Vision Therapy, Strabismus and Amblyopia

Opthalmic Dispensing II Lecture

Alvaro Cordova

Introduction:

Our ability for stereoscopic vision is essentially the basis for the human species' ability to adapt and innovate. Evolutionarily speaking, primates first developed a more forward facing eye position in response to their habitat. In order for primate survival in a forest or jungle, it was necessary to estimate distances between trees in order to jump while catching prey or to avoid being eaten. Stereopsis, or depth perception, has allowed for better hand eye coordination in humans. Any disruption to this delicate system would hinder greatly the contribution to the tribe in the case of the caveman, and in more modern times, one's quality of life (Campbell, 2000, p. 590; Saladin, 2004, p. 11). Strabismus, when left untreated, can lead to amblyopia and is an example of disruption to stereopsis. According to Grosvenor (1991), “amblyopia is known to accompany anisometropia ... and affects between 2 to 2.5% of the general population.” (p. 177). This would translate to about 5.8 to 7.2 million Americans dealing with amblyopia according to the current United States population. Anisometropic amblyopia and strabismic amblyopia, which will be discussed further in this paper, are conditions that for the most part are preventable and very debilitating to the sufferer. The good news about amblyopia is that it is not impossible to treat after the age of six or seven as was previously believed. A growing amount of evidence suggests that with several types of vision therapy, even adults can show some improvement in their visual acuity. This paper will discuss vision therapy, strabismus and amblyopia as well as its effects on the patient.

Vision and the Brain:

Before there is any discussion about the topics mentioned, it is important to know how the brain works in regards to binocular vision. From birth we are able to see, but our vision is not fully developed. The coordination of eye movement develops around the age of six or seven (Thomas, 1994, p. 8, 9) and fully develops at about the age of nine (Ross, 1997, p. 117). During development, the brain is learning “how to interpret visual messages.” (Thomas, 1994, p. 8). These visual messages are relayed by each eye to the occipital lobe where the visual cortex resides (Chalkley, 2000, pp. 3,6,9; Myers, 2004, p. 180; Saladin, 2004, p. 627). The brain now has to make one coherent image out of the two images that have come from each eye. This process is known as fusion (Caloroso, 1993, p. 5; Cassin, 2001, p. 116; Chalkley, 2000, p. 109; Thomas, 1994, p. 4). Diplopia, or double vision, is a symptom of the brain's inability to process or fuse the two images it is receiving from each eye. The process of fusion, if continuously interrupted, will invariably hinder binocular vision development when left untreated in children (Chalkley, 2000, p. 76; Thomas, 1994, p. 9).

What is Vision Therapy:

Vision therapy as defined by Caloroso is somewhat broad in its scope. She describes vision therapy as “the total treatment program for a strabismic patient, which may include optical or medical treatment options (passive therapy) and orthoptics (active therapy.” This definition involves two distinctions; active and passive therapy. Active therapy is “designed to improve visual performance by involving the patient consciously in a sequence of specific controlled visual tasks.” Passive therapy involves the use of corrective lenses, prisms or “surgical reduction in the strabismic angle.” (Caloroso, 1993, p. 127). Cassin, defines orthoptics as a treatment that deals with the “diagnosis and treatment of defective eye coordination, binocular vision and functional amblyopia by non-medical and non-surgical methods, e.g., glasses, prisms, exercises.” (Cassin, 2001, p. 196). These two definitions have a considerable overlap as well as some differences. Surgery, although not considered by Cassin to be a form of vision therapy, will be mentioned as a treatment for strabismus. The terms orthoptics, vision therapy, and vision training are used interchangeably (Caloroso, 1993, p. 127). As mentioned in the definitions, strabismus and binocular vision are treated or enhanced respectively with some form of vision therapy. In other words, binocular vision is the goal of vision therapy when it treats some form of strabismus or visual binocular problem. This paper will use Caloroso's definition for vision therapy since the active vs. passive distinction is a useful subdivision.

