Cholinergic Dysfunction after Traumatic Brain Injury – A PET Study Anna Östberg 1, Jere Virta 2, Juha Rinne 2, Vesa Oikonen 2, Pauliina Luoto 2, Olli Tenovuo* 1 1 Department of Neurology, University of Turku, Finland, 2 Turku PET Centre, Finland *presenting author

Introduction Several lines of evidence suggest that the cholinergic system plays an important role in a significant portion of TBI-related cognitive deficits. Several lines of evidence suggest that the cholinergic system plays an important role in a significant portion of TBI-related cognitive deficits. Numerous studies have suggested that cholinergic stimulation may be beneficial in chronic cognitive TBI sequels. Numerous studies have suggested that cholinergic stimulation may be beneficial in chronic cognitive TBI sequels.

Objectives To test the hypothesis of cholinergic dysfunction after TBI by examining the in vivo cholinergic system with positron emission tomography (PET), and by comparing patients with chronic cognitive TBI sequels with healthy controls. To test the hypothesis of cholinergic dysfunction after TBI by examining the in vivo cholinergic system with positron emission tomography (PET), and by comparing patients with chronic cognitive TBI sequels with healthy controls. To assess whether the acetylcholinesterase (AChE) activity of those TBI patients who have responded to a central AChE inhibitor differ from those who have failed to respond. To assess whether the acetylcholinesterase (AChE) activity of those TBI patients who have responded to a central AChE inhibitor differ from those who have failed to respond.

Subjects Inclusion criteria: chronic sequels of TBI with the presence of all four core symptoms: memory problems, fatigue, decreased initiation, and attention deficits chronic sequels of TBI with the presence of all four core symptoms: memory problems, fatigue, decreased initiation, and attention deficits more than one year post-injury more than one year post-injury mainly diffuse injury mechanism without large (> 1 cm3) local contusions mainly diffuse injury mechanism without large (> 1 cm3) local contusions an earlier treatment trial with rivastigmine with a minimum duration of one week and a minimum daily dose of 3 mg an earlier treatment trial with rivastigmine with a minimum duration of one week and a minimum daily dose of 3 mg

Subjects 38 subjects from the TBI database fulfilled the criteria of the study, 19 of them agreed to participate. 38 subjects from the TBI database fulfilled the criteria of the study, 19 of them agreed to participate. Two persons discontinued the study. Thus, the final study group consisted of 17 subjects, ten of them having shown subjective treatment response to rivastigmine, and seven of them having been non-respondents. Two persons discontinued the study. Thus, the final study group consisted of 17 subjects, ten of them having shown subjective treatment response to rivastigmine, and seven of them having been non-respondents. 12 healthy volunteers as controls 12 healthy volunteers as controls

TABLE 1. The characteristics of the study subjects. Respondents Non- respondents Controls n = 10 n = 7 n = 12 Age 44.5 ± ± ±13.9 TBI severity 1 moderate moderate 2 (20 %) 2 (29 %) severe severe 5 (50 %) 4 (57 %) very severe very severe 3 (30 %) 1 (14 %) Injury mechanism traffic traffic 9 (90 %) 5 (72 %) fall fall 1 (10 %) 2 (28 %) Years from TBI 12.0 ± ± Assessed according to the duration of posttraumatic amnesia, moderate 1 – 7 days, severe 1 – 4 weeks, very severe > 4 weeks

Methods 11 C-MP4A (acetylcholine analog with high AChE specificity) PET imaging scan 11 C-MP4A (acetylcholine analog with high AChE specificity) PET imaging scan The subjects with TBI were scanned twice - for the four weeks preceding the scan they had been either without any centrally acting drugs or had taken rivastigmine at a dose of 1.5 mg b.i.d The subjects with TBI were scanned twice - for the four weeks preceding the scan they had been either without any centrally acting drugs or had taken rivastigmine at a dose of 1.5 mg b.i.d ROIs were delineated bilaterally on the frontal cortex (FCX), lateral temporal cortex (LTC), medial temporal lobe (MTL), inferior part of the parietal lobe (LPI), occipital cortex (OCC), posterior cingulatum (PC) and putamen ROIs were delineated bilaterally on the frontal cortex (FCX), lateral temporal cortex (LTC), medial temporal lobe (MTL), inferior part of the parietal lobe (LPI), occipital cortex (OCC), posterior cingulatum (PC) and putamen

