VISUAL-FIELDS IN PATIENTS WITH POSTERIOR GPI PALLIDOTOMY

The objective of this study was to describe the incidence and types of visual field defects after posterior globus pallidus internus (GPi) p allidotomy for Parkinson's disease. The creation of the pallidotomy le sion carries a risk of damaging neighboring structures such as the opt ic tract. The reported frequency of visual field defects in patients a fter pallidotomy varies from 0 to 40%. Goldmann visual field testing w as performed on 40 patients who underwent microelectrode-guided poster ior GPi pallidotomy. The optic tract was identified during the procedu re by listening during microelectrode recording for the evoked respons es to light flashes and by assessing stimulation-induced subjective re sponses. After the first 18 patients, lesioning thresholds were increa sed from 0.5 to greater than or equal to 1.0 mA so that the lesion was placed more distant from the optic tract. The location of individual lesions was determined on postsurgical MRI. Three patients (7.5%) had visual field defects likely related to the pallidotomy. These were con tralateral homonymous superior quadrantanopias, associated in two pati ents with small paracentral scotomas. The incidence of visual field de fects with the early technique was 11% (2/18) and decreased to 4.5% (1 /22) after thresholds for lesioning were increased, Except for the loc ation of the lesion relative to the optic tract (more ventral, adjacen t to or extending into the optic tract), no other variable correlated with a post-pallidotomy visual field defect. Microelectrode-guided GPi pallidotomy is a relatively safe procedure as regards visual function even when the optic tract is used as a guide for lesion placement.