Elevated substance P levels in nasal lavage fluids from patients with chronic nonproductive cough and increased cough sensitivity to inhaled capsaicin.

BACKGROUND

The exact mechanism of a chronic nonproductive cough is sometimes unclear when patients who are without symptoms or signs indicating the major causes of chronic cough remain undiagnosed.

OBJECTIVE

We hypothesized that some neurochemical alterations in the sensory nerves in the cough reflex may occur in the upper airway of chronic nonproductive cough patients.

METHODS

We took nasal lavage fluid (NLF) specimens from 38 patients with a chronic nonproductive cough as the sole presenting symptom. All 38 had normal chest radiography, spirometry, and bronchial responsiveness. We likewise took NLF specimens from 14 healthy control subjects. We used a capsaicin cough provocation test to determine cough sensitivity and considered the value of C5 (the lowest capsaicin concentration inducing 5 consecutive coughs) as an index of cough sensitivity. We measured levels of substance P of NLF specimens by using ELISA. In addition, we evaluated the clinical response of each patient after subsequent therapeutic trials with an antihistamine and decongestant for 2 weeks.

RESULTS

By using capsaicin cough sensitivity as the basis for grouping the study subjects, we divided the patients into 2 groups: an increased cough sensitivity group (ICS, C5 <32 mumol/L) and a normal cough sensitivity (NCS) group. Patients with ICS showed an elevated SP concentration in NLF (median value, 408 pg/mL) compared with that of the NCS group (237 pg/mL) and the control subjects (138 pg/mL) (P <.01). The median value of the percentage of remnant cough after therapeutic trial compared with the cough status before treatment was significantly higher in the ICS subgroup (70%) than that of NCS (25%) (P <.05).

CONCLUSIONS

Elevated substance P contents in NLF specimens were associated with ICS in patients with chronic nonproductive cough, suggesting a neurochemical abnormality in the upper airway.