Enhanced associated movements in the contralateral limbs elicited by brisk voluntary contraction in choreic disorders

Bringing order to higher order motor disorders

Majority of movements in everyday situations are complex and involve volition, planning of the movement and selection of the motor programme, all occurring before movement execution. Higher order motor disorders may be defined as abnormal motor behaviours resulting from disruption of any of the cortical processes that precede execution of the motor act. They are common in patients with neurodegenerative disorders, psychiatric diseases and structural brain lesions. These abnormal behaviours may be overlooked in the clinic, unless specifically evoked by the examiner. We discuss clinical and pathophysiological aspects of higher order motor disorders including: (1) disorders of disinhibition, such as grasp reflex and grasping behaviour, utilisation and imitation behaviour, motor preservations and paratonia; (2) disorders of motor intention such as motor neglect and motor impersistence; (3) alien limb syndrome; and (4) motor overflow phenomena, such as mirror movements and synkinesias. A video illustration of each phenomenon is provided. We place the findings from recent neurophysiological studies within the framework of theories of motor control to provide better insight into pathophysiology of different disorders.

Mirror movements in movement disorders: a review

BACKGROUND

Mirror movements (MM) are involuntary movements of homologous muscles during voluntary movements of contralateral body regions. While subtle mirroring can be present in otherwise healthy adults, overt MM may be common in many movement disorders. Examining these collective findings may further our understanding of MM and help define their usefulness as a clinical sign.

METHODS

We sought to review English language research articles examining the presence, clinical significance, and/or pathophysiology of MM in Parkinson's disease (PD), corticobasal syndrome (CBS), essential tremor (ET), focal hand dystonia, Creutzfeldt-Jakob's disease (CJD), and Huntington's disease. When available, MM in these disorders were compared with those of healthy age-matched controls and congenital disorders such as Klippel-Feil syndrome and X-linked Kallman's syndrome.

RESULTS

Clinical presentation of MM is common in asymmetric parkinsonian disorders (early PD, CBS) and manifests differently depending on the side affected (less affected hand in PD, more affected hand in CBS, either hand in ET, and both hands in healthy adults and congenital disorders), stage of disease (early, asymmetric PD and CJD), and presence of concomitant mirror-like overflow phenomena (focal dystonia and CBS-associated alien hand). In general, uncrossed descending corticospinal projections (congenital MM) and/or abnormal activation of the motor cortex ipsilateral to the voluntary task (most acquired MM), i.e., activation of the normal crossed corticospinal pathway, are required for the generation of MM.

DISCUSSION

MM are common motor phenomena and present differently in several acquired (mostly neurodegenerative) and congenital movement disorders. Future studies on MM will enhance the clinical diagnosis of selected movement disorders and contribute to our understanding of the normal physiology of bimanual coordination.

Relative or absolute? Implications and consequences of the measures adopted to investigate motor overflow

Motor overflow is involuntary overt movement or covert muscle activity that cooccurs with voluntary movement. Overflow is present in several pathological conditions, as well as in neurologically healthy children and older adults, and can be induced in healthy young adults under effortful conditions. This motor phenomenon may provide insight into the underlying mechanisms and kinetic characteristics of voluntary and involuntary motor control in various populations. Although often measured behaviorally using force transduction techniques, different methods of calculating and presenting such overflow data have resulted in seemingly contradictory findings, with limited discussion of the advantages and limitations of different approaches. In this article, the authors examined the relevant literature to highlight significant methodological considerations for authors and readers conducting or appraising this type of research. Issues regarding the interpretation and reporting of findings are also discussed. Researchers are encouraged to continue using behavioral measures to create well-defined variables that enable the study of the kinematic characteristics of overflow, as these may offer promising new ways forward in better characterizing and understanding this intriguing movement phenomenon.

Finger muscle control in children with dystonia

BACKGROUND

Childhood dystonia is a disorder that involves inappropriate muscle activation during attempts at voluntary movement. Few studies have investigated the muscle activity associated with dystonia in children, and none have done so in the hands.

METHODS

In this study, we measured surface electromyographic activity in four intrinsic hand muscles while participants attempted to perform an isometric tracking task using one of the muscles.

RESULTS

Children with dystonia had greater tracking error with the task-related muscle and greater overflow to non-task muscles. Both tracking error and overflow correlated with the Barry-Albright Dystonia scale of the respective upper limb. Overflow also decreased when participants received visual feedback of non-task muscle activity.

DISCUSSION

We conclude that two of the motor deficits in childhood dystonia--motor overflow and difficulties in actively controlling muscles--can be seen in the surface electromyographic activity of individual muscles during an isometric task. As expected from results in adults, overflow is an important feature of childhood dystonia. However, overflow may be at least partially dependent on an individual's level of awareness of their muscle activity. Most importantly, poor single-muscle tracking shows that children with dystonia have deficits of individual muscle control in addition to overflow or co-contraction. These results provide the first quantitative measures of the muscle activity associated with hand dystonia in children, and they suggest possible directions for control of dystonic symptoms.

Mechanisms underlying functional changes in the primary motor cortex ipsilateral to an active hand

Performance of a unimanual hand motor task results in functional changes in both primary motor cortices (M1(ipsilateral) and M1(contralateral)). The neuronal mechanisms controlling the corticospinal output originated in M1(ipsilateral) and the resting hand during a unimanual task remain unclear. Here, we assessed functional changes within M1(ipsilateral) and in interhemispheric inhibition (IHI) associated with parametric increases in unimanual force. We measured motor-evoked potential (MEP) recruitment curves (RCs) and short-interval intracortical inhibition (SICI) in M1(ipsilateral), IHI from M1(contralateral) to M1(ipsilateral), and the influence of IHI over SICI using transcranial magnetic stimulation at rest and during 10, 30, and 70% of maximal right wrist flexion force. EMG from the left resting flexor carpi radialis (FCR) muscle was comparable across conditions. Left FCR MEP RCs increased, and SICI decreased with increasing right wrist force. Activity-dependent (rest and 10, 30, and 70%) left FCR maximal MEP size correlated with absolute changes in SICI. IHI decreased with increasing force at matched conditioned MEP amplitudes. IHI and SICI were inversely correlated at increasing forces. In the presence of IHI, SICI decreased at rest and 70% force. In summary, we found activity-dependent changes in (1) SICI in M1(ipsilateral), (2) IHI from M1(contralateral) to M1(ipsilateral), and (3) the influence of IHI over SICI in the left resting hand during force generation by the right hand. Our findings indicate that interactions between GABAergic intracortical circuits mediating SICI and interhemispheric glutamatergic projections between M1s contribute to control activity-dependent changes in corticospinal output to a resting hand during force generation by the opposite hand.