CONTENTS
- Basics
- Epidemiology
- History
- Symptoms
- Laboratory evaluation
- Thoracic radiology
- Bronchoscopy
- Lung biopsy
- Differential diagnosis
- Diagnostic approach
- Treatment
- Questions & discussion
abbreviations used in the pulmonary section: 1
- ABPA: Allergic bronchopulmonary aspergillosis 📖
- AE-ILD: Acute exacerbation of ILD 📖
- AEP: Acute eosinophilic pneumonia 📖
- AFB: Acid Fast Bacillus
- AIP: Acute interstitial pneumonia (Hamman-Rich syndrome) 📖
- ANA: Antinuclear antibody 📖
- ANCA: Antineutrophil cytoplasmic antibodies 📖
- ARDS: Acute respiratory distress syndrome 📖
- ASS: Antisynthetase Syndrome 📖
- BAL: Bronchoalveolar lavage 📖
- BiPAP: Bilevel positive airway pressure 📖
- CEP: Chronic eosinophilic pneumonia 📖
- COP: Cryptogenic organizing pneumonia 📖
- CPAP: Continuous positive airway pressure 📖
- CPFE: Combined pulmonary fibrosis and emphysema 📖
- CTD-ILD: Connective tissue disease associated interstitial lung disease 📖
- CTEPH: Chronic thromboembolic pulmonary hypertension 📖
- DAD: Diffuse alveolar damage 📖
- DAH: Diffuse alveolar hemorrhage 📖
- DIP: Desquamative interstitial pneumonia 📖
- DLCO: Diffusing capacity for carbon monoxide 📖
- DRESS: Drug reaction with eosinophilia and systemic symptoms 📖
- EGPA: Eosinophilic granulomatosis with polyangiitis 📖
- FEV1: Forced expiratory volume in 1 second 📖
- FVC: Forced vital capacity 📖
- GGO: Ground glass opacity 📖
- GLILD: Granulomatous and lymphocytic interstitial lung disease 📖
- HFNC: High flow nasal cannula 📖
- HP: Hypersensitivity pneumonitis 📖
- IPAF: Interstitial pneumonia with autoimmune features 📖
- IPF: Idiopathic pulmonary fibrosis 📖
- IVIG: Intravenous immunoglobulin 📖
- LAM: Lymphangioleiomyomatosis 📖
- LIP: Lymphocytic interstitial pneumonia 📖
- MAC: Mycobacterium Avium complex 📖
- MCTD: Mixed connective tissue disease 📖
- NIV: Noninvasive ventilation (including CPAP or BiPAP) 📖
- NSIP: Nonspecific interstitial pneumonia 📖
- NTM: Non-tuberculous mycobacteria 📖
- OP: Organizing pneumonia 📖
- PAP: Pulmonary alveolar proteinosis 📖
- PE: Pulmonary embolism 📖
- PFT: Pulmonary function test 📖
- PLCH: Pulmonary Langerhans Cell Histiocytosis 📖
- PPFE: Pleuroparenchymal fibroelastosis 📖
- PPF: Progressive pulmonary fibrosis 📖
- PVOD/PCH Pulmonary veno-occlusive disease/pulmonary capillary hemangiomatosis 📖
- RB-ILD: Respiratory bronchiolitis-associated interstitial lung disease 📖
- RP-ILD: Rapidly progressive interstitial lung disease 📖
- TNF: tumor necrosis factor
- UIP: Usual Interstitial Pneumonia 📖
introduction to HP (hypersensitivity pneumonitis)
- Hypersensitivity pneumonitis (HP) can be caused by nearly any organic antigen and selected chemicals. These are often named separately (e.g., farmer's lung, hot tub lung) but they are fundamentally similar.
- Among hospitalized patients, HP will tend to subside over time. However, if unrecognized, the disease will flare when patients return to the causative environment (whether that is their home or occupation).
nomenclature systems for HP
- Acute, subacute, and chronic HP have recently been renamed as non-fibrotic and fibrotic HP (figure above).
- Especially for the acute care clinician, the concepts of acute HP versus subacute HP may remain clinically useful. Unfortunately, acute HP is rare and difficult to diagnose prospectively – so many cases will likely be overlooked. Eliminating the entire nomenclature of “acute HP” will probably only exacerbate this problem in the future.
- In practice, nonfibrotic HP will consist almost entirely of subacute HP (because acute HP is rare and poorly characterized).
