Ocular motility refers to the movements of the eye in all directions of gaze and the relationship between the two eyes. Strabismus is a misalignment of the eyes, often caused by muscular imbalance. Orthoptics is a paramedical specialty that investigates the motor and sensory adaptations to strabismus. It deals with nonsurgical treatments to help patients with strabismus regain the ability to use both eyes together and obtain comfortable, binocular single vision. Orthoptics originates from the Greek words orthos (meaning “straight”) and optikos (meaning “pertaining to sight”). Orthoptists work closely with ophthalmologists in treating adults and children with eye movement and binocular vision disorders. Orthoptists tailor their approach to the examination and treatment of patients based on their age. For instance, children under 9 years of age have neural pathways that are more easily modified than teenagers. Older patients are more mature and may be easier to work with, but their established visual patterns are much more difficult to disrupt.
In the investigation of strabismus, the orthoptist is required to assess the vision or fixation of each eye, the alignment of both eyes in all directions of gaze, and the ability of the two eyes to work together. Orthoptic therapy is directed towards the elimination of suppression and amblyopia and the correction of anomalies in binocular vision. Therapy can include prescription glasses, optical correction with prisms, eye exercises, and patching. Because the brain controls visual processing and ocular muscle coordination, orthoptic therapy involves a process of mental retraining. For example, the eye’s visual capability is not strengthened by amblyopia therapy. Instead, the brain becomes readapted so that it can accept, receive, and process visual imagery received by the eye. In some cases, in addition to orthoptic therapies, ophthalmologists may prescribe medications that modify the focusing power of the eye or perform surgery for the correction of strabismus.
The work of the orthoptist is an adjunct to that of the ophthalmologist, not a substitute for it. Ultimate responsibility for the treatment of patients with strabismus rests with the ophthalmologist. It is important for an ophthalmic assistant to understand the orthoptic techniques used by orthoptists and ophthalmologists. This chapter will outline some of these tests and will introduce assistants to the more commonly used orthoptic instruments and methods.
Evaluation of strabismus
When taking a history for a patient with strabismus, the following points should be documented:
Age of onset and time course of progression
Which eye is affected, or whether either eye alternately deviates
Whether the misalignment is intermittent or constant
Whether the misalignment is more apparent with near work or when looking in the distance
Precipitating causes before the onset of strabismus, such as illness or trauma
Previous family history of strabismus
Previous therapy for treatment of strabismus
Other associated neurologic symptoms, such as headaches
Birth history, such as birth trauma or prematurity
General health and developmental history
The orderly improvement of visual acuity is greatest in the first few months of life. Normal adult acuity (6/6 or 20/20) is typically achieved by 5 years of age. The type of visual acuity testing should be chosen based on patient age and understanding. Most adults and school-age children can be tested with the conventional Snellen chart or the Early Treatment Diabetic Retinopathy Study (ETDRS) chart. The HOTV chart, LEA symbols chart, Sheridan-Gardiner test, tumbling E chart or Landolt’s broken-ring chart can be used in younger children or in patients where language may be a barrier. Picture charts, such as ones with Allen figures, are not ideal as they overestimate vision because of the lack of optotype standardization. Vision tests based on preferential looking techniques, like Teller or Cardiff acuity cards, are helpful in assessing vision in preverbal children. If a patient is too young for formal vision testing, the child’s fixation behavior can be classified using the central, steady, and maintained (CSM) designation. In this classification, “central” indicates that the corneal light reflex is centered on the pupil when a target is viewed monocularly and “steady” means that monocular fixation does not wander as it might in a patient with nystagmus. The “maintained” designation denotes that binocular fixation is sustained by each eye through a blink or through an induced smooth pursuit movement. This method for testing a young child can be accomplished by holding an interesting toy as a fixation target.
Extraocular muscle weakness causes defects in the movement of one or both eyes as they track in the different directions of gaze. The positions of gaze that are tested include the horizontal and vertical meridians and the four oblique positions. Versions are binocular eye movements in which both eyes move in the same direction. They include dextroversion (right gaze), levoversion (left gaze), supraversion (upgaze), and infraversion (downgaze). Vergence movements are binocular movements in which both eyes move in opposite directions to attain and maintain fusion. Convergence, an inward movement of each eye, and divergence, an outward movement of each eye, are required when changing fixation distances. In addition to clinical examination techniques, electrooculography is an electrophysiologic test that assesses the function of the eye muscles in normal and pathologic states. It can detect various types of nystagmus and can determine velocities of eye movements.
The Hirschberg test can determine the presence and magnitude of manifest strabismus, otherwise known as a heterotropia. The examiner directs a penlight towards a child’s eyes and notes the position of the light reflex, which normally falls centrally on the cornea or slightly nasal from the center of the pupil in each eye. If this reflex is temporally displaced in one eye and is normal in the other eye, the child has an esotropia and one eye is deviated inwards ( Fig. 29.1 ). Each millimeter of deviation from the normal position of the light reflex represents 7 degrees or 15 prism diopters, so a light reflex temporally displaced 3 mm would equate to an esotropia of approximately 20 degrees or 45 prism diopters. The Hirschberg test is not sensitive enough for the detection of small-angled strabismus, but it is the easiest method of assessing the presence of strabismus in an infant.
