Residency · Residency · Respirology

Idiopathic Pulmonary Fibrosis

Definition and Epidemiology

Definition

Idiopathic pulmonary fibrosis is a chronic, progressive fibrosing interstitial pneumonia of unknown cause that is limited to the lungs and associated with the usual interstitial pneumonia (UIP) pattern on high-resolution CT and/or histopathology. The diagnosis requires rigorous exclusion of known causes of ILD, including connective tissue disease, drug exposure, environmental exposure, and hypersensitivity pneumonitis. Without antifibrotic therapy, the median survival from diagnosis is 3-5 years, though the clinical course is highly variable, with some patients maintaining relative stability for years while others experience rapid and relentless decline.

Epidemiology

The incidence of IPF ranges from 3 to 9 per 100,000 per year, with a prevalence of 10-60 per 100,000 depending on the population studied and the diagnostic criteria applied. The disease demonstrates a clear male predominance at a ratio of approximately 2:1, and the mean age at diagnosis is 65-70 years, with IPF being rare before the age of 50. Cigarette smoking is a well-established risk factor with an odds ratio of 1.6-2.9, and environmental exposures to metal dust, wood dust, and other inhaled particulates contribute to disease risk. Gastroesophageal reflux disease and chronic viral infections, including Epstein-Barr virus and cytomegalovirus, have been implicated as potential risk factors, though the role of viral infections remains debated. Familial IPF accounts for 5-20% of cases and is associated with mutations in telomere-related genes (TERT, TERC, RTEL1, PARN) and surfactant-related genes (SFTPC, SFTPA2). The MUC5B promoter variant (rs35705950) is the strongest genetic risk factor identified to date, present in approximately 35% of IPF patients compared to 9% of controls.

Pathogenesis

Current Paradigm

The understanding of IPF pathogenesis has undergone a fundamental shift from an inflammation-driven model to an epithelial injury-driven paradigm. The current model posits that repetitive micro-injuries to the alveolar epithelium, arising from unknown triggers in genetically susceptible individuals, initiate an aberrant wound healing response. Rather than normal re-epithelialization and resolution, the injured epithelium fails to regenerate properly, leading to persistent activation of fibroblasts and their differentiated counterparts, myofibroblasts. These effector cells deposit excessive extracellular matrix composed of collagen and fibronectin, progressively destroying normal alveolar architecture. Several key molecular pathways drive this process, with TGF-beta serving as the central profibrotic mediator, supported by the WNT, Hedgehog, PDGF, VEGF, and FGF signaling cascades. Telomere shortening contributes to premature aging of alveolar epithelial cells, leading to cellular senescence and impaired regenerative capacity.

Cellular Players

Alveolar epithelial type II (ATII) cells, which normally serve as progenitor cells for epithelial regeneration after injury, undergo senescence, apoptosis, and aberrant differentiation in IPF, failing to restore the damaged epithelial barrier. Fibroblasts and myofibroblasts are the principal effector cells of fibrosis, forming fibroblastic foci at the interface between normal and fibrotic tissue, which represent the histologic hallmark of UIP. Alternatively activated (M2) macrophages promote fibrosis through the release of profibrotic mediators and represent an emerging therapeutic target. Senescent cells accumulate in the IPF lung and release a senescence-associated secretory phenotype (SASP) comprising cytokines, growth factors, and proteases that perpetuate the fibrotic process.

Diagnosis

HRCT Patterns (ATS/ERS/JRS/ALAT 2022 Update)

The 2022 ATS/ERS/JRS/ALAT guideline update refines the HRCT classification into four categories. A definite UIP pattern is defined by subpleural, basal-predominant honeycombing with or without traction bronchiectasis, demonstrating heterogeneity and the absence of features inconsistent with UIP. A probable UIP pattern shows subpleural, basal-predominant reticular abnormality with traction bronchiectasis but without honeycombing and without features inconsistent with UIP. An indeterminate for UIP pattern demonstrates fibrotic features that do not meet criteria for either UIP or an alternative diagnosis, with atypical distribution or ancillary features. An alternative diagnosis pattern shows features that suggest another specific ILD.

Features Inconsistent with UIP

Several imaging features should raise concern for an alternative diagnosis when evaluating for possible UIP. Upper or mid-lung predominance suggests hypersensitivity pneumonitis or sarcoidosis. Peribronchovascular predominance is more typical of NSIP or HP. Extensive ground glass opacity favors NSIP or HP. Profuse micronodules suggest HP or sarcoidosis. Discrete cysts away from honeycombing raise consideration of HP, LIP, or PLCH. Mosaic attenuation or air trapping is characteristic of HP. Consolidation suggests COP or aspiration.

