PURPOSE: Patients with idiopathic orthostatic intolerance often have debilitating symptoms on standing that are suggestive of cerebral hypoperfusion despite the absence of orthostatic hypotension. SUBJECTS AND METHODS: We evaluated the effects of graded head-up tilt on cerebral blood flow as determined by transcranial Doppler measurements in 10 patients with idiopathic orthostatic intolerance (nine women, one man, 22 to 47 years) and nine age- and sex-matched control subjects. RESULTS: In patients, mean (± SD) arterial pressure at 0°head-up tilt was 90 ± 11 mm Hg and was well maintained at all tilt angles (90 ± 11 mm Hg at 75°). In controls, mean arterial pressure was 85 ± 7 mm Hg at 0°and 82 ± 11 mm Hg at 75°head- up tilt. There was a substantial decrease in peak velocity with increasing tilt angle in patients (28% ± 10%) but not in controls (10% ± 10% at 75°, P <0.001). Similarly, mean velocity decreased 26% ± 13% in patients and 12% ± 11% in controls (P = 0.01). With increasing head-up tilt, patients had a significantly greater increase in regional cerebrovascular resistance than controls. CONCLUSIONS: In patients with idiopathic orthostatic intolerance, peak and mean middle cerebral artery blood flow velocity decreased in response to head-up tilt despite well sustained arterial blood pressure. These observations indicate that in this group of patients, regulation of cerebrovascular tone may be impaired and might therefore be a target for therapeutic interventions.
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