Optimizing the selection of Parkinson’s disease patients for neuromodulation using the levodopa challenge test

In Parkinson’s disease (PD), early stages are associated with a good long-duration response and as the disease advances, the short-duration response predominates. The transition between the long-duration and short-duration responses may be an important and measurable intermediate stage. A critical criterion in determining the candidature for neuromodulation is a beneficial response to an ‘off–on’ levodopa challenge test. This test is usually reserved for those that have already developed marked short-duration response and are candidates for deep brain stimulation (DBS) surgery. However, identifying those that are in transition may allow DBS to be offered earlier. The objective of the study was to determine if the transition from a long-duration to a short-duration response can be assessed on a levodopa challenge test. An ‘off–on” levodopa challenge test was done in sixty-five PD patients divided into four groups based on the disease duration. OFF motor scores increased in all groups [Mean ± STD; 22.94 ± 8.52, 31.53 ± 9.87, 34.05 ± 9.50, and 33.92 ± 10.15 in groups 1–4, respectively] while a significant response to medication was maintained on ‘off–on’ testing. The mean levodopa equivalency dose in groups 1 and 2 was significantly less than in groups 3 and 4. This transition occurred between years 7 and 9 of disease duration. Performing a regular levodopa challenge test, when levodopa dose increases substantially, should be considered to determine the ideal time for DBS in patients with Parkinson’s disease.