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To investigate patients with visual field defects following macular hole surgery to determine the cause of such defects, specifically with reference to ischemic damage versus mechanical trauma.Five patients with known visual field defects following macular hole surgery were studied with Goldmann perimetry, Humphrey automated perimetry, and multifocal electroretinography (MERG). Three patients returned at a later date for nerve fiber layer analysis.None of the five patients demonstrated evidence of a- or b-wave loss on MERG in the regions corresponding to the visual field defects. Two of three patients studied with the nerve fiber layer analyzer demonstrated significant loss of nerve fiber layer thickness in the quadrant corresponding to the field defect.The normal MERG results indicate that the possibility of an arteriolar occlusion as the principal cause for the defects is unlikely in most cases. Data suggest that the site of damage is in the nerve fiber layer, although the specific cause of this damage remains to be determined.