Convergence insufficiency, a binocular vision dysfunction, is an inability to focus both eyes simultaneously on a single point, such as a letter or word, for a sustained period of time. It is one of the most common vision disorders found in children struggling with reading. The inability to sustain the effort is key, for that is likely to be the main reason symptoms of convergence insufficiency are overlooked in so many children's routine eye examinations. Many struggling readers suffering from this binocular vision disorder actually can converge their eyes to focus on a single point; they just can't sustain the effort for very long and so the disorder isn't detected in a routine vision exam. Symptoms often show up on the second or third page of a story, when a child's reading begins to falter, becoming hesitant and increasingly inaccurate, and other signs of visual stress begin to appear.
Convergence Insufficiency Indicates a Poorly-Developed Visual Skill
The ability to converge your eyes is a skill that develops in childhood. A baby can’t do it, but by age 2 or 3, most children can. Divergence is the opposite of convergence. When you relax the muscles in your eyes, your eyes diverge and you end up focusing at a distance. During convergence you contract your eye muscles to pull your eyes together so that you can converge on a nearer point. Obviously, reading requires development of this skill, since most reading is done at a relatively close distance.
Children experiencing convergence insufficiency fail to adequately develop this skill, and if they can’t converge to a nearpoint, they will end up seeing double or will be under considerable visual stress. If they can converge to a nearpoint, but can’t sustain the effort, they might start out reading well, then gradually start falling apart, making more and more mistakes until they get a chance to take a short break. This is why it's referred to as convergence insufficiency rather than convergence inability.
Developmental optometrists are trained to assess convergence problems and recommend treatment. Sometimes the treatment can be as simple as a set of glasses with an appropriate prism in the lenses, but in my experience the treatment usually ends up being vision therapy. (If a child can't converge his eyes at all, he has convergence inability, and even this more severe condition can sometimes be addressed with vision therapy.)
Normal Development of Convergence Skill
Some of the literature on convergence insufficiency indicates that convergence skills are developed during the crawling phase of childhood when a child is required to focus on his hands to see where he is placing them. When he looks up to see where he's going, his eye muscles relax as his eyes diverge; when he looks down to see where he's placing his hand next, he uses his eye muscles to converge both eyes on his hand. As the process unfolds, the child is essentially practicing his eye convergence skill each time he raises and then lowers his head.
Similarly, it’s said that the old “monkey bars” built convergence skill because with poor convergence, depth perception is lacking and with poor depth perception, a child will miss the next bar. This is also true of the suspended horizontal ladder seen on some playgrounds. Miss the next rung and you fall or dangle helplessly, so there’s an incentive to concentrate your visual efforts on a precise point.
Be aware though, that genetics probably comes into play in all of this. If your child skipped crawling and went right to walking, he might have done so because he was so visually uncomfortable trying to look at the floor in front of him that he struggled mightily to get to his feet where the view was easier on the eyes (because he didn’t need to converge his eyes to see what he was doing.)
The Brain’s Workaround
As I’ve said elsewhere on this site, vision problems usually run in families. And, due to the relative scarcity of developmental optometrists, most adults who had vision problems like convergence insufficiency as children have never had them addressed, though they might eventually have overcome them.
Interestingly, if you tested those adults today you would probably find that many of them didn’t actually overcome convergence insufficiency, but instead developed compensating mechanisms. One of those mechanisms is to use only one eye when reading, so that the brain isn’t confused by two conflicting images. I had a parent in my office one day whose child I was certain had an untreated vision problem and I was trying to demonstrate something to this parent. When he put on a pair of polarized glasses, instead of seeing what he was supposed to see, he said that one side of the target was completely black. Without going into detail, that is what would logically happen if he was only using one eye for nearpoint work.
However, he had both eyes open, so what was happening? Well, it turns out that the brain is inventive when presented with a problem like poor convergence skill. First, it tries to solve the problem rationally, by getting both eyes into the game. It does this in a fascinating way, and in a way that can be observed using red/green glasses commonly seen in vision therapy centers. When you put a red lens in front of one eye and a green lens in front of the other, then try to read print through a plastic sheet with alternating red and green transparent striping, the print going through a red stripe can only be seen by the eye behind the red lens because red and green cancel out, resulting in a black stripe. In other words, if you closed the eye behind the green lens, you would see alternating red and black stripes because the green plastic stripe would appear black.
Using these red/green glasses, it is easy to spot a child (or an adult) who is suppressing the visual input from one eye. In the adult’s case, that suppression is very likely permanent and affects the same eye constantly. However, in the case of a young child, vision therapists often observe a phenomenon called alternating suppression, which I believe is caused by the brain attempting to first train normal convergence ability before taking the more extreme route of shutting down the input from one eye when doing nearpoint work.
Correcting Convergence Insufficiency
Because it is a skill, proper convergence ability can be developed. In fact, most children ordinarily do develop proper convergence skill. Those who don't, often the very children who struggle with reading, will usually benefit from vision therapy. If you think of vision therapy as physical therapy for the eyes, you won't be too far off, at least when it comes to correcting convergence insufficiency. Always remember, though, that vision therapy doesn't actually teach a child to read. Instead it enables a child to view a page of fine print in comfort. If your child's phonics knowledge is weak, vision therapy might be only a partial answer to his reading struggles.