Phorias and Tropias

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Eye muscle deviations. Ballyscanlon / Getty Images

Phorias and Tropias: A Matter of Alignment

Phorias and Tropias are ways of describing eye muscle deviations. Below are basic definitions:

  • Phoria: A phoria is a misalignment of the eyes that does not manifest itself outright. A phoria is the resting position that your eyes go to when covered or when fusion is broken by repetitively alternately covering each eye.
  • Tropia: A tropia is a misalignment of the eyes that is always there. A tropia manifest itself visibly even when both eyes are open and trying to work together. Sometimes, tropias can be so small that they are not noticed. Larger tropias are obvious.

    During the cover test portion of an eye examination, your eye doctor will alternately cover and uncover your eyes while you fixate on a target. If your eye moves upon being uncovered, you have a phoria. A phoria is a latent deviation, or misalignment, of the eyes that is only apparent some of the time. We all have a normal phoric resting position. Some people have a larger than normal phoria that they may be able to compensate for most of the time. However, because the phoria is much larger than what is considered normal, they can not always compensate for it when fatigued. As a result, their phoria may manifest itself and become a tropia. If a large amount of phoria exists, your eyes are not aimed at the target while at rest. Keeping both eyes fixated on the target requires an effort by you, possibly causing eyestrain or headaches.

    Another way to describe it is in two types of people that may come to the optometrist complaining of double vision.

    One person comes in and complains of double vision and one eye is completely turned in and crossed toward the nose. The eye never straightens up and is always turned it. It is quite obvious because it gives a crossed eye appearance. This person has a tropia. 

    Another person may come into see the optometrist and states that lately, they have been getting tired in the evening and they feel eye strain.

    Sometimes, they even may see double when watching TV. This person probably has a phoria. It sounds as if it presents itself, only when fatigued and when the person is unable to compensate for the deviation. 

    Eye doctors categorize phorias and tropias by direction.

    • Exotropia or exophoria - An eye turns outward
    • Exotropia or esophoria - An eye turns inward
    • Hypertropia or hyperphoria - An eye turns upward
    • Hypotropia or hypophoria - An eye turns downward

    Eye doctors also categorize them as right eye or left eye or alternating between the eyes. They can also be labeled intermittent or constant. 

    What causes a large phoria or a tropia?

    Phorias or tropias can be caused by a variety of factors. One of the most common causes is having a large amount of hyperopia or farsightedness. When young children have high amounts of farsightedness that is undiagnosed, they struggle to see clear. To compensate, they focus extra hard. Because the focusing system and the eye muscle converging system are tied together, something gives. Their brains either decide to see clear, but one eye turns in or they decide to see blurry and the eyes stay straight. This is abnormal and they may develop an esotropia. This circumstance describes what is called an accommodative esotropia.

    Also Known As: Heterophoria, orthophoria

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