Cerebral reorganisation in chronic stroke

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Early functional imaging studies were performed in recovered chronic subcortical stroke patients. These patients demonstrated relative overactivations in a number of motor-related brain regions when performing a motor task compared to control subjects.

In particular, overactivations were seen in dorsolateral premotor cortex (PMd), ventrolateral premotor cortex (PMv), supplementary motor area (SMA), cingulate motor areas (CMA), parietal cortex, and insula cortex (Chollet et al., 1991; Weiller et al., 1992, 1993; Cramer et al., 1997; Seitz et al., 1998). Such findings contributed to the notion that recruitment of these brain regions, particularly those in the unaffected hemisphere, might be responsible for recovery. However, in order to make inferences about the mechanisms underlying functional recovery it is necessary to study patients with different degrees of recovery. If one studies patients with a range of late post-stroke outcomes, it appears that those patients with the best outcome have a "normal" activation pattern when compared to normal controls, whereas those with poorer outcome show excessive activation of the primary and non-primary motor areas in both hemispheres (Ward et al., 2003a). These relative overactivations are often bilateral, involving sensori-motor, premotor, posterior parietal, prefrontal and insular cortices, SMA, CMA, and cerebellum.

When this relationship is explored formally, a significant negative linear correlation is found between the size of brain activation and outcome in a number of brain regions (Fig. 5.1). This result does not initially seem to support the notion that recruitment of these regions facilitates recovery. A key determinant of motor recovery is the integrity of fast direct motor output pathways from primary motor cortex (M1) to spinal cord motor neurons (Heald et al., 1993; Cruz et al., 1999; Pennisi et al., 1999).

(poor)Outcome score (good) (poor)Outcome score (good)

Figure 5.1. Brain regions in which there is a negative (linear) correlation between task-related BOLD signal and outcome score in a group of 20 chronic stroke patients. Results are surface rendered onto a canonical brain shown from above (left hemisphere on the left). The panels represent the same result displayed on axial slices through a canonical T1-weighted image. The plots represent task-related signal change versus outcome score for the peak voxel in BA 4p, contralesional (CL) on the left and ipsilesional (IL) on the right. Each " + " represents one patient. All voxels are significant at P < 0.05, corrected for multiple comparisons across whole brain. cs: central sulcus.

(poor)Outcome score (good) (poor)Outcome score (good)

Figure 5.1. Brain regions in which there is a negative (linear) correlation between task-related BOLD signal and outcome score in a group of 20 chronic stroke patients. Results are surface rendered onto a canonical brain shown from above (left hemisphere on the left). The panels represent the same result displayed on axial slices through a canonical T1-weighted image. The plots represent task-related signal change versus outcome score for the peak voxel in BA 4p, contralesional (CL) on the left and ipsilesional (IL) on the right. Each " + " represents one patient. All voxels are significant at P < 0.05, corrected for multiple comparisons across whole brain. cs: central sulcus.

Thus it is likely that those patients with poorer outcome have greater disruption to this cortico-motoneuronal (CMN) pathway. The work of Strick (1988) and others suggests that in primates at least, the non-primary motor system is organised as a number of neural networks or loops involving pre-motor (both lateral and medial wall), parietal and subcortical regions (see also Chapter 14 of Volume I, p. 12). These motor loops are independent of those involving M1 but crucially they are interconnected at the level of the cortex (Strick, 1988; Dum and Strick, 1991). Furthermore, each has its own direct projection to spinal cord motor neurons. In the face of disruption to the CMN pathway the generation of an output to the musculature requires an increase in signals to spinal cord motor neurons via alternative pathways. The non-primary motor loops described by Strick provide an ideal substrate. The implication is that damage in one of these networks could be compensated for by activity in another, thus explaining the recruitment of regions seen in recovered stroke patients. These projections are unlikely to completely substitute for projections from M1 as they are less numerous and less efficient at exciting spinal cord motor neurons (Maier et al., 2002). Thus patients who rely on these alternative pathways to augment or substitute for CMN pathways are unlikely to fully regain dextrous finger movements. In attempting to reconcile these results with those from early studies it seems likely that patients in many previous studies may not have been fully recovered. Some patients found the finger tapping task more effortful to perform (Chollet et al., 1991) and results in those patients were similar to results in patients with residual motor deficit in a later study (Ward et al., 2003a).

The question of the functional relevance of these additionally recruited regions remains. If one could explain differential recovery in a group of patients with identical anatomical damage by the degree to which certain areas are recruited, that would constitute direct evidence of their functional significance in relation to recovery. Due to the heterogeneous nature of infarcts such a result is very difficult to achieve. An alternative approach is to study the same chronic stroke patients before and after a therapeutical intervention. In chronic stroke patients there should be no change in anatomical connections as a result of treatment, but differences in brain activation seen in relation to improved functional status might suggest that certain regions were causally involved. Increased activity in ipsilesional (IL) PMd has been associated with therapy-induced improvement in both upper limb function (Johansen-Berg et al., 2002a) and gait (Miyai et al., 2003). There is also evidence to suggest that disruption of IL PMd (Fridman et al., 2004) and contralesional (CL) PMd (Johansen-Berg et al., 2002b) by transcranial magnetic stimulation (TMS) impairs performance of a simple motor task in chronic stroke patients but not controls. Fridman et al. (2004) however failed to find any behavioural effect from disruption of CL PMd. It is possible that the patients studied by Johansen-Berg et al. (2002b) had greater impairment, and thus were more reliant on CL PMd, but outcomes were not well characterised in either study so it is difficult to make a comparison.

