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The role of transcranial magnetic stimulation in the characterization of congenital hemiparesis
Article first published online: 22 OCT 2009
© The Authors. Journal compilation © Mac Keith Press 2009
Developmental Medicine & Child Neurology
Volume 52, Issue 2, pages 113–114, February 2010
How to Cite
STAUDT, M. (2010), The role of transcranial magnetic stimulation in the characterization of congenital hemiparesis. Developmental Medicine & Child Neurology, 52: 113–114. doi: 10.1111/j.1469-8749.2009.03516.x
- Issue published online: 15 JAN 2010
- Article first published online: 22 OCT 2009
Many patients with congenital hemiparesis possess ipsilateral corticospinal projections originating in the contralesional hemisphere and projecting to target muscles in the paretic hand. Such ipsilateral projections are apparently present in all patients with lesions disrupting the normal crossed corticospinal projections from the affected hemisphere, and also in a subgroup of patients with preserved crossed corticospinal projections from the lesioned hemisphere.
Since the seminal work by Carr et al. in 1993,1 focal transcranial magnetic stimulation (TMS) has evolved as the standard technique to determine the pattern of corticospinal organization in congenital hemiparesis. Holmström et al.2 report a further study on 16 children with congenital hemiparesis evaluated by TMS and magnetic resonance imaging as well as with a detailed assessment of both uni- and bimanual hand function. Several aspects of this study are relevant for both the clinician and the researcher dealing with congenital hemiparesis.
First, a TMS protocol similar to one suggested in a previous study3 was applied, which deviates from the ‘standard’ protocols typically applied in cooperative adult participants. Rather than determining the motor thresholds as exactly as possible (by slowly increasing the stimulator output until five out of ten stimuli elicit a motor evoked potential [MEP] of at least 50μV), the stimulator output was increased rather rapidly by increments of 5% until a reproducible response was elicited. Furthermore, corticomuscular latencies were not determined at 110% or 120% of the motor thresholds, but were measured simply using the first three MEPs reproducibly elicited. With such shortened protocols, the main question in the context of congenital hemiparesis (‘Can MEPs be elicited by TMS of the affected and/or the contralesional hemisphere?’) can be addressed with significantly fewer transcranial magnetic stimuli per session. This has clear advantages for younger and less cooperative patients,3 which outweigh the disadvantage that only estimates of the motor thresholds and corticomuscular latencies can be obtained.
Second, Holmström et al. confirm that the information obtained by TMS is clinically relevant. With their detailed assessment of uni- and bimanual hand function, they confirm previous studies1,3 that patients with ipsilateral corticospinal projections, as a group, show more severely impaired hand function than patients with contralateral projections; and that nevertheless, many patients depending on ipsilateral projections can use their paretic hand as an actively grasping, assisting hand.
In addition to this association with the severity of hand motor dysfunction, there is increasing evidence that patients depending on ipsilateral projections present a qualitatively different type of movement disorder. Holmström et al. confirm previous studies3 that only patients with this pattern of corticospinal organization show marked involuntary mirror movements, not only in the non-paretic hand (while moving the paretic hand voluntarily), but also in the paretic hand (while moving the non-paretic hand voluntarily). This inability to control the non-paretic hand independently points to a ‘dysfunctional aspect’ of the contralesional hemisphere, which can easily be explained by the representation of two hands instead of only one in its motor cortex. These mirror movements, might be only the ‘tip of the iceberg’ of – as yet poorly investigated – qualitative differences in motor performance between patients with ipsilateral and contralateral motor control. Although not explicitly addressed in the current paper, the lower dexterity of the non-paretic hand observed in many patients might also be a consequence of this ‘concordant’ representation of two hands in one motor cortex. Accordingly, the effectiveness of functional therapeutic approaches, such as constraint induced movement therapy, appears to be influenced by the pattern of corticospinal organization.4 And finally, increasing casuistic evidence suggests that, in children with hemiparesis with a preserved grasp function, the detection of an ipsilateral corticospinal organization can predict preserved grasp function after hemispherectomy.5
In conclusion, TMS evolves as an important tool in the characterization of patients with congenital hemiparesis. At this point, its application seems mandatory in potential candidates for hemispherectomy (or similar procedures) and at least advisable in any descriptive or therapeutic studies on congenital hemiparesis. In the clinical situation, TMS can certainly yield useful information concerning prognosis and the characterization of the movement disorder in an individual child. With the use of simplified and shortened protocols as described above, TMS might thus be on the verge of becoming a standard clinical procedure in congenital hemiparesis.
- 2Hand function in relation to brain lesions and cortico-motor projection pattern in children with unilateral cerebral palsy. Dev Med Child Neurol (Published online) DOI: 10.1111/j.1469-8749.2009.3496., , , et al.