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Hypersensitivity Pneumonitis: A disease of the "Leaves" of the tree.
Hypersensitivity pneumonitis (HP) is a complex syndrome where inhalation of either organic material or inorganic dusts leads to inflammation of the small airways and lung parenchyma or alveoli. One way to think of these is inflammation of the “terminal branches” and “leaves” of a tree. Hypersensitivity pneumonitis used to be traditionally known as extrinsic allergic alveolitis.
2 of the commonest types of HP are Farmer’s lung and Pigeon breeder’s lung. There have been case reports as early as 1932 when Campbell reported a case series of 5 farmers who develop acute respiratory symptoms after exposure to mouldy hay.
Risk factors of hypersensitivity pneumonitis
How do you diagnose HP?
Hypersensitivity pneumonitis is a challenging diagnosis and requires careful deliberation of the overall circumstances surrounding a patient’s presentation.
Current evidence supports a diagnosis of hypersensitivity pneumonitis based on a combination of the following criteria. Most evidence supports the use of exposure to a known offending antigen along with the following:
- Compatible clinical findings include crackles, clubbing and weight loss.
- High resolution CT (HRCT) Thorax showing specific features like upper lung predominance of patchy or geographic abnormalities. Other features include centrilobular nodules, mosaic attenuation and air trapping.
- Physiologic abnormalities including altered spirometry, reduced diffusing capacity for carbon monoxide (DLCO), or exercise-induced desaturation.
- Broncho alveolar lavage showing a lymphocytosis (20-30% lymphocytes of total leukocyte count), and usually with a low CD4 to CD8 ratio
- Histopathology showing compatible changes like poorly formed noncaseating granulomas or a mononuclear cell infiltrate.
How do you determine patient’s exposure to a known offending antigen?
Traditionally physicians relied on presence of specific antibodies in patient’s blood serum against identified antigen (serum precipitins). We now know that serum precipitins are neither specific or sensitive for HP. Studies have shown that 30% of healthy farmers have positive serum precipitins to antigens implicated in farmer’s lung without clinical evidence for disease. Similarly, 40% of healthy pigeon breeders have demonstrated positive avian precipitins without evidence of hypersensitivity pneumonitis. Notably negative serum precipitins does not exclude HP.
A detailed environmental exposure history is essential. Acute hypersensitivity pneumonitis is the easiest to recognize by symptoms developing 4 to 8 hours after exposure to a known antigen. Patients will have crackles on examination. Frequently this is confused with a bacterial or viral infection due to similar presenting symptoms and signs. A history of recurring symptoms when exposed to a similar known antigen will help diagnose HP.
Subacute HP and chronic HP can be more challenging due to the insidious presentation plus lack of clear association with the precipitating allergens. Symptoms and signs usually resolve slower after removal from the precipitating allergen and may frequently not completely return to normal. Weight loss is a key feature in subacute and chronic HP.
Diagnostic challenges in hypersensitivity pneumonitis
The lack of definite criteria to diagnose HP has led to frequent under recognition of this condition. A detailed environmental or occupational history along with temporal relationship to symptoms is essential for the diagnosis. Commonly physicians lack to consider an association or over rely on thoracic imaging or serum precipitins.
Chronic hypersensitivity pneumonitis shares similar traits with other fibrotic interstitial lung disease. It is important to not rely solely on presence of serum precipitins but use a combination of criteria listed above to avoid misdiagnosis of HP in cases of idiopathic pulmonary fibrosis (IPF) and other interstitial lung diseases.
Treatment for hypersensitivity pneumonitis
If there is clear evidence of acute hypersensitivity pneumonitis, patients usually respond to complete avoidance or exposure to the precipitating antigen.
Severely symptomatic patients should receive 0.5mg to 1mg of oral prednisolone per kilogramme body weight per day for 2 weeks to a month. This is then followed by a slow taper over 1 to 2 months.
Patients should have serial monitoring of symptoms along with serial PFTs and serial imaging to guide the treatment regimen.
Hypoxemia at rest or on exertion can benefit from oxygen despite little evidence in it reducing overall mortality.
Note bird fancier’s lung tends to have a worse prognosis than farmer’s lung. One reason implicated is there is evidence that bird antigens may persist in the home long after removal of the birds. This has been documented even after professional cleaning is done.
All patients with HP should have close monitoring long term as studies have shown nearly 3 times increased mortality in this patient cohort compared to the general population.
Future therapies for hypersensitivity pneumonitis
Due to the many similarities between chronic HP and fibrotic idiopathic pulmonary fibrosis (IPF), current trials are ongoing to see if antifibrotic medications licensed for use in IPF like pirfenidone will have a role in chronic HP.
Prevention of hypersensitivity pneumonitis in high-risk environments
Public health education is essential in informing workers in high-risk environments of exposure risks. Employers should offer appropriate respiratory protection to their workers. Despite this, in cases of confirmed hypersensitivity pneumonitis, most dust respirators do not offer complete protection.