Accommodation is one of the mechanisms that allow us to see up close. Accommodation is the focusing of the lenses in each eye. This action, combined with the convergence, allows for us to see clearly up close.
Accommodation is the result of the contraction of the ciliary bodies in the eye which allow for the lens to get thicker thereby focusing the image better in the fovea. This action also includes the constriction of the pupil which more precisely focuses the light on fovea making the image sharper. Here is video of this in action
“Its blurry up close”
When accommodation does not work, one may see blurry up close, get headaches or rub the eyes due to eye strain. The ciliary muscles attempting to make the image clear, causes this discomfort discomfort. Accommodation can be exercised to strengthen it to improve near vision. This is generally performed in conjunction with convergence exercises to improve near vision when one treats convergence insufficiency.
The Hart Chart
A simple way to improve convergence is using a Hart chart. With this activity, a grid of letters is placed at distance and one is held by the patient, near. The patient then reads a line close (or letter) then a line at distance. This is done with one eye occluded so the accommodative action is exercised as the eye focuses near then far. In my clinic, this performed while standing on balance board to further challenge the patient. This simple activity is quite effective at strengthening accommodation. A Hart chart can be purchased from Bernell, found on the internet and is included on the Vision Rehabilitation for Pediatrics Course Companion flash drive. Heres a video.
The Hart chart is one way accommodation can be strengthened. In optometric vision therapy, lenses can be used to strengthen accommodation using an activity called Accommodative Rock.
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A complete binocular vision assessment should be conducted before performing these tasks to make sure that are appropriate. Only an ophthalmologist or optometrist can accurately diagnose an accommodative problem.
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The Brock String
The humble Brock consists of a length of string (from 3 to 10 ft) with a series of beads (from 3 to 5 beads) placed at various intervals. This simple instrument is powerful tools in teaching binocular vision skills. While the configurations vary, it is elegantly simple and effective in teaching the brain how to make the eyes convergence.
How the use a Brock string
The 3 ft string is generally sufficient unless working with an athlete then a longer string my be needed. Four or five beads make for enough targets to be useful.
- With the beads evenly spaced (closest bead at about 2cm, furthest about 40 cm), have the patient hold one end of brock string to their nose.
- The therapist holds the string tight and angled slightly downward.
- Have the patient fixate on one of the further beads to begin. The patient should see one bead (indicating focus to a single point) and two strings meeting at the front of the bead (indicating both eyes are working together. It should look like the picture below.
- Have the patient alternate from bead to bead, working closer and back to exercise the convergence muscles. Look for difficulty with maintaining the fixation as the muscle fatigue.
Here is video of a brock string in action. Notice the the “bobbles” as she moves from bead to bead. This is an indication of weakness. When they see it correctly, the eyes get very still.
Why does this work?
The brock string takes advantage of “physiologic diplopia”. The eyes can only focus clearly on single point with all other things within the viewing area seen as double. This is a normal way for our eyes to work. The brock string uses this physiologic diplopia as a cue to the let brain know the eyes are working together correctly.
Remember, before the brock string, check for full extra ocular movements and exercise the eyes separately.
Brock strings are easy to make and make a great home program addition.
Learn more about this subject in a live course and webinar presented by Robert. Hosted by PESI Education.
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