INTRANASAL HISTAMINE CHALLENGE IN NORMALITY AND ALLERGIC RHINITIS

A series of investigations was performed in which histamine challenge was used to compare nasal responsiveness in 20 normal subjects and 20 with allergic rhinitis. There was found to be a lower threshold of reactivity (D100) to histamine in allergic subjects as measured by resistance changes (geometric mean, 0.53 mg/ml; normal subjects, 2.15: p = 0.022). This may represent increased number or sensitivity of histamine receptors on the nasal capacitance vessels. The loss of a laser Doppler response to a supramaximal histamine stimulus (normal subjects, 102% increase in flux at 3 minutes; p < 0.05) was observed in patients with allergic rhinitis and indicates either a down-regulation of the capillary system or an altered effect of histamine on superficial vessels, perhaps mediated by a shift in histamine receptor type. There was an observed increase in neutrophils at the mucosal surface under baseline conditions (rhinitis median, 49.6%; normal subjects, 32.72%: p < 0.05), which suggests an important primary role in the pathogenesis of this condition for this active cell. The observed increase in secretory volume response to histamine in allergic subjects, which persisted beyond 40 minutes after a single D100 challenge, may be related to an altered sensitivity of glandular tissue. There are important changes in nasal reactivity to histamine challenge in allergic rhinitis that may have implications for its pathogenesis.