Clinical Optometric Procedures: Muscle Balance Assessment

Clinical Optometric Procedures: Muscle Balance Assessment

CHAPTER 4 – MUSCLE BALANCE ASSESSMENT

• This chapter will include a review of:
• Fusional vergences
• Vergence facilities
• Stereoacuity

INTRODUCTION

The end point of refraction is to enable the patient to have clear, comfortable binocular vision for all of his visual tasks. To ensure this, the practitioner needs to investigate various visual skills. The patient must be able to align his 2 eyes and maintain alignment for sustained periods of time. In addition, the patient must have sufficient accommodation to enable him to focus on a task and sustain his accommodation comfortably. Muscle balance techniques are therefore performed to ensure that the patient’s accommodation and convergence interact appropriately. The practitioner by conducting the techniques will be able to determine if the patient’s can be corrected with lenses. If there needs to be modifications to the final lens prescription to ensure that the patient reaches the goal of clear, comfortable binocular vision, a more comprehensive binocular work-up or vision therapy is required.

It is important for the practitioner to note that a patient’s prescription will have influenced the status of the patient’s accommodative and vergence systems. The muscle balance techniques are conducted through the patient’s habitual prescription. If the practitioner determines changes, then a new Rx would be considered.

Distance and Near Lateral and Vertical Phorias The assessment of the distance and near lateral and vertical phorias are determine as outlined in section A, Chapter 4, except that this time, these tests are carried out through the patient’s Rx. This gives the practitioner an idea of how he has changed the patient’s muscle balance with the introduction of a prescription. It also assesses the vergence and accommodative system.

FUSIONAL VERGENCES

Fusional vergences are generally performed to determine through the application of prisms, the patient’s ability to use their vergence system to maintain binocularity. In this way, prisms are used to induce retinal disparity. The prism is gradually increased in magnitude, forcing the patient’s vergence system to compensate for the disparity that has been created.

Fusional vergences may be horizontal and vertical. It is more common to find someone conducting a horizontal fusional vergence as opposed to both horizontal and vertical. This is due to the fact that a normal vertical phoria can be up to ½ ∆ and the amplitude of vertical vergences to compensate is about 1∆. It has been recommended that free space methods (using a prism bar) are preferred since they mimic natural viewing conditions as opposed to using the phoropter method. You will learn more about this in the binocular vision module of the course.

Horizontal Smooth Fusional Vergences

When testing horizontal fusional vergences the following aspects are recorded:

1. Blur point: this is the point when the patient can no longer compensate the prism induced retinal disparity while maintaining stable accommodation. In other words, it’s the point at which the accommodative system can no longer assist in holding the eyes together. Only the vergence system is involved in keeping the image single. This point is not usually found in patients at distance divergence since the accommodative effort of the patient is completely relaxed with the refractive correction. If a blur value is obtained, then the practitioner should consider re-refraction.

2. Break point: this is the point at which the patient’s vergence system can no longer compensate the further increase in prism and cannot maintain the target single anymore. Fusion breaks and produces a double image.

3. Recovery point: this is point at which the patient’s vergence system can recover its fusional ability and regain single vision as the induced retinal disparity decreases.

• Equipment
• Phoropter
• A distance VA chart in which one can isolate single letters

Figure 4.1 Setup of Risley prism for positive fusional reserves Figure 4.2 Setup of Risley prism for negative fusional reserves International Centre for Eyecare Education Horizontal Smooth Fusional Vergences at Distance and Near Procedure

1. The patient must be wearing their best corrected distance or near prescription. The distance or near Pd must be set depending on the distance at which the vergences (reserves) are being measured.

2. The target is an isolated letter one line larger than the patient’s best corrected VA in the poorer eye.

3. The Risley prism is set at zero before both eyes.

4. Direct the patient to the target and instruct him to keep both eyes open. The patient should be able to see one clear image. If the patient sees 2 targets (i.e. reports diplopia), either BI or BO prism must be added before the patient to achieve fusion and this point becomes the starting point of the test (Scheiman and Wick, 2008). Carlson and Kurtz (2004) on the other hand suggest that diplopia must be recorded if the patient reports seeing two targets and the test should be abandoned.

5. The patient is instructed to look at the target and to keep it clear and single. The patient is then asked to report if the target blurs (blur point), becomes double (break point) and when the target is single once again (recovery point).

6. BI or positive fusional reserves (Fig. 4.1) are always measured before BO or negative fusional reserves (Fig.

4.2) since BO testing affects accommodation and convergence which may affect the results obtained during the determination of BI reserves.

