History:  This 6-year-old  was brought in by her mother with the complaint that the girl's left eye wanders when she is tired or inattentive and at other unpredictable times.  Visual acuity is OD 20/20 and OS 20/25.  Her cycloplegic refraction is OD +.75 and OS +.75.  She is not wearing glasses.  Prism and cover test in the primary position measures up to 30 prism diopters of exodeviation, but the left eye recovers from a larger exo shift than the right eye.  The left eye also deviates upward, but this is difficult to measure with prisms. No fusion is measured with the usual test and the girl prefers fixation with the right eye with the four light test. 

Diagnosis:  dissociated horizontal deviation

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