Changes in fiber tract integrity and visual fields after anterior temporal lobectomy

Abstract

Objective: To investigate postoperative changes in fiber tract integrity in patients with temporal lobe epilepsy (TLE) following anterior temporal lobectomy (ATL) and to determine whether postoperative changes are 1) stable vs progressive and 2) related to visual field defects.

Methods: Diffusion tensor imaging (DTI) was obtained in 7 patients with TLE before, 2 months after, and 1 year after ATL. Changes in fractional anisotropy (FA) were evaluated in a whole-brain voxel-wise analysis, as well within specific fiber tracts. Repeated-measures analysis of variance was performed to examine the time course of FA changes within ipsilateral and contralateral fiber tracts. Quantitative visual field analysis was performed to determine whether decreases in regional FA were related to the extent or location of visual field defects.

Results: Patients showed decreased FA 2 months post-ATL in ipsilateral fiber tracts transected during surgery (parahippocampal cingulum, uncinate fasciculus, inferior longitudinal fasciculus, and fornix), as well as in fiber tracts not directly transected (inferior fronto-occipital fasciculus and corpus callosum). Additional decreases in FA were not observed from 2 months to 1 year post-ATL. Visual field defects in most patients were characterized by incomplete quadrantanopsias. However, FA reductions in one patient extended into temporo-occipital cortex and the splenium of the corpus callosum and were associated with a complete hemianopia.

Conclusions: Wallerian degeneration is apparent 2 months following unilateral ATLs in ipsilateral fibers directly and indirectly affected during surgery. These changes do not appear to progress over the course of a year, but may correlate with the nature and extent of postoperative visual field defects.

Figures

Figure 1. Diffusion tensor imaging atlas of white matter tracts

Automated probabilistic fiber atlas of white matter tracts for a healthy individual. CC = corpus callosum; CST = corticospinal tract; FORX = fornix; IFOF = inferior fronto-occipital fasciculus; ILF = inferior longitudinal fasciculus; PHC = parahippocampal cingulum; SLF = superior longitudinal fasciculus; THAL = anterior thalamic radiations; UNC = uncinate fasciculus.

Figure 2. Preoperative to postoperative fractional anisotropy (FA) change and visual field defects

Voxel-wise FA change maps for each patient superimposed on their postoperative T1-weighted MRI with fiber tract overlays. FA change values from pre to 2 months post anterior temporal lobectomy (ATL). FA change maps from 2 months to 1 year post-ATL (A). FA change range from a decrease of 0.01 (pure red) to a decrease of 0.05 (pure yellow) for each individual (B). Pattern deviation probability plots (PDPPs) for each individual obtained at 2 months post-ATL (C). The measured visual field values are represented as the percentage of age-matched controls with lower sensitivity or no response to stimuli at each spatial location. The PDPPs also correct for typical vision changes in the height of the hill of vision. Shaded values range from areas of relatively normal vision (small white dots) to areas in which 99.5% of age-matched controls show greater sensitivity at that spatial location (percent

Figure 3. Fractional anisotropy (FA) change for ipsilateral and contralateral fiber tracts

Mean FA change values for each of the ipsilateral (A) and contralateral (B) fiber tracts. Negative values represent a decrease in FA and positive values represent an increase in FA. Fibers with a significant time × side interaction are denoted with *. Fibers with a significant main effect of time are denoted with +. CING = cingulum (superior part); CST = corticospinal tract; FORX = fornix; IFOF = inferior fronto-occipital fasciculus; ILF = inferior longitudinal fasciculus; PHC = parahippocampal cingulum; SLF = superior longitudinal fasciculus; THAL = anterior thalamic radiations; UNC = uncinate fasciculus.

Figure 4. Relationship between inferior longitudinal fasciculus (ILF) fractional anisotropy (FA) change and visual field mean deviation (MD) scores

Scatterplots of the relationship between pre to 2-month changes in FA of the ipsilateral ILF and the MD score for the ipsilateral (A) and contralateral (B) eyes.