History Taking

At the assistant level the history is a structure to be filled in — complaint, onset, duration, laterality. At the technician level it is an instrument. A well-taken history localizes the problem along the visual pathway and sets the tempo of the visit before a single measurement is made, and it does so through a small number of questions that are chosen rather than recited.

Three axes do most of the work: tempo, laterality, and whether the phenomenon is positive or negative. Tempo separates vascular events (seconds to minutes) from inflammatory and infectious processes (hours to days) from degenerative and refractive change (months to years). Laterality separates disease anterior to the chiasm, which is monocular, from disease behind it, which is homonymous — with the chiasm itself as the exception: compression there produces a bitemporal defect, present in both eyes and respecting the vertical midline. And a positive phenomenon — something added to the visual world — arises either from excitation of tissue, as when vitreoretinal traction fires the retina and the patient sees a flash, or from an entoptic shadow, as when vitreous opacities cast floaters on the retina; a negative phenomenon — something subtracted, such as darkness, greyout or a missing sector — arises from loss of function.