NON-COMITANT STRABISMUS

I. Introduction: Non-comitancy is a significant challenge to an optometrist. It is a difficult task to determine an accurate diagnosis of the EOM (s) affected. In addition, it is imperative for an optometrist to rule out an acute neurological dysfunction. Such cases should be sent out for a neurological work up. This lab is designed to provide you with some insight to the diagnostic tests used to quantify non-comitant strabismus.

II. A brief review of EOM diagnostic action fields: Two laws are indispensable when attempting to evaluate EOM function.

-Hering's Law of Equal Innervation: Contralateral synergist (ie. yoked) muscles are equally innervated when a conjugate eye movement is attempted. As you remember from Ocular Motility, Hering's Law most likely applies to vergence eye movements as well; but a different set of yoked muscles are used.

-Sherrington's Law: Contraction of a particular EOM is simultaneously accompanied by an equal relaxation of the ipsilateral antagonist EOM.

A. It is important to know the two dominant muscles (one for each eye) acting for each diagnostic gaze position. Please note that the diagnostic action field of an EOM is the viewing direction in which a particular EOM predominates WITH THE OTHER EOM'S ACTIVE

1. There are eight such fields to consider (in addition to primary gaze). See Griffin (Table 1A and B; pg 4 and Table 1F)for a review of these.

2. Remember that a paralyzed, paretic or obstructed muscle will have difficulty moving into it's diagnostic action field

3. Remember that, if a paretic eye is forced to take up fixation, it will result in a overaction of the contralateral synergist muscle (Hering's Law).

4. Remember that, if a paretic eye is left alone without proper therapy, it will result in a spasticity of the ipsilateral antagonist muscle (Sherrington's Law). This will eventually result in contracture of the antagonist muscle.

B. It is important to know the most likely compensation for a "damaged EOM" (ie. paralyzed, paretic or obstructed) is a corresponding head turn (torticollis).

1. For each EOM that is damaged, there is a specific compensatory head movement to reduce the deviation.

2. Just remember that the compensating head turn or tilt must be in the direction of the damaged EOM

a. A right SO palsy cannot look down and to the left...where would you point your head under these circumstances?

b. The right SO also intorts the eye (ie. rotation to the left)...where would you tilt your head under these circumstances?

C. You can obtain valuable information by carefully observing your patient from the moment they are greeted. Compensatory head turns or tilts may be noticed. You can begin your diagnostic quest, then, as you first see the patient .

III. Objective test methods for diagnosing a "damaged EOM": Most of these tests are familiar to you. When dealing with a non-comitant strabismus, however, there are new twists to the old test.

A. Cover test in the nine principle gaze positions: This test has been discussed in the last lab. Both unilateral (to detect the strabismus) and bilateral (to measure the magnitude of the strabismus) cover tests should be completed.

1. The different gaze positions can be tested by moving a fixation target or moving the patient's head (relying on the vestibular ocular reflex).

a. It is preferred to move the target.

b. It is also preferred to complete the test at a consistent testing distance.

c. This is done to avoid the potential interference of accommodation in measuring for non-comitancy.

2. A difference in the magnitude of the strabismus of 5 Æ or more is indicative of non-comitancy.

3. If a non-comitancy exists, there will be a significant (5 Æ or more) difference between the primary angle and the secondary angle of deviation.

a. The primary angle of deviation is measured when the non-suspicious eye is fixating the target and the suspect eye is behind the occluder.

i. place neutralizing prisms over the covered, suspect eye

ii. uncover that eye to see if there is any motion

iii. if not, that is the primary angle

b. The secondary angle of deviation is measured when the suspect eye is fixating the target and the non-suspicious eye is behind the occluder.

i. place neutralizing prisms over the covered, non-suspicious eye

ii. uncover that eye to see if there is any motion (you may need to cover the dominant eye to force the "damaged" eye to take up fixation)

iii. if not, that is the secondary angle

B. Duction and version testing is also important when evaluating a non-comitant strabismus.

1. The single (duction) and double (version) H test is a very quick, yet somewhat crude way to test isolated actions of specific EOM's.

2. The particular muscles' action that is measured at each position of the "H" test should be reviewed.

a. The obliques tend to be the primary elevator and depressor muscles when the eye is adducted 51 degrees.

b. The vertical recti tend to be the primary elevator and depressor muscles when the eye is abducted 23 degrees.

c. The horizontal recti tend to be the primary adductors and abductors in most gaze positions.

3. In general, things to look for when diagnosing a non-comitant angle:

a. Restriction of gaze

b. Overaction of a particular muscle (version testing)

c. Presence of nystagmus

C. If an ACUTE vertical strabismus is present, a clever objective method to isolate the EOM involved is the Three Step Method. It is nothing more than a very Sherlock Holmesian deductive approach to finding which of the eight vertical EOM's are involved.

