Lesions in early cerebellar development – do they matter?

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  • This article is a commentary on Poretti et al., pp 718-724 of this issue.

Around 80% of cortical neurons, especially those from the frontal lobe, project to the cerebellum which possesses altogether around half of the brain’s total number of neurons.1 It is increasingly recognized that the cerebellum does not only play a role in the coordination of movements and in the control of posture, but also in cognitive networks. Positron emission tomography and functional magnetic resonance imaging studies have consistently reported on cerebellar activation associated with cognitive operations such as memory retrieval, verbal fluency, language comprehension, and control of attention.2

Accordingly, the last few years have seen more and more studies and papers addressing the question of cerebellar structural impairment, not only with respect to ataxic movement disorder but also to cognition and behaviour. Concerning the role of early cerebellar impairment, ataxic cerebral palsy (CP) could be considered a model. Studies analysing structural findings of the cerebellum with respect to functional outcome in ataxic CP concluded that the presence of a cerebellar abnormality and its extent did not relate to the severity of the clinical picture. It is, however, noteworthy that ataxic CP is usually not only a motor disorder but is also characterized by quite clear cognitive impairment which may eventually dominate the clinical picture.3

Other evidence that early impairment of cerebellar development has more of an impact on motor function than on cognitive development comes from extremely preterm survivors who, in contrast to the more mature preterm and term children, are prone to have cerebellar injury which is more associated with cognitive deficits than with ataxic symptoms.4 It is striking that in these preterm children, cerebellar lesions affect the vermis and, importantly, both hemispheres.

It is less clear whether unilateral cerebellar lesion or disruptions have an impact on cognitive and motor development. Poretti et al.5 report on the outcome of children with severe unilateral cerebellar hypoplasia. They describe no supratentorial abnormalities, thus, the assumption is that the cerebellar pathology determines the clinical picture. Four of their seven patients showed rather good cognitive outcome and the other three had learning problems which they characterized as mild, one patient, however, having a very severe expressive language problem. Five of the children had some signs of truncal ataxia which was never severe; two patients did not show any ataxic symptoms. The association between the degree of cerebellar involvement and the functional outcome was less evident. Ataxia was only seen in patients showing an abnormal vermis, the two non-ataxic patients having a normal vermis structure. Relation to cognitive outcome was less clear, however. The authors had previously reported on the consequences of hemispheric cerebellar cleft on outcome6 with relatively good cognitive development when no supratentorial abnormality was therefore present (except for one patient). The preliminary conclusion, thus, could be that early lesions of only one cerebellar hemisphere do not clearly affect cognitive outcome, certainly not severely.

As discussed above, functional imaging studies have clearly shown cerebellar contribution to language and cognitive networks. It is also known that early unilateral lesions to the telencephalon may lead to the reorganization of the overall network, including the cerebellar part of the language network.7 It is, thus, tempting to think about functional studies in such patients to learn more about the young brain’s capacity for reorganization.

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