Extraocular Muscles and Nerves Associated with Strabismus:

In regards to strabismus it is important to understand the functions of the extra ocular muscles and the corresponding cranial nerves that innervate them. The eye has a muscular system that allows it to move in various directions in order to line up the object with the visual axes of the eyes.

Those muscles that aid in the movement of the eye include the medial and lateral recti, inferior and superior recti, and the inferior and superior obliques (Chalkley, 2000, p. 7; Saladin, 2004, p. 612; Thomas, 1994, p. 7). The lateral recti are located temporally, the medial recti are located nasally, the superior and inferior recti are located above and below to the eye respectively. The aforementioned rectus muscles are attached to the sclera and then proceed back into the orbit where they are attached to a bone surrounding the optic nerve called the annulus of Zinn (Cassin, 2001, p. 34; Chalkley, 2000, p. 76; Thomas, 1994, p. 5). The lateral and medial recti provide temporal and nasal movement respectively which translates to movement on the x-axis. The superior and inferior recti provide up and down motion respectively or movement on the y-axis. The superior oblique is located as its name suggests with the exception that instead of simply attaching straight back it extends over the medial wall and passes through a fibrocartilage ring called the trochlea. The inferior oblique “extends from the medial wall of the orbit to the inferolateral aspect of the eye.” (Saladin, 2004, p. 611). The oblique muscles are primarily resposible for intorting and extorting the eye (Chalkley, 2000, p. 7; Saladin, 2004, p. 612; Thomas, 1994, p. 7).

Cranial nerves are responsible for activating these muscles. Cranial nerve III (CN3), the oculomotor nerve, is perhaps the most important. CN3 innervates the medial, superior and inferior recti, the inferior oblique, the iris sphincter muscle and the ciliary muscle (Cassin, 2001, p. 68; Saladin, 2004, p. 612; Thomas, 1994, p. 6). CN4, the trochlear nerve, stimulates the superior oblique. CN6, the abducens, governs the lateral rectus muscle. Other cranial nerves, including the optic nerve or CN2, the trigeminal or CN5, and the facial nerve or CN7, govern vision, sensation, and facial muscles respectively. These muscles and their corresponding nerves are responsible for the ability of the eye to see, feel sensations and move (Cassin, 2001, p. 68; Saladin, 2004, p. 612; Thomas, 1994, p. 6).

In order to understand strabismus it is important to be able to know which muscles and nerves govern convergence. Convergence is the process of turning the eyes inward in order to maintain binocular vision as an object approaches (Caloroso, 1993, p. 6; Cassin, 2001, p. 75; Thomas, 1994, p. 3). Orthophoria is the state when the eye converges properly (Cassin, 2001, p. 196; Thomas, 1994, p. 6). Strabismus is defined as an eye misalignment or abnormal eye movement that is caused by extraocular muscle imbalance (Cassin, 2001, p. 250; Chalkley, 2000, p. 114; Sardegna, 2002, p. 218; Thomas, 1994, p. 7). The eye has trouble converging to some point in space because an extraocular muscle may be abnormally weak or strong. Usually, strabismus shows itself in the first 6 months of life if it is congenital. Strabismus that surfaces after 6 months of life is considered acquired (Cassin, 2001, p. 102). It is necessary to note that strabismus is a general term (Thomas, 1994, p. 7). There are two useful divisions of strabismus; tropias and phorias. Tropias are misalignments of the eyes that are caused by muscle imbalances (Cassin, 2001, p. 268). Phorias are latent tendencies for the eyes to deviate that is prevented by fusion (Cassin, 2001, p. 206). This paper will primarily focus on some of the causes and treatments of esotropia and exotropia.

