Hypersensitivity Pneumonitis (HP/Extrinsic Allergic Alveolitis)

Epidemiology

  • History: first described in 1932 (as Farmer’s Lung)
  • Age: most cases occur in adults (cases in children, especially associated with pigeons, may occur)
  • Tobacco Use: protective effect against HP (although >90% of patients are non-smokers at time of diagnosis)

Etiology

General Comments

  • Over 300 Different Etiologic Agents Have Been Associated with HP
    • However, Bird Fancier’s Lung Accounts for Approximately 66% of All HP Cases

Animal Products

  • Bird Fancier’s Lung (Feathers and Bird Droppings)
    • Avian Proteins (Intestinal Mucin, IgA, and IgG): these proteins are contained in both bird droppings and bloom (the waxy keratinous powder which waterproofs the feathers)
    • Associated with Multiple Types of Birds
      • Columbiformes: pigeons, doves
      • Psitacciformes: budgerigars, cockatoos, cockatiels, lovebirds, parrots
      • Passeriformes: canaries
      • Anseriformes: geese, ducks
      • Galliformes: turkeys, chickens, pheasants, partridges
  • Laboratory Worker’s Hypersensitivity Pneumonitis (Rat Fur)
    • Male Rat Urine
  • Pituitary Snuff Taker’s Disease (Pituitary Powder)
    • Bovine and Porcine Proteins

Insect Products

  • Miller’s Lung (Wheat Weevils)
    • Sitphilus Granarius

Vegetable Products

  • Bagassosis (Moldy Sugar Cane)
    • Thermophilic Actinomycetes
  • Cheesewasher’s Disease (Cheese Casings)
  • Familial/Domestic Hypersensitivity Pneumonitis (Contaminated Wood Dust)
  • Farmer’s Lung (Moldy Hay)
    • Thermophilic Actinomycetes: ubiquitous bacteria that grow in decaying matter and resemble fungi (exposure may be <750,000 spores per min)
    • Micropolyspora
  • Hot Tub Lung (Contaminated Hot Tub)
    • General Comments
      • Occurs mostly in immunocompetent patients
      • Pathology consists of well-formed granulomas (in contrast to loosely-formed granulomas in other types of HP)
      • Increased BAL CD4/CD8 lymphocyte ratios are common
    • Mycobacterium Avium Complex (see Mycobacterium Avium Complex, [[Mycobacterium Avium Complex]])
    • Cladosporium
    • Pullaria
    • Epicocucum Nigrum
  • Humidifier Lung (Contaminated Humidifiers/Dehumidifiers/Air-Conditioning Units)
    • Thermophilic Actinomycetes
    • Penicillium (see Penicillium, [[Penicillium]])
    • Cephalosporium
    • Amebae
  • Malt Worker’s Lung (Contaminated Barley)
    • Aspergillus Clavatus (see Aspergillus, [[Aspergillus]])
  • Maple Bark Disease (Contaminated Maple Logs)
    • Cryptostroma Corticale
  • Mushroom Worker’s Disease (Moldy Compost)
    • Thermophilic Actinomycetes
    • Micropolyspora
  • Paprika Splitter’s Lung (Paprika Dust)
  • Sequoiosis (Contaminated Wood)
    • Graphium
    • Pullularia
  • Suberosis (Moldy Cork)
  • Swimming Pool Lung (Mist from Pool Water, Sprays, and Fountains)
  • Wood Trimmer’s Lung (Contaminated Wood Trimmings)
    • Mucor (see Mucor, [[Mucor]])
    • Rhizopus (see Rhizopus, [[Rhizopus]])

