THERAPY AND OPTIONS
References: Griffin, Ch. 7; pg. 221-223; Ch 14; pg. 405-436
I. Introduction: Exotropias can be divided into several categories (as discussed previously). They may be intermittent or constant and usually fall into one of three broad areas:
1. Basic exotropia: approximately the same deviation at near and distance
2. Divergence excess exotropia: a significantly larger angle of deviation at distance than at near
3. Convergence insufficiency: a significantly larger angle of deviation at near than at distance
From this limited number of classifications, you may be led to think that these exotropes are a rather homogeneous bunch that can be treated with a handful of pre-packaged therapy protocols. But, that hardly would be fun, now ... would it? In general, an impression that the exotrope is somehow "easy to fix" arises from several generalities about the condition:
1. A later onset than esotropia (some authors disagree).
2. A better ability to achieve fair sensory fusion (even stereopsis).
3. A better ability to use motoric fusion.
4. A covariant ARC (if present): this means that there may be ARC when the patient allows the eye to deviate, but NRC when the pt. achieves motor fusion.
Unfortunately, exotropes are a complicated, heterogeneous bunch that most likely have many different etiologies. Therapy regimens must be specifically designed to for each individual patient . In addition, a therapist must be quite flexible to meet the changing needs of the patient as accommodative and vergence skills are gained. The Duane classification schema listed above is barely adequate for those with an intact binocular system. Exotropes tend to have an inefficient accommodative system and poor fusional skills (DUHHHH!). Often times, a initial diagnosis must be altered as a patient increases accommodative and fusional therapy.
II. Plans For Remediation Of the Basic Exotrope:
A. Many published reports state a higher success rate with constant exotropes than constant esotropes. Intermittent exotropes enjoy the highest percent "cure rate" of any strabismus type (65-85%, depending on the study).
1. Later onset may allow for some normal binocular development to occur.
a. this idea has come under a bit of fire by some investigators who claim exotropias may appear in young children (6 months- 28 months)
b. reports indicate that divergence excess cases, once thought to occur in the school aged child can appear at a much younger age (12 months-18 months)
2. Gradual onset of strabismus may allow for some normal binocular development to occur.
a. most clinicians feel that exotropia develops over a longer time course
b. in most cases, there is some fusional ability at one viewing distance or another
3. Different etiological factors could be in play with exotropias (compared to esotropias).
a. personally, I'd rather see a patient who had a constant exotropia from the age of one than a patient who had a constant esotropia from the age of one!
b. in general, exotropes a much easier group to teach vergence skills
B. Binocular Vision Work-up: It is important to establish a good idea of the strabismic "monster" you're dealing with before initiating therapy.
1. Obtain a detailed analysis of the strabismus.
a. constant vs. intermittent
b. comitant vs. non-comitant
c. does over-minusing the pt. have any beneficial effect on fusional ability
d. does BI prism neutralize the angle completely, or is there prism adaptation (ie. after a period of time, does the angle of deviation increase while wearing the "neutralizing" prism)
2. Asses fusional abilities of the pt. in free space
a. Motor fusion:
i. for an basic exotrope, you may find a point at near where a fusional effort is made as you bring a small target towards the patient's nose
ii. see if the patient maintains fusion as the target is moved towards, then away from him/her
iii. motor fusion may be assisted by the TEMPORARY AND THERAPEUTIC use of a minus over-correction
- with the minus, the small amount of accommodation additional convergence
-pt. may gain fusion with a small "boost" of minus
b. Sensory fusion:
i. illuminate a small penlight at the distance where the pt. makes a motor fusion response
ii. have the pt. view the light through r/g glasses
iii. look for a response of a single yellow/brown light with lustre (indicated bifoveal fixation and NRC
iv. see if the patient maintains lustre as the target is moved towards, then away from him/her
v. see if pt. improves with the application of minus lenses
3. Assess fusional abilities of the pt. in instruments (i.e. synoptophore)
a. Sensory fusion:
i. at the objective angle: determine sensory fusion status with all three types of fusion slides
ii. have the pt. attempt to fuse and repeat measurements around the ortho position
b. Motor fusion:
i. at the objective angle: determine motor fusion status with all three types of fusion slides
ii. have the pt. attempt to fuse and repeat measurements around the ortho position
4. DO NOT forget to analyze sensory fusion with all the tests described in the sensory adaptations to strabismus lectures.
