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Dr. Merrill is Director, Medicine Service Line, New Orleans Veterans Affairs Medical Center, and Professor of Medicine, Tulane University School of Medicine.
Correspondence to: William Merrill, MD, FCCP, Chief-Medical Service III, 1601 Perdido, New Orleans, LA 70112-1207; e-mail: william.merrill{at}med.va.gov
Ambient air contains variable amounts of respirable foreign substances. Some of these substances are capable of inciting immune responses in the host inhaling them. Hypersensitivity pneumonitis (HP) represents one possible response of the lung to the inhalation of these antigenic substances. Subjects who are exposed to respirable antigens occasionally react to this by a complex immune response, involving both T-lymphocyte and B-lymphocyte activation in the lower respiratory tract.1 Disease resulting from antigen exposure is uncommon, even among subjects with similar exposures to the relevant antigens. This variability in disease expression following exposure suggests that the genetic background of affected individuals2 or other factors such as concomitant viral infection3 may be responsible for disease expression.
Although the exact pathogenesis of HP is not known, patients with the disease have several characteristic features that allow a firm diagnosis in most cases. These features include a history of exposure to respirable antigens, the relief from symptoms with antigen avoidance, and a recrudescence of symptoms with re-exposure. The symptom complex can be quite variable and includes severe presentations that are similar to that of ARDS, subacute presentations with malaise, cough, and weight loss, and the development of chronic fibrosis. These variable forms of presentation pose a considerable challenge to the clinician. The clinician must first consider the possibility that HP may be causing the disease. Then the physician must question the patient about exposure to relevant antigens. Unfortunately, the spectrum of potential antigens is quite broad, and these antigens may be concealed in the environment. However, in most cases the amount of antigen in the inhaled air is quite large. Thus, affected patients usually have an idea that they are being exposed to something. They may notice that the air that they breathe is dusty or has an objectionable odor. In some cases, the patient’s occupation is a powerful clue to a potential exposure.
Once the possibility of exposure has been determined, support of the diagnosis may be provided by a CT scan of the chest and by lung function tests. HP is associated with the combination of ground glass changes and small nodular lesions that are visible on the CT scan.4 Pulmonary physiology usually shows a mixture of restrictive and obstructive changes. Neither a CT scan nor a lung function test provides a diagnosis, but both provide soft support. Subsequently, the clinician may proceed by attempting to identify immune responses against the probable antigens. The easiest test to perform is the search for precipitating antibodies. The antigens responsible for common forms of HP such as farmer’s lung5 and pigeon-breeder’s disease6 are well-characterized and can be tested by commercial laboratories. In other cases, antibodies may be difficult to detect, and previously unknown antigens cannot be assessed by standard antibody methods in commercial laboratories. If antibodies cannot be detected, the patient may be exposed to the potential antigen in a controlled environment. This type of exposure can cause significant physiologic reactions including fever and lung function changes. Fortunately, these reactions are usually well-tolerated.7 The physiologic response to challenge may be quite helpful in arriving at a firm diagnosis.
The intrapulmonary reactions associated with HP are quite predictable. Deceased subjects have a significant increase in the number of lymphocytes in the lower respiratory tract. The percentage of lymphocytes is increased threefold to fivefold, and the total number of cells recovered by BAL is increased twofold to threefold compared to control subjects who have a similar smoking status. The composition of lymphocytes in the respiratory tract also is altered. The normal respiratory tract has a lymphocyte composition similar to that of circulating cells. Specifically, the ratios of helper to suppressor cells are consistent with those ratios in the blood. Patients with HP have a significant increase in the number of suppressor/cytotoxic cells recovered. This results in a significant reduction in the helper/suppressor cell ratio. These lavage findings are seen in few other diseases and provide significant support for the diagnosis.1
If the diagnosis is still in doubt, an open or thoracoscopic lung biopsy can assist by demonstrating changes that are compatible with the diagnosis. Importantly, other diseases may be excluded by their absence in biopsy material.8
Once a diagnosis has been made, the treatment of choice is antigen avoidance. This can be achieved by several fairly simple steps. A high-efficiency filter mask can protect the wearer from the great bulk of antigens in inspired air. This may be adequate for many workplace or hobby exposures. Living places can be protected by high-efficiency particle filters for the ambient air.9 In some cases, a good house cleaning has been reported to yield benefits.10 Corticosteroid therapy can significantly reduce acute symptoms. However, there is some concern that steroid-treated patients are more likely to relapse than those who are untreated.11 Thus, steroids should be reserved for patients with severe symptoms or for those who fail to respond to antigen avoidance.
In this issue of CHEST, the article by Tsushima et al (see page 1085) is not the first description of HP resulting from the inhalation of fungal spores in mushroom workers.12 13 However, it is a nice description of and methodology for the diagnosis of HP. These investigators have used techniques that would be available to the average clinician. These techniques (ie, CT scan, lung function testing, BAL, and biopsy) should be available in most hospitals. The only study not readily available as a clinical test was the lymphocyte transformation assay. In this case, the assay was necessary for a clear determination that the fungal material had induced immune response in the exposed workers because antibody levels could not be measured. This type of assay might well be necessary in a workplace incident in which compensation to the affected employees was being considered. However, I believe that most clinicians would accept the evidence of HP in the absence of this type of test. The positive exposure test provides similar information and would have convinced me of the causal nature of the mushroom exposure.
In summary, the greatest challenge in these cases is for the clinician to consider that HP is among the possible diagnoses. Once this is considered, a panel of tests can be performed. These tests, without biopsy in most cases, will be adequate. Biopsy should be considered for patients whose presentation is puzzling or for those who fail to respond appropriately to antigen avoidance.
References
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