VT and Amblyopia
Amblyopia is clinically defined as the difference in the best corrected acuity between the two eyes of two lines or more in Snellen Acuity. Amblyopia is also known as visual acuity poorer than 20/20 in the absence of underlying structural or pathological anomalies, but with at least one of it causes occurring before age six, according to Dr. Ken J Ciufredda. The most common causes for amblyopia are strabismus, anisometropia, and very high amounts of isometropia. The isometropia has to be high enough and close to equal in each eye so that a clear image has never been received, or have unilateral or bilateral astigmatism.
Strabismus, which is often considered to be the most common cause of amblyopia, is the deviation of the alignment of one eye in relation to the other. When unable to align the eyes on a single point, the brain learns to actively inhibit the deviated eye to avoid diplopia, or double vision. This adaptation causes the deviated eye to be under stimulated, and often times the brain cells responsible for vision in the strabismic eye do not develop normally.
Refractive amblyopia caused by anisometropia is defined as the two eyes existing in an unequal refractive state. Usually there must be a significant difference in refractive states for amblyopia to develop in this case, to facilitate eliminating a blurry or poor quality image from the affected eye. It is thought to occur more frequently in hyperopia than in myopia. Often times this can be most overlooked form of amblyopia for the reason that the child appears perfectly normal cosmetically, although one eye is becoming amblyopic. Because there is no visible eye turn and the child doesn’t complain of blurry vision, parents feel their child is fine and do not see a reason to have a comprehensive eye examination.
In our office, the approach to amblyopia, whether strabismic or refractive, is always slow and cautious. It is the doctor’s responsibility to set guidelines and treatment protocol for such complications as eccentric fixation and anomalous correspondence (if strabismus is also present – there is no AC if there is no strabismus). Strabismic amblyopia, in particular, presents the possibility of intractable diplopia if handled incorrectly, and therefore Vision Therapists should always remain in constant communication of the patient’s progress with their doctor.
The similarities and differences between refractive amblyopia and strabismic amblyopia will dictate the paths of Vision Therapy. Patients with strabismic amblyopia tend to have imperfections in their spatial awareness and spatial maps. This is to say they have challenges determining where an object is in space in relation to them, as well as difficulty determining the spatial differences from one object to another. Patients with refractive amblyopia may have small imperfections in their spatial awareness; however, due to their lack of a deviation and potential for binocularity, their abilities for simultaneous perception allow for the building of generally accurate spatial maps. Refractive amblyopia patients tend to be more influenced by the blur factor, or the lack of resolution.
In our office, Vision Therapy for strabismic amblyopes begins with gross motor movements and integration of primitive reflexes, if necessary. Concurrently, we will focus the general ocular motor skills of accommodative function, pursuits and saccades on a monocular basis. The strabismic amblyope will also begin to work on gross peripheral fusion, peripheral awareness, and basic spatial awareness activities. Depending on the needs of the patient, the doctor may also recommend bi-nasal occlusion if the patient has an esotropia.
Monocular fixation in a binocular field, also known as MFBF, comes next. The purpose of MFBF is to encourage the patient to equalize their input, while keeping their periphery open to enhance peripheral fusion. It also allows the affected eye to assert cortical representation under conditions where it normally would have been suppressed by the “good” eye. This is usually done with a red/green target. The red/green anaglyphs, in combination with red/green filters, allow the Vision Therapist to enhance the signal to the amblyopic eye to the patient, while maintaining the binocular field. MFBF also gives the patient the opportunity to equalize their own input signal, without allowing them to revert back into their amblyopia.
If indicated, bi-ocular activites will be next. This can be accomplished with dissociated prisms and split vectograms, among other options. The idea with bi-ocular work is to split the images beyond the point of fusion, to allow the patient the opportunity to appreciate and further equalize the signal inputs from both eyes, without the demand or possibility of fusion.
The final steps of Vision Therapy for a patient with strabismic amblyopia will include binocular techniques involving depth equations (SILO awareness) such as vectograms (Clown and Spirangle), spatial estimation and localization techniques (such as distance estimation and pointer in straw) on the X, Y, and Z axis. These techniques will always require a loading task such as auditory integration with a metronome, interference of the Vision Therapist’s choice, and well as other peripheral stimulus for the patient to either integrate into the activity or actively suppress as unneeded information. The “loading” portion of the sequence becomes extremely valuable because it allows the patient to employ his/her new visual skills in a “real life” stress situation. This situation also will evidence any of the patient’s residual tendencies to revert back to the “old system” under stress.
Refractive amblyopia therapy will generally follow the same path, though more emphasis will be placed on monocular fixation activities and high contrast targets first, then working peripheral fusion followed by high contrast third degree fusion targets. The change in hierarchy of approach is necessary because the refractive amblyope will have comparatively better spatial localization when attempting spatial judgments through the amblyopic eye, but will have difficulty identifying detail.
Variations to how amblyopia may include foveal tagging, such as working with the MIT to help eliminate eccentric fixation, and/or after-image transfer activities to help eliminate anomalous correspondence.
Assuming the patient is still foveal in the amblyopic eye, Vision Therapy helps to “re-tune” the connections brain, thereby encouraging the brain to accept equal input from both eyes and thus negating the need to actively inhibit the weaker eye.