a. ARC may be present when the pt. is strabismic
b. NRC may be present when the pt. is able to fuse the target
c. this covariance of ARC is very handy and because of it, we rarely must go to great lengths to eliminating ARC before we begin the rehabilitation at the binocular pattern stage
5. Obtain detailed information regarding the pts. monocular skills.
a. motiliy skills may be reduced
i. look for pursuit and saccade dysfunction
ii. look for some limitations of pursuit ranges: for an exotrope, adduction deficits may be noted
iii. this is secondary to co-contracture of the ipsilateral (to the non-fixating eye's) lateral rectus
iv. it should be noted that esotropes are far more likely to experience limitation of pursuit ranges (ie. the medial rectus is a stronger muscle)
b. accommodation may be reduced and there may be a difference in skills between the two eyes
6. CYCLOPLEGE ALL EXOTROPES. It will provide valuable refractive information.
a. traditional wisdom is to cut plus from the Rx for an exotrope
b. in some cases, however, PLUS is indicated at near....it may actually help eliminate the exotropia!
7. Asses the prognosis:
C. Passive modes of therapy for a basic exotrope:
1. Minus lens over correction:
a. some optometrists believe that over-minusing a young pt. is one way to establish binocularity (even if it utilizes accom. convergence over fusional convergence). I usually reserve over-minusing for therapy only and not for full time wear especially in the case of a constant exotrope
b. they either base it on the child's calculated or gradient AC/A ratio (if the pt. is able to fuse intermittently) or prescribe 2 diopters of minus over BVA
c. application of an add is considered if the AC/A ratio is high, or if the patient shows any signs of near point stress including:
i. accommodative dysfunction
ii. esophoria @ near
iii. asthenopic complaints
d. the minus should be reduced in half diopter steps every 2 weeks
e. it is somewhat controversial to Rx the minus for full time wear in the basic exotrope, and is used in our clinic as a last resort
f. it is very useful, however, to use over-correction of minus IN OFFICE THERAPY ONLY
i. gets the pt. to "feel" convergence
ii. get "more convergence for the buck"
- a liitle accommodative "boost to convergence" may cause more fusional convergence to take place
- it is crucial in these pts. to attain motor fusion abilities
2. Application of BI prism for full time wear: rarely used in the case of a basic exotropia (especially a constant exotropia)
a. there are problems with prism adaptation
b. the patient will usually have a heavy suppression zone which will not respond to optical correction for the deviation
i. often, these patients need to make fusional movements in order to eliminate their deep and large suppression zone
ii. it may be used, however, in therapy when trying to build sensory fusion
3. Oppenheimer strips (bi-temporal occlusion): these were devised by an actual exotrope (named Oppenheimer, oddly enough) who found that if a thin band was placed 1 mm lateral to the edge of the pupil (in the dark) of each eye UNDER MONOCULAR CONDITIONS, it controlled his deviation better
a. basically the strips served as a biofeedback mechanism to tell him when the eye deviated
b. perception of the black band as the eye moved out is enough, in some cases to elicit a fusional response
D. The Monocular Phase of basic exotropia therapy is quite important and is usually worked 3-4 sessions before the other phases begin. ARC therapy is rarely necessary for exotropes (particularly intermittent XT's), but would be administered concurrent with this phase.
1. Accommodative skills should be equal and sufficient in both eyes.
2. Oculomotility should be equal and sufficient in both eyes.
a. both gross and fine pursuits should be established
i. small pursuits in the opposite direction of the eye turn (adduction for an exotrope) is important
ii. extension of any limitations of pursuits should be worked on
b. both gross and fine saccades should be established
E. Rehabilitation of a Binocular Pattern. At this phase initial free space sensory fusion, anti-suppression, monocular fixation in a binocular field accommodative rock and biocular accommodative rock are included. It is initiated only after sensory fusion has been evaluated.