Thus "alternative" brain regions appear to be recruited after focal damage in those patients with greatest need. In addition, there is evidence that IL PMd takes on an executive motor role, such that task-related BOLD signal increases linearly as a function of hand grip force in chronic stroke patients with significant impairment, but not in good recov-erers or in controls. Thus it is unlikely that the response to focal injury involves the simple substitution of one cortical region for another, as nodes within a remaining motor network may take on new roles, that is there is true lesion-induced reprogramming in the human central nervous system.

The studies discussed so far have been on chronic stroke patients. It appears that the relationship between size of brain activation and outcome in the late post-stroke phase holds true for patients in the early post-stroke phase also, at least when considering the primary and non-primary motor regions discussed above (Ward et al., 2004). Thus patients with greater initial deficit recruited more of the primary and non-primary motor network during hand grip. This result suggests that rather than slowly being recruited over time, brain regions in non-primary motor loops can participate in motor action very early after stroke in those patients that have the greatest need. Such rapid cerebral reorganisation can also be seen in normal people after repetitive TMS (rTMS) to the hand area of M1, which reduces M1 cortical excitability without altering task performance. Lee et al. (2003) demonstrated an immediate increase in recruitment of ipsilateral PMd following rTMS to M1 suggesting that this compensation allowed maintenance of task performance.

In other brain regions there is an interaction between outcome and time after stroke. In CL middle intraparietal sulcus, CL cerebellum, and IL rostral premotor cortex there is a negative correlation between size of activation and outcome in the early but not late post-stroke phase (Fig. 5.2) (Ward et al., 2004). In other words patients with poorer outcome scores recruit these areas only in the early and not in the late post-stroke phase suggesting that those with greater deficit engage attentional networks more in the early compared to late post-stroke phase. Attention may no longer be a useful tool for optimising motor performance in the late post-stroke phase. Alternatively, increasing the degree to which a motor task is attended to by chronic stroke patients might facilitate performance by enhancing detection of discrepancies between predicted and actual consequences of any action. These findings need to be explored further, but in those patients with most to gain from rehabilitation, different therapeutical approaches may be required at different stages after stroke.

The role of CL (ipsilateral to the affected hand) M1 in recovery of motor function after stroke remains controversial. Anatomical studies suggest that both direct (corticospinal) and indirect (corticoreticu-lospinal) pathways from ipsilateral M1 end in projections to axial and proximal stabilising muscles rather than hand muscles (Brinkman and Kuypers, 1973; Carr et al., 1994). However, repetitive TMS to

Figure 5.2. A plot of task-related signal change in posterior contralesional (CL) intraparietal sulcus versus outcome/recovery score for a group of early phase patients (10-14 days post stroke) and a group of late phase stroke patients (over 3 months post stroke). The peak voxel in intraparietal sulcus is shown on canonical coronal T1-weighted brain slice (P < 0.05, corrected for multiple comparisons across whole brain).

Figure 5.2. A plot of task-related signal change in posterior contralesional (CL) intraparietal sulcus versus outcome/recovery score for a group of early phase patients (10-14 days post stroke) and a group of late phase stroke patients (over 3 months post stroke). The peak voxel in intraparietal sulcus is shown on canonical coronal T1-weighted brain slice (P < 0.05, corrected for multiple comparisons across whole brain).

M1 results in errors in both complex and simple motor tasks with the ipsilateral hand (Chen et al., 1997) suggesting that ipsilateral M1 may play a role in planning and organisation of normal hand movement. CL M1 recruitment not present in normal controls has been described in some chronic stroke patients (see Calautti and Baron, 2003, for review), but its contribution to functional recovery remains controversial. A negative correlation between size of activation and outcome, similar to that described in non-primary motor regions, has been demonstrated for CL posterior M1 (Brodmann area, BA 4p) but not anterior M1 (BA 4a) (Ward et al., 2003a). Studies using TMS to disrupt local cortical function have failed to find any functional significance of increased CL M1 activation after stroke (Johansen-Berg et al., 2002b; Werhahn et al., 2003). TMS to the motor hot spot for hand muscles (corresponding to M1) may affect predominantly BA 4a, rather than BA 4p, and so it remains plausible that parts of CL M1 can generate a motor output to an affected hand in patients with a significant deficit, in whom a dependency on alternative motor projections has developed.

For chronic stroke patients with preserved IL M1 shifts in the peak IL sensorimotor activations have been found by comparison to control subjects (Weiller et al., 1993; Pineiro et al., 2001). As with other motor regions, this recruitment depends on the final outcome of the patient. A negative correlation between size of brain activation and outcome is seen in IL BA 4p and in BA 4a, ventral to the peak hand region of M1 (Ward et al., 2003a). Overall, these data suggest that there is a certain amount of remapping of hand representation in M1, even though undamaged, which may result from functionally relevant changes in both its afferent and efferent connections. Changes in somatotopical representation in non-primary motor regions might result in stronger connections with different (e.g. more ventral or caudal) regions of M1 in order to facilitate access to intact portions of the direct corticospinal pathway. Shifts in somatotopic representation in non-primary motor regions might also facilitate recruitment of surviving ischaemia-resistant small diameter myelinated corticospinal fibres, such as those arising from pre-motor cortex, to compensate for loss of large diameter fibres. In our studies, parts of SMA, CMA, and PMd that were outside regions normally activated by the task in the control group, were recruited by chronic patients with poorer outcome (Ward et al., 2003 a). Thus shifts in the hand representation in M1 as well as in non-primary motor regions occurred primarily in patients with greatest deficit and presumably with the most significant damage to CMN pathways. Support for lesion-induced changes in connectivity comes from the observation that rTMS to M1 leads to the stimulated part of M1 becoming less responsive to input from PMd and SMA, as well as increased coupling between an inferomedial portion of M1 and anterior motor areas (Lee et al., 2003).

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