VT and Lenses
Considering the physiological connection between accommodation and vergence, both plus and minus lenses can be used to strengthen accommodative vergence and fusional vergence, as well as increase the overall flexibility between the two systems. Plus lenses decrease the amount of accommodative demand (relax accommodation), thereby asking the patient to stimulate fusional convergence to maintain a clear and single image. Conversely, minus lenses increase the amount of accommodative demand (stimulate accommodation), while asking the patient to relax fusional convergence to maintain a clear and single image.
In our office, plus and minus lenses are used in Vision Therapy only after the patient has successfully completed a range of real space activities, including a series of near to far jumps (i.e. Bull’s Eye Activities). These activities are designed to help the patient understand the spatial aspect of accommodation, the differences in feeling tone between convergence and divergence, as well as the differences in feeling between stimulation and relaxation of accommodation. Once these activities have been mastered, plus and minus lenses may be introduced.
In most cases, our patients will begin with monocular lens work. Our starting point in terms of lens diopters for these monocular activities in our office is +/- 0.50; however, this determination is dependent on age, relative competency of their accommodative system, as well as other complications such as strabismus and amblyopia. These determinations are made by the doctor. As the patient improves in stamina and speed, the level of difficulty is increased in ½ diopter steps until the patient reaches the desired range, which in our office is +2.00/-6.00 (unless otherwise defined by the doctor for a given patient). When lenses are used monocularly, the patient is able to stimulate or relax accommodation without the need of a precise vergence response. Under these monocular circumstances the vergence system will still be active; however, the patient is not able to gather information under binocular conditions. For example, if a patient is having difficulty with stimulation of accommodation, he may be asked to perform monocular tasks with a near target while placing and removing a minus lens from in front of his eye (Wach’s Mental Minus). The patient’s primary concern and goal in this activity is getting a clear image with ease and speed both with the lens on and off. Feeling tone, lens feeling, i.e. SILO, and the understanding of what the eye is doing under the patch is also of importance to improve spatial awareness and integrating accommodation and vergence together. If a patient is having trouble specifically with relaxation of accommodation, the same monocular conditions are arranged with a plus lens. The patient is asked to perform monocular tasks with a near target while placing and removing a plus lens from in front of their eye. Again, the patient’s primary concern and goal in this activity is to rapidly gain a clear image both with the lens on and off. Therefore, the patient is encouraged to practice relaxing accommodation without concern for fusional vergence (diplopia).
Another phase of lens work may include some bi-ocular activities, such as a bifocal rock activity. Using minus lenses (usually in the -4.00 to -6.00 range) we work the feeling tone and awareness aspects of accommodation. This allows for working the accommodative system monocularly, while still asking the patient to maintain a binocular field. Typically, our office does not do disassociated prism work for the purposes of monocular alternating rock activities which would be applicable prior to binocular lens work.
The final step in our Vision Therapy program is for binocular lens work. Binocular lens work differs from monocular lens work due to the addition of vergence demand. The demand of both plus and minus lenses is far greater and requires coordination and integration of the accommodative and vergence system under binocular conditions. Degrees of freedom between accommodation and vergence allow the patient to have greater range of clear and single vision. For instance, if a patient is performing a binocular reading task with a pair of +/- 2.00 flippers, i.e. binocular accommodative rock, the following demands will be made of the patient in order to achieve clear and single vision:
• PLUS LENSES- the accommodative system will be relaxed and will reflexively relax vergence, thereby requiring the patient to actively stimulate vergence to stay on the plane of regard in order to maintain clear and single vision
• MINUS LENSES- the accommodative system will be stimulated and will reflexively stimulate accommodation, thereby requiring the patient to actively relax vergence to stay on the plane of regard in order to maintain clear and single vision
The more frequent the “flip” from minus lenses to plus lenses, the greater demand on the patient to maintain flexibility between accommodation and vergence and the greater need for increased stamina in shifting from one posture to the other.When beginning binocular accommodative rock procedures, suppression controls are introduced to monitor for binocular instability. The American Heritage Dictionary defines binocularity as “Relating to, used by, or involving both eyes at the same time”. In order to ensure that the patient is truly using both eyes equally and at the same time to gather information, a suppression control is necessary. The methods for suppression control that are used in our Vision Therapy office are red/green bar readers with red/green glasses or stereo strips with a pair of orthogonally arranged, polarized lenses. With either of the aforementioned anti-suppression methods, the patient is able to receive direct feedback as to whether he/she is using both eyes equally. If this is not the case, the patient will be unable to see the corresponding area with eye that they are neglecting. Also, if the patient is having difficulty meeting the vergence demand, he/she may experience diplopia, which is more easily determined when using the above-mentioned suppression controls. This is important because as the accommodative vergence demand is raised and lowered through the use of plus and minus lenses, the involvement of both eyes together is imperative to building strong binocularity for a patient that will transfer into the real world. In order to ensure these skills do transfer, our goals, as therapists should be increasing the accommodative vergence flexibility while building stamina within the accommodative vergence system.
Regardless of diagnosis, the expectations of the patient’s endpoint, for instance for a significant esophoria versus a low exophoria at near, should not differ, though their areas of weakness will. Our starting point in terms of lens diopters for binocular activities in our office is +/- 0.50. As the patient improves in stamina and speed, the level of difficulty is increased in ½ diopter steps until the patient reaches a reasonable endpoint, which in our office is +/-2.50 assuming a 6:1 AC/A ratio in ideal circumstances. This process builds degrees of freedom between accommodation and vergence so one system is not excessively driving the other. Depending on the patient’s natural posture, we may expect different areas of difficulty going into these procedures; however, our approach to these activities should generally be the same. Since plus lenses have base in properties we would expect esophoric patients to have an easier time with plus lenses (reflexively the eyes diverge, actively the eyes converge where eso’s are naturally strong), and a more difficult time with minus lenses, which have base out properties (minus lens activities cause the eyes to diverge to keep the target clear and single). The opposite will be true for most exophoric patients who would have an easier time with minus lenses.
As with prisms, lenses can be a valuable tool in any VT room and proper use will offer many benefits to our patients. Lenses are considered medical devices and their use should be dictated and managed by a Developmental Optometrist, or other licensed eye care physician.