19 - evaluating the deviation

what is strabismus a break down in?

1 or more normal BV components

what are the anomalies of strabismus divided into?

1. sensory anomalies

2. motor anomalies

or BOTH

what is heterophoria?

latent ocular deviation

what kind of viewing conditions do you measure heterophoria in?

under dissociated viewing conditions → fusion disrupted

what are the techniques to measure heterophoria?

1. ACT

2. von graefe

3. maddox rod

4. modified thorington

what is a fixation disparity?

small ocular misalignment where foveal images fall within Panum's area → thus no diplopia

what kind of viewing conditions do you measure fixation disparity?

under fused or associated viewing conditions

what is fixation disparity typically?

what is the limit of fixation disparity to be within panum's area?

what are the techniques to measure fixation disparity?

1. borish card

2. saladin card

3. wesson card

what type of ocular deviation is strabismus?

manifest ocular deviation

what viewing conditions does strabismus present in?

under fused or associated viewing conditions

what are the techniques to measure strabismus?

1. UCT

2. hirschberg

what are the risk factors for strabismus?

1. prematurity/low birth weight

2. maternal smoking during pregnancy

3. hyperopia → esotropia

4. anisometropia and astigmatism → exotropia

5. family Hx

6. cerebral palsy + down syndrome

what anatomical abnormalities cause strabismus?

1. EOM

2. abnormal content of orbits

3. abnormal ligaments, tendons (brown's, grave's, etc.)

what optical abnormalities cause strabismus?

1. anisometropia

2. high RE

what age is more likely to develop ET?

young children

what age are more likely to develop XT?

older children and adults

what are the innervational (muscle tonus) causes of strabismus?

intoxication, medications, trauma, disease

congenital, high/low AC/A

what are the characteristics of the angle of deviation?

1. direction

2. frequency

3. magnitude

4. laterality

5. AC/A ratio

6. cosmesis

7. comitancy

how to describe direction of angle of deviation?

eso

exo

vertical

cyclotorsional

combination

how to describe frequency of angle of deviation?

heterophoric

intermittent

constant

what is a micro angle of deviation?

0-5pd

what is a small angle of deviation?

6-20pd

what is a moderate angle of deviation?

21-40pd

what is a large angle of deviation?

>40pd

how to describe laterality of angle of deviation?

left or right

alternating

how much strabismus is not cosmetically noticeable?

how much strabismus may be cosmetically noticeable?

10-20pd

how much strabismus is cosmetically noticeable?

>20pd

what factors give an esotropic appearance?

negative angle kappa

wide bridge

+ epicanthus

small IPD

what factors give an exotropic appearance?

positive angle kappa

narrow bridge

- epicanthus

large IPD

what is comitant?

magnitude is the same in all fields of gaze

what is noncomitant/incomitant?

magnitude is not the same in all fields of gaze

what should you directly observe in strabismic patients?

lids

head turn/tilt/tip

how to do hirschberg test?

pt (binocular) fixates transilluminator at 50 cm

use position of corneal reflexes relative to center of pupil

compare to angle kappa

who is hirschberg good to use on?

young patients → minimal cooperation necessary

what conditions is angle kappa done in?

monocular

what is the most common angle kappa?

(+) 0.5 mm nasal

what are the disadvantages of angle kappa?

1. poor sensitivity to EF measurement

2. may simulate, conceal or exaggerate strabismus

how much is 1 mm equivalent to in hirschberg? what is +/-?

1 mm = 22pd

+ = nasal

- = temporal

what does each displacement indicate in hirschberg?

temporal → ET

nasal → XT

inferior → hyper

superior → hypo

how to do krimsky test?

place prism over fixating eye

change prism power until reflexes are symmetrical

who is krimsky test good to use on?

young pts

pts with very reduced vision in one eye

how to do bruckner test?

patient fixates ophthalmoscope light at 1 m

observe reflexes

what does the brighter light indicate in bruckner test?

deviating eye

why? - brighter reflection b/c less pigmented portion of retina

when do you get better results in bruckner test?

pre-dilation

what is important about the target during CT?

make sure pt keeps target clear → accommodative target

in CT, which eye is the fixating eye?

eye NOT behind prism

how to adjust target for amblyopic patients during CT?

adjust target to within threshold

how to do UCT?

instruct patient to keep target clear

observe the uncovered eye

allow patient to re-fixate

how to do simultaneous prism CT?

simultaneously place prism over the deviating eye as you cover the fixating eye

add prism to neutralize deviating eye until there is no movement → reversal

what is the advantage of simultaneous prism CT?

identifying small angle strabismus

when do you stop increasing prism on ACT?

first reversal → record high neutral reading

why should you repeat ACT with strabismic eye fixating?

record primary and secondary angle of deviation

what is the primary angle of deviation?

amount of deviation when the normal eye is fixating in primary gaze → prism over paretic eye

what is the secondary angle of deviation?

amount of deviation when the paretic eye is fixating in primary gaze → prism over non-paretic eye

Why might the secondary angle of deviation be larger in magnitude than the primary angle?

secondary angle requires the paretic eye to fixate → Herring's law → extra innervation to fixate → higher deviation

what amount of deviation can most examiners reliably detect?

~2 pd

how to do ACT on a nystagmus patient?

frosted (opaque) occluder

high plus lens

how to do ACT on a rebound saccade?

repeat ACT several times

rebound saccade - eye overshoots multiple time and saccades back and forth to fixation

what may impact the measured magnitude of ACT?

1. eccentric fixation

2. horizontal and vertical components of strab

3. fixation distance (near and far)

4. high power and anisometropic lenses

5. examiner accuracy

what are the signs in true angle of deviation equation?

nasal EF and eso → +

temporal EF and exo → -

what is the equation for true angle of deviation?

how does power of stacked prism lenses compare to addition of power of lenses?

power of stacked prisms is higher than addition of powers of lenses → add 10 pd to 10 pd would have power of 21 pd

what happens to error as power of stacked prism lenses increases?

error increases

what to do if large amounts of loose prism?

place prism in front of each eye

is stacking horizontal and vertical prism okay?

yes

how does a short fixation distance affect the magnitude?

decreases XT

increases ET → esp if high AC/A ratio

how does the power of lenses affect the direction of deviation?

plus → decreases measured

minus → increases measured

how does prism position affect magnitude?

as prism rotates → power changes

what is the proper way to hold prism?

back surface in frontal place position perpendicular to fixation object → may need to rotate inward for near CT

what is a primary vertical deviation?

present when eyes are straight

what are secondary vertical deviations?

present only when eyes are deviated

who are secondary vertical deviations common in?

XT

what kind of testing can help differentiate between primary and secondary vertical deviations?

associated phoria testing → Borish, Wesson, Saladin

what is the AC/A ratio equation?

IPD (cm) + fixation distance (m) (

eso/BO = +

exo/BI = -

what is the AC/A ratio if distance = near?

IPD

what is the AC/A ratio if high eso at near?

AC/A > IPD

what is the AC/A ratio if high exo at near?

AC/A < IPD