There are hundreds of different vision therapy activities, each specifically designed to help a certain ocular condition. One of the most common disorder vision therapy patients have is strabismus, or a misalignment of the eyes, in which both eyes do not line up in the same direction. A very popular tool optometrists use when dealing with a strabismus case is the Brock string. A Brock string is a long string made up of 3 beads, all a different color, and these beads are able to change its position by moving up and down the string.
How does a Brock string work and how can it help realign the eyes? The patient starts off with holding one end of the string at eye level and the doctor holds the other end to make sure that the string is parallel to the floor. The first bead, in this case, the blue bead, should be placed around a comfortable near distance away from the patient’s nose. This is about an inch further from normal reading distance, or the distance a book is from the eyes when reading, which is approximately 14 inches. The middle bead should be about 30 inches away and the third bead about 5 feet away from the patient’s nose. These distances don’t have to be exact.
The patient is instructed to first look at the middle bead. When his/her eyes are both looking at the red bead and are focused, they should see 1 red bead versus 2 blue and 2 green ones, like the picture below. Some patients have difficulty with this. Since one of their eyes are misaligned, both eyes may be unable to look at the center bead at the same time. If this happens, a paddle or a 3×5 card is placed over their dominant eye and they are re-instructed to focus on the center bead once again. This forces their misaligned eye to look directly at an object and to help re-align it. While the patient is looking at the red bead, the card or paddle is slowly moved away from the dominant eye and is instructed to continue looking at the red bead. If the patient is successful in keeping their strabismic eye at the center bead when their dominant eye is uncovered, they should be able to see the below picture.
Now the patient will shift their focus to the bead furthest away from them, or, in this case, the green bead. The procedure with the index card/paddle is repeated again to help the turned eye work together with the dominant eye to focus on the bead. If both eyes are staring at the green bead and are focused, it should look like the diagram below on the left. The doctor will then have the patient look at the closest bead. The procedure is once again repeated until he/she sees the X-shaped figure below to the right.
As the patient improves in their focusing ability, the near blue bead will be moved closer to their nose. This pushes their eyes to work a bit harder to converge together to make the blue bead a single image. Over multiple sessions, the blue bead will be moved closer and closer until convergence is comfortable 6 to 8 inches away from the nose.
The main goal of patients working with a Brock string is to be able to focus on any of the 3 beads and create a clear single image of that bead and double of the other ones while using both eyes. Now, one may wonder why the doctors want the patients to see the X-like figure with the Brock string? This is known as physiological diplopia and is a normal phenomenon. Any objects not in our field of focus will appear double. This is due to the fact that our eyes are on opposite sides of our nose and see different angles of the same object. Our brain and both eyes will form a single image of an object we are focusing on and everything else will appear double. This is different than what a strabismic patient sees. They may have constant double vision and the doctors want to re-train their eyes so their area of fixation is single.
The Brock string is a physiological diplopia exercise. When you fixate on a bead, that bead is single while the string and beads become double in the front and back, forming an X. This is a normal response and we want all our patients to achieve this! We not only enjoy using the Brock string at Insight Vision Center Optometry because it offers instant feedback to patients and let’s them know if their eyes are working together to focus on an object at different distances, but also because both children and adults can get use them as well!
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