Reactive Simple Chemicals

  • Diphenylmethane Diisocyanate HP (see Diphenylmethane Diisocyanate, [[Diphenylmethane Diisocyanate]])
    • Altered Proteins
  • Epoxy Resin Lung
    • Heated Epoxy Resin: Phthalic Anhydride
  • Toluene Diisocyanate HP (see Toluene Diisocyanate, [[Toluene Diisocyanate]])
    • Altered Proteins and Others
  • Trimellitic Anhydride HP (see Trimellitic Anhydride, [[Trimellitic Anhydride]])
    • Altered Proteins
  • Cobalt (see Cobalt, [[Cobalt]])
    • “Hard Metal”: Tungsten Carbide + Cobalt
  • Pyrethrum Pneumonitis
    • Pyrethrum Insecticide (see Pyrethrum, [[Pyrethrum]])
  • Pauli’s Reagent Alveolitis
    • Sodium Diazobenzene Sulfate: lab reagent
  • Water-Based Metal Working Fluid HP: pure petroleum oils, emulsion of petroleum in a water base (semi-synthetic fluids), or emulsion of synthetic oils in water (synthetic fluids) -> used in many industries (auto machinists, etc) to decrease heat from machine tools and the object in production
    • Unknown Antigen: although biocides are added to metal working fluids, resistant microorganisms survive, leading to selection of organisms such as Mycobacterium Immunogenum
      • Unlike hot tub lung, mycobacteria are not cultured from BAL fluid (measurement of M immunogenum-specific cell-mediated immunity using detection of specific antibodies against recombinant M immunogenum antigens by ELISA may be helpful)

Drugs


Physiology

  • Organic dust exposure with deposition in distal lung (injury mainly involves type 3, type 4 hypersensitivity)
  • Organic dusts as immunologic adjuvants (induce humoral and cell-mediated immunity):
    • Micropolyspora Faeni antigens induces IL-1 and TNF-α from macrophages (anti-TNF-α infihibits pulmonary inflammation)
    • Thermophilic Actinomycetes antigens activate alveolar macrophages and alternative complement (promoting PMN chemotaxis to lung)
    • Ag’s contain enzymatic substances, endotoxins, histamine releasers, and inducers of non-specific precipitins
    • Increased mast cells in BAL in normal farmers and in Farmer’s Lung (correlated with disease activity/ but no evidence for IgE-mediated type 1 mechanisms exists)
    • Micropolyspora Faeni Ag, complement, mononuclear cells, and Ig present in bronchial wall of Farmer’s Lung (possible type II cytotoxic component)
    • Sensitized lymphocyctes occur in both blood and lung
    • Animal models of HP show that repeated exposure may lead to improvement (possibly due to antigen-specific T-suppressor cells)
    • Type 3 immune-complex injury plays a major role in HP: immune complexex can induce IL-1 and TNF-alpha release from BAL cells, in-duce pulmonary granuloma formation, activate alveolar macrophages
      • Precipitating Ab are present to specific Ag (but these are related to environmental exposure, not necessarily to disease)
      • However, serum complement does not decrease with Ag exposure and histologic vasculitis is absent in lung tissue
    • Type 4 delayed hypersensitivity injury plays a major role:
  • Inhibitors of type 4 reaction (cortisone, CSA, antimacrophage sera, etc.) inhibit animal model granuloma formation
  • Host Factors
    • Less than 10% of exposed people develop HP
    • No association between HLA types and susceptibility
    • No higher incidence of HP in atopic patients

Pathologic Patterns

  • Acute HP Pattern
    • Usually resolves without sequelae
    • Lymphocytes, plasma cells, activated alveolar macrophages/foamy macrophages/giant cells/obstructive bronchiolitis/absence of vasculitis
  • Subacute HP Pattern
    • Absence of vasculitis
    • Granulomas: bronchocentric distribution/with giant cells
    • Cellular bronchiolitis: chronic inflammation around airways with or without BO
    • Interstitial pneumonitis: infiltrated with macrophages, plasma cells, and lymphocytes/may have UIP-like features in some cases

Diagnosis

Diagnostic Challenges

  • Natural challenge: avoidance x 72 hrs with return to suspected site, monitor clinical and PFT’s x 24 hrs
    • Leads to recurence of symptoms (may not be positive in chronic cases with only brief exposure)
    • Does not reveal specific agent
  • Lab challenge (although antigens are not usually available): nebulized challenge in PFT lab, monitor clinical and PFT’s x 24 hrs
    • Bronchospasm may precede expected response 4-6 hrs later
    • Caution: endotoxin, etc. in challenge material may cause febrile reactions in some people (mimicking HP)

