By: Alan P Pearson OD MEd PhD FCOVD
©2017 Vision Clinics of Development & Learning
Binocular stability is the skill by which binocular vision and eye focusing are coordinated, maintained, and adapt to stress. Good binocular stability results in continuously clear and single vision during a visually demanding task such as reading. Binocular instability refers to problems in maintaining visual clarity and/or single vision in response to visual demands.
Binocular vision involves precise fine-motor alignment of the eye muscles so that the line of sight of each eye points at the same object. With precise alignment the brain can then fuse the images from each eye into a single image. Binocular vision allows us to appreciate 3D depth. When the two eyes are not aligned, we may experience double vision.
Eye focus, also called accommodation, involves a fine-motor muscle surrounding a lens right behind the pupil. The muscle is relaxed and the lens flat when looking in the distance. The muscle contracts and the lens bulges when focusing up close. Without the proper muscle and lens accommodation, we experience visual blur.
When involved in a complex visual task such as reading, we have to coordinate both binocular vision and eye focus over a long period of time while the eyes are dynamically moving across the pages. A book is a flat two dimensional page with high contrast black and white symbols bunched close together. When looking at a word chunk, the line of sight of each eye has to point at the same word chunk, and the eyes need to simultaneously focus for the distance of the word chunk. Then, the eyes make small jumps called saccades from one word chunk to the next word chunk while maintaining the eye pointing and eye focusing appropriately.
In binocular instability, there are moments when binocular vision and/or eye focusing are inaccurate, inefficient, or uncoordinated. Because there are neurological connections between binocular vision and eye focusing, sometimes there occurs a viscous cycle between the tasks of single vision and clear vision. For example, trying to make the words clear by focusing the eye can send signals to the eye muscles that cross the eyes, but if this is allowed to happen it will lead to double vision, so eye pointing needs to be relaxed which then sends signals to relax eye focusing which makes the words blurry. This is frustrating, and can be the root of symptoms such as visual fatigue, headaches, words jumping and moving, words going double at times, words going in and out of focus, losing place, skipping words, poor attention, avoidance behaviors, and poor comprehension.
A Functional Vision Evaluation is necessary to diagnose binocular instability. Regular routine eye exams emphasize eye health and eye optics. In order to function well, the eyes need to be healthy and any need for eyeglasses taken care of first. With healthy eyes and necessary eyeglasses, a Functional Vision Evaluation will take an in-depth assessment of binocular vision, eye focusing, and eye tracking in the context of the visual challenges the individual faces in development, school, work, or recreation. A Functional Vision Evaluation looks at abilities to maintain single and clear vision over time when involved in a dynamic task. Being able to clearly identify small letters on a card means eye focus is happening at that moment, but it doesn't mean that eye focusing is working well over a 20 minute reading task.
Only an eye doctor (Optometrist or Ophthalmologist) has the tools to perform a Functional Vision Evaluation and diagnose binocular instability. Not all eye doctors have the interest or training, however, to perform a Functional Vision Evaluation. The following directories can be helpful in locating a doctor who can perform a Functional Vision Evaluation:
This Binocular Stability MAP is a best practices treatment protocol. It is not the only way to treat the condition and every patient is a unique individual with varied needs. Therefore, this Binocular Stability MAP should only be used when prescribed, provisioned, and monitored by an eye doctor. Parents and other professionals may participate and deliver therapeutically meaningful activities at home, clinic, or school with proper instructions and collaborative monitoring by the eye doctor and his/her vision therapy staff. Communications via mySplashboard will allow for inevitable clarifications, problem solving, and individualization of the treatment plan and activities.
The long range goal of the Binocular Stability MAP is:
Patient will be able to maintain single and clear binocular vision in a sustained near point visual task. Patient will demonstrate quick and precise adjustments to varied fixation distances such as distance, intermediate, and near. Patient will eliminate or minimize symptoms of visual fatigue and vision based performance deficits.
Achieving this goal is determined by the eye doctor with appropriate functional testing during a progress evaluation.
The following short term goals are usually prescribed:
These goals and their associated activities are described in detail in each section of the Binocular Stability MAP.
There are many other visual dysfunctions that can occur simultaneously with binocular instability. For example, reflex problems, visual-vestibular problems, fixation and orientation problems, rhythm and timing problems, body knowledge/coordination problems, and visual perception and processing problems. The Binocular Stability MAP is not meant to address these problems. Some of these problems will need to be addressed before the Binocular Stability MAP is appropriate. Some of the problems can be addressed simultaneously with the Binocular Stability MAP and multiple MAPs may be posted to a patient's treatment plan. The eye doctor will determine the best strategy, sequence, and emphasis in MAP and activity selection.