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Heterophoria is a type of eye condition in which the directions that the eyes are pointing at rest position, when ''not'' performing binocular fusion, are not consistent with each other. It is also known as 'latent squint' because tendency of the eyes to deviate is kept latent by fusion. A person with two normal eyes has single vision (usually) because of the combined use of the sensory and motor systems. The motor system acts to point both eyes at the target of interest; any offset is detected visually (and the motor system corrects it). Heterophoria only occurs during dissociation of the left eye and right eye, when fusion of the eyes is absent. If you cover one eye (e.g. with your hand) you remove the sensory information about the eye's position in the orbit. Without this, there is no stimulus to binocular fusion, and the eye will move to a position of "rest". The difference between this position, and where it would be were the eye uncovered, is the heterophoria. In contrast, fixation disparity is a very small deviation of the pointing directions of the eyes that is present while performing binocular fusion. Heterophoria is usually asymptomatic. This is when it is said to be "compensated". When fusional reserve is used to compensate for heterophoria, it is known as compensating vergence. In severe cases, when the heterophoria is not overcome by fusional vergence, sign and symptoms appear. This is called decompensated heterophoria. Heterophoria may lead to squint or also known as strabismus. ==Cross-Cover test== The cross-cover test, or alternating cover test is usually employed to detect heterophoria. One eye is covered, and then the cover is moved immediately over to the other eye. With heterophoria, when the cover is moved to the other eye, the eye that has just been uncovered can be seen to move from a deviated point. The difference between heterotropia and heterophoria can be easily understood as follows. With heterotropia, a correcting movement of the eye can be detected already by the simple cover test; with heterophoria, such correcting movement only takes place in the cross-cover test. The reason is that subjects with heterophoria are able create and maintain binocular fusion through vergence, and the cross-cover test purposely breaks this fusion, making the latent misaligment visible. Whereas the cross-cover test allows a qualitative assessment to be done, a quantitative assessment of latent eye position disorders can be done using the Lancaster red-green test. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heterophoria」の詳細全文を読む スポンサード リンク
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