Autonomic nervous system function in chronic dizziness

Objective: The goals of this study were to validate the clinical diagnosis of autonomic dizziness as a cause of chronic nonvertiginous dizziness that may be exacerbated by physical exertion or orthostatic challenges, estimate its prevalence in a tertiary referral population, and investigate the usefulness of three autonomic challenges as objective tests for this condition. Study Design: Laboratory investigation of autonomic activity. Setting: Tertiary care balance center. Patients: Fifteen men and women with symptoms indicative of autonomic dizziness. Subjects with other causes of dizziness, histories of syncope, or psychiatric disorders were excluded. Interventions: Autonomic tests included 45 minutes of head upright tilt (HUT), 20 minutes of 5% CO2 inhalation and then HUT, and 2 minutes of voluntary hyperventilation and then HUT. Main Outcome Measures: Patterns of cardiovascular and respiratory responses and subjective ratings of dizziness, autonomic symptoms, and anxiety during autonomic challenges. Results: Twelve subjects had evidence of autonomic dysfunction, including 10 with abnormal heart rate, blood pressure, or respiratory responses to HUT. Two other subjects had prolonged hypocarbia after voluntary hyperventilation. Many of these abnormalities would have been missed by current autonomic testing paradigms. In one subject, CO2 inhalation revealed latent anxiety. In two subjects, the presence of high symptom ratings without objective autonomic dysfunction prompted a successful search for other diagnoses. Conclusion: Study results validated the clinical syndrome of autonomic dizziness. Autonomic testing protocols may have to be updated to detect clinically relevant abnormalities in patients with dizziness.