Eduovisual
Respiratory
Hypersensitivity pneumonitis: workup and exposure mitigation
— Mechanism: mixed type III (immune complex, acute) and type IV (T-cell mediated, chronic) hypersensitivity producing lymphocytic alveolitis and poorly formed non-necrotizing granulomas.
— Current ATS/JRS/ALAT 2020 classification splits HP into non-fibrotic (inflammatory) and fibrotic phenotypes — the older acute/subacute/chronic scheme is being phased out on exams but still appears.
— Recurrent "flu-like" episodes with cough, dyspnea, fever 4–8 hours after a specific exposure (returning home, entering a barn, using a hot tub) that resolve when away.
— Subacute or chronic progressive dyspnea and cough in a patient with bird, mold, humidifier, hot tub, or occupational dust exposure — often misdiagnosed as recurrent pneumonia, asthma, or COPD.
— New ILD pattern on CT in a non-smoker without rheumatologic disease, especially with a centrilobular nodular or mosaic attenuation pattern.
— Farmer's lung → thermophilic actinomycetes in moldy hay
— Bird fancier's/pigeon breeder's lung → avian proteins (droppings, feathers, down comforters)
— Hot tub lung → Mycobacterium avium complex in aerosolized water
— Humidifier/HVAC lung → bacteria, amoebae in standing water
— Chemical worker's lung → isocyanates (auto paint, polyurethane foam)
— Cheese washer's, malt worker's, woodworker's lung → various molds
Board pearl: A non-smoking middle-aged adult with progressive dyspnea, new pet birds at home, and ground-glass plus mosaic attenuation on HRCT is HP until proven otherwise — pursue exposure history before ordering a lung biopsy.
— Onset 4–8 hours after high-intensity antigen exposure; resolves over hours to days with avoidance.
— Symptoms: fever, chills, malaise, dry cough, dyspnea, myalgias — easily mistaken for viral illness or atypical pneumonia.
— Pattern recognition: symptoms recur on Mondays at work ("Monday fever" in occupational HP) or every weekend at the farm/cabin; improve on vacation.
— Insidious dyspnea on exertion and chronic cough over months to years from low-grade continuous exposure (e.g., a single indoor bird, hidden mold behind drywall, contaminated central humidifier).
— Weight loss, fatigue, clubbing in advanced disease — overlaps clinically with idiopathic pulmonary fibrosis (IPF).
— May lack a clear temporal trigger; the patient often does not connect symptoms to exposure.
— Home: birds (even neighbors'), down bedding/pillows, mold (water damage, basement, leaks), humidifiers, hot tubs/indoor pools, wind instruments (clarinet, saxophone — "saxophone lung" from biofilm).
— Occupational: farming, dairy, poultry, lumber/sawmill, metalworking fluids, paint shop (isocyanates), cheese/wine/malt industry, lifeguarding indoors.
— Hobbies: pigeon racing, mushroom growing, woodworking.
— Timing: "Do symptoms improve on vacation or when you leave the house?" — a positive answer is highly suggestive.
— Medications: rare drug-induced HP (nitrofurantoin, methotrexate, amiodarone) is a differential rather than classic HP.
Key distinction: Acute HP mimics community-acquired pneumonia but lacks a clear infectious prodrome and recurs with re-exposure; chronic fibrotic HP mimics IPF but typically has a younger non-smoking host and an identifiable antigen.
Step 3 management: Document a structured exposure inventory in the chart at the first visit — this single note often determines whether HP is diagnosed years earlier or after irreversible fibrosis develops.
— Acute episode: febrile, tachypneic, may appear toxic; resolves rapidly after removal from exposure.
— Chronic/fibrotic: cachectic in advanced disease, resting hypoxemia, exertional desaturation often dramatic on 6-minute walk test (6MWT).
— Bibasilar fine inspiratory crackles ("Velcro crackles") — present in both acute and fibrotic disease, indistinguishable from IPF on auscultation alone.
— "Squawks" or mid-to-late inspiratory short squeaks — relatively specific for HP and other bronchiolitis-centered ILDs; highly testable physical finding.
— Wheezing is uncommon; if prominent, reconsider asthma or ABPA.
— Clubbing in ~20–50% of fibrotic HP (less than IPF) — its presence in a non-smoker with ILD should raise suspicion for fibrotic HP.
— Loud P2, RV heave, elevated JVP, peripheral edema → pulmonary hypertension and cor pulmonale in advanced fibrotic disease.
— Resting tachycardia common during acute episodes.