The Krimsky test also measures the angle of strabismus and is more accurate than the Hirschberg test. This test is useful in the assessment of infants or those with poor vision in one or both eyes, which limits fixation on a target. A prism placed in front of the fixating eye optically shifts the visual target and the fixating eye subsequently moves to pick up fixation. This draws the fellow deviated eye into a straighter position so that its corneal reflex is centered. The strength of the prism required to center the corneal reflection in the strabismic eye is equal to the amount of deviation present.
A heterotropia, or tropia, is a constant manifest ocular deviation. Heterotropias must be differentiated from heterophorias, or phorias. A heterophoria is a latent ocular deviation kept in check by the power of fusion and made intermittent by disrupting fusion. Heterophorias are classified in a fashion similar to heterotropias. Esophoria is the tendency of the eyes to turn in, exophoria is the tendency of the eyes to turn out, hyperphoria is the tendency of one eye to turn up and hypophoria is the tendency of one eye to turn downwards. Any of these conditions may occur normally when fusion is disrupted.
The cover–uncover test is a reliable and easy method to detect and measure the angle of a heterotropia. This test is conventionally performed in the primary straight-ahead position, with and without glasses, with a viewing target situated at both distance and near. Subsequently, each of the eight other diagnostic positions of gaze are tested. To ensure fixation in young children, a flashing picture or a video displayed in the distance helps to capture their attention. A near fixation object should be an interesting and detailed article, such as a brightly-colored toy or a toy with a squeaker. Once the examiner is certain the child is looking at the fixation object, an occluder is used to cover one eye and the clinician looks for movement of the fellow uncovered eye. One of three situations may occur:
A manifest strabismus is revealed if the uncovered eye moves to take up fixation when the cover is placed over the fellow eye. If a child has manifest strabismus and the sound eye is occluded, the deviating eye may move horizontally or vertically. An outward movement of a nasally deviated eye is noted in patients with esotropia, whereas an inward movement of the laterally deviated eye is noted in those with exotropia. Likewise, a downward movement of the superiorly deviated eye can be observed in patients with hypertropia, whereas an upward movement of the inferiorly deviated eye is noted in those with hypotropia.
The uncovered fellow eye may wander, indicating that the fixation of the eye is defective or absent. This may occur in patients with significant amblyopia or blindness.
There may be no movement of the uncovered fellow eye, indicating that the eye is straight.
The alternating cover test, used to detect heterophorias, is performed by alternately covering each eye without allowing the patient to regain binocularity. The examiner looks for movement of the eye at the moment it is being uncovered. The occluder disrupts fusion and any latent tendency of the eye to drift is revealed if the eye deviates under the cover. The alternating cover test can also be used to measure the magnitude of strabismus. This is done by placing a prism over the strabismic eye with the apex of the prism oriented in the direction of the eye turn ( Fig. 29.2 ). The prism displaces the image of the fixation target onto the fovea of the deviated eye so that during the alternating cover test, there is no movement of either eye to take up fixation. Prisms of gradually increasing magnitude are introduced before one eye, as the other is occluded. The patient must maintain fixation on an accommodative target, such as a letter or number for an older child or an interesting toy or picture for a young child. For patients with limited cooperation, the examiner must exhibit ingenuity and patience to attract and maintain the patients’ attention on a fixation target.
On occasion, a child is referred who appears to have an ocular deviation but has no detectable strabismus on further testing. This condition is called pseudostrabismus. Pseudostrabismus can be differentiated from true strabismus by means of the cover test. With pseudostrabismus, neither eye moves to pick up fixation with alternate occlusion because the eyes are straight. In most instances, the appearance of strabismus is caused by the presence of prominent epicanthal folds that extend from the upper lid to cover the inner canthal region. The child’s eyes appear to be turned in because only a small amount of the sclera is visible medially when compared to the larger amount visible laterally. This false impression of an inward eye deviation (pseudoesotropia) is augmented when the child looks to either side, as the medial aspect of the adducting eye is further hidden by the epicanthal fold. Once pseudoesotropia has been detected, parents can be reassured that the appearance of the eye misalignment will improve over time as the child’s face grows.
The Maddox rod is comprised of a series of red cylinders that distort a point of light into a fine red band, thereby changing the size, shape, and color of the point of light perceived by the eye. When a patient views the fixation light, one eye sees the light and the other eye, over which the Maddox rod is placed, sees a fine red line. The eyes assume a fusion-free position because these disparate images cannot be combined. The direction of the red line is perpendicular to the direction of the Maddox rod cylinders. If the Maddox rod is held so that the red cylinders are running horizontally before one eye, the vertical red line will appear either through the point of light (orthophoria) or to either side of the light (esophoria or exophoria). If the Maddox rod is held vertically, the red line will appear as a horizontal band either through the light (orthophoria), above it (hypophoria), or below it (hyperphoria). Measurement of a phoria’s magnitude is determined by the amount of prism required to displace the red line so that the patient sees it running through the point of light. The Maddox rod can be used to measure the magnitude of vertical or horizontal phorias at either distance or near, although it is not as accurate as the alternating cover test.