- This chapter will use both nomenclature systems. For now, familiarity with both sets of terminology may remain useful.
overall epidemiology
- HP is uncommon among patients who are actively smoking (which seems to have a protective effect against the development of HP). Smokers with HP-like radiologic features are more likely to have respiratory bronchiolitis (RB-ILD). (Walker 2019)
- HP is most likely in older individuals (with an average age of diagnosis in the 50s-60s). (32706311)
causative agents
There are hundreds of potential causative agents, including a broad variety of organic antigens. Below are some of the more common agents, but this list is not exhaustive.
occupational exposure
- Farmer's lung (moldy hay or grain silage).
- Machine operator's lung (metalworking fluids).
- Malt worker's lung (Aspergillus from moldy barley).
- Miller's lung (dust-contaminated grain, with wheat weevil).
- Cheese worker's lung.
- Pauli's reagent alveolitis (use of Pauli's laboratory reagent).
- Chemical worker's lung (polyurethane foams, spray paints, glues, elastomers).
- Animal handler's lung, Lab worker's lung (proteins from rat or gerbil fur/urine).
- Woodworker's lung (Alterna spp in wood dusts; especially moldy oak, cedar, and mahogany).
residential exposure
- Hot tub lung, lifeguard's lung (Mycobacterium avium complex).
- Humidifier/air conditioner lung (contaminated forced air systems or water reservoirs; Thermoactinomyces species).
- Compost lung (Aspergillus spp. in compost)
- Summer-type pneumonitis (old houses contaminated with Trichosporon cutaneum).
- Duvet lung (avian feathers in pillows, down blankets or coats).
- Basement lung (Cephalosporium or Penicillium spp. in a flooded basement).
recreational exposure
- Bird fancier's lung (droppings/feathers from various birds).
- Gerbil keeper's lung.
- Wind instrument lung (Fusarium spp, Penicillium spp, nontuberculous mycobacteria; may contaminate trombone, bagpipe, or saxophone).
medications
- Amitriptyline
- Atorvastatin.
- Bupropion.
- Carbamazepine.
- Cephalosporins.
- Ciprofloxacin.
- Cyclophosphamide.
- Docetaxel.
- Fluoxetine.
- Interferon-alpha.
- Mesalamine.
- Methotrexate.
- Nitrofurantoin.
- NSAIDs.
- Paclitaxel.
- Penicillin.
- Phenytoin.
- Rituximab.
- Sertraline.
- Sirolimus.
- Sulfonamides. (Fishman 2023)
exposure history is critical
- Ask about unusual exposures (e.g., occupations, hobbies, animal exposures).
- A formal exposure questionnaire may improve thoroughness (one produced by CHEST is available here: 📄).
- However, it's important to recognize that in up to half of patients diagnosed with HP at referral centers, no causative antigen can be found. (Murray 2022) Therefore, lack of any specific causative agent doesn't exclude HP.
try to determine if there is a linkage between symptoms and exposure
- Symptoms following exposure are more likely after a heavy exposure than low-level exposure.
- Do symptoms resolve during vacations? During hospital admission?
acute HP
- Following an intense exposure, symptoms may begin ~4-8 hours later, peak after ~8-12 hours, and last for 12-48 hours (although some residual dyspnea may persist for weeks). Clinically this may initially resemble a pneumonia or asthma, sometimes leading to hospitalization. Fortunately, patients will rapidly recover following removal from the causative environment. However, re-exposure may lead to a recrudescence of illness.
- Symptoms may include:
- Dry cough.
- Chest tightness, dyspnea.
- Respiratory distress (which may require hospitalization).
- Constitutional symptoms (e.g., chills, fever, malaise, headache, flu-like syndrome).
- Examination may reveal end-inspiratory crackles or squeaks.
subacute HP
- With less intense and more prolonged exposure, symptom onset is insidious and not readily associated with exposure. Clinically this may mimic chronic bronchitis, posing a greater diagnostic challenge.
- Symptoms may include:
- Chronic cough that may be productive of mucoid sputum.
- Exertional dyspnea.
- Constitutional symptoms may occur (e.g., malaise, anorexia, fatigue, weight loss).
- Chest auscultation may be notable for diffuse crackles and squeaks.
chronic HP (fibrotic HP)
- Clinically, this will resemble IPF (idiopathic pulmonary fibrosis).