Complete Blood Count (CBC) (see Complete Blood Count, [[Complete Blood Count]])

  • Leukocytosis (see Leukocytosis, [[Leukocytosis]]): with increased neutrophils/mild lymphopenia
  • Peripheral Eosinophilia (see Peripheral Eosinophilia, [[Peripheral Eosinophilia]]): typically absent

C-Reactive Protein (CRP) (see C-Reactive Protein, [[C-Reactive Protein]])

  • May Be Elevated

Erythrocyte Sedimentation Rate (ESR) (see Erythrocyte Sedimentation Rate, [[Erythrocyte Sedimentation Rate]])

  • Normal

Precipitating Antigen-Specific Antibodies

  • Technique: ELISA is usually the preferred method to detect precipitins or total IgG antibodies (however, even the ELISA method lacks standardization)
    • Other Methods: immunodiffusion, immunoelectrophoresis, electrosyneresis
  • HP Cannot Be Ruled In Based Solely Upon Positive Antibodies or Ruled Out Based on Negative Antibodies [MEDLINE]
    • Example: asymtomatic pigeon breeders (40% of cases) and farmers (105% of cases) may have significant antibody levels
    • Example: specific antibody reactivity is lacking in Many Patients with HP
  • Data from the HP Study Group: Significant Predictors of Hypersensitivity Pneumonitis [MEDLINE]
    • Positive Precipitating Antibodies: Odds Ratio 5.3 (95% CI: 2.7-10.4)

Serum Immunoglobulin E (IgE) (see Serum Immunoglobulin E, [[Serum Immunoglobulin E]])

  • Normal

Pigeon Ag Skin Test

  • Wheel and flare seen in most Pigeon Breeder’s Lung (10% of these have bronchospasm on Ag challenge, possibly due to IgG4-mediated type 1 reaction)
  • Skin tests: non-specific (not used)

Angiotensin Converting Enzyme (ACE) Level

  • May be elevated (also seen in organic dust-exposed normals)

Arterial Blood Gas (ABG) (see Arterial Blood Gas, [[Arterial Blood Gas]])

  • Acute HP: hypoxemia with hypocapnia
  • Subacute HP:
  • Chronic HP:

Clinical

General Comments

  • Clinical Pattern is Determined by Intensity and Frequency of Antigen Exposure
  • The Traditional 3-Group Classification System Below is Controversial: since the subacute group is difficult to define, some authors suggest that the clinical data better fits a 2-group classification system (Chest, 2012) [MEDLINE]

Hypersensitivity Pneumonitis Study Group: Significant Predictors of Hypersensitivity Pneumonitis (Am J Respir Crit Care Med, 2003) [MEDLINE]

  • Exposure to Known Offending Antigen: Odds Ratio 38.8 (95% CI: 11.6-129.6)
  • Symptoms 4-8 hrs After Exposure: Odds Ratio 7.2 (95% CI: 1.8-28.6)
  • Positive Precipitating Antibodies: Odds Ratio 5.3 (95% CI: 2.7-10.4)
  • Inspiratory Crackles: Odds Ratio 4.5 (95% CI: 1.8-11.7)
  • Recurrent Episodes of Symptoms: Odds Ratio 3.3 (95% CI: 1.5-7.5)
  • Weight Loss: Odds Ratio 2.0 (95% CI: 1.8-28.6)

Acute Hypersensitivity Pneumonitis

Characteristics

  • Exposure: intermittent short duration exposure
  • Symptom Onset: occur 4-6 hrs after challenge, last for 18-24 hrs after exposure ceases
    • Presents like Acute Bacterial or Viral Pneumonia, But May Persist if Exposure Persists
  • Course: reversible

