Mirror movements are involuntary movements of one side of the body that accompany and "mirror" (reflect) intentional movements on the opposite side of the body (also known as imitation synkinesis). They are usually symmetrical and most often seen when using distal muscles of the upper limb. Mirror movements are frequently present in young children but prevalence decreases with age. Persistence of mirror movements into adult life ("congenital mirror movements") is pathological, as is acquisition in adult life. These movements are uncommon after acquired brain lesions with no relationship to specific anatomical areas.
Congenital mirror movements are associated with skeletal developmental abnormalities, especially of the atlanto-occipital region, such as Klippel-Feil syndrome. They are also seen in 85% of patients with X-linked Kallmann syndrome (hypogonadotrophic hypogonadism and anosmia). Acquired mirror movements have been described following thalamic lesions, and in association with spastic paraparesis, extrapyramidal disorders, Friedreich’s ataxia, phenylketonuria, and affecting hemiparetic limbs following stroke in young children.
There is some neurophysiological evidence from patients with Xlinked Kallmann syndrome for the existence of an ipsilateral corticospinal pathway, consistent with other evidence that the congenital condition is primarily a disorder of axonal guidance during development. Concurrent activity within ipsilateral and contralateral corticospinal pathways may explain mirroring of movements. Alternatively, a failure of transcallosal inhibition, acquired at the time of myelination of these pathways, may contribute to the genesis of mirror movements. Loss of joint position sense following thalamic lesions may be of relevance. A deficit of sustained attention has also been postulated as the cause of mirror movements.
Farmer SF, Harrison LM, Mayston MJ, Parekh A, James LM, Stephens JA. Abnormal cortex-muscle interactions in subjects with X-linked Kallmann’s syndrome and mirror movements. Brain 2004; 127: 385-397
Mayston MJ, Harrison LM, Quinton R, Stephens JA, Krams M, Bouloux P-MG. Mirror movements in X-linked Kallmann’s syndrome.
I. A neurophysiological study. Brain 1997; 120: 1199-1216