- Patients may evolve from acute/subacute HP to eventually develop chronic HP. However, other patients may initially present with chronic HP (without any history of clinically notable acute/subacute HP).
- Patients present with persistent and progressive dyspnea, and often dry cough.
- There are usually no systemic symptoms (although some patients may have anorexia and weight loss).
- Examination may reveal digital clubbing and fine bibasilar crackles. Inspiratory squawks may persist in HP (correlating to bronchiolitis and small airways involvement). (Murray 2022)
general laboratory tests
- Laboratory tests are generally nonspecific (e.g., leukocytosis without eosinophilia may occur in acute HP).
- Acute HP may cause an elevation of acute phase reactants, including C-reactive protein. However, procalcitonin levels are generally quite low (<0.1 ng/mL in 89% of patients). (27140709)
serum antibodies
- ⚠️ Detection of IgG antibodies (precipitins) against various antigens can support a diagnosis of HP, but this process is fraught with hazard.
- False-positive results may result if patients are sensitized to the antigen, but not clinically experiencing actual pneumonitis. For example, ~40% of pigeon breeders may have positive results, but this often merely reflects exposure to the antigen (rather than clinical disease).
- False-negative results:
- False-negative results often occur because the patient's HP is due to an antigen that isn't tested for. Some panels of antibodies are available, but none are exhaustively complete against all antigens.
- In patients with chronic HP who lack recent antigen exposure, antibody levels may wane over time. (Fishman 2023)
- Tests vary widely:
- Different laboratories often create their own antibody panels.
- Tests and panels lack adequate standardization.
- (⚠️ Skin tests to detect an immediate wheal and flare response to various antigens are not useful. Such skin tests evaluate for IgE antibodies and type-I hypersensitivity, which are not involved in the pathogenesis of hypersensitivity pneumonitis.) (Fishman 2023)
- CT scans often aren't performed on these patients (due to the acute onset and rapid clinical resolution).
- Potential radiological findings may include:
- Diffuse ground-glass opacities and consolidation (this would be the expected finding for an acutely ill, hospitalized patient).
- Normal CT scan may occur.
- Centrilobular nodules may be seen.
There are three characteristic findings:
#1/3) Ground glass opacities (~80%)
- Hypersensitivity pneumonia is one of the most common causes of isolated diffuse ground glass opacities among immunologically normal hosts.
- Ground glass opacities are generally symmetric. They may be diffuse or patchy.
#2/3) Mosaic attenuation +/- three-density (head cheese) sign
- Obstruction of some airways may cause hyperinflation of the distal lung tissue. Expiratory imaging will highlight the obstruction of some small airways.
- Rarely, expiratory air trapping may be the only radiologic sign of subacute HP.
- A mixture of increased attenuation areas (due to ground glass opacification), normal areas, and reduced attenuation areas (due to air trapping) may create the three-density (aka head cheese) sign.
#3/3) Centrilobular nodules (~60%)
- Ill-defined, small ground-glass nodules may be seen within the center of pulmonary lobules. Nodules may appear as clusters. (Walker 2019)
- This appearance may be relatively suggestive of hypersensitivity pneumonitis, although other processes can also cause it, especially RBILD (respiratory bronchiolitis interstitial lung disease).
other features:
- Distribution is variable:
- Abnormalities are generally diffuse.
- Upper and middle lung zones may be most affected. Less often, there is middle and lower predominance (with sparing of the bases). (Walker 2019)
- Costophrenic recesses are usually spared.
- Cysts can occur in ~13% of patients with subacute HP (~1-15 thin-walled cysts, 3-25 mm in size, in a random distribution). (Walker 2019)
- Pneumomediastinum or pneumothorax may occur (presumably related to regional hyperinflation causing alveolar rupture).
ATS/JRS/ALAT guideline for CT interpretation in nonfibrotic HP
CT scan is typical of nonfibrotic HP if the following criteria are met:
- (a) At least one abnormality indicative of parenchymal infiltration, which may include:
- GGO (ground glass opacities).
- Mosaic attenuation created by GGO (ground glass opacities).
- (b) At least one abnormality indicative of small airway disease, which may include:
- Ill-defined, centrilobular nodules <5 mm on inspiratory images.
- Air trapping on expiratory CT scan.
- (c) Distribution of both should be diffuse.
- Craniocaudal distribution should be diffuse (although some basal sparing may occur).
- Axial distribution should be diffuse.
CT scan is compatible with nonfibrotic HP if:
- Above criteria are not fully met.
