Phorias And Tropias Flashcards
Define heterotropia (strabismus).
The foveal line of sight fails to intersect the object of fixation
Associated conditions
Define heterophoria.
The relative position of the lines of sight of light under dissociated viewing conditions
Dissociated (inadequate stimulus to fusion)
Define meter angle.
Reciprocal of the target in meters from center of rotation (13mm behind the cornea)
What is the equation for vergence in prism diopters?
Vergence= MA x IPD = 100tan(vergence in degrees)
Define tonic vergence.
Position of the eyes under stimulus free conditions (totally dark)
0.50 to 1 MA
Define accommodative convergence (AC). What is the normal range of AC/A?
Stimulated when the eyes converge
1D accommodation = 4PD of accommdation convergence to give AC/A of 4PD/D
Normal= between 2 and 6PD/D
What is proximal vergence?
Caused by awareness of nearness of an object
Define disparity or fusional vergence.
Vergence to place the 2 retinal images (OD and OS) onto corresponding retinal points
Corresponding points have the same visual direction
What is the result of images falling on non corresponding points? How can this be eliminated?
Diplopia
Disparity (fusional) vergence movement
What criteria can we use to classify an oculomotor imbalance?
Tropia or phoria
Eso, exo, R hyper, L hyper, hypo
Distance, near, both
Which eye (for tropia)
Constant or intermittent (for tropia)
Unilateral or alternating (for tropia)
Compensated or uncompensated (phoria)
What are the symptoms of oculomotor balance?
None
Headache
Tired eyes
Intermittent blurred vision
Diplopia
Dizziness
Head tilt
Stereopsis problems
General irritation
What are the causes of ocular-motor imbalance?
Refractive (uncorrected refractive error)
Anatomical (abnormal orbit)
Myogenic (muscle)
Neurological
Pathology
How can we treat heterophoria/heterotropia?
Change working distance (viewing distance, duration)
Prisms
Refractive correction
VT
Surgery
Other medical treatment to correct underlying pathology/cause
Occlude on eye (only one to definetley work)
What objective tests can we use to identify a heterophoria/heterotropia? Which will distinguish between phoria and tropia?
Unilateral CT (only to distinguish between phoria and tropia)
Alternating CT
Hirschberg Test
What subjective tests can we use to diagnose a heterophoria/heterotropia?
Subjective CT
Maddox Rod
Von Graefe
Modified thorington
Howell Test
Maddox wing
Etc
Describe a unilateral, alternating and subjective cover test.
Unilateral: only tests to distinguish between phoria and tropia
Alternating: used to measure size of deviation
Subjective: used when no other movement was seen with unilateral or alternating
How do we perform unilateral cover test for tropia vs phoria?
For tropia: watch movement of uncovered eye as cover is introduced
For phoria: watch for movement of covered eye as cover is removed
When do we use hypotropia?
Unilateral, downward deviating tropics
Describe a unilateral vs alternating tropia.
Unilateral same eye is deviated
Alternating sometime the right is deviated sometimes the left is deviated
Distinguish by whether the uncovered eye turns out when you remove the cover (doesn’t move= alternating)
Describe an alternating L HT.
In both cases the left visual axis is always higher than the right
ow can we cause a larger deviation in alternating cover test?
Prolonged dissociation allows a larger deviation to be measured
Occlude should be left for 3-5 seconds
Observe unoccluded eye
How do prisms neutralize a deviation?
Do not move eyes
Relocate the image onto corresponding retinal points so the patient no longer has to make a disparity vergence eye movement to compensate for the deviation
Eliminate need for patient to compensate for their phoria
Is there a deviated eye under associated conditions?
No the eyes appear straight
The eye that is covered turns out
What is the correcting prism for:
ET or EP
XT or XP
R HT or R HP
L HP or L HT
Base out
Base in
Base down OD and/or base up OS
Base down OS and/or Base up OD
What deviation are indicated by the subjective cover test?
Exo- with
Eso- against
Right hyper- downwards (cover moved right to left), upwards (cover moves left to right)
Left hyper- upwards (cover moved right to left), downwards (cover moved left to right)
What is the Hirschberg test?
Determine position of visual axis by comparing corneal reflex (1st purkinje image)
Penlight at 40cm
Compare location of reflex
What prism diopter value can be given to each 1mm of relative reflex displacement?
1mm of relative reflex displacement is equivalent to approximately 22 prism diopters (11 degrees)
What is angle kappa?
The angle formed at the intersection of the pupillary axis and the visual axis at the nodal point of the eye.
May cause the corneal reflex to not be images in the center of the pupil even if no tropia is present
What is the pupillary axis?
Imaginary line normal to the anterior surface of the cornea, passing through the center of the entrance pupil, real pupil and the center of curvature of the cornea
What is the visual axis?
Imaginary line from point of fixation through nodal point to the fovea
Where is the location of the corneal reflex?
Where the visual axis intersects the cornea
May not be centered even without tropia
What should we do if a deviation is observed binocularly?
Test should be repeated monocularly to establish angle kappa and to identify deviated eye
Why does having a phoria matter?
Under associated condition patient must exert disparity vergence to place images onto corresponding retinal points
Compensate for the phoria may cause symptoms
How does vergence change in a myope taking off their glasses to read?
Become more exo
Always check near phoria before telling a patient they can read unaided
What deviation is experienced with the onset of presbyopia?
More exo at near due to loss of accommodative vergence