Diagnostic Pathway (2022 Guidelines)

The 2022 diagnostic pathway represents an important evolution in IPF management. A definite UIP pattern on HRCT in the appropriate clinical context, specifically a patient older than 60, male, with a smoking history and no identifiable cause, permits the diagnosis of IPF without surgical lung biopsy. A significant change in the 2022 update is the conditional recommendation that a probable UIP pattern on HRCT may also be sufficient to diagnose IPF without biopsy when MDD agreement supports the diagnosis. An indeterminate for UIP pattern should prompt consideration of BAL, where lymphocytosis would suggest an alternative diagnosis, followed by surgical lung biopsy or transbronchial lung cryobiopsy if the diagnosis remains uncertain. An alternative diagnosis pattern directs the workup toward the specific diagnosis suggested by the imaging features.

Histopathology (When Biopsy Performed)

When biopsy is performed, the UIP pattern is characterized by spatial and temporal heterogeneity, with dense fibrosis and honeycombing alternating with areas of normal lung. Fibroblastic foci at the interface between normal and fibrotic tissue are the hallmark finding. The absence of features suggesting an alternative diagnosis, such as granulomas, prominent interstitial inflammation, organizing pneumonia, or hyaline membranes, is required to confirm UIP.

<image>A detailed HRCT comparison showing four imaging patterns side by side in axial and coronal views: (1) Definite UIP - prominent basal honeycombing with traction bronchiectasis, subpleural predominance, and spatial heterogeneity; (2) Probable UIP - basal reticular pattern with traction bronchiectasis but no honeycombing; (3) Indeterminate for UIP - fibrotic features not meeting UIP criteria; (4) Features suggesting alternative diagnosis - ground glass with subpleural sparing (NSIP) or mosaic attenuation (HP). Include arrows pointing to key distinguishing features. Add a diagnostic confidence scale below each pattern showing likelihood of IPF diagnosis.</image>

Management

Antifibrotic Therapy

Two antifibrotic agents are approved for IPF, and both have been shown to reduce the rate of FVC decline by approximately 50%, though neither reverses established fibrosis nor improves lung function. Pirfenidone possesses anti-fibrotic, anti-inflammatory, and antioxidant properties, inhibiting TGF-beta signaling and collagen synthesis. It is titrated over 14 days to a target dose of 801 mg three times daily (2403 mg total daily dose) taken with food. The ASCEND trial demonstrated a 48% reduction in the composite endpoint of FVC decline of 10% or more or death at 52 weeks, and a pooled analysis with the CAPACITY trials showed a mortality benefit. The principal side effects include nausea in 36% of patients, rash and photosensitivity in 30%, diarrhea, anorexia, and elevated liver function tests requiring monitoring every 3 months for the first 6 months and every 3-6 months thereafter. Photosensitivity management requires consistent use of sunscreen with SPF 50 or higher, protective clothing, and avoidance of midday sun exposure. Nintedanib is a triple tyrosine kinase inhibitor targeting VEGFR, PDGFR, and FGFR, administered at 150 mg orally twice daily with food. The INPULSIS 1 and 2 trials demonstrated a reduction in annual FVC decline by approximately 50%, corresponding to a difference of approximately 125 mL per year, with a consistent treatment effect regardless of baseline FVC. The TOMORROW trial established the dose-response relationship. The most common side effect is diarrhea, occurring in 62% of patients, along with nausea, decreased appetite, elevated liver function tests, and increased bleeding risk. Diarrhea management involves loperamide initiated at symptom onset, adequate hydration, and dose reduction to 100 mg twice daily if intolerance persists. No head-to-head randomized controlled trial has compared pirfenidone and nintedanib directly, and the choice between them is guided by side effect profile and patient preference. Combination therapy with both agents has limited evidence and is not currently recommended.

When to Start Antifibrotic Therapy

The ATS 2022 guidelines provide a conditional recommendation to initiate antifibrotic therapy in patients with IPF regardless of FVC, including those with preserved FVC above 80% predicted. Earlier treatment is associated with better preservation of lung function over time, and there is no evidence that treatment can be started too early. Delays in initiating therapy may result in irreversible decline that could have been mitigated.

Non-Pharmacologic Management

Supplemental oxygen should be prescribed for resting or exertional hypoxemia, although no randomized controlled trial specific to IPF supports this practice. Pulmonary rehabilitation improves exercise capacity and quality of life, though benefits may be less durable than those observed in COPD. Vaccination against influenza, pneumococcus, and COVID-19 is recommended. Gastroesophageal reflux disease management is complicated by evolving evidence: proton pump inhibitors were previously conditionally recommended, but the 2022 guideline update removed this recommendation due to uncertainty. Palliative care should be integrated early in the disease course, with attention to symptom management including low-dose morphine at 2.5-5 mg every 4 hours as needed for refractory dyspnea, and advance care planning should be initiated before crisis situations arise.

Lung Transplant

Referral for lung transplant evaluation should be considered when DLCO falls below 40% predicted, FVC declines by 10% or more over 6 months, six-minute walk test desaturation falls below 88%, or hospitalization for respiratory worsening occurs. IPF is the most common indication for lung transplant worldwide. Bilateral transplantation is increasingly preferred over single lung transplant due to better survival and avoidance of complications in the native lung. Median post-transplant survival is approximately 5-6 years.