- Additional features that may be seen include:
- Airspace consolidation.
- Lung cysts.
- Uniform and subtle GGO (ground glass opacities).
- Distributions that may be seen include:
- Craniocaudal: diffuse, or lower lobe predominant.
- Axial: diffuse, or peribronchovascular.
CHEST guideline for CT interpretation in nonfibrotic HP
(Editor's note: These are superior to the ATS/JRS/ALAT guidelines.)
typical nonfibrotic HP
- (1) Any of the following:
- Profuse poorly defined centrilobular nodules of ground glass opacity affecting all lung zones.
- Inspiratory mosaic attenuation with three-density sign.
- Inspiratory mosaic attenuation and air trapping associated with centrilobular nodules.
- (2) Lack of features suggesting an alternative diagnosis
compatible with nonfibrotic HP
- (1) Any of the following:
- Centrilobular nodules of ground glass attenuation that are not profuse or diffuse, and not associated with mosaic attenuation or lobular air trapping.
- Patchy or diffuse ground glass opacity
- Mosaic attenuation and lobular air trapping without centrilobular nodules or ground glass opacity.
- (2) Lack of features suggesting an alternative diagnosis.
overall appearance
- Chronic HP may mimic other fibrosing interstitial lung diseases, with the following features:
- Reticular pattern (interlobular septal thickening).
- Traction bronchiectasis.
- Honeycombing will eventually occur (although this is usually minimal). Unlike IPF, honeycombing seldom has basal predominance. (Walker 2019)
- Distribution is variable:
- Upper or middle lung predominance is often seen in HP, whereas this is unusual for most interstitial lung diseases (more on distribution: 📖). If seen, this may help distinguish HP from other interstitial lung diseases.
- Unfortunately, HP can be basilar predominant, with a distribution mimicking UIP or NSIP. (Shepard 2019)
- Involvement is often heterogeneous, with sparing of some of the secondary pulmonary lobules. Patchy, asymmetric fibrosis may suggest HP.
features of subacute HP are often seen
- If visible, these may help differentiate HP from other interstitial lung diseases.
- Ground glass opacities are seen in most patients.
- Lobular air trapping is seen in ~80% of patients.
- If air trapping on expiratory CT scan is seen bilaterally and in three lobes, this is inconsistent with a diagnosis of IPF (idiopathic pulmonary fibrosis).
- If a three-density (head cheese) sign is present, this is even more suggestive of HP.
- Centrilobular nodules are seen in ~50% of patients (but this may be subtle).
other potential findings
- Cysts may occur. (Walker 2019)
- Mild lymphadenopathy may occur. However, usually only 1-2 lymph nodes are involved and they are <15 mm in size. (Walker 2019)
- Rarely, chronic hypersensitivity pneumonia may cause emphysema rather than fibrosis (especially farmer's lung). (Shepard 2019)
ATS/JRS/ALAT guideline for CT interpretation in fibrotic HP
CT scan is typical of fibrotic HP if the following criteria are met:
- (a) CT pattern of lung fibrosis
- Irregular linear opacities and/or coarse reticulation with lung distortion.
- Traction bronchiectasis and honeycombing may be present, but should not predominate.
- (b) Fibrosis should occur in one of the following distributions:
- Random, both axially and craniocaudally.
- Mid-lung zone predominant.
- Relative sparing of the lower lung zones.
- (c) At least one abnormality that is indicative of small airway disease:
- Ill-defined, centrilobular nodules.
- Air trapping on expiratory CT scan (including the head cheese sign).
CT scan is compatible with fibrotic HP if the following criteria are met:
- The above criteria are not fully met, due to deviations as described below:
- (a) Variant patterns of lung fibrosis:
- May resemble a UIP (usual interstitial pneumonia) pattern.
- May involve extensive GGO (ground glass opacities ) with superimposed subtle features of fibrosis.
- (b) Variant distributions of lung fibrosis:
- Axial: peribronchovascular, or subpleural areas.
- Craniocaudal: upper lung zones (if seen, this suggests HP rather than IPF, but it is seen in only <10% of patients). (32706311)
- (c) At least one abnormality that is indicative of small airway disease is still required:
- Ill-defined, centrilobular nodules.
- Air trapping on expiratory CT scan (including the head cheese sign).
CT scan is indeterminate for fibrotic HP if:
- Literally everything else: Any CT scan that doesn't meet either of the above sets of criteria.