1. Determine the sensory fusion abilities and work to expand them.
a. establish a pseudo-centration point: a point in free space where the exotrope is able to obtain bifoveal fixation of a small target. Sensory fusion at this point should be measured.
i. this is an objective measure
ii. work both convergence and divergence from this point
iii. this can be done with a light at near point (ie a reduced environment) with the pt. wearing r/g glasses; look for luster responses for indication of sensory fusion without ARC
iv. eventually, you may work with more structured targets, like superimposition of the ring and spot stimuli for the Hess Lancaster screen at the pt's. pseudo centration point
b. the pseudo-centration point can be established with minus lenses during therapy: a general rule of thumb will help you determine the amount of minus required:
i. start with -1.00 lenses
ii. cover/ uncover the non-dominant eye (after occlusion for 2-3 seconds)
iii. look for a fusional recovery; if it is normal, you have enough minus to stimulate convergence during therapy
c. it may be established with BI prism, although this is rarely done
i. prism adaptation
ii. prism rarely initiates learning of fusional convergence skills
2. Anti-suppression techniques are necessary with exotropes, because their suppression zones are usually quite large and deep.
a. free space techniques include Doell's mazes, etc. (for a review of theses techniques, see lab 6)
b. in instrument techniques include the BU series; cards 1-6 and the AN Star series; cards 1-5 (for a review of theses techniques, see lab7)
3. Monocular fixation in a binocular field accommodative rock and biocular accommodative rock are an important component to establishing some binocular cooperation (for a review of theses techniques, see lab 5)
F. The Binocular Phase includes work on the various stages of sensory fusion and motoric fusion abilities. This phase is usually worked on throughout therapy, but is emphasized after success with the rehabilitation of a binocular pattern phase therapy.
1. As you build fusional ranges under these very reduced conditions in the rehabilitation of a binocular pattern phase of therapy, it may be useful to begin binocular phase therapy with in instrument techniques.
a. the synoptophore: remember that the synoptophore is set at optical infinity, and may be difficult for some pt's at first
i. start with moderate sized superimposition targets
ii. have the pt. fuse the targets at the objective angle or fuse targets at ortho, if the pt. has that ability (there should be no movement on bilateral cover test)
iii. slowly decrease the BI demand or increase the BO demand (ie. force the pt. to converge
iv. extend BI and BO ranges
v. go to a smaller superimposition target
vi. try flat fusion and stereo targets
b. stereoscopes (mirror and Brewster):
i. start with superimposition cards of the BU series (for Brewster stereoscopes) or the superimposition paired stereocards (for the mirror stereoscopes)
ii. start at objective angle or have the pt. fuse the targets at ortho (if they have that ability)
iii. extend BI and BO ranges
iv. try flat fusion and stereo targets
2. It is important to eventually work in some free space techniques to the regimen (the artificial visual environment that the instruments create do not establish a strong enough binocular pattern to survive "natural" viewing demands).
a. Brock string: place near ball at the pt's pseudo-centration point. Work convergence (smooth) and divergence from this point
b. vectograms: it is best to start with fixed vectograms and tranaglyphs again at or near the pt's pseudo-centration point
i. the compass points fixed vectogram is a good starting point
ii. eventually, work the vectogram back to a normal (ie. 40-50 cm.) working distance
iii. progress to other vectograms including variable vectograms
3. Build sensory and motor fusion with all three types of fusion targets. Usually for the basic exotrope, progress will be made following the dictates of the British orthoptic model:
ii. flat fusion
4. Increase difficulty level as skills improve.
5. If the angle is quite large ( ie the pt. must compensate for a large exotropia angle), relieving prisms may be indicated as the pt. improves sensory fusional ability.
a. relieving prisms are not neutralizing prisms; their purpose is to allow the patient to use less convergence to fuse the targets
b. prismatic amounts up to 15 Æ (split between the eyes; as long as there is not amblyopia present) may be ground into the lens to help the pt. maintain fusion
c. with additional therapy, it would be hoped that the amount of the prism could be reduced as the pt. gains better control over their vergence system
d. one must watch for prism adaptation, however
6. If the angle remains quite large after long term therapy, in spite of establishing good sensory and motoric fusion ranges with NRC, it may be necessary to refer for surgical reduction of the fusional demand. This is an option to be discussed with the patient and is only viable if the amount of relieving prism needed to help the pt. is above 25Æ. This is rarely necessary for an exotrope, however.
