Mannitol provocation enhances laryngoscopic diagnosis of suspected inducible laryngeal obstruction

Background: Inducible laryngeal obstruction (ILO) is confirmed by observing paradoxical vocal fold movement (PVFM) on laryngoscopy, but test sensitivity is reduced by its intermittent nature. Specificity of isolated expiratory PVFM is also unclear, possibly denoting a physiological response to lower airway obstruction. Objective: To clarify laryngoscopic diagnosis in suspected ILO through mannitol provocation. Methods: In patients with suspected ILO, we assessed rates of laryngoscopic PVFM, both at baseline and after mannitol provocation, defined as any inspiratory adduction, more than or equal to 50% expiratory adduction, or both. We also studied accentuation of laryngoscopic findings after mannitol provocation, defined as new or increased PVFM. We explored relationships between isolated expiratory PVFM, lower airway obstruction on spirometry, and bronchial hyperresponsiveness to mannitol. We also studied healthy volunteers. Results: Among 80 patients with suspected ILO, PVFM rates were 42 (52.5%) at baseline and 58 (72.5%) after mannitol. Mannitol accentuated laryngoscopic findings in 45 of 80 (56%), with new PVFM in 17 of 80 (21%) and increased PVFM in 28 of 80 (35%) and 28 of 42 (67%) of patients with PVFM at baseline. Among patients with baseline isolated expiratory PVFM, 21 of 30 had accentuation by mannitol and there was no relationship with airway obstruction or bronchial hyperresponsiveness. Among healthy volunteers, PVFM rates were identical at baseline and after mannitol (4/15, 27%, all 4 with isolated expiratory PVFM); none (0/15) had accentuation by mannitol. Conclusion: Accentuation of laryngoscopic findings after mannitol provocation is more useful than PVFM at baseline laryngoscopy in distinguishing patients with suspected ILO from healthy volunteers. Isolated expiratory PVFM without accentuation by mannitol can be a normal finding and unrelated to bronchial obstruction or hyperresponsiveness.