This article continues our series exploring what is thought to be happening in the brain when a person has dystonia. The first article covered the role of the basal ganglia, and the second the role of the cerebellum and surround inhibition in dystonia. This article explores the sensory abnormalities that are thought to be contributing factors in the development of primary dystonia.

Initially it might seem strange to connect subtle changes in the sensory system with the onset of dystonia. However, muscle movement is only possible when there is sensory input from the eyes, touch, or sensory receptors such as proprioceptors (the sensors that tell the brain where a limb is positioned). The sensory system provides the major drive to the motor system and the basal ganglia plays an important role in the central processing of somatosensory (sensory/motor) input.

In a person who does not have dystonia, the brain is able to manage and control movement through the integration of all the messages / signals it receives from receptors and proprioceptors within the skin, muscles, joints etc. This information notifies the brain about where a limb is in relation to other limbs as well as where it is in space (spatial awareness), as well as muscle length and tension. Through these messages the brain is able to understand where a specific body part is in relation to another (proprioception) and which muscles need to relax or contract to create the desired movement. In people with dystonia it has been identified that the brain is not receiving or integrating all sensory messages correctly and in particular the proprioceptive messages (sensory information) from muscles are misprocessed in the brain.

The reason that this is relevant to sensory abnormalities in dystonia is because the inhibitory messages that should be occurring between agonist and antagonistic muscles have been found to be abnormal within the sensory-motor cortex of the brain (see Fig.1.). This coupled with the abnormal processing of the sensory proprioceptive messages may cause inefficient sensory and motor integration, which is believed to be contributing to the generation of dystonic movements.


Symptoms and signs

Mild sensory symptoms such as general discomfort in the neck, ill-defined bodily feelings (discomfort, pain, or abnormal sensations of body position, weight or movement) can occur weeks or even months before the development of dystonia, or indeed after the development of dystonia. Such abnormal feelings prior to the onset of the motor symptoms of dystonia may include symptoms including irritation of the throat prior to the development of spasmodic dysphonia: or in the case of blepharospasm irritation or dry eyes prior to onset. Sensitivity to light is also frequently reported by people with blepharospasm, indeed studies suggest that blepharospasm may be more common in region with higher sunlight, although whether this may be a trigger or part of an underlying cause is uncertain.


Experimental research evidence

A number of experimental techniques have illustrated that the process of integrating sensory inputs before executing motor movements (sensorimotor integration) may in fact be impaired in dystonia. In some people with adult-onset primary focal dystonia, these techniques have identified higher temporal and spatial discrimination thresholds. In other words people found it harder to recognise two stimuli rather than one when they are close together in time (temporal discrimination) or to recognise that two nearby objects touching the skin are truly two distinct points, not one (spatial discrimination). Spatial discrimination testing assesses how the sensory and motor systems integrate.

Abnormal temporal and spatial discrimination thresholds have been found in both the affected and unaffected body regions of people with focal dystonia. This evidence suggests the existence of a problem in sensori-motor integration. Interestingly, temporal discrimination can be impaired both in people with cervical dystonia and in some of their unaffected first-degree relatives, which suggests that there may be a gene/s involved in causing the problem and unaffected people with abnormal temporal determination thresholds are carriers of this.

Abnormal spatial and temporal discrimination has been demonstrated in a number of types of focal dystonia including cervical dystonia, blepharospasm and focal hand dystonia, suggesting it is an underlying problem regardless of the clinical type of dystonia.

To support these findings abnormal temporal discrimination has also been reported in some genetic forms of dystonia, for example in DYT1 dystonia, in both affected people and those carrying the abnormal DYT1 gene but who have not developed dystonia. Again this suggests that sensory abnormalities can be an inherited characteristic that is associated with dystonia but is not a direct symptom of having dystonia.

Fig. 1 MAPS OF THE MOTOR AND SENSORY CORTEX IN THE BRAIN (Image amended from http://home.haugnett.no/arvefahlvik/klem/article4/Article4_html_m1b93870a.jpg)


Evidence of sensory changes in task specific dystonia

The brain has distinct areas that control different actions within the body: the cortex (the surface area of the brain) is responsible for sensory and motor movement signals. There is a map for each area of the body, which can be seen in Figure 1. In task specific dystonias it has been identified that that these somatosensory maps can become altered, as measured by magnetic stimulation of these areas. This can produce blurred/altered representations of a person’s finger/s in focal hand dystonia. It is not known whether the sensory abnormalities are a cause or a result of the motor abnormalities, but successful treatment of the dystonic hand spasms with botulinum toxin can reverse the cortical abnormalities. These changes in cortical representation are a referred to as plasticity and it is believed one of the underlying problems in dystonia is excessive cortical plasticity.


Conclusion

The sensory aspects of dystonia include intrinsic sensory problems and abnormal integration of sensory information into control of the motor output of the brain. The basal ganglia, cerebellum, thalamus, and their connections, coupled with altered sensory input to these pathways, seem to play a key part in producing abnormal sensorimotor integration.

The presence of these sensory abnormalities, coupled with evidence of impaired sensory and motor processing and loss of so-called surround inhibition (please see previous article), provide strong support for the idea that dystonia is not only a motor but also a sensory disorder. However, despite all these findings, an important gap remains in the translation of these abnormalities into an understanding of the fundamental changes that underlie dystonic symptoms, and how to alleviate them.