III. Remediation of a Divergence Excess Exotropia: these cases present a unique personality type and a unique method for remediation
A. Binocular vision work-up is usually the same as for a basic XT.
B. Passive modes of therapy (those that do not involve active therapy procedures) for a divergence excess exotrope:
1. Over-correcting minus is once again a controversial point of exotropia therapy for the divergence excess case. I usually reserve over-minusing for therapy only and not for full time wear, but many clinicians use minus religiously. If it is done, however, please monitor the situation (ie. eso @ near) and remember that the pt. must be weaned off the minus, and replace accommodative convergence effort with fusional convergence effort.
a. an add may be required in the divergent excess case to prevent esophoria at near
b. the minus should be reduced in half diopter steps every 2 weeks
2. BI prism is even less useful for a divergence excess case.
a. the pt. is usually a heavy suppressor and has hARC when deviated and NRC when fused.
b. optical correction of the angle does not help establish a binocular pattern; it enhances the suppression and ARC because behind the prism, the exotropic eye is still deviated
3. Oppenheimer strips are sometimes extremely useful and is worth a try in divergent excess cases
C. Monocular and Establishment of a binocular pattern phases are similar to those for a basic exotropia. remember that nearpoint activity will be close to normal: anti-suppression work must be extended to distant targets to be beneficial to these pts.
1. It is important to establish awareness of physiological diplopia with these exotropes. In particular at distance viewing, these pts. need some "hints" that they are fused and not suppressing.
2. Some investigators recommend awareness of pathological diplopia (ie remove suppression even when the eye is deviated so that the pt. will have diplopia if not fused) for cases of intermittent exotropia of the divergence excess type. I'm not brave enough to try this, however!!!
3. Use of r/g T.V. trainers (r/g filters over the T.V.) helps the pt. become aware of suppression at distant targets (ie. part of the screen will turn black!)
D. The Binocular phase of therapy is unique in that it is often better to start therapy with moderate sized stereo targets (ie 3° fusion). To understand this reasoning it may be necessary to look a bit closer at the abilities of the divergent excess case:
1. Ability @ nearpoint: usually the pt. is ortho or exophoric at near. Therefore, they have developed normal binocular skills @ near
2. Difficulty with distance targets (ie as stereo cues are less important for identification and spatial localization of targets): at distance, these exotropes run into more problems.
a. as discussed in class, these pts. have little reason to maintain fusion at distance where the fringe benefits of fusion (ie stereopsis) are less significant
b. the pt. rather have a situation where they have single vision (ie. they suppress the fovea and temporal retina of the deviating eye) and a panoramic view (i.e. nasal retinal of the deviating eye not suppressing)
c. even at distance, however, the pt. has fusional abilities
d. if prompted, most intermittent divergence excess cases can voluntarily fuse
e. if ignored or left untreated, the exotropia will become more constant at distance and much more difficult to remediate successfully (ie. do not wait for an intermittent strabismus become more constant before treating...EVER)
f. cases of intermittent exotropia of the divergence excess type degrading to a constant, basic exotropia have been reported
3. The Brock/Flax method for remediation of a divergence excess type exotropia: basically follows the idea that you should always initiate therapy with something the pt. can complete successfully (ie must give them incentives to maintain fusion)
a. start with medium sized stereo targets with a peripheral fusion lock (ie a circle bordering the target seen by both eyes)
b. work your way towards flat fusion targets * superimposition targets
d. work your way to light target superimposition
e. as skills improve, start working through this same sequence at increasing working distances
i. eventually, project vectograms on an overhead projector
ii. you may also project stereo, flat fusion and superimposition targets at distance with a machine called a Brock Stereo-Motivator (essentially, a overhead projector and a set of glass slides (like synoptophore slides)