Predicting speech fluency and naming abilities in aphasic patients

Abstract

There is a need to identify biomarkers that predict degree of chronic speech fluency/language impairment and potential for improvement after stroke. We previously showed that the Arcuate Fasciculus lesion load (AF-LL), a combined variable of lesion site and size, predicted speech fluency in patients with chronic aphasia. In the current study, we compared lesion loads of such a structural map (i.e., AF-LL) with those of a functional map [i.e., the functional gray matter lesion load (fGM-LL)] in their ability to predict speech fluency and naming performance in a large group of patients. The fGM map was constructed from functional brain images acquired during an overt speaking task in a group of healthy elderly controls. The AF map was reconstructed from high-resolution diffusion tensor images also from a group of healthy elderly controls. In addition to these two canonical maps, a combined AF-fGM map was derived from summing fGM and AF maps. Each canonical map was overlaid with individual lesion masks of 50 chronic aphasic patients with varying degrees of impairment in speech production and fluency to calculate a functional and structural lesion load value for each patient, and to regress these values with measures of speech fluency and naming. We found that both AF-LL and fGM-LL independently predicted speech fluency and naming ability; however, AF lesion load explained most of the variance for both measures. The combined AF-fGM lesion load did not have a higher predictability than either AF-LL or fGM-LL alone. Clustering and classification methods confirmed that AF lesion load was best at stratifying patients into severe and non-severe outcome groups with 96% accuracy for speech fluency and 90% accuracy for naming. An AF-LL of greater than 4 cc was the critical threshold that determined poor fluency and naming outcomes, and constitutes the severe outcome group. Thus, surrogate markers of impairments have the potential to predict outcomes and can be used as a stratifier in experimental studies.

Keywords: aphasia; diffusion tensor imaging; fluency; functional MRI; lesion size/volume; outcome; therapy.

Figure 1

Canonical AF, Functional gray matter (fGM), and Combined AF-fGM maps. (A) First column is the canonical probabilistic map of AF tract derived from DTI overlaid onto a normalized averaged T1 brain in MRIcon. (B) Second column shows the canonical map of functional gray matter map. (C) Third column displays combined canonical map of combined AF-fGM map.

Figure 2

CIUs/min vs. AF-, fGM-, and Combined AF-fGM lesion loads. Lesion load is displayed on the X-axis, and CIUs/min is displayed on the Y-axis. AF-LL is shown in red, fGM lesion load is shown in blue, and combined AF-fGM-LL is shown in purple with corresponding regression curves. All regressions are significant (p < 0.01), and AF-LL significantly predicted better for speech efficiency through multiple regression analysis (p < 0.01).

Figure 3

Words/min vs. AF-, fGM-, and Combined AF-FGM Lesion Loads. Lesion load is displayed on the X-axis, and Words/min is displayed on the Y-axis, and lesion loads are as labeled in Figure 1. All regressions are significant (p < 0.01), and AF-LL predicted words/min significantly better than fGM-LL (p < 0.05).

Figure 4

Naming Ability vs. AF-, fGM-, and Combined AF-fGM Lesion Loads. Lesion load is displayed on the X-axis, and naming ability is displayed on the Y-axis; lesion loads are as labeled in Figure 1. All regressions are significant (p < 0.01), and AF-LL significantly predicted better for naming ability through multiple regression analysis (p < 0.05).

Figure 5

Speech Fluency ROC curve shows prediction from AF-LL and lesion volume for speech fluency. AF lesion load (in red) was the best at 96% in accuracy predicting severe and moderately/mildly affected groups at threshold at 3.75 cc.

Figure 6

Naming Ability ROC curve shows prediction from AF-LL and lesion volume for naming ability. AF-LL was best at predicting naming with 90% accuracy, and a threshold for severe group at 4 cc.

Figure 7

Speech Fluency Classification of severe and moderately/mildly affected speech fluency outcome (shown by red horizontal line) at 15 CIUs/min. The vertical line is at 3.75 cc of AF-LL determined by the ROC curve as the threshold for classifying severe outcome, which is highlighted in blue.