Hemifield slide diplopia from altitudinal visual field defects.

We report two cases in which heteronymous, altitudinal visual field defects resulted in loss of fusion and transient overlap of preserved hemifields. This phenomenon resulted in complaints of diplopia similar to that described as "hemifield" slide previously described by temporal hemianopsias. These cases illustrate that generalized loss of fusion, and not the particular type of hemianopic field defect, is responsible for the symptoms known as hemifield slide.