— SpO2 at rest and with ambulation — exertional desaturation <88% is a key trigger for supplemental O2 evaluation and pulmonary rehab referral.
— 6MWT for baseline and longitudinal monitoring; distance walked and lowest SpO2 are prognostic.
— Body weight and BMI trends — unintentional weight loss tracks with progression.
— Should be normal — rashes, synovitis, Raynaud, sclerodactyly, or mechanic's hands point toward connective tissue disease–associated ILD instead.
Board pearl: Inspiratory squawks + Velcro crackles in a bird owner = fibrotic HP on the boards; the same crackles in a 70-year-old male smoker without exposure = IPF.
Step 3 management: Order a 6MWT with continuous pulse oximetry at the diagnostic visit — it informs oxygen prescribing, rehab referral, and disability documentation in one test.
— CBC: mild leukocytosis with neutrophilia during acute episodes; lymphocytosis is not characteristic.
— ESR/CRP: nonspecifically elevated in acute HP.
— Serum-specific IgG ("precipitins") against suspected antigens (avian, Aspergillus, thermophilic actinomycetes): supports exposure, not disease — high false-positive rate among asymptomatic exposed individuals. A negative panel against the suspected antigen lowers but does not exclude HP.
— Serologies to exclude mimics: ANA, RF, anti-CCP, myositis panel, ANCA — should be negative or non-contributory in primary HP.
— HIV and hypersensitivity-mimicking infection workup as clinically indicated.
— Chest X-ray often normal, especially in non-fibrotic HP — a normal CXR does not rule out HP.
— High-resolution CT (HRCT) with inspiratory and expiratory cuts is the cornerstone imaging study.
· Non-fibrotic HP: diffuse ground-glass opacities, ill-defined centrilobular nodules, and mosaic attenuation with air trapping on expiration (the "three-density" or "headcheese" sign when ground-glass, normal lung, and air trapping coexist — highly specific).
· Fibrotic HP: reticulation, traction bronchiectasis, with mosaic attenuation and air trapping persisting — upper/mid lobe predominance favors HP over IPF (basal/subpleural).
· Honeycombing can occur in fibrotic HP and overlaps with usual interstitial pneumonia (UIP) pattern.
— Restrictive pattern with reduced TLC, FVC, and FEV1 with preserved or increased FEV1/FVC ratio.
— Reduced DLCO — often the earliest abnormality.
— Mixed obstructive-restrictive pattern can occur due to bronchiolar involvement.
Key distinction: Mosaic attenuation with expiratory air trapping + centrilobular nodules favors HP; basal subpleural honeycombing without air trapping favors IPF.
Step 3 management: Order HRCT with inspiratory and expiratory phases explicitly — radiologists may not include expiratory imaging unless requested, and air trapping is a key diagnostic feature.
— Lymphocytosis >30% (often >40–50%) is the hallmark BAL finding in HP — among the highest yield ancillary tests.
— CD4/CD8 ratio is variable and no longer considered specific (low ratio classically taught but not reliable).
— BAL also excludes infection (cultures for bacteria, mycobacteria, fungi; PCP in immunosuppressed).
— Highest yield in non-fibrotic HP; less sensitive in established fibrotic disease.
— Transbronchial cryobiopsy is increasingly preferred over surgical lung biopsy for ILD characterization due to lower morbidity.
— Surgical lung biopsy (VATS) reserved for cases where less invasive workup is non-diagnostic and management hinges on the result.
— Classic histology: lymphocytic bronchiolocentric inflammation + poorly formed non-necrotizing granulomas + giant cells; fibrotic HP adds peribronchiolar fibrosis and may show airway-centered UIP-like changes.
— Rarely performed; only at specialized centers; risk of severe reaction. Not a Step 3 routine test.
— ATS/JRS/ALAT guidelines emphasize a multidisciplinary diagnosis integrating exposure history, HRCT pattern, BAL lymphocytosis, and biopsy when needed.
— Confidence categories: definite, high-confidence, moderate-confidence, low-confidence, or not excluded HP — drives whether to treat empirically or pursue biopsy.
— Baseline PFTs (FVC, DLCO), 6MWT, echocardiogram if pulmonary hypertension suspected, vaccinations (influenza, pneumococcal, COVID-19, RSV when eligible) — especially before immunosuppression.
Board pearl: BAL lymphocytosis >40% in a patient with compatible HRCT and a credible antigen exposure can establish a high-confidence HP diagnosis without biopsy.
