Altered eyeblink reflex conditioning in restless legs syndrome patients

Sensory aspects of restless legs syndrome: Clinical, neurophysiological and neuroimaging prospectives

Restless Legs Syndrome (RLS) is a complex sensorimotor disorder, classified among the sleep-related movement disorders. Although sensory symptoms appear as key features of the disorder, they are still poorly characterized from a clinical perspective and conceptualized from a pathophysiological point of view. In this review, we aim to describe the clinical and functional substrates of RLS, focusing mainly on its sensory symptoms and on their neurophysiological and anatomical correlates. Knowledge of both subjective sensory symptoms and objective sensory signs are still controversial. Current data also indicate that the sensory component of RLS seems to be subserved by anomalies of sensorimotor integration and by mechanism of central sensitization. Overall, electrophysiological findings highlight the involvement of multiple generators in the pathogenesis of RLS, eventually resulting in an increased nervous system excitability and/or alterations in inhibition within the somatosensory and nociceptive pathways. Structural and functional neuroimaging data show the involvement of several crucial areas and circuits, among which the thalamus appears to play a pivotal role. A holistic approach looking at brain connectivity, structural or functional abnormalities, and their interplay with molecular vulnerability and neurotransmitter alterations is warranted to disentangle the complex framework of RLS.

Central and peripheral nervous system excitability in restless legs syndrome

Neurophysiological techniques have been applied in restless legs syndrome (RLS) to obtain direct and indirect measures of central and peripheral nervous system excitability, as well as to probe different neurotransmission pathways. Data converge on the hypothesis that, from a pure electrophysiological perspective, RLS should be regarded as a complex sensorimotor disorder in which cortical, subcortical, spinal cord, and peripheral nerve generators are all involved in a network disorder, resulting in an enhanced excitability and/or decreased inhibition. Although the spinal component may have dominated in neurophysiological assessment, possibly because of better accessibility compared to the brainstem or cerebral components of a hypothetical dysfunction of the diencephalic A11 area, multiple mechanisms, such as reduced central inhibition and abnormal peripheral nerve function, contribute to the pathogenesis of RLS similarly to some chronic pain conditions. Dopamine transmission dysfunction, either primary or triggered by low iron and ferritin concentrations, may also bridge the gap between RLS and chronic pain entities. Further support of disturbed central and peripheral excitability in RLS is provided by the effectiveness of nonpharmacological tools, such as repetitive transcranial magnetic stimulation and transcutaneous spinal direct current stimulation, in transiently modulating neural excitability, thereby extending the therapeutic repertoire. Understanding the complex interaction of central and peripheral neuronal circuits in generating the symptoms of RLS is mandatory for a better refinement of its therapeutic support.

Sensory aspects of movement disorders

Movement disorders, which include disorders such as Parkinson's disease, dystonia, Tourette's syndrome, restless legs syndrome, and akathisia, have traditionally been considered to be disorders of impaired motor control resulting predominantly from dysfunction of the basal ganglia. This notion has been revised largely because of increasing recognition of associated behavioural, psychiatric, autonomic, and other non-motor symptoms. The sensory aspects of movement disorders include intrinsic sensory abnormalities and the effects of external sensory input on the underlying motor abnormality. The basal ganglia, cerebellum, thalamus, and their connections, coupled with altered sensory input, seem to play a key part in abnormal sensorimotor integration. However, more investigation into the phenomenology and physiological basis of sensory abnormalities, and about the role of the basal ganglia, cerebellum, and related structures in somatosensory processing, and its effect on motor control, is needed.

Dopa responsive burning mouth syndrome: restless mouth syndrome or oral variant of restless legs syndrome?

Burning mouth syndrome (BMS) is an oral dysesthesia presenting as a burning sensation of the tongue and other oral and perioral mucosae. A painful symptomatology in different bodily regions (extraoral) may also be a common feature in patient with BMS. The management of BMS is challenging and there is no clear guideline for the management of idiopathic BMS. Herein, we describe a group of patients (5 patients) in whom symptoms of BMS responded to levodopa. In parallel, four patients fulfilled the criteria for restless legs syndrome (RLS). Family history of RLS was positive in two patients. We reviewed the literature and noted a marked overlap between BMS and RLS. Overlaps were noted in epidemiological profiles, pattern of clinical features and even in neurophysiological observations (alterations in the striatal dopaminergic system). We suggest that a subset of patients with BMS may be a phenotypic variant of RLS and a trial of dopaminergic drugs should be given in patients with BMS who has a history suggestive of RLS or in a patient who do not show a response to usual therapies for BMS.

The conditioned eyeblink reflex: a potential tool for the detection of cerebellar dysfunction in multiple sclerosis

The delayed conditioned eyeblink reflex, in which an individual learns to close the eyelid in response to a conditioned stimulus (e.g. a tone) relies entirely on the functional integrity of a cerebellar motor circuitry that involves the contingent activation of Purkinje cells by parallel and climbing fibres. Molecular changes that disrupt the function of this circuitry, in particular a loss of type-1 metabotropic glutamate receptors (mGlu1 receptors), occur in Purkinje cells of patients with multiple sclerosis and in mice with experimental autoimmune encephalomyelitis as a result of neuroinflammation. mGlu1 receptors are required for cerebellar motor learning associated with the conditioned eyeblink reflex. We propose that the delayed paradigm of the eyeblink conditioning might be particularly valuable for the detection of subtle abnormalities of cerebellar motor learning that are clinically silent and are not associated with demyelinating lesions or axonal damage. In addition, the test might have predictive value following a clinically isolated syndrome, and might be helpful for the evaluation of the efficacy of drug treatment in multiple sclerosis.