Chapter VIIITHE MONOCULAR DUCTION MUSCLE TEST
Whilethe previously described binocular muscle test made with the phorometer and Maddox rod, only determines the existence and amount of esophoria, exophoria, and hyperphoria, neither the faulty nor the deviating muscle is located, hence amonocular muscle testis essential in order to determine whether the muscles of the right or left eye are faulty. Furthermore, an imbalance may possibly be due to either a faulty muscular poise, or lack of nerve force in one or both eyes. A “duction test” should accordingly be made of each muscle of each eye separately, followed by a comparison of the muscular pull of both eyes collectively.
These tests are commonly termed adduction, abduction, superduction and subduction, and are defined in the order named. They include tests of the vertical and horizontal muscles of each eye, made individually by means of the rotary prisms, each being placed before the eye undergoing the test.
The phorometer and the Maddox rod should be removed from operative position, discontinuing the use of the muscle-testing spot-light, employed in the previously described binocular test. The optical correction, if one is required, should be left in place, while the patient’s attention should be directed, with both eyes open, to the largest letter on the distant test chart; or if preferable, the Greek cross in the Woolf ophthalmic, chimney may be used. Either one, however, should be located on a plane with the patient’s head. As a guide for the operator, it might be well to remember that when the handle of the rotary prism is in a horizontal position, the lateral or horizontal muscles are being tested. On the other hand, when the handle is in a vertical position, the vertical muscles are undergoing the test.
Adduction, or relative convergence, is the power of the internal muscles to turn the eyes inward; prism power base out and apex in, is employed.
Fig. 22—To test adduction, base out is required. Rotary prism’s line or indicator should be rotated from zero outwardly.To test abduction, base in is required. Indicator should be rotated inwardly from zero.
Fig. 22—To test adduction, base out is required. Rotary prism’s line or indicator should be rotated from zero outwardly.
To test abduction, base in is required. Indicator should be rotated inwardly from zero.
To test adduction of the patient’s right eye, the rotary prism should be placed in position before the right eye, the red line or prism indicator being registered at zero upon the prism upper scale. The two cyphers (0) should be placed in a vertical position with the handle pointed horizontally (Fig. 21). The rotary prism should then be rotated so that its red line or indicator is rotated outward from zero until the large letter—preferably the largest letter, which is usually “E”—on the distance test-type or the Greek cross previously referred to, first appears to double in the horizontal plane. The reading on the scale of measurements should accordingly be noted. This test should be repeated several times, constantly striving for the highest prism power that the patient will accept without producing diplopia. The prismequivalent thus obtained will indicate the right adduction and should be so recorded, as designated inFig. 24. The amount of adduction ranges from 6 to 28, prism diopters, the normal average being 24.
Abduction is the relative power of the external muscles to turn the eyes outward. Prism power base in and apex out is employed. To determine abduction, or the amount of divergence of the external rectus muscle of the right eye, prism power with base in or toward the nasal side should be employed. The rotary prism will therefore remain in the same relative position as in making the adduction test (Fig. 22), with the two cyphers (0) or zero graduations vertical, but the indicator or red line should be rotatedinwardfrom zero, or towards the patient’s nose.
With the patient’s attention again directed to the large letter “E,” or the Greek cross, this inward rotation should be continued until diplopia or double vision occurs. Like the former, this test should be repeated several times, the refractionist continuing to strive for the highest prism power which the eye will accept. This will indicate abduction of the right eye and should be so recorded as designated inFig. 24. The amount of abduction ranges from 3 to 10 prism diopters. The normal average is 8.
The ratio of adduction to abduction is normally rated at about three to one. In other words, it is conceded that the power of the eye to converge is normally three times as great as its power to diverge, the usual measurements being eight to twenty-four respectively. While applicable in most instances, this may vary in different cases.
Superduction, sometimes termed sursumduction, is the relative power of the superior recti to turn the eyes upward. Prism power base down and apex up is employed. To test superduction, the rotary prism should be placed in position with the two cyphers lying horizontally, with the handle pointed vertically (Fig. 23). The patient’s attention should again be directed to the large letter “E”, and the indicator or red line should be rotated downward from zero. The highest prism power that the patient will accept before the object appears to double in the vertical plane will indicate the degree of right superduction. Thisshould be recorded accordingly. Conditions of this kind do not usually exceed two or three degrees. The test, however, should be repeated several times before the final result is recorded, as indicated inFig. 24. The amount of superduction ranges from 1 to 4 prism diopters. The normal average is 2.
Fig. 23—To test superduction, base down is required. Rotary prism’s line or indicator should be rotated downward from zero.To test subduction, base up is required. Indicator should be rotated upward from zero.
Fig. 23—To test superduction, base down is required. Rotary prism’s line or indicator should be rotated downward from zero.
To test subduction, base up is required. Indicator should be rotated upward from zero.
Subduction, sometimes termed infraduction or deorsumduction, is the relative power of the inferior recti to turn the eyes downward. Prism power base up and apex down is employed. To test subduction, the rotaryprism should be operated with zero graduations placed horizontally, as in the superduction test (Fig. 23), but the indicator should be slowly rotated in the reverse direction, or upward from zero. With the patient’s attention again directed to the large letter “E,” or the Greek cross, the strongest degree prism thus secured without diplopia will indicate the right subduction. The amount of subduction ranges from 1 to 4 prism diopters. The normal average is 2.