Step 3 management: Refer to pulmonology and, when available, an ILD center for MDD before initiating chronic immunosuppression — definitive classification changes both treatment and prognosis (HP vs IPF have opposite responses to steroids).
— Outcomes correlate more strongly with successful exposure remediation than with pharmacotherapy.
— Even in fibrotic HP, ongoing exposure accelerates decline; antigen avoidance can stabilize disease.
— Antigen is identified in only ~30–50% of fibrotic HP cases — this does not preclude treatment, but lack of identifiable antigen is an independent poor prognostic marker.
— Mild/early non-fibrotic HP with full antigen removal: often improves without immunosuppression; observe with serial PFTs.
— Symptomatic or progressive non-fibrotic HP: systemic corticosteroids.
— Fibrotic HP: immunosuppression (steroids ± steroid-sparing agent) and consider antifibrotic therapy if progressive despite immunosuppression.
— Advanced disease (FVC <50%, DLCO <40%, resting hypoxemia, pulmonary hypertension): refer for lung transplant evaluation early.
— Fibrotic phenotype on HRCT (especially honeycombing)
— Unidentified antigen
— Older age, lower baseline FVC/DLCO
— Ongoing exposure
— Clubbing
— Pulmonary hypertension on echo
— Smoking history
— Most HP is managed outpatient.
— Hospitalize acute HP with hypoxemia (SpO2 <90% on room air), severe dyspnea, or inability to remove exposure (rare — e.g., shared housing with persistent contamination).
Key distinction: In IPF, corticosteroids are harmful (PANTHER-IPF trial); in HP, corticosteroids are first-line for symptomatic disease — getting the diagnosis right matters therapeutically.
Step 3 management: Before starting steroids, schedule an environmental/occupational health consult or home assessment — drugs won't work if the antigen remains in the bedroom or workplace.
— Prednisone 0.5–1 mg/kg/day (typically 40–60 mg) for 2–4 weeks, then taper over 2–3 months guided by symptoms, PFTs, and HRCT.
— Reassess at 4–6 weeks with PFTs; if no objective improvement (e.g., FVC or DLCO gain ≥5–10%), reconsider diagnosis or escalate.
— Acute non-fibrotic HP often responds dramatically; fibrotic HP shows variable response.
— Mycophenolate mofetil 1–1.5 g BID — generally preferred first-line steroid-sparer; improves DLCO and reduces prednisone dose.
— Azathioprine 1–2 mg/kg/day — alternative; check TPMT or NUDT15 activity before initiation to avoid life-threatening myelosuppression.
— Rituximab and calcineurin inhibitors used in refractory cases at ILD centers.
— Nintedanib is FDA-approved for progressive pulmonary fibrosis (PPF), including fibrotic HP that progresses despite standard therapy (INBUILD trial).
— Pirfenidone has supportive data, less robust regulatory positioning for HP specifically.
— Progressive pulmonary fibrosis criteria: ≥2 of — worsening respiratory symptoms, FVC decline ≥5% in 1 year, DLCO decline ≥10%, or imaging progression — despite appropriate management.
— Supplemental oxygen for resting SpO2 ≤88% or exertional desaturation.
— PJP prophylaxis (TMP-SMX) when prednisone ≥20 mg/day for ≥1 month, especially with concurrent immunosuppressant.
— Bone health: calcium, vitamin D, DEXA, bisphosphonate per FRAX/glucocorticoid-induced osteoporosis guidelines.
— GI prophylaxis (PPI) when indicated; glycemic monitoring; vaccinations updated.
Board pearl: Nintedanib slows FVC decline in fibrotic HP with a PPF trajectory — it does not reverse fibrosis or replace antigen avoidance and immunosuppression.
Step 3 management: Check TPMT/NUDT15 before azathioprine, baseline LFTs before nintedanib (hepatotoxicity), and pregnancy status before either — common Step 3 pre-prescription checkpoints.
— Indication for advanced fibrotic HP refractory to medical therapy.
— Refer to a transplant center when FVC <65–70% predicted, DLCO <40%, significant exertional desaturation, pulmonary hypertension, or progressive decline despite therapy — early referral is emphasized because waiting list times can be long.
— Pre-transplant evaluation: age (relative, generally <70–75), comorbidities, functional status, psychosocial assessment, adherence, substance use, vaccinations.
— HP has reasonable post-transplant outcomes comparable to or better than IPF; recurrence in the allograft is reported if antigen exposure persists — exposure remediation continues post-transplant.