- This category includes, for example, a UIP (usual interstitial pneumonia) pattern with zero features of HP. The term “indeterminate” may actually be somewhat misleading in some situations, because some CT scans classified as “indeterminate” may in fact strongly suggest that the patient does not have HP.
CHEST guidelines for CT interpretation in fibrotic HP
typical fibrotic HP
- (1) CT signs of fibrosis with either of the following:
- Profuse, poorly defined centrilobular nodules of ground glass opacity affecting all lung zones.
- Inspiratory mosaic attenuation with three-density sign
- (2) Lack of features suggesting an alternative diagnosis.
compatible with fibrotic HP
- (1) CT signs of fibrosis with any of the following:
- Patchy or diffuse ground glass opacity.
- Patchy, nonprofuse centrilobular nodules of ground glass attenuation.
- Mosaic attenuation and lobular air trapping that do not meet criteria for typical fibrotic HP.
- (2) Lack of features suggesting an alternative diagnosis.
overall use of bronchoscopy
- The primary role of bronchoscopy is to exclude alternative diagnoses (especially infection). This is often more relevant among patients with suspected nonfibrotic HP. (32706311)
- Bronchoscopy has a higher yield in nonfibrotic HP, as compared to fibrotic HP.
the role of lymphocytosis in BAL (bronchoalveolar lavage)
- Lymphocytosis may support the diagnosis of HP (e.g., >30% lymphocytes in a nonsmoker, or >20% in a smoker).
- Lymphocytosis has been shown to help differentiate HP from IPF or sarcoidosis (which have fewer lymphocytes). However, the ability of lymphocytosis to differentiate HP from other interstitial lung diseases is largely unclear. (32706311)
- Sensitivity may be decreased in some situations:
- Patients with fibrotic HP are less likely to show lymphocytosis (since they may lack active alveolar inflammation).
- Lymphocytosis may take >5 days to develop following an acute exposure. (Fishman 2023)
- Lymphocytosis is most sensitive for patients with nonfibrotic HP. In this context, absence of lymphocytosis lowers the probability enough to exclude HP. (35140104)
- Specificity:
other potential findings on bronchoalveolar lavage
- Elevated plasma cells may be highly suggestive of HP if this is seen.
- Foamy macrophages can occur.
- Neutrophilia can occasionally occur:
- Acute HP may cause striking neutrophilia within two days of an exposure, generally normalizing within a week thereafter. (31916970)
- In fibrotic HP, neutrophilia may occur. However, in general neutrophilia would be more suggestive of IPF (idiopathic pulmonary fibrosis).
type of biopsy
transbronchial forceps biopsy
- Diagnostic yield appears to be ~35% for patients with fibrotic HP versus ~65% among patients with nonfibrotic HP. (32706311)
- Diagnostic yield may relate to the findings on CT scan (e.g., diffuse ground glass opacities may imply an acute and diffuse process with higher yield, whereas a scan that resembles idiopathic pulmonary fibrosis may have more heterogeneous pathology that defies simple characterization).
cryobiopsy
- Diagnostic yield is ~85%. (32706311)
- Cryobiopsy carries a higher risk than bronchoscopic forceps biopsy, but with a much higher yield.
- Risks and contraindications are discussed further in the chapter on IPF: 📖
surgical lung biopsy
- Diagnostic yield is ~95%. (32706311)
- Surgical lung biopsy is the most definitive and also the most invasive strategy.
- Procedural mortality is ~2%. (32706311)
- Risks and contraindications are discussed further in the chapter on IPF: 📖
histopathological findings
histologic features of nonfibrotic HP:
- Typical nonfibrotic HP demonstrates four key features:
- (1) Small airway involvement (aka bronchiolocentric accentuation).
- (2) Uniform cellular interstitial inflammation.
- (3) Predominantly lymphocytic infiltrate.
- (4) At least a single, poorly formed granuloma and/or multinucleated giant cell. Granulomas are typically small and poorly formed, with or without giant cells.
- Other common features:
- Small foci of organizing pneumonia (Masson bodies).
- Foamy macrophages.
- Cholesterol clefts.
- Schaumann bodies.
- Calcium oxalate crystals.
- Widespread peribronchiolar metaplasia.
histologic features of fibrotic HP:
- Typical fibrotic HP has three key features:
- (1) Airway-centered fibrosis with or without widespread peribronchiolar metaplasia.
