Dictionary of Allergies .. Airway hyperresponsiveness (AHR)

Airway hyperresponsiveness (AHR)

Hyperresponsiveness of the airways of the lung is a state of abnormal sensitivity to a wide range of stimuli as agonists drugs, chemical irritants, osmolarity and cold air. The principle response is increased airflow limitation or bronchoconstriction and it is characteristic of asthma and responsible for many of its features. Airway hyperresponsiveness (AHR), often called "the hallmark of asthma", is known to be an important risk factor for the development of asthma [1] and to be indicative of asthma severity with respect to symptomatic asthma [2]. AHR is also reported to be a marker for the subsequent development of asthma in patients with allergic rhinitis [3]. It is tempting therefore to speculate that the degree of AHR in patients with cough-variant asthma may be an indication of increased susceptibility to the development of classic asthma. AHR is usually defined as an increased sensitivity of the airways to inhaled histamine or methacholine [4]. It can be demonstrated by bronchial provocation tests with histamine or methacholine. The sensitivity of the airways to these agents is commonly expressed as the provocative concentration causing a 20% fall (PC20) in forced expiratory volume in 1 s (FEV1). However, there is accumulating evidence that AHR is a more complex functional abnormality that comprises more than just hypersensitivity [5]. When exposed to high concentrations of inhaled histamine or methacholine, asthmatic patients show excessive narrowing of the airway, as reflected by an elevated or absent maximal response plateau. It can be argued that the latter is clinically a more relevant component of AHR than the former per se because it reflects the potential severity of airway obstruction in each individual [6].

Epidemiological studies have shown the close relationship between the presence of airway hyperresponsiveness and atopy, the later defined as one or more positive skin prick tests to common inhalant allergy. Environmental factors are central to the development of airway hyperresponsiveness, [7]. (See also: Airway inflammation and airway hyperresponsiveness). Factors influencing development of airway hyperresponsiveness may be: 1.Toxic (viral, respiratory tract infections, toxic variation in airway calibre, chemicals. 2. Endogenous (circadian). 3. Exogenous (sulphur dioxide, histamine, low molecular weight chemicals, exercise, allergy, complex biological molecules, histamine) [8].

References

1. Hopp RJ, Townley RG, Biven RE, Bewtra AK, Nair NM. The presence of airway reactivity before the development of asthma. Am Rev Respir Dis 1990;141:2-8.

2. Murray AB, Ferguson AC, Morrison B. Airway responsiveness to histamine as a test for overall severity of asthma in children. J Allergy Clin Immunol 1981;68:119-124.

3. Braman SS, Barrows AA, de Cotiis BA, Settipane GA, Corrao WM. Airway hyperresponsiveness in allergic rhinitis. A risk factor for asthma. Chest 1987;91:671-674.

4. Hargreave FE, Dolovich J, O'Byrne PM, Ramsdale EH, Daniel EE. The origin of airway hyperresponsiveness. J Allergy Clin Immunol 1986;78:825-832.

5. Sterk PJ, Bel EH. Bronchial hyperresponsiveness: the need for a distinction between hypersensitivity and excessive airway narrowing. Eur Respir J 1989;2:267-274.

6. Woolcock AJ, Salome CM, Yan K. The shape of the dose-response curve to histamine in asthmatic and normal subjects. Am Rev Respir Dis 1984;130: 71-75.

7. Koh,Y.Y., Park,Y. & Kim, C. K. The importance of maximal airway response to methacholine in the prediction of wheezing development in patients with cough-variant asthma. Allergy 2002:57(12):1165-1170.
8. Davies, R.J.: Environmental allergy. Allergy 1988:43(8):23-25.