— Structured 8–12 week program; improves dyspnea, exercise tolerance, and quality of life in ILD.
— Indicated for any symptomatic HP patient with functional limitation — Step 3 favorite referral.
— Long-term continuous O2 for resting SpO2 ≤88% or PaO2 ≤55 mmHg (≤59 mmHg with cor pulmonale/erythrocytosis).
— Ambulatory/exertional O2 for activity-induced desaturation <88%.
— Pulmonary hypertension: vasodilator therapy (e.g., inhaled treprostinil for PH-ILD per INCREASE trial) only at specialized centers; avoid empirical pulmonary vasodilators in ILD-PH outside indications — risk of worsening V/Q mismatch.
— Transbronchial cryobiopsy requires balloon occlusion for bleeding control; performed at experienced centers.
— Surgical lung biopsy carries 2–5% mortality in ILD — weigh risk/benefit; avoid in patients with severe baseline impairment.
CCS pearl: For a hospitalized acute HP patient — admit, remove from exposure, supplemental O2, IV methylprednisolone 1 mg/kg if severe, pulmonology consult, PFTs and 6MWT before discharge, outpatient HRCT follow-up at 3 months, pulmonary rehab referral, and document exposure mitigation plan in the discharge summary.
— Higher baseline rate of fibrotic phenotype and pulmonary hypertension; presentation often mimics IPF or "old age dyspnea."
— Steroid toxicity is amplified: osteoporotic fracture, hyperglycemia/new diabetes, delirium, infection, skin fragility, cataracts, glaucoma — start lowest effective dose and taper faster when feasible.
— Polypharmacy review before adding immunosuppression; check drug interactions (mycophenolate–antacid, azathioprine–allopurinol/febuxostat — azathioprine + allopurinol can cause fatal pancytopenia).
— Fall risk assessment, vision/hearing screening, advanced care planning, code status discussion before initiating therapy.
— Mycophenolate: no dose adjustment in CKD but monitor for GI and hematologic toxicity; avoid in severe renal failure when alternatives exist.
— Azathioprine: reduce dose in CKD; toxicity risk increases.
— Nintedanib: not recommended in severe renal impairment (CrCl <30); limited data.
— TMP-SMX prophylaxis: monitor potassium and creatinine, dose-adjust for CrCl.
— Nintedanib is contraindicated in moderate-severe hepatic impairment (Child-Pugh B/C); baseline and monthly LFTs in the first 3 months, then periodically. Hold/reduce for AST/ALT >3× ULN.
— Pirfenidone also requires LFT monitoring; dose adjust or discontinue with significant elevations.
— Azathioprine and mycophenolate require dose review in hepatic dysfunction.
Key distinction: Azathioprine plus allopurinol/febuxostat is a high-yield, life-threatening drug interaction — always check the gout medication list before starting azathioprine, particularly in elderly comorbid patients.
Step 3 management: Start a steroid-sparing agent early in elderly HP patients to minimize cumulative glucocorticoid exposure — bone-protective therapy and glycemic monitoring should accompany any prednisone course ≥3 months.
— HP is uncommon during pregnancy; physiologic dyspnea and restrictive changes can confound assessment.
— Avoid teratogens: mycophenolate is category D/contraindicated (pregnancy prevention program required); nintedanib and pirfenidone are contraindicated in pregnancy.
— Acceptable options: corticosteroids (prednisone preferred over dexamethasone due to placental metabolism), azathioprine (used in transplant pregnancies; benefit can outweigh risk), supplemental O2, antigen avoidance.
— Pre-conception counseling: discontinue mycophenolate ≥6 weeks before conception; switch to azathioprine if continued immunosuppression needed; multidisciplinary care with MFM and pulmonology.
— Lactation: prednisone compatible; mycophenolate/nintedanib/pirfenidone contraindicated.
— Less common; often related to bird exposures, indoor mold, or wind instruments in adolescents.
— Presentation may mimic asthma — recurrent cough and exertional dyspnea unresponsive to bronchodilators; clubbing in chronic cases.
— Diagnosis follows similar principles with pediatric pulmonology MDD; growth and developmental monitoring are critical.
— Farm workers: thermophilic actinomycetes in moldy hay/silage — engineering controls (silo design, mechanical ventilation), N95 or PAPR during high-exposure tasks.
— Bird breeders/pet shop workers: avian protein remediation; commonly cannot continue hobby/occupation.
— Metalworking fluid/auto paint workers: isocyanate and bacterial contamination of fluids — OSHA exposure limits; surveillance spirometry.