- (2) Fibrosing interstitial pneumonia (which might have the appearance of fibrotic NSIP, UIP, isolated peribronchiolar fibrosis, or fibrotic lung disease that defies a specific classification)
- (3) Poorly formed granulomas.
histopathological features that may suggest an alternative diagnosis:
- Prominent lymphoid aggregates (especially with secondary germinal centers) may suggest underlying connective tissue disease (CTD-ILD) or immunodeficiency.
- Well-formed granulomas may suggest sarcoidosis, infection, or aspiration. However, tighter granulomas may be seen in HP due to NTM (nontuberculous mycobacteria); for example, in hot tub lung.
Different forms of HP may mimic a variety of disorders. Acute HP may be confused with the following entities:
- Metal fume fever (flu-like syndrome caused by inhalation in electric arc welding).
- Occupational asthma.
- Silo filler's disease (exposure to nitrogen dioxide in silos filled with organic material may cause bronchitis, bronchiolitis, or ARDS).
- Organic dust toxic syndrome (ODTS)
- Exposure to bacterial endotoxins or mycotoxins in agricultural settings causes a transient flu-like syndrome with normal oxygen saturation and normal chest x-ray.
- Prognosis is favorable, with patients generally recovering fully without sequelae within 24 hours. (ERS handbook 3rd ed.)
- Allergic bronchopulmonary aspergillosis (patients may also have antibodies against Aspergillus fumigatus).
- Psittacosis.
overall approach
- Diagnosis is based largely on three domains:
- (1) Exposure identification (e.g., clinical history, serum IgG testing against potential antigens).
- (2) CT scan (including inspiratory and expiratory scans).
- (3) Bronchoalveolar lavage lymphocytosis / histopathological features.
- Diagnosis is generally approached with the use of a multidisciplinary ILD conference.
- A low-confidence diagnosis may be adequate for some patients, including the following situations:(32706311)
- Alternative diagnoses have been reasonably excluded, such that further investigation is unlikely to change management.
- Invasive testing exposes the patient to unacceptable risks.
diagnosis of nonfibrotic HP
- Diagnosis of nonfibrotic HP is often easier for several reasons:
- There is more often an identifiable exposure.
- CT scans are easier to interpret, without the presence of overlying fibrosis. For example, profuse centrilobular ground-glass nodules on CT scan may be extremely suggestive of HP.
- If bronchoscopy is performed, it is more likely to yield a lymphocytic predominant BAL (bronchoalveolar lavage).
noninvasive diagnosis of HP
- Guidelines disagree about whether HP can be confidently diagnosed in a patient with identified antigen exposure and a CT scan that is typical of HP:
- The ATS/JRS/ALAT guideline shown above suggests that this combination may lead to a diagnosis of HP with only moderate confidence (70-79% likelihood). (32706311)
- CHEST guidelines suggest that this combination may lead to a definite diagnosis of HP (>90% likelihood). Indeed, these guidelines specifically recommend against further invasive testing in this scenario. (33865835)
- This illustrates that guidelines merely provide a general conceptual schema for approaching the diagnosis, rather than pathways that must be rigidly followed.
- Ultimately, diagnosis will depend on the specific clinical and radiological features of any specific patient. For example, a textbook CT scan showing diffuse ground-glass centrilobular nodules is extremely suggestive of nonfibrotic HP in the appropriate clinical context.
avoidance of exposure
- This is the most important treatment.
- Depending on the type of exposure, avoidance may be challenging. For example, occupational medicine may be helpful for investigating and avoiding occupational exposures.
immunosuppressive therapy
- No controlled studies are available regarding immunomodulation in the treatment of HP. It's dubious whether immunosuppression truly affects long-term outcomes, or merely causes a temporary reduction in symptoms. (ERS handbook 3rd ed.)
- Steroid is currently considered the primary therapy for acute HP. A dose of ~0.5 mg/kg/day prednisone might be utilized for ~4 weeks, followed by a gradual taper off over perhaps 1-2 months. (Shah 2019; Fishman 2023)
- Treatment response should be monitored (including symptoms, pulmonary function testing, and radiographic studies). If steroid doesn't cause improvement, it should be discontinued.
- Azathioprine and mycophenolate mofetil have been utilized for chronic HP, but there is no clear data regarding this.
antifibrotic therapies
- In the INBUILD trial, nintedanib slowed the rate of disease progression (including within the subgroup of patients with HP). (31566307, 32145830)
lung transplantation
- Indications for transplantation are similar to those for other patients with fibrotic interstitial lung disease.
