bronchial provocation testing may be employed for a variety of purposes , but it is most commonly used to assess the tendency of the bronchi to narrow in response to exogenous stimuli . testing can be done by direct and indirect challenges using a number of different agents   ( table  1).table  1agents commonly used in bronchial provocation testing indirect challengesdirect challengesadenosine monophosphate ( amp)acetylcholineeucapnic voluntary hyperventilation ( evh)carbacholexercisehistaminehypertonic salinemethacholinemannitolnot yet approved in the us agents commonly used in bronchial provocation testing   not yet approved in the us methacholine aerosol inhalation is the prototype of a direct inhalation challenge . bronchoconstriction following inhalation results from the direct action of methacholine on acetylcholine receptors in airway smooth muscle . methacholine bronchoprovocation challenge is highly sensitive in identifying bronchial hyperreactivity , and a negative test is often used to exclude asthma . indirect challenges simulate airway responses to specific physiologic situations such as exercise [ 1 , 2 ] . the most recent indirect challenge is the mannitol ( man ) dry powder challenge , which is available as a standardized test kit in some countries . it is currently in review , but not approved , by the food and drug administration for use in the us . the kit includes prefilled capsules with mannitol in escalating doses , to be administered in an easy - to - use , handheld dry powder device . the safety and efficacy of man dry powder inhalation challenge have been established in large phase iii trials involving healthy subjects and patients with asthma [ 3 , 4 ] . mannitol is a hypertonic stimulus , and inhalation is thought to exert an osmotic effect within the airway that subsequently leads to the release of inflammatory mediators from mast cells , basophils , and human lung fragments . in asthmatic patients this leads to airway narrowing similar to that observed with hypertonic saline and exercise challenge . a positive response to mannitol is used to identify patients who have exercise - induced bronchoconstriction ( eib ) , with or without chronic symptoms of asthma . a negative response suggests that asthma is not present or , in some cases , that it is mild with intermittent symptoms or well - treated [ 3 , 4 , 6 ] . the simplicity of the mannitol challenge suggests that there may be some advantages over other bronchoprovocation tests . mannitol is given by a handheld dry powder inhaler ( dpi ) , so there is no specialized equipment required for the doctor s office . use of the dpi also makes the test easy for patients to learn and permits the test to be conducted quickly [ 4 , 6 ] . these advantages were recently confirmed in a study assessing the sensitivity and specificity of mannitol to identify eib as a manifestation of bronchial hyperreactivity in a highly selected population of possible asthmatics with a normal fev1 . mannitol and methacholine were therapeutically equivalent to identify eib and establish a clinician s diagnosis of asthma . however , in addition to not requiring specialized equipment , the mannitol challenge was more reproducible and took less time to perform than the methacholine test . only 43.5% of the subjects had a positive response to exercise as defined by at least a 10% reduction in fev1 on at least one of two challenges . to better understand the mechanism associated with a mannitol challenge , brannan et al . studied 12 asthmatic and 9 nonasthmatic subjects for evidence of mast cell activation and leukotriene release . they measured urinary excretion of leukotriene ( lt ) e4 and prostaglandin ( pg ) d2 metabolites ( and mast cell markers ) n - methylhistamine and 9,11 , pgf2 at 60  min before and 90  min after mannitol inhalation . airway narrowing provoked by mannitol challenge in asthmatic subjects was associated with increased urinary excretion of 9,11 , pgf2 . excretion of this pgd2 metabolite is an index of mast cell activation , and following mannitol challenge it is probably secondary to increased osmolarity of the airway surface liquid . the data support earlier work showing changes in airway sensitivity to mannitol in the presence of various mediator antagonists . interestingly , in subjects with asthma the histamine antagonist fexofenadine ( 2    180  mg over 14  h ) reduced airway sensitivity to mannitol , while the leukotriene antagonist montelukast ( 3    10  mg over 36  h ) caused a faster recovery of lung function to baseline post - challenge but had no effect on sensitivity . the same mediators and time sequence of release have been reported with exercise - induced asthma , suggesting a similar mechanism of action . the mast cell stabilizer nedocromil sodium ( 8  