Phoria vs Tropia

• A patient who does not demonstrate a deviation of eye alignment is termed "ortho" or "orthophoric".
• A tropia is an eye turn or fixed deviation that the patient has no, or very little, control over. The patient is unable to keep the eye straight with the power of fusion. This deviation is called heterotropia, or if the direction is specified, exotropia, esotropia, or hypertropia. Even though there is such a thing as an intermittent tropia, a tropia is a manifest deviation, meaning it is evident upon inspection and is not hidden. The cover test helps to evaluate this.
• A phoria  “comes and goes”; there is a tendency for the eyes to drift out of alignment. A phoria is a muscle imblance that is hidden by fusion.  The eyes remain straight as long as fusion is present.  The phoric deviation only becomes evident when fusion is disrupted. Therefore, an eye can deviate when it is covered, an eso- or exophoria. The uncover and the alternate cover test looks for this.
• In practice the cover and uncover tests are performed together. ie. cover left eye, watch right eye for tropia. Uncover left eye, watch left eye for phoria. Rpt in other eye.

http://www.eyetec.net/ce/M5S2.htm