— Healthcare/lab workers: rare animal handler HP, lab rodent allergens.
Board pearl: A pregnant patient newly diagnosed with HP — stop nintedanib, pirfenidone, and mycophenolate; bridge with prednisone ± azathioprine; coordinate MFM care; treat the antigen exposure as you would in any other patient.
Step 3 management: Always ask about workplace exposures and report suspected occupational HP to the patient's employer health program and, when applicable, to OSHA — this is both clinical and public health practice.
— Progression to pulmonary fibrosis — the most consequential outcome; converts a potentially reversible disease into a chronic, progressive illness.
— Acute exacerbation of HP (AE-HP): sudden worsening dyspnea with new diffuse ground-glass on HRCT, often without identifiable trigger; high mortality (similar to AE-IPF). Treat with high-dose steroids, exclude infection, supportive care.
— Pulmonary hypertension (group 3 PH): present in 20–40% of advanced fibrotic HP; portends worse prognosis and right heart failure.
— Respiratory failure: acute (severe acute HP, AE-HP) or chronic hypoxemic; may require non-invasive ventilation or transition to comfort care depending on goals.
— Lung cancer: increased risk in fibrotic lung disease, especially with smoking history — maintain screening per USPSTF and have low threshold for evaluating new nodules.
— Glucocorticoid toxicity: hyperglycemia/steroid-induced diabetes, osteoporosis and fragility fractures, weight gain, mood changes, infection (especially PJP and reactivation TB), adrenal suppression, cataracts, skin atrophy, myopathy.
— Mycophenolate: GI upset, leukopenia, infection, teratogenicity.
— Azathioprine: myelosuppression, hepatotoxicity, pancreatitis, increased non-melanoma skin cancer risk; lethal interaction with allopurinol.
— Nintedanib: diarrhea (most common — dose reduce or hold; loperamide), hepatotoxicity, bleeding risk (caution with anticoagulants), arterial thromboembolic events, GI perforation (rare).
— Pirfenidone: photosensitivity (sunscreen counseling), GI upset, hepatotoxicity.
— Loss of livelihood (occupational HP), forced relinquishment of beloved pets (bird-related HP), housing remediation costs — significant patient burden affecting adherence and depression risk.
Board pearl: Sudden worsening in a fibrotic HP patient → think AE-HP, infection, pulmonary embolism, or pneumothorax — order CT angiography and rule out reversible causes before attributing to acute exacerbation alone.
Step 3 management: Pre-emptively start PJP prophylaxis, calcium/vitamin D, and consider a bisphosphonate for any patient expected to receive ≥3 months of prednisone ≥7.5 mg/day.
— Acute respiratory failure: SpO2 <88% on supplemental oxygen, PaCO2 rising, accessory muscle use, altered mental status → consider HFNC or NIPPV; intubate if refractory.
— Severe acute HP episode with hypoxemia and inability to remove from exposure → hospitalize for monitored oxygen, IV methylprednisolone, exposure remediation planning.
— Acute exacerbation of fibrotic HP: admit, ICU evaluation, broad infectious workup (PJP, viral, bacterial, atypical, fungal), pulse-dose methylprednisolone, supportive ventilation.
— Hemodynamic instability from right heart strain/cor pulmonale: cardiology and pulmonary vascular consult.
— Massive hemoptysis or pneumothorax (rare): chest imaging, tube thoracostomy, IR/thoracic surgery.
— Pulmonology/ILD specialist: all suspected HP cases; required for MDD.
— Occupational medicine/environmental health: for occupational and complex home exposure cases.
— Allergy/immunology: rarely useful for HP itself (precipitin panels can be obtained without consult); consider if mimics like ABPA suspected.
— Rheumatology: if CTD-ILD on differential.
— Cardiothoracic transplant team: early referral for advanced fibrotic HP.
— Palliative care: symptomatic dyspnea, advanced disease, goals-of-care discussions — appropriate alongside disease-directed therapy, not only at end of life.
— Stable on outpatient oxygen requirement, tolerating oral steroids, ambulating at baseline, follow-up pulmonology appointment within 2 weeks, documented antigen mitigation plan, vaccinations updated, PJP prophylaxis initiated when indicated, education on red-flag symptoms.
CCS pearl: On the CCS interface for an acute HP admission: order O2 to keep SpO2 ≥90%, IV methylprednisolone, pulmonology consult, HRCT, BAL when stable, social work for environmental remediation, pulmonary rehab referral, and schedule pulmonology follow-up at 2 weeks and PFTs at 4–6 weeks.
