Hypersensitivity pneumonitis differential diagnosis

Overview
The differential diagnosis of hypersensitivity pneumonitis is, primarily, a group of diseases known as idiopathic interstitial pneumonias. This group of diseases includes idiopathic pulmonary fibrosis (IPF) (which manifests histologically as usual interstitial pneumonia (UIP)), idiopathic non-specific interstitial pneumonia (NSIP) and cryptogenic organizing pneumonia, among others. There are several important clinical syndromes that occur as a result of inhalation of organic agents but are not true forms of Hypersensitivity Pneumonitis.

Conditions mimicing Hypersensitivity pneumonitis
Other diseases that are secondary to inhalation of organic agents but are not true forms of HP are as follows:


 * 1) Inhalation fever: Patients present with fever, chills, headache, and myalgias however there are not pulmonary findings (although mild dyspnea may occur). Onset is 4-8 hours following exposure. There are no long-term sequelae occur.
 * 2) Organic dust toxic syndrome: This syndrome is the result of exposure to bioaerosols contaminated with toxin-producing fungi (mycotoxins). Patients present with fever, chills, and myalgias 4-6 hours after exposure. In contrast to inhalation fever, the chest X ray may show diffuse opacities. Bronchiolitis or diffuse alveolar damage may be present on lung biopsy specimens. This is not a true form of HP because no prior sensitization is required.
 * 3) Chronic bronchitis: This can result from chronic obstructive pulmonary disease, which is the most common respiratory syndrome among agricultural workers. The prevalence of chronic bronchitis is much higher at 10%, compared with 1.4% for HP.
 * 4) Exposure to aerosolized Mycobacterium avium complex (MAC): A hypersensitivity pneumonitis like syndrome has been described in patients exposed to aerosolized Mycobacterium avium complex (MAC). Hot tub lung is a term used to describe these hypersensitivity pneumonitis-like cases because they have generally been associated with hot tub use. The syndrome has been linked to the high levels of infectious aerosols containing MAC organisms found in the water. Whether this syndrome represents a true MAC infection or classic HP remains controversial (Marras, 2005).

Differential Diagnosis
By frequency of Interstitial Lung Diseases (Xaubet, 2004):


 * 1) Idiopathic pulmonary fibrosis (38.6%)
 * 2) Sarcoidosis (14.9%)
 * 3) Cryptogenic organizing pneumonia (10.4%)
 * 4) Interstitial lung disease associated with collagen vascular diseases (9.9%)
 * 5) Hypersensitivity Pneumonitis (HP) (6.6%)
 * 6) Unclassified (5.1%)

In alphabetical order:


 * Air-conditioner lung
 * Aspergillus clavatus
 * Bagassosis
 * Breeder's lung
 * Bird fancier's lung
 * Cheese worker's lung
 * Chemical worker's lung
 * Cladosporium
 * Farmer's lung
 * Grain handler's lung
 * Humidifier lung
 * Malt worker's lung
 * Micropolyspora faeni
 * Shell hypersensitivity
 * Mucor stolonifer
 * Paprika splitter's lung
 * Penicillium casei
 * Saccharopolyspora rectivirgula
 * Sitophilus granarius
 * Thermoactinomyces sacchari
 * Thermoactinomyces vulgaris