Causes and Descriptions of Esotropia and Exotropia:

Five percent of children have some type of strabismus (Chalkley, 2000, p. 76). One of the more common types of strabismus is esotropia. Esotropia is a definite turning of the eye inward or nasally (Caloroso, 1993, p. 249; Cassin, 2001, p. 101; Grosvenor, 1991, p. 124; Sardegna, 2002, p. 218; Thomas, 1994, p. 7). This condition is what is socially referred to as crossed-eyes (Cassin, 2001, p. 101; Chalkley, 2000, p. 79; Grosvenor, 1991, p. 125; Thomas, 1994, p. 7). Several causes for esotropia may exist. Generally, a weak lateral rectus muscle in the strabismic eye is the primary cause. Conversely, an overly active or tense medial rectus muscle may also be involved (Chalkley, 2000, p. 78; Thomas, 1994, p. 7). There may also be a problem with the accommodative response. As mentioned before, the oculomotor nerve is responsible for several muscles that provide the following actions: Accommodation, the process of changing the shape of the crystalline lens by the ciliary muscles in order to change its dioptric power and adjust for objects at various distances, convergence, the inward turning of the eyes, and the dilation and contriction of the pupil. Since CN3 has such a strong relationship between convergence and accommodation, there may be an overactive convergence response if there is a significant amount of refractive error. This is most commonly found in hyperopic children. Uncorrected hyperopia will cause the child to accommodate in order to see far away. This then leads to over accommodation and to over convergence (Caloroso, 1993, p. 249; Cassin, 2001, p. 102; Grosvenor, 1991, p. 124; Thomas, 1994, p. 8). This type of esotropia is categorized as accommodative esotropia. (Caloroso, 1993, pp. 249, 250). Esotropia can be classified as either intermittent or constant and unilateral or alternating (Caloroso, 1993, p. 12; Cassin, 2001, p. 102). Generally, asthenopia, or eye discomfort arising from eye use, is a common complaint among those with intermittent strabismus (Caloroso, 1993, p. 12; Cassin, 2001, p. 40). Moreover, genetic influences can't be ruled out as a possible cause (Ross, 1997, pp. 123,128; Sardegna, 2002, p. 218; Thomas, 1994, p. 8).

Exotropia, although less common than esotropia, is no less debilitating. Exotropia is a definite turning of the eye temporally (Caloroso, 1993, p. 223; Cassin, 2001, p. 104; Chalkley, 2000, p. 79; Sardegna, 2002, p. 218; Thomas, 1994, p. 8). This is what is socially referred to as wall-eyes (Cassin, 2001, p. 104). This form of strabismus may be caused by a weakness in the medial rectus muscle (Thomas, 1994, p. 8). Statistically, myopes are more prone to exotropia. A direct correlation between myopia and exotropia has not been found as of yet (Thomas, 1994, p. 8). Depending on actions taken to correct these types of exodeviations, it is not uncommon to reclassify the patient as the development of the eye changes (Caloroso, 1993, p. 223). Exotropia may also be alternating or unilateral, intermittent and may have a genetic cause as well (Cassin, 2001, p. 104; Thomas, 1994, p. 8).

Vertical Strabismus and Phorias:

Hypertropia and hypotropia are upward and downward deviations of the eye respectively and are categorized as vertical strabismus (Caloroso, 1993, p. 278; Cassin, 2001, pp. 135,136). A strong inferior oblique may also be present in vertical strabismus and is generally present in esotropia and exotropia (Caloroso, 1993, p. 278). Phorias are related closely to their tropic cousins. In the case of esophoria and exophoria similar muscle imbalances are present but only partially (Thomas, 1994, p. 9).

Amblyopia:

Strabismus, when left untreated in children under 7, may cause the sufferer to eliminate one of the two images. This process is known as suppression and is an unconscious response to diplopia (Caloroso, 1993, p. 159; Thomas, 1994, p. 9). Once the eye has undergone suppression the eye will effectively become amblyopic. Amblyopia, according to Sardegna, is a condition in which the eye provides poor vision even though there isn't any pathology or damage to the retina or visual pathways. (Caloroso, 1993, p. 175; Cassin, 2001, p. 30; Ross, 1997, p. 117; Sardegna, 2002, p. 9; Stevens, 1999, p. 111).

