base Out RE
LXOT?
Base in LE
Right hypertropia?
Base down RE
XOP
BI either eye
How do you know if patient has crossed or uncrossed diplopia when they have a tropia, explain this for RSOT?
If they have RSOT then the fixation target stimulates the nasal retina of the RE and fovea of the LE
Nasal retina projects temporally
So px has uncrossed diplopia
Describe how you would use prism cover test to measure/correct a RSOT
To alleviate diplopia, base opposite direction of deviation so base out in front of RE
Which way does the eye move with a prism infornt?
Eye moves towards apex, image moves towards base
Describe PCT routine
Head erect, measure VA, line above worst VA then discover what type of deviation it is
Perform alternating cover test slowly
Put prism of estimated strength in front of they eye with tropia or either eye if its a phoria
Increase prism till you see opposite movement, always look at the ye behind the prim, so if eso then continue increasing till exo is seen.
What do you record?
Record prism dioptres the one before reversal
How do you record results?
Near with rx. 10 prism dioptres BI
What are the advantages of PCT?
Accurate measurement down to 2 prism dioptres
Measures full angle
What are the disadvantages of PCT?
Dependant on px fixating properly, unreliable with poor VA, can’t be used in young children, can’t measure cyclodeviations
What is simultaneous prism cover test used for? (how does it differ from PCT)
Measures manifest deviation only
How do you do a simultaneous prism cover test?
- determine type of deviation
- place estimated prism strength infront of deviated eye whilst doing alternating cover test
- Keep increasing prism until reversal seen
- Record one before reversal
How can you estimate the angle of a deviation?
Corneal reflections
What are 3 ways of ESTIMATING the angle of deviation using corneal reflections?
- Bruckner
- Hirschberg
- Krimsky
Describe the Hirschberg test
Patient fixates of pentorch at 33cm
Compare corneal reflections in both eyes
1mm corneal reflection displacement equals to how much prism of deviation?
1mm corneal reflection displacement= 20-22 prism dioptres deviation approximately
Describe the krimsky test
Patient fixates of pentorch at 33cm
Compare corneal reflections in both eyes
Estimated prism strength placed infront of FIXING eye, base opposite deviation
Increase prism until corneal reflections are equal in both eye
What are the advantages in using corneal reflections?
Good for uncooperative patients, infants, learning difficulties,
Good if blind or one eye poor VA
What are the disadvantages in using corneal reflections?
Can not measure latent deviations (phorias)
Will not detect microtropias
Accommodation not controlled
What is different in subjective PCT?
Same as objective PCT but ask patient if they notice image moving from
- side to side or
- up to down
Keep increase prism until movement of images neutralised
When is subjective PCT useful?
In small symptom producing vertical deviations
What is maddox rod?
Place striated red filter (can be clear or blue) in front of either eye if latent (not normally used with manifest)
Other eye views spot light in dark room and prism placed infront of it, apex of prism in the direction you want the spot to move
Increase prism until spot and image is coincident
Why is it done in dark room?
As other light sources produce extra streaks
When spot and image is coincident what does this mean?
Prism at this point is equal to the angle of deviation
What are the advantages of maddox rod?
Easy to use
Detects and measure cyclodeviations
What are the disadvantages of maddox rod?
Needs good VA, accommodation not controlled, can’t have suppression or ARC, slight head tilt may simulate vertical deviation