Step 3 management: Early palliative care co-management in advanced fibrotic HP improves symptom control and aligns care with patient goals without precluding transplant referral.
— Older male smokers; basal subpleural honeycombing on HRCT (UIP pattern); no identifiable exposure; no air trapping typically.
— Steroids harmful; treat with antifibrotics (nintedanib, pirfenidone).
— Fibrotic HP can mimic IPF — careful exposure history is the differentiator.
— Young to middle-aged adults; bilateral hilar lymphadenopathy + upper lobe perilymphatic nodules; non-caseating granulomas with well-formed clusters (vs poorly formed in HP); often asymptomatic or with extrapulmonary features (skin, eye, cardiac, hypercalcemia).
— BAL: lymphocytosis with elevated CD4/CD8 ratio (HP variable).
— Often CTD-associated (scleroderma, polymyositis/antisynthetase); HRCT shows basal ground-glass and reticulation with subpleural sparing; biopsy distinguishes.
— Migratory patchy consolidation, often peripheral or peribronchial; responds dramatically to steroids; can be drug-induced or post-infectious.
— Nitrofurantoin, amiodarone, methotrexate, bleomycin, immune checkpoint inhibitors — temporal relationship to drug, often improves with cessation ± steroids.
— Rheumatoid arthritis, scleroderma, dermatomyositis, Sjögren — extrapulmonary features and positive serologies; manage with rheumatology.
— Peripheral and BAL eosinophilia >25%; dramatic steroid response; "photographic negative of pulmonary edema" in chronic eosinophilic pneumonia.
Key distinction: Sarcoid granulomas are well-formed and non-caseating with perilymphatic distribution; HP granulomas are poorly formed, bronchiolocentric, with surrounding lymphocytic alveolitis — testable histology pair.
Board pearl: Mosaic attenuation with expiratory air trapping is one of the most specific HRCT features differentiating HP from IPF, NSIP, and sarcoidosis.
— Atypical or viral pneumonia (Mycoplasma, Chlamydia, influenza, COVID-19, CMV): acute febrile presentation overlaps with acute HP; sputum/NAAT testing, CXR/HRCT pattern, and lack of recurrent temporal exposure pattern help differentiate.
— Pulmonary tuberculosis and non-tuberculous mycobacterial (NTM) infection: especially relevant in hot tub lung, where MAC is both the antigen (immune response) and a potential pathogen — sometimes treated as overlap. Obtain AFB smears and cultures from BAL.
— PJP in immunocompromised patients on steroids — important to exclude with new diffuse infiltrates in a treated HP patient.
— Fungal pneumonias (histoplasmosis, blastomycosis, coccidioidomycosis): geography, travel, exposure history; serology and antigen testing.
— Heart failure with preserved or reduced EF — bilateral infiltrates, dyspnea; differentiate via BNP, echo, and response to diuresis.
— Mitral stenosis with pulmonary edema — uncommon but considered with restrictive physiology and dyspnea on exertion.
— Pneumoconioses (silicosis, coal worker's, asbestosis): nodular or reticular changes with characteristic exposure histories; no antigen-specific immune mechanism.
— Berylliosis (chronic beryllium disease): granulomatous lung disease mimicking sarcoid and HP; aerospace/electronics workers; diagnose with beryllium lymphocyte proliferation test (BeLPT).
— Acute inhalational injury (chlorine, ammonia, smoke): acute and event-related.
— Lymphangitic carcinomatosis — septal thickening, often unilateral or asymmetric; known primary; rapid progression.
— Lymphoma involving lung — rare mimicker of nodular HP.
Board pearl: Hot tub lung blurs the line between infection and hypersensitivity — treatment is removal from the hot tub plus steroids; antimycobacterial therapy is usually unnecessary unless true MAC infection (cavitary disease, persistent positive cultures) is established.
Step 3 management: Always include an infectious workup (BAL cultures, AFB, fungal, PJP when applicable) before committing to chronic immunosuppression for presumed HP — missed infection mimicking HP is a board favorite.
— Bird-related HP: complete removal of birds from the home, professional deep cleaning of HVAC and carpets/upholstery, removal of down bedding/comforters/pillows, washing of bird-exposed clothing separately. Persistence of avian antigens in dust for months means partial measures are inadequate.
— Mold/moisture HP: identify and remediate water intrusion (roof, plumbing, basement humidity); HEPA filtration; replace moldy drywall and insulation; address indoor humidity (<50%); clean or replace humidifiers; service or replace HVAC.