Diagnosis

  • Chest X-Ray (see Chest X-Ray, [[Chest X-Ray]]): acute pulmonary edema pattern
  • Chest CT (see Chest Computed Tomography, [[Chest Computed Tomography]]): ground-glass infiltrates
  • High-Resolution Chest CT (HRCT) (see High-Resolution Chest Computed Tomography, [[High-Resolution Chest Computed Tomography]]): due to characteristic clinical manifestations and rapid resolution in acute HP, HRCT is not usually performed in this subset of HP patients
    • There are Case Reports of Normal HRCT in Biopsy-Proven HRCT: this may be related to the time interval between removal from the offending antigen and the performance of the HRCT
      • HP Study Group [MEDLINE]: only 4% of patients had a normal HRCT
    • Patchy or Diffuse Bilateral Ground-Glass Opacities: may be asymmetric in some cases
      • Ground-glass opacities in acute HP may be similar to those seen in desquamative interstitial pneumonia (see Desquamative Interstitial Pneumonia, [[Desquamative Interstitial Pneumonia]])
    • Small Poorly-Defined Ground-Glass Centrilobular Nodules (see Bronchiolar Disorders, [[Bronchiolar Disorders]]): characteristic feature
    • Lobular Areas of Decreased Attenuation and Vascularity: inspiratory HRCT
    • Focal Airspace Consolidation: may occur in some cases, due to organizing pneumonia or less commonly, diffuse alveolar damage
    • Areas of Decreased Attenuation and Vascularity Indicating Air Trapping (Represent Bronchiolar Obstruction): expiratory HRCT
    • Hilar/Mediastinal Lymphadenopathy (see Mediastinal Mass, [[Mediastinal Mass]]): occasionally seen
    • Typical Absence of Effusions/Solitary Nodules/Cavitation/Calcification/Atelectasis
  • Pulmonary Function Tests (PFT’s) (see Pulmonary Function Tests, [[Pulmonary Function Tests]]): PFT’s have no discriminative ability to distinguish HP from other interstitial lung diseases
    • General Comments: may be normal if done after acute symptoms resolve
    • Restrictive Pattern with Decreased DLCO: typical pattern
      • However, Data from the HP Study Group Demonstrate that the DLCO is Normal in 22% of Cases (Chest, 2012) [MEDLINE]
    • Normal Raw
  • Induced Sputum: contains increased total cells and lymphocytes (cell count differential suggest that induced sputum and BAL reflected different compartments of inflammation)
    • The Clinical Utility of Induced Sputum in the Diagnosis of Acute HP is Unclear
  • Bronchoscopy with Bronchoalveolar Lavage (BAL) (see Bronchoscopy, [[Bronchoscopy]]): BAL technique is not standardized
    • BAL Lymphocytosis (>30% for Non-Smokers/Ex-Smokers, >20% for Current Smokers) is Mandatory for the Diagnosis of HP: a normal number of BAL lymphocytes rules out all but residual disease
    • However, BAL Lymphocytosis Can Also Be Seen in Exposed Asymptomatic Subjects
    • CD4/CD8 Ratio <1
      • Increased BAL CD4/CD8 Ratio Has Been Classically Associated with Sarcoidosis, But High CD4/CD8 Ratios Can Also Be Seen in HP (J Allergy Clin Immunol, 1991) [MEDLINE] (Eur Respir J, 1997) [MEDLINE]: making differentiation of these entities difficult by CD4/CD8 ratio alone
  • Open Lung Biopsy: not usually required for diagnosis
    • Neutrophilic Infiltration of the Respiratory Bronchioles and Alveoli
    • Diffuse Alveolar Damage and Temporally Uniform, Nonspecific, Chronic Interstitial Pneumonitis May Be Seen in Some Cases

Clinical Features

  • Fever/Chills(see Fever, [[Fever]]): fever may be up to 104°
  • Dyspnea (see Dyspnea, [[Dyspnea]])
  • Cough (see Cough, [[Cough]])
    • Common: due to bronchocentric involvement
    • Usually Dry, But May Produce Sputum
  • Diffuse Crackles
  • Constitutional Symptoms
    • Fatigue (see Fatigue, [[Fatigue]])
    • Headache (see Headache, [[Headache]])
    • Malaise
    • Myalgias (see Myalgias, [[Myalgias]])
  • Clinical Features Which May Differentiate HP from Sarcoidosis [MEDLINE]
    • Inspiratory Crackles: HP more commonly presented with inspiratory crackles than sarcoidosis (87% vs 15%)
    • Hilar/Mediastinal Lymphadenopathy: hilar and/or mediastinal lymphadenopathy was more commonly observed in sarcoidosis than in HP (46% vs 2%)

