Objective: To assess whether there are changes in short interval intracortical inhibition (SICI) that reflect changes in motor function and plasma levodopa levels in patients with advanced Parkinson’s disease (PD).

Background: SICI has been investigated in both the OFF and ON-medication states in PD but the findings have been inconsistent. A number of studies have reported reduced SICI in the untreated or OFF-medication state particularly in those with more advanced disease, and further that the reduced SICI normalises with dopaminergic therapy, however other studies have failed to detect similar changes.

Methods: Six patients who have been established on levodopa carbidopa intestinal gel (LCIG) therapy for more than 12 months underwent assessment of their SICI, objective measure of bradykinesia and plasma levodopa levels. Studies were performed under two conditions, i.e. with the LCIG pump ON then OFF with serial measurements being made every 30 minutes. SICI was assessed using a threshold tracking paired-pulse TMS with the conditioning stimulus intensity set to 70% of resting motor threshold. Motor evoked potentials were recorded over the abductor pollicis brevis. Hand tapping rate was measured with a timed tapping task in which two electronic touch pads positioned 20 cm apart were tapped as fast as possible for 60 seconds.

Results: In the LCGI pump OFF condition, there was a rapid decline in plasma levodopa levels as expected but there was no significant reduction in hand tapping rate. SICI was comparable with controls in the LCGI pump ON condition, but became progressively reduced at subsequent time points in the OFF condition, being highly significant at 180 minutes (P<0.000). There was a statistically significant correlation between mean SICI and mean levodopa concentration (r = 0.900, P=0.037), but no correlation between mean hand tapping speed and either mean SICI or mean levodopa concentration.

Conclusions: The results from this study suggest that SICI is reduced in patients with severe PD and in addition SICI positively correlates with plasma levodopa levels. The findings suggest that SICI is influenced by the disease severity as well as plasma levodopa levels. Further, the findings raise the possibility that long term LCIG therapy may re-establish the long duration response given that hand tapping speed was maintained despite low plasma levodopa levels in advanced PD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/cortical-excitability-in-patients-with-parkinsons-disease-on-levodopa-carbidopa-intestinal-gel-thereapy/