Impaired Limb Proprioception in Adults With Spasmodic Dysphonia

Abstract

Objectives: Focal dystonia of the head and neck are associated with a loss of kinesthetic acuity at muscles distant from the dystonic sites. That is, while the motor deficits in focal dystonia are confined, the associated somatosensory deficits are generalized. This is the first systematic study to examine, if patients diagnosed with spasmodic dystonia (SD) show somatosensory impairments similar in scope to other forms of focal dystonia.

Methods: Proprioceptive acuity (ability to discriminate between two stimuli) for forearm position and motion sense was assessed in 14 spasmodic dystonia subjects and 28 age-matched controls using a passive motion apparatus. Psychophysical thresholds, uncertainty area (UA), and a proprioceptive acuity index (AI) were computed based on the subjects' verbal responses.

Results: The main findings are as follows: first, the SD group showed significantly elevated thresholds and UAs for forearm position sense compared with the control group. Second, 9 of 14 dystonia subjects (64%) exhibited an AI for position sense above the control group maximum. Three SD subjects had a motion sense AI above the control group maximum.

Conclusions: The results indicate that impaired limb proprioception is a common feature of SD. Like other forms of focal dystonia, spasmodic dystonia does affect the somatosensation of nondystonic muscle systems. That is, SD is associated with a generalized somatosensory deficit.

Keywords: Basal ganglia; Focal dystonia; Human; Kinesthesia; Somatosensation.

Fig. 1

A. Experimental setup with passive motion apparatus. The arm was rotated horizontally towards flexion or extension (axis of rotation = dashed line). EMG of the biceps and triceps brachii was recorded to monitor muscle activation. B. Exemplar psychometric function obtained during motion sense testing. The JND corresponds to the stimulus size at 75% correct response level. The uncertainty area is the standard deviation of the Gaussian function fit.

Fig. 2

A. Exemplar acuity functions of one control and two SD subjects. Note that with respect to the control, the functions of the SD subjects are shifted to the right giving rise to higher thresholds. Also note that next to an elevated threshold, subjects may also exhibited less certainty in discriminating stimuli around the threshold resulting in a larger uncertainty area (e.g. SD 27). B. JND as a function of UA for position sense. Each data point reflects a single subject. Note that the SD sample revealed a left and upward shift with respect to the controls. Eight SD subjects showed UA and/or JND values above the maximum of the control group.

Fig. 3

A. Mean thresholds for position and motion sense for both groups. Mean JND was significantly larger for position, but not for motion sense. Error bars represent 1 standard deviation (* = p < 0.01; n.s. = not significant). B. Mean JND and mean UA for position and motion sense in relation to the tested standards (10° and 1.5°/s). The comparison stimuli for position sense testing were always smaller than 10°, for motion sense testing stimuli were always larger than 1.5°/s.

Fig. 4

Proprioceptive acuity index for position sense. Data points represent PAI values of each SD subject (for better readability sorted in ascending order). The shaded area represents range of observed PAIs for the control group. Note that 9 SD subjects had PAIs above the maximum of the control group. No SD subject had PAI values below the control group median, indicating that the SD sample revealed an overall elevated shift in acuity.