Subacute Hypersensitivity Pneumonitis

Characteristics

  • Exposure: continuous but relatively short duration exposure
  • Symptom Onset: insidious onset of above symptoms with progression over days-weeks

Diagnosis

  • High-Resolution Chest CT (HRCT) (see High-Resolution Chest Computed Tomography, [[High-Resolution Chest Computed Tomography]])
    • There are Case Reports of Normal HRCT in Biopsy-Proven HRCT: this may be related to the time interval between removal from the offending antigen and the performance of the HRCT
      • HP Study Group [MEDLINE]: only 4% of patients had a normal HRCT
    • Patchy or Diffuse Bilateral Ground-Glass Opacities: may be asymmetric in some cases
    • Small Poorly-Defined Ground-Glass Centrilobular Nodules (see Bronchiolar Disorders, [[Bronchiolar Disorders]]): may be only finding in some subacute HP cases
      • Irregular nodules >10 mm are unusual (they usually represent areas of focal organizing pneumonia)
    • Lobular Areas of Decreased Attenuation and Vascularity: inspiratory HRCT
    • Focal Airspace Consolidation: may occur in some cases, due to organizing pneumonia or less commonly, diffuse alveolar damage
    • Areas of Decreased Attenuation and Vascularity Indicating Air Trapping (Represent Bronchiolar Obstruction): expiratory HRCT
      • Seen in up to 90% of cases
    • Cysts (3-25 mm) (see Cystic-Cavitary Lung Lesions, [[Cystic-Cavitary Lung Lesions]]): occur in 13% of patients with subacute HP
      • Usually Associated with Ground-Glass Opacities
      • Cysts Resemble Those of Lymphoid Interstitial Pneumonia: like the cysts of lymphoid interstitial pneumonia, they are presumably caused by partial bronchiolar obstruction by the peribronchiolar lymphocytic infiltrates
    • Hilar/Mediastinal Lymphadenopathy (see Mediastinal Mass, [[Mediastinal Mass]]): occasionally seen
    • Typical Absence of Effusions/Solitary Nodules/Cavitation/Calcification/Atelectasis
    • Patchy Interstitial Infiltrates: less common pattern
    • “Hilar Haze” Pattern: may mimic acute pulmonary edema
  • Pulmonary Function Tests (PFT’s) (see Pulmonary Function Tests, [[Pulmonary Function Tests]]): PFT’s have no discriminative ability to distinguish HP from other interstitial lung diseases
    • Restrictive (and Sometimes Obstructive) Pattern
      • Obstructive Pattern is Especially Common in Farmer’s Lung
    • Methacholine Challenge: may be positive
    • Decreased DLCO
      • However, Data from the HP Study Group Demonstrate that the DLCO is Normal in 22% of Cases (Chest, 2012) [MEDLINE]
  • Bronchoscopy with Bronchoalveolar Lavage (BAL) (see Bronchoscopy, [[Bronchoscopy]]): BAL technique is not standardized
    • BAL Lymphocytosis (>30% for Non-Smokers/Ex-Smokers, >20% for Current Smokers) is Mandatory for the Diagnosis of HP: a normal number of BAL lymphocytes rules out all but residual disease
    • However, BAL Lymphocytosis Can Also Be Seen in Exposed Asymptomatic Subjects
    • CD4/CD8 Ratio <1
      • Increased BAL CD4/CD8 Ratio Has Been Classically Associated with Sarcoidosis, But High CD4/CD8 Ratios Can Also Be Seen in HP (J Allergy Clin Immunol, 1991) [MEDLINE] (Eur Respir J, 1997) [MEDLINE]: making differentiation of these entities difficult by CD4/CD8 ratio alone
  • Open Lung Biopsy
    • Cellular Bronchiolitis, Non-Caseating Granulomas, and Bronchiolocentric Interstitial Pneumonitis with a Predominance of Lymphocytes
    • Areas of Organizing Pneumonia May Be Seen

