Phorias are a common visual phenomenon where the eyes tend to drift out of alignment. This deviation remains hidden when both eyes work together to focus on an object, a process known as binocular vision or fusion. A phoria becomes noticeable only when this fusion is disrupted, for example, by covering one eye. This condition differs from a tropia (strabismus), which is a constant and visible misalignment of the eyes. A phoria is a latent deviation, while a tropia is a manifest one.
Types of Phorias
Phorias are classified based on the direction an eye tends to drift when its connection with the other eye is interrupted. Esophoria describes an inward turning tendency of the eye. Conversely, exophoria refers to an outward turning tendency. Both are horizontal deviations.
Phorias can also involve vertical tendencies. Hyperphoria indicates an upward drift, while hypophoria denotes a downward drift. These tendencies are typically overcome by the brain’s natural ability to fuse images from both eyes into a single, clear perception.
Causes and Symptoms
Several factors can contribute to phorias, often involving imbalances in the eye’s muscular system or how the brain coordinates eye movements. Genetic factors also play a role, as phorias can run in families. Uncorrected refractive errors, such as farsightedness (hyperopia), can increase the likelihood of esophoria. Prolonged visual tasks, like extensive screen time or reading, can also exacerbate phorias due to increased eye stress. Neurological factors, brain injuries, or trauma to the eye area may contribute.
Many individuals with phorias experience no symptoms, as their brain effectively compensates for the latent deviation. When symptoms do occur, they frequently include eye strain, particularly after extended visual tasks. Headaches, often after prolonged near work, are another common complaint. Other sensations might involve eye fatigue, occasional blurred vision, or fleeting double vision, especially during moments of tiredness or stress.
Detection and Diagnosis
Eye care professionals use specific tests to identify phorias. The Cover Test is a primary diagnostic tool to reveal latent deviations. In this test, the practitioner covers one eye and observes the movement of the other eye.
If the uncovered eye shifts to pick up fixation, it indicates a manifest deviation (tropia). If it remains steady, the covered eye is then observed as the cover is removed. If the previously covered eye moves to re-align, it signifies a phoria, as the deviation was only present when fusion was interrupted.
The Prism Cover Test quantifies the magnitude of the phoria. This test involves placing prisms of increasing strength in front of the eye during the cover test until no movement is observed, indicating the amount of prism needed to neutralize the deviation.
When Phorias Require Attention
Phorias are categorized as either compensated or decompensated. A compensated phoria means the brain can effectively manage the eye’s tendency to drift without causing noticeable symptoms. Many people have small phorias that never cause problems because their visual system is robust enough to maintain comfortable alignment. A decompensated phoria occurs when the brain’s ability to maintain comfortable alignment is overwhelmed, leading to noticeable symptoms. This can happen if the phoria is particularly large, or if the individual is fatigued or stressed.
Management options for symptomatic or decompensated phorias aim to alleviate the strain on the visual system. Corrective lenses, such as glasses or contact lenses, can address underlying refractive errors. Prism lenses are a specialized type of corrective lens that can redirect light before it enters the eye, reducing the effort required by the eye muscles to maintain alignment.
Vision therapy involves exercises designed to improve eye teaming, focusing skills, and the brain’s ability to coordinate eye movements. In rare cases where other treatments are ineffective, surgical intervention on the eye muscles may be considered to adjust eye alignment. A comprehensive eye examination by an optometrist or ophthalmologist is important for diagnosis and management.