Results – SPM analyses The baseline AChE activity (= without medication) was significantly lower in TBI subjects than in controls in several areas of the neocortex The baseline AChE activity (= without medication) was significantly lower in TBI subjects than in controls in several areas of the neocortex

Results – SPM analyses A significant difference between the respondents and non-respondents was seen bilaterally in the frontal cortex, the respondents having significantly lower AChE activity A significant difference between the respondents and non-respondents was seen bilaterally in the frontal cortex, the respondents having significantly lower AChE activity

Results – SPM analyses The SPM analysis showed a widely significant lowering of the AChE activity in the neocortex after rivastigmine, in both respondents and non- respondents without any significant difference between them. The SPM analysis showed a widely significant lowering of the AChE activity in the neocortex after rivastigmine, in both respondents and non- respondents without any significant difference between them.

Results – ROI analyses AChE activity in the non-respondent group did not differ significantly from the controls except in the inferior part of the parietal lobe (-8.4 ± 2.6 %, p = 0.050) and the occipital cortex (-12.6 ± 6.8 %, p = 0.013). AChE activity in the non-respondent group did not differ significantly from the controls except in the inferior part of the parietal lobe (-8.4 ± 2.6 %, p = 0.050) and the occipital cortex (-12.6 ± 6.8 %, p = 0.013).

Results – ROI analyses In the respondents, the AChE activity was significantly reduced in several cortical areas, including the lateral frontal cortex (-10.9 ± 9.0%, p = 0.003), lateral temporal cortex (-8.4 ± 6.4%, p = 0.032), inferior part of the parietal cortex ( ± 10.0%, p = 0.007), occipital cortex (-9.0 ± 21.8 %, p = 0.047), anterior cingulate cortex (-8.3 ± 4.6 %, p = 0.014), and posterior cingulate cortex (-13.6 ± 7.8 %, p = 0.001) In the respondents, the AChE activity was significantly reduced in several cortical areas, including the lateral frontal cortex (-10.9 ± 9.0%, p = 0.003), lateral temporal cortex (-8.4 ± 6.4%, p = 0.032), inferior part of the parietal cortex ( ± 10.0%, p = 0.007), occipital cortex (-9.0 ± 21.8 %, p = 0.047), anterior cingulate cortex (-8.3 ± 4.6 %, p = 0.014), and posterior cingulate cortex (-13.6 ± 7.8 %, p = 0.001)

Results – ROI analyses Within the subjects with TBI, those responding to rivastigmine treatment showed significantly lower AChE activity compared to the non- respondents in the lateral frontal cortex (-9.4 ± 9.2 %, p = 0.023). Within the subjects with TBI, those responding to rivastigmine treatment showed significantly lower AChE activity compared to the non- respondents in the lateral frontal cortex (-9.4 ± 9.2 %, p = 0.023).

Conclusions The main finding was a widely lowered cortical AChE activity in TBI subjects compared to controls. The main finding was a widely lowered cortical AChE activity in TBI subjects compared to controls. A more profound cholinergic dysfunction in the respondents contrast to non-respondents when compared to controls supports the concept that AChE-inhibitors may be beneficial for some patients with chronic TBI sequels A more profound cholinergic dysfunction in the respondents contrast to non-respondents when compared to controls supports the concept that AChE-inhibitors may be beneficial for some patients with chronic TBI sequels

Conclusions Our study also shows that the respondents have a significantly lower AChE activity in the lateral frontal cortex in direct comparison with the non- respondents. As frontal dysfunction often lies behind the most disabling symptoms of TBI, the finding of a more profound frontal cholinergic dysfunction in the respondents suits nicely into this entirety. Our study also shows that the respondents have a significantly lower AChE activity in the lateral frontal cortex in direct comparison with the non- respondents. As frontal dysfunction often lies behind the most disabling symptoms of TBI, the finding of a more profound frontal cholinergic dysfunction in the respondents suits nicely into this entirety.