Clinical Features

  • Fever/Chills(see Fever, [[Fever]]): fever may be up to 104°
  • Dyspnea (see Dyspnea, [[Dyspnea]])
  • Cough (see Cough, [[Cough]])
    • Common: due to bronchocentric involvement
    • Usually Dry But May Produce Sputum
  • Diffuse Crackles
  • Weight Loss (see Weight Loss, [[Weight Loss]])
  • Constitutional Symptoms
    • Fatigue (see Fatigue, [[Fatigue]])
    • Headache (see Headache, [[Headache]])
    • Malaise
    • Myalgias (see Myalgias, [[Myalgias]])
  • Clinical Features Which May Differentiate HP from Sarcoidosis [MEDLINE]
    • Inspiratory Crackles: HP more commonly presented with inspiratory crackles than sarcoidosis (87% vs 15%)
    • Hilar/Mediastinal Lymphadenopathy: hilar and/or mediastinal lymphadenopathy was more commonly observed in sarcoidosis than in HP (46% vs 2%)

Chronic Hypersensitivity Pneumonitis

Characteristics

  • Exposure: long duration exposure

Diagnosis

  • High-Resolution Chest CT (HRCT) (see High-Resolution Chest Computed Tomography, [[High-Resolution Chest Computed Tomography]]): radiographic changes in chronic HP may not allow accurate differentiation from idiopathic pulmonary fibrosis (with usual interstitial pneumonia pathology)
    • Reticulation and Traction Bronchiectasis and Bronchiolectasis Due to Fibrosis Superimposed on Findings of Acute/Subacute HP
      • Reticulation Be Patchy or Random
      • Reticulation May Have a Predominantly Subpleural/Peri-Bronchovascular Distribution
      • Reticulation Typically Spares the Lung Bases
    • Subpleural Honeycombing: occurs in a minority of chronic HP cases
    • Hilar/Mediastinal Lymphadenopathy (see Mediastinal Mass, [[Mediastinal Mass]]): occasionally seen
    • Emphysematous Changes: for unclear reasons, emphysematous changes are more common than interstitial fibrosis in chronic HP associated with farmer’s lung (without a smoking history) [MEDLINE]
    • Radiographic Features Which Best Differentiate Chronic HP from Idiopathic Pulmonary Fibrosis (IPF) and Non-Specific Interstitial Pneumonia (NSIP) [MEDLINE]
      • Lobular Areas with Decreased Attenuation and Vascularity
      • Centrilobular Nodules (see Bronchiolar Disorders, [[Bronchiolar Disorders]])
      • Absence of Lower-Zone Predominance of Abnormalities
  • Pulmonary Function Tests (PFT’s) (see Pulmonary Function Tests, [[Pulmonary Function Tests]]): PFT’s have no discriminative ability to distinguish HP from other interstitial lung diseases
    • Restrictive (and Sometimes Obstructive) Pattern
      • Obstructive Pattern is Especially Common in Farmer’s Lung
    • Decreased DLCO
      • However, Data from the HP Study Group Demonstrate that the DLCO is Normal in 22% of Cases (Chest, 2012) [MEDLINE]
  • Bronchoscopy with Bronchoalveolar Lavage (BAL) (see Bronchoscopy, [[Bronchoscopy]]): BAL technique is not standardized
    • BAL Lymphocytosis (>30% for Non-Smokers/Ex-Smokers, >20% for Current Smokers) is Mandatory for the Diagnosis of HP: a normal number of BAL lymphocytes rules out all but residual disease
    • However, BAL Lymphocytosis Can Also Be Seen in Exposed Asymptomatic Subjects
    • CD4/CD8 Ratio <1
      • Increased BAL CD4/CD8 Ratio Has Been Classically Associated with Sarcoidosis, But High CD4/CD8 Ratios Can Also Be Seen in HP (J Allergy Clin Immunol, 1991) [MEDLINE] (Eur Respir J, 1997) [MEDLINE]: making differentiation of these entities difficult by CD4/CD8 ratio alone
  • Open Lung Biopsy: may be required

