Esophoria is defined as an inward deviation of the eyes. This can be thought of as a natural over-convergence of positioning. The eyes don't always perfectly point in the same direction, rather they have a natural inward position. The result of an inward eye (phoric) posture is that the eyes and brain must constantly coordinate to rotate and move and diverge the eyes to a straight-ahead position. If eye teaming and coordination is lost, the eyes move out outward out of alignment and a person sees double. Doubled images are often horizontal if no additional vertical phoria is present. When a patient's ability to fuse and maintain single vision is lost and vision is doubled, the condition is termed a tropia or visible eye turn.
Eye positioning is tested using a cover test. A clinician covers one eye at a time, and then alternates between the eyes to disrupt fusion and watch how the eyes react. When fusion is broken (by moving a hand or cover paddle from one eye to the next), the eyes assume a position of rest, called the phoric or tropic posture. This position may be:
To better define the movement, the ending -phoria or -tropia is added. As stated above, if the deviation only occurs when fusion is broken, the ending is -phoria. If the deviation is visible even without breaking fusion, the ending is -tropia.
The lines can get a bit blurred here - a phoria can break down into a tropia. This means that a patient is able to keep both eyes teamed together, but at times one eye deviates. This is a common cause of double vision and is termed an intermittent tropia since it doesn't always occur. During a vision exam an eye specialist may repeat eye position testing at different distances, in different positions of gaze, or with different targets or lenses to see if the patient's phoria breaks down and becomes an intermittent tropia.
In short, it's not completely clear what the exact cause for esophoria is. The most common thought is that esophoria is due to a combination of anatomical, mechanical, and innervation factors. Anatomical may be eye position, orbit size and shape, eye size, tissue/fat within the orbit, etc. Mechanical refers to the action of the extraocular muscles. Innervation refers to the neural impulses that guide eye movement. The end result is an abnormal balance between convergence and divergence.
The focusing system of the eyes can also affect eye posture. If over-stimulated, the focusing (or accommodation) system can cause the eyes to converge.
Symptoms of esophoria may include:
If you're experiencing any of these symptoms, it may be time for an eye examination. To best evaluate an esophoria, an eye specialist will perform an eye health examination as well as a binocular vision examination.
The key difference in these conditions is the distance at which the esophoria is most noticeable.
The ultimate goal of treatment of esophoria (and other conditions related to esophoria) is to improve the patient's ability to maintain fusion when converging and diverging. The end result is improved binocular vision as well as improved visual comfort.
Glasses may have some effect on a patient's eye position, especially if there is a high glasses prescription (often a far-sighted or hyperopic prescription) or a significant imbalance between the two eyes. Correction of a significant refractive error (the fancy term for a need for glasses) is often the very first step of treatment for any binocular vision disorder. Glasses may also help relax and balance the focusing system of the eyes. Occasionally a bifocal, trifocal, or progressive lens is prescribed to reduce the stress placed on the focusing system - even in children!
Vision therapy is often very effective in improving fusion (or fusional range). The goal of therapy is to help a patient learn to team both eyes together quickly and efficiently. Vision therapy for esophoria has a similar appearance for exophoria, but the key focus is working on divergence. This is done through a series of guided tasks or exercises that work on critical skills such as recognition of poor eye coordination, improving convergence ability, improving divergence ability, and improving the ability to rapidly change between convergence and divergence. Stressing the focal system of the eyes (the accommodative system) through the use of distance and near targets or with various lenses is also common. The goal is to help the patient better control both convergence/divergence skills and focal skill independent of one another AND coordinated together.
Numerous tools may be used to help train these skills, such as computer or virtual reality games, vectograms, lens/prism flippers, loose lenses of various powers, and a Brock string. Active vision therapy done under the supervision of an eye care provider has been shown to produce better results than simple convergence activities at home (sometimes called pencil push-ups or pencil pull-aways). A vision therapy practicioner can help build a treatment plan that best fits your needs.
Prism lenses are sometimes effective for esophoria, and some patients may significantly benefit from prismatic lenses. Prisms are often oriented in a base-out direction, which decreases the divergence demand placed on a patient's visual system. The problem with prism is that often prism lenses act as a temporary solution to a larger problem - the patient still has poor divergence ability. Prism lenses are sometimes use as part of therapy for esophoria as well.