Chapter XVMOVEMENTS OF THE EYEBALLSAND THEIR ANOMALIES
Aftera careful study of the foregoing chapters, the refractionist may desirefurtherknowledge concerning muscular imbalance—a matter in which the Ski-optometer plays an exceptionally important part.
It should be remembered that it is only the general utility of the instrument,plusone’s knowledge of refraction and individual diagnosis that enables the refractionist to attain maximum efficiency in every examination, a fact which largely accounts for the following chapter.
When we view an objectdirectly, so that it appears to be more distinct than surrounding objects, we are said to “fix” or “fixate” it.
As the fovea is normally the most sensitive part of the retina, affording by far the most distinct vision, “fixation,” in the great majority of cases, is so performed that the image of the object that is “fixated” falls upon the fovea of the eye that is “fixing.” This is known as central or muscular “fixation.”
When central vision is absent, however, the patient is compelled to seewith a portion of the retinaoutsideof the fovea. The eye must then be so directed as to cause the image of the object to fall on this outlying portion of the retina. This is termed “eccentric fixation,” and usually denotes that vision is exceptionally poor.
The ability to “fix” is apparently acquired in early infancy by constant practice in looking at objects. Any marked interference with vision, particularly with central vision—present at birth or soon thereafter—will tend to prevent the acquisition of this ability, and inextremecases the eye does not learn to “fix” at all, but aimlessly wanders in all directions.
We habitually use the eyes together, fixating with both at once; that is, we direct the eyes in such a way that the image of the object to which the attention is directed falls on the fovea of each eye.
Where both eyes are accurately directed to an object at which one or both are looking, the condition is known as “binocular fixation,” which is commonly understood to mean thatboth eyes are straight.
The ability to produce and maintain binocular fixation—to keep botheyes directly straight—is acquired early in life. The impulse to maintain it grows with exercise, and soon becomes so strong that after the age of infancy binocular fixation is present in the great majority of persons, and in most of them is present all the time.
Binocular fixation must be distinguished through three conditions—orthophoria,heterophoriaandsquint.
This is the condition in which both eyes look straight at the same object, whether both see it or not. There is not the slightest tendency of deviation.
This is the condition in which both eyes keep looking straight at the same object so long as bothseeit; but as soon as one eye is excluded from vision (as by a screen) that eye deviates. This is then a tendency of deviation which is strong enough to become manifest when either eye is covered, but which is abolished or overcome by the compelling impulse of binocular fixation as soon as both eyes are used for seeing. A heterophoria thus produces a maximum deviation. The deviation is also said to belatent, since it is absent underordinary conditions and is brought to light only under special conditions. A common though improper term for heterophoria is “insufficiency.”
Squint is the condition in which there is so great a tendency to deviation that even when both eyes are uncovered, one deviates and only one “fixes.” It differs, therefore, from heterophoria in that the deviation it produces is obvious under ordinary conditions.
Squint is also called strabismus, or heterotropia. In other words, in orthophoria there is binocular fixationall the timeand under all conditions; in heterophoria it is present only when the two eyes are uncovered, so that both see the object looked at; while in squint it is not present at all.
Or, in still plainer terms, in orthophoria both eyes are straight all the time; in heterophoria both are straight, but only so long as both are uncovered; and in squint only one eye is straight, no matter whether both eyes are uncovered or not.
In squint, while binocular fixation is altogether absent, the ability to perform monocular fixation is almost always preserved; i.e., the squinting eye will “fix” at once if the other eye is covered. It isonly when there is marked amblyopia, particularly as the result of a central scotoma (or spot on the cornea in the line of vision) that the squinting eye loses its power to fix at all, and wanders uncertainly about, receiving impressions now on one, now on another portion of the retina.
The term imbalance is often used to denote the two conditions opposed to orthophoria; i.e., to denote collectively heterophoria and squint.
