Epidemiology of ABPA
Aspergillus hypersensitivity (AH) is defined by the presence of an immediate-type cutaneous hypersensitivity to A fumigatus antigens, and it is the first stepÂ in the development of ABPA. Only a minority ofÂ patients with AH develop the complete clinicalÂ picture of ABPA. The population prevalence ofÂ ABPA in asthma, generally referred to as 1 toÂ 2%, is based on the inference of only threeÂ studies (one peer-reviewed and two non-peer-reviewed studies). In the only peer-reviewedÂ study, 14 patients with allergic bronchopulmonaryÂ mycosis were identified from a total of 1,390 newÂ referrals in a catchment area population of half aÂ million, estimating a period prevalence of just aboveÂ 1%. The other two non-peer-reviewed questionnaire-based studies suggested a maximum prevalence of ABPA of 1% in the United States. In aÂ recent metaanalysis, we demonstrated a prevalenceÂ of AH and ABPA in asthma of 28% and 12.9%,Â respectively. The limitation noted in this review wasÂ that all the studies were performed in specializedÂ clinics and may not be representative of the generalÂ population. Thus the exact population prevalence ofÂ ABPA remains speculative but is likely to be fairly
806 high in patients attending asthma clinics. Table 1Â summarizes the prevalence of ABPA in patients withÂ asthma reported in various studies over theÂ last two decades. The prevalence of ABPA in patients admitted with acute severe asthma is evenÂ higher. In a recent study of 57 patients with acuteÂ severe asthma admitted in the respiratory ICUs, weÂ demonstrated the prevalence of AH and ABPA to beÂ around 51% and 39%, respectively. The occurrenceÂ of AH and ABPA was significantly higher in patientsÂ with acute asthma compared to the outpatient bronchial asthma (around 39% and 21%, respectively).
Pathogenesis of ABPA
The susceptibility of asthmatic patients to develop ABPA is not fully understood (Fig 1). Some authorsÂ have reported that exposure to large concentrationsÂ of spores of A fumigatus may cause ABPA.
Most present with low-grade fever, wheezing, bronchial hyperreactivity, hemoptysis, or productiveÂ cough. Expectoration of brownish black mucus plugs is seen in 31 to 69% of patients. The symptoms of hemoptysis, expectoration of brownish black mucus plugs, and history of pulmonary opacities in anÂ asthmatic patient suggests ABPA. Patients can occasionally be asymptomatic, and the disorder isÂ diagnosed on routine screening of asthmatic pa-tients. Physical examination can be normal orÂ may reveal polyphonic wheeze. Clubbing is rare,Â seen in only 16% of patients. On auscultation, coarseÂ crackles can be heard in 15% of patients. PhysicalÂ examination can also detect complications such asÂ pulmonary hypertension and/or respiratory failure. During exacerbations of ABPA, localized findings ofÂ consolidation and atelectasis can occur that needs toÂ be differentiated from other conditions.
Aspergillus Skin Test: The Aspergillus skin test is performed using an A fumigatus antigen, eitherÂ commercial (eg, Aspergillin; Hollister-Stier Laboratories; Spokane, WA) or locally prepared. The test isÂ read every 15 min for 1 h, and then after 6 to 8 h.Â The reactions are classified as type I if a wheal andÂ erythema developed within 1 min, reaches a maximum after 10 to 20 min, and resolves within 1 to 2 h.Â A type III reaction is read after 6 h, and any amountÂ of subcutaneous edema is considered a positiveÂ result. An immediate cutaneous hypersensitivity to AÂ fumigatus antigens is a characteristic finding ofÂ ABPA and represents the presence A fumigatus-specific IgE antibodies, whereas a type III skinÂ reaction probably represents the immune complexÂ hypersensitivity reaction, although its exact significance remains unclear. The test can be performedÂ using either a skin-prick test or intradermal injectionÂ with the latter being more sensitive. A skin-prick test should be performed for Aspergillus skinÂ testing, and if the results are negative should beÂ confirmed by an intradermal test. There is noÂ difference on the outcome of the test and the type ofÂ antigen (locally prepared or commercial) used forÂ performance of the test.
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