Hess screen test, Lancaster red-green test, and Lees screen test
The Hess and Lees screen tests and Lancaster red-green test are similar in purpose but differ in the type of screen used and the method of charting. They are useful in the detection of paretic strabismus and are based on the fact that foveae of straight eyes project to the same point in space. In patients with strabismus, the foveae do not project to the same point in space and the measurement of this difference is a measure of the deviation.
The Hess screen is a three-foot square grid with small red lights at points in the grid such that the lights make up two squares, one inside the other. Each red light is individually controlled by the examiner and the patient, wearing red-and-green glasses, holds a flashlight that projects a green light. The patient fixates on the examiner’s red light with the eye viewing through the red lens and projects the green flashlight in the direction towards which the eye under the green lens is pointing. The patient then tries to place the green light over each red light. If the eyes are not straight, the displacement of the green light in relation to the red light is a measure of the deviation ( Fig. 29.3 ). The Lancaster red-green test is similar to the Hess screen test, only the examiner uses a flashlight to project a linear red light at various points in the grid. The patient, wearing red-and-green glasses, tries to superimpose his or her linear green light overtop of the examiner’s red light. The Lancaster red-green test can detect, but not measure, torsional differences between the eyes.
The Lees screen test is plotted in the same way as the Hess screen test, although it is not as dissociating because the patient does not wear red-and-green glasses. The patient sits in front of two screens that are at right angles to one another. The eyes are dissociated via a two-sided mirror with an attached chin rest bisecting the junction of the two screens. The patient views the images from each fovea as though they are straight ahead. The patient is shown targets on one screen and asked to point with his or her light to localize the points on the second screen while looking into the mirror. The displacements of the patient’s perceived points from their true locations are proportionate to degree of muscle imbalance.
Sensory adaptation to strabismus
Patients with strabismus typically experience diplopia, or double vision, because their foveae are not directed to the same position in space. Because young children demonstrate neural plasticity, those with strabismus develop one of two sensory adaptations to overcome their double vision and the confusion of competing foveal images. Sensory adaptations to strabismus include suppression or anomalous retinal correspondence. In a child with strabismus, the unconscious habit of continuous suppression eventually leads to strabismic amblyopia and the loss of binocular visual function. In the past, antisuppression devices were used to make the patient aware of diplopia to assist the patient in overcoming suppression. Some examples of these devices, which are no longer in use include the stereoscope, the Tibbs binocular trainer, the amblyoscope, red-and-green glasses, and the reading bar.
Normally, the foveae of the two eyes point in the same visual direction. This is known as normal retinal correspondence (NRC). Anomalous, or abnormal, retinal correspondence (ARC) is a faulty sensory adaptation to a strabismus in which the fovea of one eye projects to the same point in space as an extrafoveal point on the retina of the other deviating eye. When anomalous retinal correspondence develops, it typically occurs in patients with long-standing monocular strabismus. ARC is a sensory adjustment on the part of the strabismic eye. The fovea of the misaligned eye is suppressed to avoid the confusion of disparate foveal images and diplopia. Instead, a nonfoveal point on the retina assumes the function of the fovea to match the projection of the fovea of the other eye. In a sense, the patient develops a gross form of binocular vision. Three tests used to identify the presence of ARC include the Worth 4-dot test, the Bagolini striated glasses test, and the afterimage test.
Worth 4-dot test
The Worth 4-dot test detects the presence of fusion, diplopia, or suppression of one eye. It consists of four lights in a diamond formation, housed in a flashlight for near testing or on a screen or panel for distance testing. The two lateral lights are green, the upper light is red, and the lower light is white. The patient wears red-and-green glasses and is asked to report the number and color of the visible lights. When one eye is dominant, the white light is perceived as red or green, depending on the color of the lens over the dominant eye. Patients with fusion and single binocular vision see four lights: the red light above, the two green lights on either side, and the white light as either a combination of red and green or alternating between red and green. Patients with diplopia see five lights comprised of three green and two red lights. If a patient is suppressing one eye, only the colored lights observed by the other eye will be seen: either two red or three green lights. Patients may have one response when tested at distance and an entirely different response when tested at near.
Bagolini striated glass test
The Bagolini striated glass test is the least dissociative of the three tests for ARC. In this test, two lenses with striations at 45 and 135 degrees are placed in front of each eye. When the patient looks at a point of light, he or she perceives a line perpendicular to the striations of the lens. A cover test is then performed by the examiner. Normal retinal correspondence exists if the patient has no manifest deviation on cover test and sees a perfect “X” with a light at the point where the two lines intersect. Anomalous retinal correspondence exists if the patient has a manifest deviation on cover test and sees a perfect “X”. An absent line indicates suppression of one eye while a break in one line indicates central, or foveal, suppression. Patients with diplopia and normal retinal correspondence see two separate streaks that form more of a “V” or an inverted “V” ( Fig. 29.4 ).