Hess/Lees charts Flashcards
What is the principle of how Hess and Lees charts work?
for investigation of strabismus - the eyes are dissociated and the position of the non-fixing eye is plotted as the fixing eye adopts different positions of gaze determined by the examier
How is Hess testing performed?
dissociation is achieved by colour filters
* red (appearance) filter before fixing eye
* green (appearance) filter before non-fixing eye
* examiner presents red or white target
* subject perceives with fixing eye (only) target and grid
* subject perceives their green target with non-fixing eye (only)
* position of non-fixing eye is revealed by projection of green target and plotted
* procedure repeated, reversing laterality for other eye
What visual angle is subtended by the horizontal and vertical lines of the grid in Hess testing?
5 degrees
What is the size of each square in the Hess chart?
5 degrees
What area size is represented by the inner field of the Hess chart?
15 degrees
What area is represented by the outer field of the Hess chart grid?
30 degrees
What is the patient asked to do in the Hess chart testing?
asked to superimpose their green light on the examiner’s red light
Can the Hess chart differentiate between manifest and latent deviations?
no
What is seen by each eye in Hess chart testing?
eye under red goggle sees red light, eye under green goggle sees green light
What is the fixing eye and what is the indicating eye when the examiner shines a red light in Hess chart testing?
the eye under the red goggle is the fixing eye, the other eye will be the indicating eye from which deviation is measured
What is directly represented by what is mapped on a Hess chart?
the position of the eye
What is the rule for retinal correspondence for Hess testing and why?
patient MUST have normal retinal correspondence (NRC) - if have ARC, measurement of deviation will be incorrect
What are 3 contraindications to Hess testing?
- abnormal retinal correspondence (ARC)
- suppression
- red-green colour blindness
How far should the patient sit from the chart in Hess testing?
50cm
What are the 4 conditions for assessing gaze in Hess testing?
- fixing R and L eye
- in all positions of gaze
- for inner (15 degree) field
- for outer (30 degree) field - if required
Which order should Hess chart testing be performed in?
- assess left eye first - fix right eye (red filter on R eye + red light)
Which goggle should be used for fixing?
where possible should always use red light for fixing - i.e. examiner moves red light and patient moves green light over it
Which goggle is over which eye when assessing L vs R eye?
L eye: red filter over R eye, green filter over L eye
R eye: red filter over L eye, green filter over R eye
When is it useful to use the outer 30 degrees of gaze with Hess chart testing?
for small/subtle under/overactions
What principle does Hess testing rely on that means that the mapped gaze corresponds to the position of the eye?
foveal projection
How is Lees testing performed?
- dissociation achieved by mirror between subject’s eyes
- mirror bisects two facing Lees screens 90 degrees apart
- operator presents target to fixing eye
- subject’s non-fixing eye position is revealed by their projected perception of a corresponding point in the opposite Lees screen
What are 3 advantages of Lees testing over Hess testing?
- constant illumination L vs R
- greater contrast
- easier test for usbject to understand and perform
What does the smaller field of movement for Hess and Lees charts usually indicate?
smaller field of movement indicates affected eye
What does inward displacement indicate in Hess and Lees testing?
underaction