Acute Exacerbation of IPF (AE-IPF)

Definition

An acute exacerbation of IPF is defined as an acute, clinically significant respiratory deterioration occurring over a period typically shorter than one month, characterized by new bilateral ground glass opacity or consolidation superimposed on the underlying UIP pattern. Identifiable causes including infection, pulmonary embolism, heart failure, and other recognizable etiologies must be excluded, although the distinction between triggered and idiopathic acute exacerbations is recognized. The in-hospital mortality of AE-IPF is devastating, ranging from 50-80%, and survivors experience accelerated decline in lung function.

Management

Management of AE-IPF is largely supportive and includes supplemental oxygen and non-invasive ventilation when appropriate, though many clinicians advocate for an early goals-of-care discussion before proceeding to intubation given the extremely poor prognosis. High-dose corticosteroids, typically methylprednisolone 500-1000 mg intravenously daily for 3 days followed by prednisone at 1 mg/kg, are commonly administered, though evidence supporting their efficacy is limited and based primarily on expert opinion. Antifibrotic therapy should be continued or initiated during the acute episode. Empiric broad-spectrum antibiotics are administered until infection is excluded. For appropriate transplant candidates, urgent listing may be considered.

<image>A prognostic assessment infographic for IPF showing the GAP (Gender, Age, Physiology) staging system. Display a scoring table: Gender (Female = 0, Male = 1), Age (<60 = 0, 60-65 = 1, >65 = 2), FVC % predicted (>75 = 0, 50-75 = 1, <50 = 2), DLCO % predicted (>55 = 0, 36-55 = 1, <=35 = 2, unable to perform = 3). Show resulting stages: Stage I (0-3 points, 1-year mortality 5.6%), Stage II (4-5 points, 1-year mortality 16.2%), Stage III (6-8 points, 1-year mortality 39.2%). Below, include a longitudinal FVC trajectory graph showing the typical pattern: gradual decline with potential acute exacerbation drops. Overlay antifibrotic treatment effect showing slowed decline rate.</image>

Emerging Therapies and Pipeline

Clinical Trials

The IPF therapeutic pipeline includes several promising agents targeting novel mechanisms. Inhaled treprostinil demonstrated improved six-minute walk distance in IPF-associated pulmonary hypertension in the INCREASE trial and is approved for PH-ILD, though not for IPF specifically. BI 1015550 (nerandomilast), a selective PDE4B inhibitor, is currently in the phase 3 FIBRONEER-IPF trial as an oral agent with a targeted mechanism. Anti-lysophosphatidic acid (LPA) receptor antagonists are being evaluated in multiple clinical trials. Senolytics, including the combination of dasatinib and quercetin, target senescent cells and are in early phase trials. Pentraxin-2 (PRM-151/pamrevlumab) has shown mixed results in clinical trials. Anti-CTGF (pamrevlumab) failed to meet its primary endpoint in the ZEPHYRUS-1 phase 3 trial in 2023.

Key Clinical Pearls

• A definite or probable UIP pattern on HRCT in the appropriate clinical context can establish an IPF diagnosis without surgical lung biopsy (2022 guideline update)
• Both pirfenidone and nintedanib reduce FVC decline by ~50% but do NOT reverse fibrosis or improve FVC; the goal is slowing progression
• FVC decline >= 10% absolute or DLCO decline >= 15% over 6-12 months predicts mortality and should prompt transplant referral evaluation
• Acute exacerbation of IPF carries 50-80% in-hospital mortality; early goals-of-care discussion is essential for all IPF patients
• The MUC5B promoter variant (rs35705950) is the strongest genetic risk factor for IPF but, paradoxically, is associated with slower disease progression in those who develop IPF

References

1. Raghu G, Remy-Jardin M, Richeldi L, et al. Idiopathic Pulmonary Fibrosis (an Update) and Progressive Pulmonary Fibrosis in Adults: An Official ATS/ERS/JRS/ALAT Clinical Practice Guideline. Am J Respir Crit Care Med. 2022;205(9):e18-e47.
2. Richeldi L, du Bois RM, Raghu G, et al. Efficacy and Safety of Nintedanib in Idiopathic Pulmonary Fibrosis. N Engl J Med. 2014;370(22):2071-2082. (INPULSIS)
3. King TE Jr, Bradford WZ, Castro-Bernardini S, et al. A Phase 3 Trial of Pirfenidone in Patients with Idiopathic Pulmonary Fibrosis. N Engl J Med. 2014;370(22):2083-2092. (ASCEND)
4. Ley B, Ryerson CJ, Vittinghoff E, et al. A multidimensional index and staging system for idiopathic pulmonary fibrosis. Ann Intern Med. 2012;156(10):684-691. (GAP index)
5. Collard HR, Ryerson CJ, Corte TJ, et al. Acute Exacerbation of Idiopathic Pulmonary Fibrosis. An International Working Group Report. Am J Respir Crit Care Med. 2016;194(3):265-275.

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