Exophoria is a common type of heterophoria, defined as a tendency for the eyes to turn outward toward the temples when the visual system is at rest or when binocular fusion is interrupted. The term “exo” refers to this outward deviation, while “phoria” indicates the misalignment is latent, meaning it is not constantly visible. This condition involves eye coordination and is often noticed during tasks requiring sustained near vision.
Understanding the Mechanism of Exophoria
Exophoria is a misalignment tendency apparent only when the eyes are prevented from working together, such as during an eye exam. In this condition, the eyes’ natural resting position is slightly deviated outward. This differs from exotropia, which is a constant, visible misalignment of the eye, also known as manifest strabismus.
The brain constantly counteracts this outward pull using fusional vergence, the involuntary movement of the eyes to maintain binocular vision. The brain must continuously signal the extraocular muscles to converge, or turn inward, to overcome the exophoric tendency and keep the eyes aligned. While successful alignment results in clear, single vision, this constant effort can lead to fatigue. If the brain’s effort fails, the eye momentarily drifts outward, causing physical symptoms.
Triggers and Underlying Causes
The cause of exophoria results from an imbalance in the system controlling eye alignment, involving mechanisms that manage both convergence (inward turning) and divergence (outward turning). Contributing factors are categorized into anatomical, mechanical, and neural components.
Exophoria is frequently linked to convergence insufficiency, where the eyes have difficulty turning inward to focus on close objects. This weakness requires the visual system to work harder to maintain fusion. Prolonged near-point activities, such as reading or using digital screens, are primary triggers that worsen symptoms by taxing the convergence system.
Other factors include uncorrected refractive errors, though exophoria is often developmental and appears in childhood as visual demands increase. Fatigue, stress, and general illness can lower the brain’s ability to maintain control, causing the latent deviation to manifest. A family history of eye alignment problems may also be associated with the development of exophoria.
Recognizing the Symptoms
Symptoms of exophoria arise when the brain’s compensatory effort to maintain single vision begins to fail intermittently. A common complaint is eye strain, soreness, or fatigue, especially after extended periods of reading or computer use. This discomfort results directly from the extra muscular effort required to keep the eyes from drifting outward.
Patients frequently report frontal headaches that worsen later in the day after sustained visual tasks. The struggle to maintain alignment can also lead to temporary blurred vision or the sensation that words are moving on a page, affecting concentration. The most definitive symptom is occasional diplopia, or double vision, which occurs when the eyes momentarily lose fusion.
These visual complaints are most noticeable when a person is tired, stressed, or engaged in activities demanding close focus. Children may exhibit symptoms by avoiding reading, rubbing their eyes frequently, or closing one eye to achieve a single image.
Treatment and Management
Managing exophoria aims to reduce symptoms and improve the patient’s ability to maintain stable binocular vision. Treatment begins with correcting any underlying refractive errors with eyeglasses, though this may not resolve the coordination issue. For symptomatic cases, the primary non-surgical approach is vision therapy, a personalized program of eye exercises.
Vision therapy specifically targets the improvement of convergence skills and fusional reserves. Exercises often include techniques like pencil push-ups, Brock strings, and specialized tools such as vectograms to train the eyes and brain to work together efficiently. Office-based training under professional supervision is more effective than home exercises alone.
Another option is the use of prism lenses, incorporated into eyeglasses to shift the light entering the eye. Base-in prisms are used for exophoria to decrease the convergence effort required by the eye muscles. These lenses relieve symptoms of eye strain and double vision by reducing visual demand. Surgical intervention is rare and reserved for severe cases that do not respond to non-surgical treatment or when the deviation becomes a constant exotropia.