1.Classification According to Direction of Deviating Eye: Heterophoria and squint may be classified according to the direction assumed by the deviating eye. Thus we have the following varieties of heterophoria:
In rare cases of vertical heterophoria, each eye has either an upward tendency (anophoria) or a downward tendency (cataphoria). These cases must not be confused with anatropia and catatropia. In anaphoria andcataphoria, there is binocular fixation when both eyes are uncovered, while in anatropia and catatropiaoneof the eyes squints. This shows the following squint condition:
In addition to these lateral and vertical deviations, conditions exist in which the vertical meridian of one eye, instead of maintaining its parallelism with the vertical meridian of the other, either forms (or tends to form) an angle with it (cyclotropia), but is kept in position through muscular effort (cyclophoria.)
Cyclotropia is usually due to paralysis of one of the ocular muscles, causing the vertical meridian of the affected eye to be tilted out or toward the temple (extorsion) or in toward the nose (intorsion). A tilting of the vertical meridian toward the right is also called dextrotorsion (or positive declination); and to the left, levotorsion or negative declination.
2.Constant, Intermittent and Periodic Deviations: A deviation, whether squint or heterophoria, may be present at all times (constant), or occasionally present and occasionally absent (intermittent). In this case we may have heterophoria alternating with orthophoria, or heterophoria alternating with squint; or squint alternating with orthophoria. We also find variations such as a squint for near and a heterophoria or orthophoria for distance; or a heterophoria for near and orthophoria for distance; or a constant squint for near and an intermittent squint for distance, etc. Again, a deviation may be periodic, in that its amount for distance may greatly exceed that for near, or vice versa.
Opposed to a periodic deviation is one which is present, and in about equal amount, both for distance and near. Such a deviation, whether squint or heterophoria, is called “continuous.”
3.Alternating and Uniocular Squint: An alternating squint is one in which when both eyes are uncovered, so that both have a chance to “fix”; sometimes the right eye will deviate, sometimes the left. In uniocular (less properly monocular) squint, under the same conditions, one eye, either the right or the left, always “fixes” and the other always deviates. A uniocular squint is denoted as right or left, according to whether it is the right or left eye which deviates.
A Typical Refraction Room—The Woolf Sanitary All-Metal EquipmentInstallation comprising: Ophthalmic Chair, complete with Ski-optometer, Test Letter Cabinet, Asceptic Trial-Case Cabinet, Muscle Testing and Skioscopic Lamp, Ophthalmometer, Perimeter, Adjustable Tables, Adjustable Stool.
A Typical Refraction Room—The Woolf Sanitary All-Metal EquipmentInstallation comprising: Ophthalmic Chair, complete with Ski-optometer, Test Letter Cabinet, Asceptic Trial-Case Cabinet, Muscle Testing and Skioscopic Lamp, Ophthalmometer, Perimeter, Adjustable Tables, Adjustable Stool.
A Typical Refraction Room—The Woolf Sanitary All-Metal Equipment
Installation comprising: Ophthalmic Chair, complete with Ski-optometer, Test Letter Cabinet, Asceptic Trial-Case Cabinet, Muscle Testing and Skioscopic Lamp, Ophthalmometer, Perimeter, Adjustable Tables, Adjustable Stool.
4.Comitant and Non-Comitant Deviations: In some varieties of heterophoria and squint, the amount of deviation is the same in all directions of the gaze, so that the angle between the visual line of one eye and that of the other remains the same, no matter which way the eyes are turned. Such deviations are called comitant or non-comitant, because one eye accompanies and keeps pace with the other in all its movements. In other cases, the deviation changes as the eyes are moved in different directions, so that the angle between the two visual lines constantly varies. Such deviations are termed concomitant. Usually in a non-comitant squint the angle of deviation increases in a regular way as the eyes are moved in one direction and decreases as they move in the direction opposite.
In cases of long standing, however, the squinting eye, particularly when very amblyopic, wanders in an uncertain way and apparently quite without reference to the movements of the other eye.