Heterophoria (from other Greek: другойτερος “other” + φορέω “wear”) is a form of strabismus , an eye condition in which the directions of eyes in a resting position without the need for binocular fusion are not consistent with each other. A person with two normal eyes has a single field of view (usually) due to the combined use of sensory and motor systems. The propulsion system directs both eyes to the target of interest; any displacement is determined visually (and the propulsion system corrects it). Heterophoria occurs only during dissociation of the left and right eyes, when there is no synchronization of the eyes. If you close one eye (for example, with your hand), you exclude sensory information about the position of the eye in orbit. Without it, there is no incentive for binocular fusion, and the eyes will move to a "rest" position. The difference between this position and the position of open eyes is called a heterophore.
| Heterophory | |
|---|---|
| ICD-10 | H 50.5 |
| ICD-9 | 378.40 |
In contrast, fixed mismatch is a very small deviation of the eye directions that is present when binocular fusion is performed.
Heterophoria is usually asymptomatic. This is when they talk about "compensated heterophory." When fusion reserve is used to compensate for heterophory, it is known as compensation for convergence. In severe cases, when heterophory is not overcome by fusional convergence, then signs and symptoms appear. This is called "decompensated heterophory." Heterophoria can lead to strabismus .
Hypophoria (from other Greek: ὑπό “down”) is a type of heterophory with a tendency to deviate the gaze down.
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Cross Cover Test
A cross-cover test , or alternative overlap test , is commonly used to detect heterophory. One eye is closed, and then the lid is quickly moved to the other eye. With heterophory, when the lid moves to the other eye, in the eye that has just been opened, you can see the movement from the point of deviation. The difference between strabismus and heterophory can be easily understood as follows. With strabismus, eye movement correction can be detected even in a single closure test; in heterophory, such corrective movement occurs only in a cross-test. The reason is that subjects with heterophory can create and maintain binocular fusion through convergence and a cross-cover test specifically breaks this fusion, making latent skew visible.
While the cross-cover test provides only a qualitative assessment, you should also make a quantitative assessment of latent violations of the position of the eyes using the red-green Lancaster test.
Etiology of heterophory
In order to understand how heterophoria occurs, we must understand how the eye can maintain proper gaze fixation with an undirected visual axis. Heterophoria is actually a displacement of the optical axis in both eyes. In other words, one or both eyes do not properly fix the direction to the object of interest. However, we must know that the eye has a fusional vergent system that corrects this bias.
The manifestation of heterophory
When the fusional vergent system can no longer restrain heterophory, phoria appears. In this state, the eyes deviate from fixing the direction to the object.
Notes
Links
- Classification of heterophory in children
- Ophthalmology: tropias versus phorias
- Animation at mrcophth.com
- http://eyes.cochrane.org/glossary Template: Eye pathology