- Outcomes among patients with chronic HP are superior to those with IPF. The risk of recurrence within the allograft appears low. (Murray 2022)
- More information on when to refer for transplant evaluation is here: 📖
poor prognostic factors
- Epidemiologic factors:
- Older age.
- Male sex.
- Exposures:
- Persistent antigenic exposure and/or inability to identify an inciting agent.
- History of smoking.
- Physiology:
- Low FVC.
- Low DLCO.
- Decline in FVC over time.
- Lack of lymphocytosis on BAL (bronchoalveolar lavage).
- Radiology/pathology:
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References
- 31916970 Nakamatsu E, Morita T, Morishima A, Tagawa H, Furukawa M, Matsushita M, Yamane H. A 78-Year-Old Man With Repeated Dyspnea and Neutrophilia in Peripheral Blood and BAL. Chest. 2020 Jan;157(1):e17-e20. doi: 10.1016/j.chest.2019.07.034 [PubMed]
- 32706311 Raghu G, Remy-Jardin M, Ryerson CJ, Myers JL, Kreuter M, Vasakova M, Bargagli E, Chung JH, Collins BF, Bendstrup E, Chami HA, Chua AT, Corte TJ, Dalphin JC, Danoff SK, Diaz-Mendoza J, Duggal A, Egashira R, Ewing T, Gulati M, Inoue Y, Jenkins AR, Johannson KA, Johkoh T, Tamae-Kakazu M, Kitaichi M, Knight SL, Koschel D, Lederer DJ, Mageto Y, Maier LA, Matiz C, Morell F, Nicholson AG, Patolia S, Pereira CA, Renzoni EA, Salisbury ML, Selman M, Walsh SLF, Wuyts WA, Wilson KC. Diagnosis of Hypersensitivity Pneumonitis in Adults. An Official ATS/JRS/ALAT Clinical Practice Guideline. Am J Respir Crit Care Med. 2020 Aug 1;202(3):e36-e69. doi: 10.1164/rccm.202005-2032ST [PubMed]
- 33470673 Huntley CC, Walters GI. An update on hypersensitivity pneumonitis: what a clinician wants to know. Curr Opin Pulm Med. 2021 Mar 1;27(2):95-104. doi: 10.1097/MCP.0000000000000763 [PubMed]
- 33865835 Fernández Pérez ER, Travis WD, Lynch DA, Brown KK, Johannson KA, Selman M, Ryu JH, Wells AU, Tony Huang YC, Pereira CAC, Scholand MB, Villar A, Inase N, Evans RB, Mette SA, Frazer-Green L. Executive Summary: Diagnosis and Evaluation of Hypersensitivity Pneumonitis: CHEST Guideline and Expert Panel Report. Chest. 2021 Aug;160(2):595-615. doi: 10.1016/j.chest.2021.03.067 [PubMed]
- 35140104 Hamblin M, Prosch H, Vašáková M. Diagnosis, course and management of hypersensitivity pneumonitis. Eur Respir Rev. 2022 Feb 9;31(163):210169. doi: 10.1183/16000617.0169-2021 [PubMed]
Books:
- Shah, P. L., Herth, F. J., Lee, G., & Criner, G. J. (2018). Essentials of Clinical pulmonology. In CRC Press eBooks. https://doi.org/10.1201/9781315113807
- Shepard, JO. (2019). Thoracic Imaging The Requisites (Requisites in Radiology) (3rd ed.). Elsevier.
- Walker C & Chung JH (2019). Muller’s Imaging of the Chest: Expert Radiology Series. Elsevier.
- Palange, P., & Rohde, G. (2019). ERS Handbook of Respiratory Medicine. European Respiratory Society.
- Murray & Nadel: Broaddus, V. C., Ernst, J. D., MD, King, T. E., Jr, Lazarus, S. C., Sarmiento, K. F., Schnapp, L. M., Stapleton, R. D., & Gotway, M. B. (2021). Murray & Nadel’s Textbook of Respiratory Medicine, 2-Volume set. Elsevier.
- Fishman's: Grippi, M., Antin-Ozerkis, D. E., Cruz, C. D. S., Kotloff, R., Kotton, C. N., & Pack, A. (2023). Fishman’s Pulmonary Diseases and Disorders, Sixth Edition (6th ed.). McGraw Hill / Medical.