7. Prism is gradually increased from a zero point either on the phoropter or a prism bar until the patient reports the first perceptible blur. Prism is added at a rate of 2 per second. The patient should be encouraged to determine the point of sustained blur (i.e. when the patient can no longer clear the target at the point of blur).

This point is recorded as the blur point. Make a mental note of the amount of prism at this point. If no blur point is reported, then an “x” is recorded.

8. The prism is then further increased until the patient reports that the target first becomes double. This point is taken as the break point. Increase the prism (overshoot) until a point of sustained double is determined.

9. After the double point has been determined, the practitioner must slowly reduce the amount of prism until the patient reports that the two images have become single again. This is the recovery point. Make a mental note of the prism at this point.

10. The values that are noted are taken from both eyes. For example if you have 2 in front of one eye and 3 before the other, then the total value is added together making a total of 5 for either the blur, break or

• recovery.
• Recording the fusional reserves
• The test distance must be recorded, i.e. Distance or Near.
• Each result should have 3 values, viz. a blur / break / recovery
• If no blur value is obtained, then an “x” is recorded
• Negative fusional reserves or BO reserves: X / 14 / 10
• Positive fusional reserves or BI reserves: 12 / 18 / 10
• e.g. Distance: BI x / 10 / 4 Near: BI 13 / 18 / 8

Table 4.1 Expected findings for smooth vergence testing Adapted from Morgan’s table of expected findings in an adult, clinical population (Scheiman and Wick, 2008) Distance Value Std deviation

BO Blur 9 ±4

Break 19 ±8

Recovery 10 ±4

• BI Break 7 ±3

Recovery 4 ±2

• Near

BO Blur 17 ±5

Break 21 ±6

Recovery 11 ±7

• BI Blur 13 ±4

Break 21 ±4

Recovery 13 ±5

Interpretation of findings

The values obtained for fusional reserves must always be compared to the normal range of fusional reserves.

The determination of fusional reserves is conducted to determine if a patient has the ability to overcome or compensate for his/her phoria.

A patient with an exophoria obviously displays a tendency for the eyes to turn outward and therefore one would have to consider the BO reserves or positive fusional reserves to determine if the eyes have the capability to exert enough effort to overcome the tendency for the eyes to deviate outward. The determination of the required amount of reserves for a patient to have comfortable single clear binocular vision (compensated) is given by Sheard’s and Percival’s rules.

Sheard’s criterion states that: the compensating vergence should be at least twice the heterophoria amplitude.

• This means a patient with a 5
• BI (exophoria) at near, will be compensated and comfortable at near if his positive
• (BO) fusional reserve is at least 10

or greater.

Vertical Fusional Vergences

Blur points are not reached when measuring vertical fusional reserves. One will be able to measure vertical fusional reserves by placing BU prism before one eye and BD prism before the other. It is also possible to determine the reserve by placing either a BU or BD prism before one eye since both eyes work relative to each other, i.e. a supravergence in front of one eye is equal to an infravergence in the other eye.

• Figure 4.3 Setup of Risley prisms for BD or supravergences

Procedure

1. The patient is directed to an appropriate letter and asked to report the break and recovery points are recorded with changes in prism.

• 2. Prism is changed before the eye at a rate of 1

before the eye.

3. If BU is placed before the RE, then it indicates the practitioner is determining right infravergence. The practitioner must note the amount of prism before the right eye when the patient detects break.

• 4. Once the break is determined, the practitioner may overshoot the value by about 2-3
• and then reduce the

prism until the patient observes the recovery point at which the target is single again.

5. These steps are repeated with BD prism over the RE, indicating right supravergence (Fig. 4.3).

Recording the fusional reserves

• Recording of vertical fusional reserves are similar to that of horizontal. Only 2 values will be recorded,
• namely, break and recovery
• e.g. Distance vertical vergences: OD infra 4 / 2, supra 2 / 1

Interpretation of findings

While the reserves may be compared to population norms (Appendices), it should be compared to the specific vertical phoria that the patient may present with.

Note:

Vertical fusional vergences are not commonly performed by practitioners since the vertical amplitudes (reserves) are usually between 1-2

• . The eye can only tolerate a vertical deviation of ½. However, if the phoria shows up

as being greater than 2

vertically, then it is worth investigating the vertical fusional vergences to determine International Centre for Eyecare Education compensation of the phoria.

• Expected findings for smooth vergence testing

(Carlson and Kurtz, 2004)

Break: 3

± 4 Recovery: 1.5 ± 2

VERGENCE FACILITY

The purpose of vergence facility testing is to evaluate the performance of the disparity of the vergence system within a specified time period to determine the vergence system’s speed and resistance to fatigue. It is a test that can be performed at distance and/or near.