There are three useful classifications for amblyopia: anisometropic amblyopia, strabismic amblyopia and anisostrabismic amblyopia (Caloroso, 1993, p. 175; Choi, 2001, p. 1052). These distinctions are based upon the cause of the amblyopia. In the case of anisometropic amblyopia, one of the eyes contains a significant refractive error, anisometropia, and becomes amblyopic. In many cases anisometropic amblyopia may not have any visible deviation and may go undiagnosed. Strabismic amblyopia is amblyopia that originates because of strabismus. Anisostrabismic amblyopia is a form of amblyopia that is combined because of both anisometropia and strabismus are present. What is interesting about amblyopia is that the type of amblyopia may have different neurophysiological mechanisms. A study of 14 patients, 8 anisometropic and 6 strabismic, were shown checkerboards of various sizes while undergoing an MRI. Choi found that calcarine activity was more suppressed at higher spatial frequencies for anisometropic amblyopia and was more suppressed at lower spatial frequencies in the case of strabismus (2001, p. 1052). The calcarine fissure is the part that separates the upper and lower parts of the brain and are where the optic radiations end (Cassin, 2001, p. 56). When the anisometropic amblyope viewed the smaller checkerboard there was less activity in the calcarine fissure whereas the strabismic amblyope had less activity viewing larger checker sizes (Choi, 2001, p. 1052). Although this study had a small sample, it is nonetheless a significant find and further reinforces the classification system for amblyopia.

Amblyopia is better treated before the age of six or seven years of age (Chalkley, 2000, p. 76; Thomas, 1994, p. 9). This is commonly called the “critical” period. Previously, it was commonly believed that after this age it was “impossible” to regain any vision. It turns out that this critical period can extend beyond what was previously believed (Birnbaum, 1977, p. 269; Wick, 1992, p. 866). Begley notes that vision therapy is one of the “first to exploit the brain's rewiring capabilities, known as neuroplasticity” and further adds the following.

For decades scientists had thought that the brain undergoes very little change after childhood. They knew that the adult brain could form the new connections that underlie learning and memory, but believed that its basic structure was immutable and fixed, or “hard wired.” (Begley, 2005)

Also, there seems to be a reasonable explanation for the disparity between Wick, Birnbaum's finding and Thomas, Chalkley's findings about the critical period; the use of occlusion therapy, which will be discussed in further detail. As Stevens notes, occlusion therapy is almost always the treatment of choice for amblyopia and treatment is considered complete once visual acuity does not improve (Stevens, 1999, p. 111). Wick notes that occlusion therapy “in older patients is not often recommended because of the difficulty in re-establishing normal visual acuity in the amblyopic eye.” and reasons that “after 10 years of age occlusion alone is not a very successful treatment for patients with anisometropic amblyopia.” (Wick, 1992, p. 876). Wick performed a study in which 19 patients, all over the age of 6, were studied over 6 years to see how the effects of full refractive correction, using lenses or prisms to align the visual axes, 2 to 5 hours of occlusion therapy and active vision therapy could be used to reverse the affects of amblyopia. Wick found that 8 of the 19 patient's conditions were able to be reversed completely and the patient with the least amount of improvement improved 75 percent and was 49 years old. Finally, Wick concluded with the following.

Such results might also explain why many clinicians use the terms critical period and plastic period interchangeably because they would have observed limited improvement when (occlusion) therapy was instituted after 10 years of age and then concluded that the critical (development) period and plastic (treatment) period were the same. (Wick, 1992, p. 877)

Perhaps with further study better methods for treating amblyopia in adults may be found.

Treatments for Strabismus and Amblyopia:

Correcting strabismus or amblyopia may take many forms. The use of surgery, optical devices and active vision therapy are essential and in many cases the optician may play a pivotal role in the success of the treatment. Some of the treatments for esotropia and exotropia include surgery, but it is considered a last resort. A surgeon could either strengthen or weaken muscles by reattaching them to a different area on the sclera in order to correct the imbalance. In some cases it is necessary to perform the procedure more than once or vision therapy may be implemented for any residual imbalance. (Chalkley, 2000, p. 78; Ross, 1997, p. 132; Sardegna, 2002, p. 219; Thomas, 1994, p. 13). In the case of accommodative esotropia, full correction for distance and near would be used. Bifocals would short circuit the over convergence response due to excessive accommodation by providing all the plus they need. With bifocals it is important to bisect the pupil to ensure that the segment is used (Caloroso, 1993, p. 257; Thomas, 1994, p. 13). The use of prisms may be used initially to encourage fusion (Caloroso, 1993, p. 259).

