Preservation of the upper nasal field in the chiasmal syndrome: an anatomic explanation.

IT IS WELL ESTABISBHED CLINICALLY that the lower temporal chiasmal fibers serving the upper nasal visual fields are the last to fail in patients with intrasellar tumors. In the initial stages and during early progression, the pattern of field loss is often quite atypical.' 2 Notching of the chiasm by overlying arteries may even reverse the progression of field defects.3 Nevertheless, the defects most commonly start in the upper temporal fields, and the last quadrant of the field to be lost is the upper nasal quadrant.