mg , 10  min prechallenge ) , also has been reported to significantly reduce bronchoconstriction to mannitol inhalation in subjects with exercise - induced asthma . more recently , both cromolyn sodium ( 40  mg , 15  min prechallenge ) , another mast cell stabilizer , and formoterol ( 24  mcg , 15  min prechallenge ) , a long - acting bronchodilator , were shown to protect against the change in 9,11 , pgf2 concomitant with a reduction in the mannitol - induced fall in fev1 ( by 35 and 95% , respectively ) . one of the early observations in the development of the mannitol challenge was that it has a tussive effect . like other indirect bronchoprovocation challenges , asthmatics cough more than healthy controls in response to mannitol , and the tussive effect is independent of the bronchoconstrictive effect , suggesting different physiological mechanisms . however , the simplicity of the mannitol challenge compared to other indirect bronchoprovocation tests makes it a potentially useful tool for evaluating airways hyperresponsiveness and cough sensitivity . a pilot study looked at the dose of mannitol needed to provoke two or five coughs in 13 subjects with nonasthmatic chronic cough compared to 16 healthy subjects . the subjects with chronic cough showed a heightened cough response to challenge ( i.e. , less mannitol needed to provoke cough ) ; none had airway responsiveness to mannitol . the findings support the hypotheses that ( 1 ) mannitol - provoked cough is increased in patients with nonasthmatic chronic cough , and ( 2 ) mannitol - provoked cough is independent of mannitol - provoked bronchoconstriction . the cough response may involve indirect activation of mast cells in the superficial airway and subsequent release or mediators that , in turn , activate local cough receptors . the separate observation that nedocromil sodium failed to attenuate mannitol - induced cough , in contrast to mannitol - induced bronchoconstriction , supports the lack of direct involvement of mast cell pathways . similar findings have been reported for capsaicin , a common test for studying induced cough . capsaicin , the  hot  ingredient in red pepper , is known to stimulate unmyelinated c fibers in the sensory nervous system . studies have shown that coughing can be induced by capsaicin inhalation in a dose - dependent manner in healthy subjects and those with mild asthma , and that capsaicin challenge does not cause dyspnea and has no effect on the fev1 . it has been demonstrated that capsaicin - induced cough can be blocked by local anesthetic [ 1416 ] but not by pretreatment with the mast cell stabilizer cromolyn sodium . these data support the separation of neuronal pathways of coughing from bronchoconstrictive airway responses directly related to inflammatory mediators . mannitol inhalation challenge may be useful for diagnosing and/or managing at least three types of patients described in the literature . koskela et al .   analyzed the cough response to three airway challenges in order to clarify whether the recording of the provoked cough would be beneficial in the management of asthma . they used isotonic histamine , hypertonic saline solution , and hypertonic histamine , all delivered by an ultrasonic nebulizer using the 2-min tidal breathing method . coughing during isotonic histamine challenge seemed to be a manifestation of bronchoconstriction , but recording the cough did not provide additional information to airflow measurements . sensitivity to the cough - provoking effect of hypertonic challenges was enhanced in patients with asthma and was unrelated to airway hyperresponsiveness . these investigators , therefore , advocated that cough assessment during hypertonic challenges could help to identify those patients with cough - variant asthma . johansson et al .   investigated the relationship between cough sensitivity to inhaled capsaicin and odor sensitivity using a chemical sensitivity scale for sensory hyperreactivity ( css - shr ) , dividing subjects into four groups with different odor sensitivity according to their css - shr score . of these , 137 individuals randomly received a capsaicin inhalation test in which the number of coughs encountered 10  min from the start of each inhalation was registered . more than 80% of subjects with positive capsaicin inhalation scores also had positive css - shr scores ; only 5% of subjects with negative css - shr scores had a positive capsaicin inhalation test . the direct relationship between sensitivity to inhaled capsaicin and a high css - shr score makes it possible to relate subjective data to objective findings . mannitol challenge of patients with odor insensitivity and dyspnea may support the diagnosis and lead to early and appropriate treatment . mannitol - induced coughing was evident