Active vision therapy may also play a role in correcting the esotrope or exotrope. Hart Chart procedures are examples of accommodative therapy. Two charts with black letters are printed on a white background; one containg large letters and the other small, for far and near respectively. The larger lettered set would be placed at a distance while the smaller lettered could be held. A set of plus and minus lenses assist in the process of changing the amount of accommodation necessary to see the letters clearly. The purpose of these exercises is to either increase the amplitude of accommodation or to improve the efficiency of accommodation (Caloroso, 1993, p. 296).

The Aperture-Rule trainer is an instrument used to train vergence. The instrument can be used with either one or two apertures. An image is placed some distance behind the aperture(s). The single aperture is used to encourage convergence and the double aperture is used to stimulate divergence. This technique has the advantage of dissociating vergence, or eye movements, and accommodation (Caloroso, 1993, p. 300).

Bar readers are tools used as an anti-suppression therapy. The bars are either polarized or composed of anaglyphic, or red and green material. The reader is used with different filters and helps to partially dissociate the image. The patient may or may not see words when the filters are applied, and may be instructed to blink rapidly, or to shake the bars when suppression occurs. This method stimulates the continuous perception of two distinct objects for a short duration of time (Caloroso, 1993, p. 302).

Prism therapy may be used monocularly in order to provide the patient with awareness of his or her eye movements. The patient notices jumps in the images when the eye moves. The technique is used to encourage fixation of the amblyopic eye as well as vergence ranges when used on both eyes (Caloroso, 1993, p. 339).

The Brewster stereoscope is another approach for stimulating binocular vision. There are two distinct images in which the patient attempts to fuse while looking through the scope. The objects can be placed at a variety of distances apart. The distances are based upon the type of vergence that is desired. This instrument can also be used as a form of anti-suppression therapy. The patient points to a control and blinks rapidly when suppression occurs. This is by no means an exhaustive overview of the many active vision therapy techniques available, but is simply given to provide an understanding of the methodology behind some techniques (Caloroso, 1993, pp. 296, 309).

The use of occlusion therapy has been successful in the treatment of amblyopia for centuries and is still the primary approach to treating amblyopia (Wick, 1992, p. 870). In occlusion therapy, the eye without the amblyopia is covered with an occluder of some type. This forces the amblyopic eye to work and develop (Caloroso, 1993, p. 113; Thomas, 1994, p. 13). Several types of occluders exist and will occlude partially or completely depending on the needs of the patient. Generally, full and complete occlusion will be the method of choice since there exists several types of patches and bandage occluders that are more resistant to non-compliance than partial occluders. Full occlusion guarantees that the patient can not use the eye in any way shape or form without removing the occluder. Unfortunately, many children will find occluders uncomfortable and may simply avoid using them. In such cases, the use of cycoplegic eye drops may be necessary such as atropine, homatropine and others. These drugs have side-effects that must be weighed carefully and are generally used as a last resort before proceeding to surgery (Caloroso, 1993, pp. 113,142; Thomas, 1994, p. 13).

Conclusion:

Stereopsis is an advantage that mankind has inherited through natural selection. Without this advantage, our ability as humans to use or make tools would be hindered considerably. Such considerations would bear heavily on humanity's use of technology. Consider the task of driving a vehicle without the ability to determine the proximity of pedestrians. It is undeniable that binocular vision can have a considerable impact on one's quality of life. Many careers such as that of a pilot, require fully developed depth perception. Sadly, strabismus and amblyopia can shrink the opportunities available to the sufferer. Strabismus can be caught early with a simple eye exam. Waiting after the age of six or seven to treat strabismus or amblyopia may put the patient at risk for permanent vision loss despite tailored vision therapy as in the case of the 49 year old in Wick's study. Only with further research will the process of suppression be better understood with the hope of yielding techniques for treating or reversing strabismus and amblyopia.

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