Clinical Features

  • Fever/Chills (see Fever, [[Fever]]): fever may be up to 104°
  • Dyspnea (see Dyspnea, [[Dyspnea]])
  • Cough (see Cough, [[Cough]])
    • Common: due to bronchocentric involvement
    • Usually Dry But May Produce Sputum
  • Diffuse Crackles
  • Weight Loss (see Weight Loss, [[Weight Loss]])
  • Constitutional Symptoms
    • Fatigue (see Fatigue, [[Fatigue]])
    • Headache (see Headache, [[Headache]])
    • Malaise
    • Myalgias (see Myalgias, [[Myalgias]])
  • Pulmonary Hypertension/Cor Pulmonale (see Pulmonary Hypertension, [[Pulmonary Hypertension]]): due to advanced interstitial lung disease
  • Absence of Clubbing: usually

Treatment

Avoidance of Antigen/Exposure

  • Cessation of exposure is usually adequate for milder, acute cases

Corticosteroids (see Corticosteroids, [[Corticosteroids]])

  • Corticosteroids are usually used for more severe cases
  • Prednisone 60 mg/day usually x 2-3 weeks, then taper over 3-6 weeks
  • Typically result in rapid clinical response (over days)
  • Subacute/Chronic HP: may require >8 week course
  • Farmer’s Lung: steroids hasten acute recovery but do not have long-term clinical benefit
  • Hot Tub Lung: treatment with steroids OR treatment aimed at MAI appear to be equally effective -> dual therapy is currently recommended

Treatment of Bronchospasm

  • Albuterol and cromolyn are useful to treat obstructive component in some studies

Inhaled Corticosteroids (see Corticosteroids, [[Corticosteroids]])

  • Have not been shown to be useful (but high dose inhaled steroids have not been studied)

Avoidance of Exposure/Respiratory Protection

  • Symptoms usually resolve
  • Masks probably do limit exposure adequately though

Immunotherapy

  • Contraindicated (risk of causing immune-complex vasculitis)
  • Possible future role of Ag-specific inhalational or IV desensitization

Prognosis

  • Morbidity: 5-year morbidity is 30% (due to pulmonary fibrosis)
  • Overall, there is an increased mortality in patients with HP compared with the general population (hazard ratio, 2.98), even though these individuals are less likely to smoke [MEDLINE]

References

  • Difference in the phenotypes of bronchoalveolar lavage lymphocytes in patients with summer-type hypersensitivity pneumonitis, farmer’s lung, ventilation pneumonitis, and bird fancier’s lung: report of a nationwide epidemiologic study in Japan. J Allergy Clin Immunol. 1991;87(5):1002-1009 [MEDLINE]
  • Lung and blood T-cell receptor repertoire in extrinsic allergic alveolitis. Eur Respir J. 1997;10(4):772-779 [MEDLINE]
  • Hypersensitivity pneumonitis. AJR Am J Roentgenol. 2000;174(4):1061-1066 [MEDLINE]
  • High-resolution computed tomographic characteristics in acute farmer’s lung and in its follow-up. Eur Respir J 2000; 16:56–60 [MEDLINE]
  • Hypersensitivity pneumonitis: spectrum of high-resolution CT and pathologic findings. AJR Am J Roentgenol. 2007;188(2):334-344 [MEDLINE]
  • Extrinsic allergic alveolitis: incidence and mortality in the general population. QJM. 2007;100(4):233-237 [MEDLINE]
  • HP Study Group. Clinical diagnosis of hypersensitivity pneumonitis. Am J Respir Crit Care Med. 2003;168(8):952-958 [MEDLINE]
  • Chronic hypersensitivity pneumonitis: differentiation from idiopathic pulmonary fibrosis and nonspecific interstitial pneumonia by using thin-section CT. Radiology. 2008;246(1):288-297 [MEDLINE]
  • Recent advances in hypersensitivity pneumonitis. Chest 2012;142:208-217 [MEDLINE]
  • Bird fancier’s lung: a state-of-the-art review. Clin Rev Allergy Immunol. 2012 Aug;43(1-2):69-83. doi: 10.1007/s12016-011-8282-y [MEDLINE]