Subthalamic Nucleus Stimulation and Somatosensory Temporal Discrimination in Parkinson's Disease
Abstract & Commentary
By Panida Piboolnurak, MD
Assistant Professor, Department of Neurology and Neuroscience, Weill Medical College, Cornell University. Dr. Piboolnurak is on the speaker's bureau for Allergan.
Synopsis: Although subthalamic nucleus stimulation can improve motor functions in Parkinson's disease, this study showed that it degrades somatosensory temporal discrimination possibly by modifying central somatosensory processing.
Source: Conte, A., et al. Subthalamic nucleus stimulation and somatosensory temporal discrimination in Parkinson's disease. Brain 2010:133;2656-2663.
Although there are many studies on the effects of subthalamic nucleus deep brain stimulation (STN DBS) on motor functions in Parkinson's disease (PD), there are a few studies investigating the effects of STN DBS on the sensory system. This study tested the effects of STN DBS on somatosensory temporal discrimination (STD) in PD.
Thirteen patients with advanced PD and 13 age-matched healthy controls were recruited. The indications for DBS were severe motor fluctuations, dyskinesias, and motor deficits. Inclusion criteria were treatment with STN DBS for at least 12 months, mini-mental state examination (MMSE) > 24, absence of clinical sensory deficits with DBS switched on, and no changes in stimulation or medications within the past 3 months. The patients were evaluated in four conditions (off-med/off-stim, off-med/on-stim, on-med/off-stim, and on-med/on-stim). The experimental conditions were randomly assigned and counterbalanced across the patients.
MMSE, attentive matrices, Raven's Progressive Matrices, Corsi's test, Rey's test, and verbal fluency test did not significantly change across conditions. STD threshold was tested in the index finger (hand), periorbital region (eye), and neck. The thresholds were higher in the patients compared to healthy subjects, with the highest STD thresholds in the hand. There was no significant correlation between the individual UPDRS scores for the hand, neck, and face with their respective STD thresholds in all experimental conditions. The thresholds were lower in the on-med state compared to the off-med state. In the on-med state, the thresholds were higher in the on-stim than in the off-stim condition. However, in the off-med state, the thresholds were not different in the off- or on-stim condition. These findings suggest that dopaminergic therapy improves STD processing whereas STN DBS degrades the process. Given that cognition and attention were not different among four conditions, worsening in STD processing in the on-stim state cannot be explained by the DBS potential effects on cognition and attention.
The study also showed that the amplitudes of the upper limb somatosensory evoked potential (SEP) parietal component were larger in the off-stim than in the on-stim condition. There was no significant correlation between the changes in SEP component amplitudes and changes in STD thresholds in on-med/on-stim and on-med/off stim states.
The authors explained that DBS might decrease SEP amplitudes by interfering with subcortical somatosensory pathways. However, given the finding that STD thresholds changed to a similar extent on both body sides in patients with unilateral as well as those with bilateral DBS, it was unlikely that DBS interfered with the nearby somatosensory pathways. The authors proposed that DBS-induced chronic STN inactivation actively elicits changes in central somatosensory processing, possibly by altering the interplay between cortical (pre-supplementary motor area and somatosensory primary areas) and subcortical neural circuits (thalamus, striatum, subthalamic nucleus, and cerebellum). Concerning the effects of dopaminergic therapy, the authors explained that dopamine might directly modulate the neural circuits for STD process.
Commentary
STN DBS is an effective treatment for PD patients with motor fluctuations and dyskinesias. Given the findings suggestive of central somatosensory processing interference by STN DBS, further studies on the clinical implications are essential. Another finding that needs a further investigation is the increase in STD thresholds by DBS only in the on-med state. Because the thresholds remained unchanged in the off-med/on-stim condition compared to those in the off-med/off-stim condition, this indicates that DBS did not worsen STD, but rather decreased the benefit of dopaminergic therapy on STD. Future studies on an interaction between DBS and dopaminergic therapy are required to explain this finding.