'Hemispherical asymmetry in the Meyer's Loop': a prospective study of visual-field deficits in 105 cases undergoing anterior temporal lobe resection for epilepsy.

OBJECTIVES: Visual-field deficits following temporal lobe surgery have been reported in the literature. In this prospective study, the authors analyse their experience of visual-field deficits in 105 consecutive cases undergoing temporal-lobe surgery performed by a single surgeon, with particular consideration to the laterality of the deficit and its functional implications.

METHODS: 105 consecutive patients undergoing an anterior temporal lobe resection for epilepsy, between March 1998 and June 2004, were selected. The patient population had a mean age of 35 years (range 19-60 years); 53 had a left-sided resection and 52 a right-sided resection. 91 patients had mesial temporal sclerosis, three gangliogliomas, four dysembryoplastic neuroepithelial tumours (DNETs), two neurocytomas and two cavernomas, and in three cases the histology was inconclusive. Pre- and postoperative visual-field tests were obtained using the Humphrey Esterman binocular functional test for all cases. The test was set to stimulus white III, with a single intensity of 10 DB on the background of 31.5 ASB for all patients. A minimum follow-up period of 12 months postsurgery was employed. Postoperative MRI scans were carried out on all patients. 60 scans were randomly selected, and the extent of temporal lobe resection calculated manually for each.

RESULTS: Of the 105 cases, 16 patients had a visual-field deficit postoperatively which was not present preoperatively: 12 following a left and four following a right-sided resection. The OR for incurring a postoperative visual-field defect following left versus right-sided surgery was 3.51 (95% CI 1.05 to 11.73, p=0.04). In four patients, the deficit was severe enough to preclude them from driving in the UK (three left- and one right-sided resection). There was no association between the extent of tissue resection and the incidence of postoperative visual-field deficits.

CONCLUSIONS: This study suggests left-/right-hemispherical asymmetry in the Geniculocalcarine tracts with field deficits being 3.5 times more likely following left-sided anterior temporal lobe resections compared with right-sided resections. This has significant implications on counselling patients for these procedures. MR tractography may provide an anatomical substrate for these clinical findings, perhaps revealing a more anterior course of the optic radiations within the temporal lobe in one hemisphere versus the other.