Some reflections on mirror movements

An understanding of a child’s motor development is a key part of paediatric training. We are taught when ‘motor milestones’ should be acquired and how associated movements diminish as a child matures. Mirror movements are a particularly fascinating type of associated movement. They describe unintentional movements that are seen in homologous muscles when only one side is voluntarily activated, for example with attempted individual finger movements. They may be divided into three types: 1 physiological mirror movements of normal development; 2 pathological revealed mirror movements; seen in dystonia or conditions such as stroke or Parkinson’s disease; and 3 congenital mirror movements which occur in neurodevelopmental conditions, such as X-linked Kallmann syndrome, agenesis of the corpus callosum, Klippel–Feil syndrome, and also in congenital hemiplegia. Physiological and revealed mirror movements are characterized by temporal imprecision where movements of the mirroring and non-mirroring sides are not precisely time-locked. Their mechanisms are likely to be due to immaturity or pathological (largely interhemispheric) failure of inhibition. In contrast, congenital mirror movements are time-locked to the voluntary movements and there is evidence of an aberrant corticospinal tract. It has long been known that physiological mirror movements occur in young children and typically wane as the child enters adolescence. An elegant study by Koerte et al. systematically quantifies the distribution of mirror movements in a large group of neurologically healthy participants, aged between 3 and 96 years. Using both clinical and computerbased assessments; the authors show a strong non-linear relationship between mirror movements and age. A rapid decline in mirror movements is observed during childhood and adolescence with a gradual increase starting in the third decade. Some task effect was noted, with more controlled movements evoking stronger mirror movements. Interestingly, throughout the age span mirror movements were more intense in females. By providing normative data, this study will provide clarity in distinguishing physiological from pathological mirror movements. The U-shaped relationship with age may reflect the processes of increasing inhibition during childhood and adolescence with the later increase suggesting an age-related loss of inhibition. Since it is known that mirror movements can represent an early pathological sign of Parkinsonism and cerebrovascular disease, it will be of interest to know the fate of some of the older participants in this study. Neurophysiological studies in atypical development have provided important insights into the underlying processes involved in motor control. The precise time-locking of congenital mirror movements is demonstrated by analysing the timing of electromyographic (EMG) bursts in the voluntarily activated and the mirroring muscles. Cross-correlation and coherence analysis of EMGs and electroencephalograph in individuals with congenital mirror movements show that in contrast to non-mirroring individuals and in those with physiological and revealed mirror movements, there is temporal synchrony of EMGs. Transcranial magnetic stimulation has shown that in these individuals there are abnormal fastconducting ipsilateral corticospinal pathways which originate alongside the crossed corticospinal pathways, thus providing bilateral drive to the left and right motoneurone pools. Finally, tractography studies have shown in detail the aberrant corticospinal tracts. Recent progress in our understanding has been made in two groups with congenital mirror movements. First, in children with congenital hemiplegia it has been shown that motor function may be maintained through ipsilateral corticospinal activation from the undamaged cortex, albeit at the expense of producing mirror movements. In animal studies this process has been shown to result from activity dependent processes. Ipsilateral corticospinal pathways are normally withdrawn in the perinatal period but persist if there is functional inactivation of the contralateral corticospinal tract. In a second group with familial mirror movements who are otherwise neurodevelopmentally normal, a candidate gene has recently been identified where a frameshift mutation leads to truncated DCC (deleted in colorectal carcinoma) protein. This receptor protein is normally involved in guiding developing central nervous system axons across the midline so it is likely that mutations of this protein result in reduced or absent axonal fibre crossing and abnormal persistence of ipsilateral neural connections. Abnormal neuronal signalling is also a likely mechanism in other congenital mirror movements such as X-linked Kallmann syndrome. Mirror movements, though often of little clinical significance, deserve our attention. Not only do they tell us about normal human development but they may be a harbinger of neurological illness in older participants and in their most florid form they lead us to consider basic mechanisms of corticospinal tract development and neuronal plasticity.