— Hot tub lung: discontinue use; if continuing, disinfect, change filters, ensure proper chlorination, and avoid indoor enclosed hot tubs.
— Farmer's lung: dry hay before storage; mechanized handling; avoid moldy silage; PAPR/N95 respirator during high-exposure activities; engineering controls in barns.
— Occupational HP: OSHA-compliant ventilation, substitution of safer agents (water-based vs solvent-based paints), enclosed processes, fit-tested respirators, periodic surveillance spirometry. Job change may be necessary when engineering and PPE controls are insufficient — engage occupational medicine.
— Annual influenza, pneumococcal (PCV20 or PCV15+PPSV23 per current ACIP), COVID-19 boosters, RSV (age 60+ per ACIP), Tdap, zoster (Shingrix, age 50+ or immunocompromised), hepatitis B if not immune.
— Avoid live vaccines while on significant immunosuppression.
— Smoking cessation (always); GERD treatment (may contribute to lung injury); pulmonary rehab; nutrition; physical activity within tolerance.
— Steroid taper to lowest effective dose; transition to steroid-sparing agent for fibrotic disease; consider antifibrotic if PPF criteria met.
Board pearl: Patient counseling — "If you keep the bird, the prednisone won't keep working." Antigen avoidance is non-negotiable for disease modification.
Step 3 management: Document the specific antigen mitigation plan in the chart and provide written instructions — exposure remediation is both a medical and a behavioral intervention that requires reinforcement at every visit.
— 2–4 weeks after diagnosis or treatment initiation: tolerability, side effects, symptom response, repeat exposure history, reinforce mitigation.
— 4–6 weeks: repeat PFTs (FVC, DLCO) to assess therapy response; HRCT only if clinically indicated or unclear response.
— 3, 6, 12 months: PFTs and 6MWT; HRCT typically at 6–12 months or sooner if decline.
— Stable fibrotic HP: PFTs every 3–6 months long-term; HRCT annually or with progression.
— On nintedanib/pirfenidone: LFTs monthly × 3, then every 3 months; assess GI tolerability and dosing.
— On mycophenolate/azathioprine: CBC and CMP every 2 weeks initially, then monthly to quarterly.
— Symptoms: mMRC dyspnea scale, cough frequency, exercise tolerance.
— Objective: FVC, DLCO trends; 6MWT distance and lowest SpO2; resting oximetry.
— Imaging: HRCT for progression suspicion or treatment change.
— Echocardiogram annually or with new signs of pulmonary hypertension.
— Refer all symptomatic patients; improves dyspnea, exercise capacity, and HRQoL.
— Components: supervised exercise, breathing techniques, education, nutritional and psychosocial support.
— Action plan for symptom flares: when to call, when to come in, antibiotic stewardship.
— Mental health screening for depression/anxiety, especially with lifestyle disruption (pet relinquishment, job change).
— Advance care planning conversations early in fibrotic disease, revisited annually.
— Per USPSTF in eligible smokers; low threshold for evaluating new nodules in fibrotic lungs.
Board pearl: A 10% decline in FVC or 15% decline in DLCO over 12 months in fibrotic HP defines progression and triggers escalation — add or switch immunosuppressant, consider antifibrotic, and re-refer for transplant evaluation.
Step 3 management: Build a longitudinal monitoring plan into the EHR with PFTs at 3-6 month intervals — Step 3 questions often hinge on recognizing decline before patients are symptomatic.
— Discuss the risk–benefit of long-term immunosuppression (infection, malignancy, fertility) before initiation; document the conversation.
— Antifibrotics: explain that they slow but do not reverse decline; financial toxicity is real (high cost, prior authorization) and warrants social work involvement.
— Lung biopsy consent: explicitly discuss mortality (2–5% for surgical lung biopsy) and the alternative of cryobiopsy or empirical management based on MDD.
— Discuss the diagnosis with the patient before recommending workplace changes; respect the patient's right to weigh livelihood against health.
— Workers' compensation and disability documentation — physician's responsibility to provide accurate diagnostic and exposure information; do not understate or overstate severity.
— Report sentinel cases of occupational HP to appropriate occupational surveillance systems; some states have mandatory reporting for occupational lung disease.
— Identification of contaminated buildings (HVAC, humidifiers) may obligate notification of building management or public health, especially in multi-tenant or healthcare settings.
— Hot tub lung in commercial facilities (gyms, hotels) may require public health reporting.
