The field defects of anterior temporal lobectomy: a quantitative reassessment of Meyer's loop

Temporal lobectomy is often complicated by superior quadrantanopia. The relation of field loss to sagittal resection length can inform us about the functional anatomy of Meyer's loop, with ramifications for surgical planning. However, the literature has produced highly variable results. We studied 29 patients with anterior temporal lobectomies using Goldmann perimetry. 24 patients had post-operating neuroimaging, with which we assessed resection length relative to each patient's temporo-occipital dimensions. For the field defect we calculated the proportion of area lost for three isopters. We found a significant correlation between resection size and field loss for both nasal and temporal defects. Linear regressions suggested an anterior limit of Meyer's loop at 24 to 28 mm from the anterior temporal pole, and involvement of the lower quadrant when resections reached 70 to 79 mm, with significant inter-subject variability. The nasal defect was 15% greater than the temporal defect for all degrees of quadrantanopia, with no difference between right and left hemispheres. Macular involvement began when field defects reached 61% of quadrant area, corresponding to a resection of about 58 mm. Patterns of field loss showed that the lower margins were most often horizontal or with a slight slope towards fixation, rather than true wedge defects. We conclude that field loss is related to resection length and that Meyer's loop extends more anteriorly than estimated in traditional surgical studies, in agreement with modern MRI and dissection studies. The patterns of field loss support a revised retinotopic model in which the most anterior fibers of Meyer's loop represent the superior field, not the vertical meridian as traditionally proposed.