Orthostatic; Hypotension, Parkinsonism

The objective of this study is to find a more objective and accurate way to assess the efficacy of the treatment for neurogenic orthostatic hypotension. For this purpose, the investigators will use an activity monitor to determine the amount of time patients spend in the upright position (standing and walking; upright time) during 1 week of placebo (a pill with no active ingredients) and 1 week of their regular medication for orthostatic hypotension (midodrine or atomoxetine at their usual doses). Total upright time (i.e. tolerance to standing and walking) will be compared between placebo and active treatment to test the hypothesis that it can be used to assess the efficacy of the treatment for orthostatic hypotension and whether this outcome is superior to the assessment of symptoms using validated questionnaires.