— Sensitively addressed; offer social work, rehoming resources; recognize grief response; do not coerce — provide accurate medical recommendation and document the patient's decision.
— Hospital-to-outpatient handoff: medication reconciliation (steroid taper specifics, PJP prophylaxis, bone protection, immunosuppressant labs), follow-up appointment within 2 weeks, written discharge instructions including red flags and exposure mitigation, and a clear primary contact for questions.
— Avoid prednisone "abrupt stop" prescriptions; explicit taper schedule.
— Environmental HP disproportionately affects lower-income tenants in poorly maintained housing; advocate for remediation and connect with legal aid when landlords are unresponsive.
Board pearl: A clinician's signature on a workers' compensation form documenting occupational HP can change a patient's career and benefits — accuracy, supporting documentation (HRCT, PFTs, exposure history), and pulmonology consultation matter.
Step 3 management: At discharge, explicitly write the steroid taper schedule, prophylaxis plan, lab monitoring intervals, and exposure mitigation steps — incomplete transitions of care are a top cause of HP relapse and a frequent Step 3 patient-safety vignette.
— Thermophilic actinomycetes / moldy hay → Farmer's lung
— Avian proteins / bird droppings, feathers → Bird fancier's, pigeon breeder's lung
— Mycobacterium avium complex / hot tub aerosol → Hot tub lung
— Aspergillus clavatus / malting barley → Malt worker's lung
— Penicillium / moldy cheese → Cheese washer's lung
— Wood dust / oak, maple, cedar → Woodworker's lung
— Isocyanates / spray paint, polyurethane → Chemical worker's lung
— Bacteria/amoebae / humidifier, HVAC → Humidifier lung
— Reed/woodwind biofilm → Saxophone/clarinet lung
— Down feathers → "Feather duvet lung"
— Ground-glass opacities + centrilobular nodules + mosaic attenuation with expiratory air trapping ("headcheese" sign).
Board pearl: "Returns home from vacation, symptoms reappear within hours, pet bird at home, mosaic attenuation on HRCT" — pattern recognition gets the answer in 10 seconds on Step 3.
A non-smoking middle-aged adult has had three episodes of fever, cough, and dyspnea over six months, each resolving with a course of antibiotics. Each episode follows weekends at the family farm. CXR is normal between episodes. Next best step? → Detailed exposure history + HRCT (with expiratory phase); consider BAL.
A 55-year-old woman with progressive dyspnea over 18 months, dry cough, and a new pet parakeet acquired two years ago. HRCT shows mid-lung mosaic attenuation, centrilobular nodules, and mild reticulation. BAL with 45% lymphocytes. Best initial management? → Remove the bird from home + deep cleaning + corticosteroids; consider steroid-sparing agent.
A 40-year-old man who uses an indoor hot tub daily presents with cough and dyspnea over 3 months. HRCT: diffuse centrilobular nodules and ground-glass. BAL grows Mycobacterium avium complex. Treatment? → Discontinue hot tub use; corticosteroids; antimycobacterials usually unnecessary unless cavitary/invasive disease.
A 62-year-old non-smoker with progressive dyspnea, basal Velcro crackles, HRCT with mid-upper lobe reticulation, traction bronchiectasis, and mosaic attenuation with expiratory air trapping. Bird owner. Diagnosis? → Fibrotic HP, not IPF — air trapping and exposure history are the keys.
Fibrotic HP patient stable on azathioprine develops gout; primary care adds allopurinol. Most concerning outcome? → Profound pancytopenia from azathioprine toxicity (xanthine oxidase inhibition).
Patient on mycophenolate for fibrotic HP is planning conception. Best step? → Discontinue mycophenolate ≥6 weeks before conception; switch to azathioprine if continued immunosuppression needed.
Fibrotic HP patient on prednisone + mycophenolate has FVC declining 8% over 12 months despite antigen removal. Next step? → Add nintedanib (progressive pulmonary fibrosis indication) and consider transplant referral.
About to start azathioprine. What test must be checked first? → TPMT (and/or NUDT15) activity.
Board pearl: Watch for the "vacation history" clue — Step 3 stems often plant the diagnostic key in the social history paragraph.
Hypersensitivity pneumonitis is an antigen-driven interstitial lung disease whose diagnosis depends on linking exposure history with HRCT (mosaic attenuation, centrilobular nodules, expiratory air trapping) and BAL lymphocytosis, and whose long-term outcome depends far more on complete antigen avoidance than on any pharmacologic therapy.