Unusual chiasmal visual field defects.

Masses beneath the chiasm usually cause superiorly denser field defects. This report showed two very rare field defects related with infrachiasmal compression, monocular inferotemporal quadrantanopia in Case 1 and monocular temporal and inferonasal field defect in Case 2. We presume that these very rare visual field defects might be caused by a compression between the mass and the anterior cerebral artery. The detailed anatomic relations of the compressed optic chiasm between the mass and the anterior cerebral artery are best revealed on a coronal T2-weighted image with a section thickness of 3 mm or less, and a routine brain MRI with a larger section thickness may not reveal such findings. Therefore, appropriate resolutions and a meticulous review of MR imaging may provide the clues to the etiology of these visual defects. In conclusion, the clinician should be aware that such unusual visual field defects could also be caused by a compression between the mass and anterior cerebral arteries.