Any difference between superduction and subduction, usually denoting the existence of hyperphoria, should be given careful consideration.
As previously explained, after a duction test of each of the four muscles of the right eye, the rotary prism before that eye should be placed out of position and the procedure for adduction, abduction, superduction and subduction repeated by means of the rotary prism before the left eye. In case of an existing imbalance, after testing the muscle of both right and left eyes, the refractionist can quickly determine which muscle or muscles may be lacking in strength (Fig. 24). In practically every instance muscle exercises or correcting prisms maythen be prescribed with definite knowledge of requirements, as further described in the following paragraphs.
A binocular muscle test made with the phorometer, Maddox rod and distant muscle-testing point of light might quickly indicate six degrees of exophoria, both before and after the optical correction is made. While this would doubtless be the correct amount of the manifest imbalance, it would be a difficult matter to ascertain which muscles caused the disturbance. To determine this important question, the monocular or duction test should be invariably employed.
Assuming, for example, a specific case where six degrees of exophoria was determined in the binocular test that the muscle findings in the duction test show right adduction of twenty-four degrees, with an accompanying abduction of eight degrees; likewise a superduction and subduction of two degrees for each eye. With the aid of a chart or diagram—which should be made in every case—a comparison of these figures would indicate an exophoria of approximately six degrees, with a corresponding weak left internus (Fig. 24). This not only shows themuscle pull of each eye individually, but a comparison of the two eyes as indicated by the dotted lines. Thus the relationship of the two eyes, and their corresponding muscles is quickly indicated.
Fig. 24—Duction chart should be made in every case. Above readily shows existence of muscular imbalance and proves subduction and superduction for both eyes are equal; otherwise hyperphoria would be disclosed. Also note abduction for both right and left eye are equal, otherwise esophoria would be disclosed. Also note adduction for right eye is 24° while left is but 18°, proving a case of 6° of exophoria with a left weak internus.
Fig. 24—Duction chart should be made in every case. Above readily shows existence of muscular imbalance and proves subduction and superduction for both eyes are equal; otherwise hyperphoria would be disclosed. Also note abduction for both right and left eye are equal, otherwise esophoria would be disclosed. Also note adduction for right eye is 24° while left is but 18°, proving a case of 6° of exophoria with a left weak internus.
A glance at the above diagram discloses the following three important facts, all of which should be known to the refractionist before a single thought can be devoted to the correcting of the case:
1. 6° exophoria is the amount of the insufficiency.2. 18° adduction (which should be 24°).3. Left weak internus.
1. 6° exophoria is the amount of the insufficiency.
2. 18° adduction (which should be 24°).
3. Left weak internus.
As previously stated, the power to converge is normally rated 3 to 1, or 8 to 24, as shown above, while the power of the eye to look upward, is equal to the power to look downward. The diagram accordingly proves that the muscles of the right eye are in perfect balance, having equal muscular energy.
A comparison of the left eye shows adduction of 18 degrees with an abduction of 8 degrees, proving a lateral insufficiency because the ratio is less than 3 to 1; and the muscles of the left eye are at fault. The power of 2 degrees superduction and 2 degrees subduction, proves that no weakness exists in the vertical muscles.
After making the duction test for each eye individually, a comparison of both eyes in relationship to each other may be more readily determined by following the dotted lines (Fig. 24).
As previously stated, it is the inability of the two eyes to work together that causes the imbalance, so that if both eyes were normal, the adduction, abduction, superduction and subduction of the two eyes would agree.
The duction chart (Fig. 24.) also shows that the corresponding musclesof each eye agree—with the exception of the adduction of the right eye and the left eye. This proves that the left internus is weak, measuring only 18 degrees instead of 24 degrees; it further proves the 6 degrees of exophoria in the monocular test, as was quickly and more readily determined in the binocular test.
Likewise, in cases of esophoria, hyperphoria, or cataphoria, the making of definite muscle measurements independently through the prescribed method would show through the merest glance at a similar diagram which muscle or muscles were relatively out of balance. Heterphoria of almost any type, or tendencies other than normal, may be fully investigated by making a thorough and separate test of each muscle.
Where an imbalance exists, a rapid test may be employed to distinguish a pseudo or false condition from a true condition. This is accomplished by first placing the two Maddox rods (both the red and white) so that the rods lie in a vertical position. If the two lines fuse, we have determined the existence of a false condition caused by a possible error of refraction or nerve strain. If the lines separate, we have determined a true muscular condition, and then only should the second method of muscular treatment follow.
Ski-optometer Model 205Same as Model 215 but Automatic Cylinder Arrangement omitted.Embodying Spherical Lenses Combined with Appliances for Testing and Correcting Muscular Imbalance.
Ski-optometer Model 205Same as Model 215 but Automatic Cylinder Arrangement omitted.Embodying Spherical Lenses Combined with Appliances for Testing and Correcting Muscular Imbalance.
Ski-optometer Model 205
Same as Model 215 but Automatic Cylinder Arrangement omitted.
Embodying Spherical Lenses Combined with Appliances for Testing and Correcting Muscular Imbalance.