in 93% of 419 subjects studied for suspected eib as diagnosed by mannitol and methacholine challenges . of these , 204 ( 49% ) had occasional cough that did not interfere with the challenge , 178 ( 42% ) had frequent cough that delayed administration of the next dose , and 9 ( 2% ) had severe cough that interfered with the challenge . the studies cited above have shown heterogeneity in the cough response to mannitol . using a cough counter supplied by karmel sonics - israel , my coworkers and two individuals had no cough at any time during the challenges , while the other had a cough throughout ; all three had positive bronchoprovocation testing with drops in fev1 by more than 20% at premaximal doses . the variation in the cough responses probably reflects differences in the underlying mechanisms of airway hyperresponsiveness in these individuals , but more patients should be studied before drawing any conclusions . similar findings have been reported by dicpinigaitis et al .   who reviewed the safety of capsaicin cough challenges over 20  years of clinical experience . like the initial results to date with mannitol , no serious adverse events were associated with capsaicin cough - challenge testing , only minor and transient descriptions of throat irritation . however , heterogeneity in the cough response was observed : asthmatics without cough did not differ from healthy volunteers in terms of cough reflux sensitivity to capsaicin , though differences were evident in airway reactivity . the results from mannitol and capsaicin thus support the suggestion that cough and bronchial responsiveness are distinct phenomena mediated through unique neural pathways . cough monitoring during bronchial challenges may help sort out subtypes of asthma and airway reactivity.fig . cough monitoring from four studies on three asthma patients undergoing bpt demonstrates the cough - response variability among patients . the patients were monitored with the pulmotrack ( model 3010-cc , karmelsonix ltd , haifa , israel ) throughout the test . a , b patient whose exposure to mannitol triggered extensive coughing that diminished after albuterol ( arrow ) . all patients had positive bpt with reductions in fev1  >  20% at premaximal dose cough monitoring during mannitol bronchial provocation test ( bpt ) . cough monitoring from four studies on three asthma patients undergoing bpt demonstrates the cough - response variability among patients . the patients were monitored with the pulmotrack ( model 3010-cc , karmelsonix ltd , haifa , israel ) throughout the test . a , b patient whose exposure to mannitol triggered extensive coughing that diminished after albuterol ( arrow ) . all patients had positive bpt with reductions in fev1  >  20% at premaximal dose koskela et al .   analyzed the cough response to three airway challenges in order to clarify whether the recording of the provoked cough would be beneficial in the management of asthma . they used isotonic histamine , hypertonic saline solution , and hypertonic histamine , all delivered by an ultrasonic nebulizer using the 2-min tidal breathing method . coughing during isotonic histamine challenge seemed to be a manifestation of bronchoconstriction , but recording the cough did not provide additional information to airflow measurements . sensitivity to the cough - provoking effect of hypertonic challenges was enhanced in patients with asthma and was unrelated to airway hyperresponsiveness . these investigators , therefore , advocated that cough assessment during hypertonic challenges could help to identify those patients with cough - variant asthma . johansson et al .   investigated the relationship between cough sensitivity to inhaled capsaicin and odor sensitivity using a chemical sensitivity scale for sensory hyperreactivity ( css - shr ) , dividing subjects into four groups with different odor sensitivity according to their css - shr score . of these , 137 individuals randomly received a capsaicin inhalation test in which the number of coughs encountered 10  min from the start of each inhalation was registered . more than 80% of subjects with positive capsaicin inhalation scores also had positive css - shr scores ; only 5% of subjects with negative css - shr scores had a positive capsaicin inhalation test . the direct relationship between sensitivity to inhaled capsaicin and a high css - shr score makes it possible to relate subjective data to objective findings . mannitol challenge of patients with odor insensitivity and dyspnea may support the diagnosis and lead to early and appropriate treatment . mannitol - induced coughing was evident in 93% of 419 subjects studied for suspected eib as diagnosed by mannitol and methacholine challenges . of these , 204 ( 49% ) had occasional cough that did not interfere