Procedure:

1. Direct the patient to a distance target that is large enough. This may comprise a vertical column of 20/30 letters or better. In some cases however, the entire chart may be used as a fixation target. If performing the test at near, a vertical column of letters or block of letters may be an ideal target.

• 2. The patient views the target through the habitual Rx under normal room illumination.
• 3. BO prisms are used in powers of 4
• and 6 to 8 and 12.

4. The prisms may be introduced in flippers or loosely held by each hand.

5. Prism is introduced with BI first followed by BO prism, while continuing this cycle of introduction for about 4 cycles. Each cycle is a direct change from BI prism to BO prism without a delay between the presentation of either type of prism.

6. The test may be performed objectively by the practitioner observing the patient’s fusional response with the introduction of the prism or subjectively when the patient reports double vision upon introduction of the prism and then single vision when there is adaptation to the prism before the eye.

7. The practitioner must record the amount of time for each cycle or the total time take for 4 cycles.

• Expected values for jump vergences
• According to Griffin in Binocular anomalies, diagnosis and vision therapy (2002):

Average for 16

jump vergences is 3-5 seconds per jump or 6-10 seconds per cycle. These expected values may differ with the age of the patient. It should be noted that the larger the prism range chosen, the slower the expected cycle time.

• Research by Gall et al (1998), 15 cycles per minute is the preferred rate using 3
• BI /12 BO for near testing.

Less than or equal to 12 cycles per minute are indicative of binocular problems.

STEREOACUITY

Stereoscopic vision is the individual’s ability to appreciate relative depth (nearness/farness). Stereoscopic vision results from the retinal disparity that exists between integration of monocular images. The stereoacuity test measures the smallest amount of horizontal retinal image disparity between each eye that gives rise to the sensation of relative depth and is measured in seconds of arc.

Normal stereopsis is required for certain vocations such as airline pilots, police, and stereophotogrammers.

Stereopsis measurements may thus be helpful in determining future vocation options.

Stereopsis measurement is important in the diagnosis of variable binocular anomalies. Patients with certain forms of strabismus (e.g. constant) or refractive anomalies (e.g. anisometropia) may not be able to appreciate stereopsis.

Stereoacuity measurements may also be important for predicting the prognosis for successful treatment of a specific binocular anomaly. The prognosis for the improvement of binocular anomalies is better when the stereopsis is appreciated (i.e. the better the stereoacuity, the better the prognosis).

Finally, stereopsis measurements may be useful in the detection of certain neurological conditions, particularly right posterior cerebral lesions.

Procedure

Many stereoscopic tests are available for clinical purposes. The most common clinical method uses linear polarized tests such as the Titmus or Randot tests. The tests may differ in their properties and some may test contour stereopsis while others test global stereopsis.

Contour or local stereopsis is produced by two similar targets that are laterally displaced (Fly, Wirt circles, animals). These have the disadvantage of offering monocular cues. A patient with a constant strabismus may appreciate up to 70 sec. of arc.

Global stereopsis is produced by random dot targets that have no monocular cues (RDE, randot targets). A constant strabismic could not pass a 660 sec. target. Global stereopsis is generally preferred for clinical use.

• Method
• 1. The test distance is 40cm.
• 2. Best Rx must be worn. For presbyopic patients, a near Rx must be worn.
• 3. No rotation is allowed or monocular cues will be given.

4. The monocular cues are tested first (letters R & L) 5. The circles are then tested and if impossible the larger forms (lower disparity) are tried 6. Guessing must be encouraged

Recording

• Stereo scores are noted from the scores provided on the test charts employed and noted in sec. of arc (e.g.
• 30”)
• Expected value
• 20 sec. arc
• Scoring Charts for the Titmus & Randot Stereo Clinical Tests

Table 4.2 Stereo test (Titmus and Randot)

Test Correct Answer Stereo Angle

• at 40cm.
• Shepard

Percentage Verhoff Distance

• A Cat 400 sec. 15% .1
• B Rabbit 200 sec. 30% .2
• C Monkey 100 sec. 50% .3
• Test TITMUS Stereo Angle
• at 40cm. RANDOT Stereo Angle at 40cm.
• 1 Bottom 800 sec L 400 sec
• 2 Left 400 sec R 200 sec
• 3 Bottom 200 sec L 140 sec
• 4 Top 140 sec M 100 sec
• 5 Top 100 sec R 70 sec
• 6 Left 80 sec M 50 sec
• 7 Right 60 sec L 40 sec
• 8 Left 50 sec R 30 sec
• 9 Right 40 sec M 25 sec
• 10 R 20 sec

Figure 4.4 Randot figuresInternational Centre for Eyecare Education