1. Check which eye is hypertropic in primary gaze. This usually limits the possible candidates to four.

a. If the right eye is hyper: either the 2 muscles that depress O.D.or the 2 muscles that elevate O.S. (ie. RIR, RSO, LSR, LIO)

b. If the left eye is hyper: either the 2 muscles that depress O.S. or the 2 muscles that elevate O.D. (ie. LIR, LSO, RSR, RIO)

2. Check what lateral gaze position (L or R) does the deviation increase. This usually reduces the possible candidates to two.

a. It is necessary to decide which two of the four remaining muscles have their diagnostic action field of action in that direction.

b. If the right eye is hyper:

i. if the right hyper is worse in right gaze; either the muscle that depresses the right eye on abducting OR the muscle that elevates the left eye on adduction is the culprit (ie. RIR or LIO)

ii. if the right hyper is worse in left gaze; either the muscle that depresses the right eye on adducting OR the muscle that elevates the left eye on abduction is the culprit (ie. RSO or LSR)

c. If the left eye is hyper:

i. if the left hyper is worse in right gaze; either the muscle that depresses the left eye on adducting OR the muscle that elevates the right eye on abduction is the culprit (ie. LSO or RSR)

ii. if the left hyper is worse in left gaze; either the muscle that depresses the left eye on abducting OR the muscle that elevates the right eye on adduction is the culprit (ie. LIR or RIO)

3. Check what effect a head tilt (L or R) does the vertical deviation increase. This usually limits the candidate list to one (...elementary, Watson!)

a. This is the most difficult step to determining what muscle is damaged.

b. It is necessary to remember that with a right head tilt, O.D. must intort and O.S. must extort. The opposite is true for a left head tilt.

c. It is also pretty dang important to remember that two EOMs serve to cyclorotate the eye (when it is in primary position) for each direction head tilt.

i. the SR and SO serve to intort the eye

ii. the IR and IO serve to extort the eye

4. If one of the obliques is the culprit:

i. if it is the SO, the only muscle left to intort the eye is the SR

- the SR is much better at elevation of the eye, hence the hypertropia of the affected eye will increase

ii. if it is the IO, the only muscle left to extort the eye is the IR

- the IR is much better at depression of the eye, hence the hypotropia of the affected eye will increase

5. If one of the recti muscles is the culprit:

i. if it is the SR, the only muscle left to intort the eye is the SO

- the SO has a secondary movement from primary gaze,depression. Hence, without the elevation characteristics of the SR , hypotropia of the affected eye will increase

ii. if it is the IR, the only muscle left to extort the eye is the IO

- the IO has a secondary movement from primary gaze, elevation. Hence, without the depression characteristics of the IR , hypertropia of the affected eye will increase

IV. Subjective methods for measuring comitancy: A very sensitive method for detecting ACUTE non-comitant deviations is the Hess-Lancaster Screen. It has been previously described in class. This method measures the subjective angle of deviation. It will also detect a non-comitant strabismus. As a rule, always have the patient wear the red lens over the right eye

A. The RIGHT EYE FIELD is measured when the patient controls the red light.

B. The LEFT EYE FIELD is measured when the patient controls the green light.

C. The patient and the doctor each control either the green spot (which is seen by the eye covered with the green lens) or the red annulus (which is seen by the eye covered with the red lens) . For this test, have the pt. wear the red lens over the right eye ONLY.

1. The patient superimposes his target onto the target held by the doctor in all nine gaze positions (marked on the screen).

2. The RIGHT EYE FIELD is measured when the patient controls the red light.

3. The LEFT EYE FIELD is measured when the patient controls the green light.

4. When evaluating the RIGHT EYE FIELD, you are measuring the subjective angle of deviation with:

a. the left eye fixating the various positions of the green light (controlled by the doctor).

b. the right eye deviating and attempting to superimpose the red light (controlled by the patient) onto the green light.

5. When measuring the LEFT EYE FIELD, you are measuring the subjective angle of deviation with:

a. the right eye fixating the various positions of the red light (controlled by the doctor).

b. the left eye deviating and attempting to superimpose the green light (controlled by the patient) onto the red light.

6. Any significant changes in the subjective angle at the different gaze positions can be considered non-comitant. The pattern of the plot will isolate the suspect muscle.

7. To make a diagnosis from this plot, it is required to know that:

a. The paretic muscle, when asked to move into it's diagnostic action field and SUPERIMPOSE onto the fixation target (which is seen by the non-paretic eye) will UNDERSHOOT

b. The paretic muscle, when asked to FIXATE the target controlled by the doctor will have to work extra hard. This will result in...

c. The contralateral synergist (yoked) muscle, will OVERSHOOT (following Hering's Law).

8. One of the problems with conducting this test with a chronic or congenital non- comitant deviation:

a. Secondary contracture of the ipsilateral (homolateral) antagonist of the damaged muscle

i. the apparent over-action of the ipsilateral antagonist may beome the most salient feature of the test results!

b. On the contralateral side, the yoked (contralateral synergist) muscle will over shoot. Over time, there may be a secondary under-action of the ipsilateral antagonist of the yoked muscle as well

9. Therefore, a total of FOUR muscles may well be involved in an "older" non-comitant deviation:

UNDER-ACTING MUSCLES OVER-ACTING MUSCLES

1. The paretic muscle 1. The ipsilateral antagonist of the paretic muscle

2. The ipsilateral antagonist to 2. The yoked muscle