with the challenge , 178 ( 42% ) had frequent cough that delayed administration of the next dose , and 9 ( 2% ) had severe cough that interfered with the challenge . the studies cited above have shown heterogeneity in the cough response to mannitol . using a cough counter supplied by karmel sonics - israel , my coworkers and two individuals had no cough at any time during the challenges , while the other had a cough throughout ; all three had positive bronchoprovocation testing with drops in fev1 by more than 20% at premaximal doses . the variation in the cough responses probably reflects differences in the underlying mechanisms of airway hyperresponsiveness in these individuals , but more patients should be studied before drawing any conclusions . similar findings have been reported by dicpinigaitis et al .   who reviewed the safety of capsaicin cough challenges over 20  years of clinical experience . like the initial results to date with mannitol , no serious adverse events were associated with capsaicin cough - challenge testing , only minor and transient descriptions of throat irritation . however , heterogeneity in the cough response was observed : asthmatics without cough did not differ from healthy volunteers in terms of cough reflux sensitivity to capsaicin , though differences were evident in airway reactivity . the results from mannitol and capsaicin thus support the suggestion that cough and bronchial responsiveness are distinct phenomena mediated through unique neural pathways . cough monitoring during bronchial challenges may help sort out subtypes of asthma and airway reactivity.fig . cough monitoring from four studies on three asthma patients undergoing bpt demonstrates the cough - response variability among patients . the patients were monitored with the pulmotrack ( model 3010-cc , karmelsonix ltd , haifa , israel ) throughout the test . a , b patient whose exposure to mannitol triggered extensive coughing that diminished after albuterol ( arrow ) . all patients had positive bpt with reductions in fev1  >  20% at premaximal dose cough monitoring during mannitol bronchial provocation test ( bpt ) . cough monitoring from four studies on three asthma patients undergoing bpt demonstrates the cough - response variability among patients . the patients were monitored with the pulmotrack ( model 3010-cc , karmelsonix ltd , haifa , israel ) throughout the test . a , b patient whose exposure to mannitol triggered extensive coughing that diminished after albuterol ( arrow ) . all patients had positive bpt with reductions in fev1  >  20% at premaximal dose as a hypertonic agent , mannitol inhalation may have a beneficial effect on mucus elimination in patients with cf . an initial study demonstrated an improvement in lung function related to small airway obstruction and a significant improvement in respiratory symptoms and quality of life after a 2-week treatment with mannitol in 38 patients . the suggested mechanism for the positive action of mannitol in cf is the same as that for nebulized hypertonic saline treatment , i.e. , osmotically induced water influx into the bronchial lumen thereby increasing the hydration of airway mucus so that it is cleared more effectively and expectorated . enhanced ciliary clearance of mucus also should result in a sustained reduction in mucus load , thus providing less opportunity for proliferation of bacteria and infection leading to less antibiotic use and fewer hospitalizations . long - term studies have been proposed and are presently being assessed by investigative protocols . in summary , mannitol is a simple , easy - to - administer indirect , inhalation challenge test . sensitivity to mannitol is increased in patients with asthma and in patients with chronic cough , but mannitol - provoked cough occurs via different pathways than mannitol - provoked bronchoconstriction . mannitol challenge testing may help sort out the heterogeneity in cough and bronchoconstrictive responses in certain patient populations . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
bronchial provocation testing uses a variety of direct and indirect inhalational challenges to evaluate airway hyperreactivity . 
 mannitol , a simple , easy - to - administer hypertonic stimulus available in many countries , is currently under review by the fda in the us . 
 healthy subjects show no airway response to inhaled mannitol ; asthmatic patients respond with airway narrowing similar to challenges with hypertonic saline and exercise . 
 mannitol challenge also has a tussive effect that is independent of bronchoconstriction , suggesting different physiologic pathways . 
 patients with chronic cough show increased sensitivity to mannitol , and mannitol testing may be useful for evaluating heterogeneity in the cough response .