A man in his early 60s presented to his GP with a six-month history of dry cough and increasing exertional dyspnoea. There were no constitutional symptoms and he had never smoked. On auscultation of his chest, there were fine inspiratory crackles in the mid-zones. There was no finger clubbing.
A chest X-ray was arranged, which showed volume loss and bilateral hazy shadowing in the mid and upper zones – the report suggested chronic hypersensitivity pneumonitis. On further questioning, the man reported that his wife had kept budgies for the last 20 years and that he cleaned their cage every week. The patient was referred to his chest physician, where a high-resolution CT scan (HRCT) confirmed chronic hypersensitivity pneumonitis, attributed to the budgies (‘bird fancier’s lung’). Serum-specific IgG antibodies to avian proteins were not detected. The birds were rehoused and the patient’s cough resolved. His dyspnoea improved, although he was left with residual breathlessness on walking uphill.
In the UK, the most common forms of extrinsic allergic alveolitis (or ‘hypersensitivity pneumonitis’) are bird fancier’s lung, from exposure to antigens on budgies, parrots and pigeons, and ‘farmer’s lung’, due to inhaled fungal antigens on mouldy hay. Domestic causes, for example, household moulds or feather-duvet-related hypersensitivity pneumonitis, are probably under-recognised.1 ‘Metal worker’s lung’, which occurs from microbial contamination of metal working fluid, is now the most common occupational cause. Outbreaks, particularly in occupational settings, have been reported.2
A study using a UK primary care register identified 271 new cases of hypersensitivity pneumonitis between 1991 and 2003, an incidence of one per 100,000 total population. In the 65 to 74-year-old age group the rate was
2.4 per 100,000, with little difference between males and females.3 But the estimated UK prevalence in those who keep birds in their homes is up to 7%.4
Hypersensitivity pneumonitis has a variable presentation and natural history, and can be easy to miss, particularly as symptoms may mimic other more common respiratory diseases.
Hypersensitivity pneumonitis is caused by an abnormal immune response to an inhaled agent, resulting in diffuse inflammation, predominantly in the lung parenchyma. There is no finite list of causes and more than 300 different aetiological agents have been reported. They include fungi and bacteria, animal (bird) proteins and a few chemical reagents.5,6 As such, it is important to take a meticulous occupational and environmental history from patients with suggestive symptoms.
Clinical presentation is traditionally categorised according to the pattern and duration of illness.
Acute hypersensitivity pneumonitis
Typically develops between four and six hours after heavy exposure to a causative agent. Symptoms mimic flu with fever, malaise, weight loss, cough and headache, in addition to chest tightness and dyspnoea. Improvement usually occurs spontaneously from 12 hours to several days after exposure avoidance.
Sub-acute hypersensitivity pneumonitis
Presents with a more gradual history of malaise, weight loss and anorexia, in addition to dyspnoea and a productive cough. This is seen in patients with repeated acute attacks.
Chronic hypersensitivity pneumonitis
This usually presents with insidious onset of cough, dyspnoea, fatigue and weight loss. This occurs due to ongoing low-level exposure or recurrent unrecognised acute and sub-acute episodes. Bird fancier’s lung typically presents in this way.5,6
Clinical examination findings are of tachypnoea and fine bi-basal inspiratory (‘velcro’) crackles on auscultation. In acute and sub-acute hypersensitivity pneumonitis, fever may be present. In chronic hypersensitivity pneumonitis, there may be weight loss, muscle wasting and 50% of patients will have finger clubbing.7 Until relatively recently, a key finding was the identification of antigen-specific precipitating IgG antibodies. These tests have been largely replaced and instead, the presence of antigen-specific IgG antibodies (not just the precipitating fraction) is now sought. While these tests can be highly useful, there is a risk of false positive results in asymptomatic exposed individuals. Similarly, the tests may be negative if the causative agent has not been correctly identified, if the patient is no longer exposed and in chronic disease (as in the case history above).1
Diagnosing hypersensitivity pneumonitis
• Chest radiograph – appearance varies according to stage of disease
• High-res CT scan – changes often diagnostic of hypersensitivity pneumonitis with a supporting history
Pulmonary function test
Non-specific but commonly show restrictive spirometry
Can help secure the diagnosis
The following have been identified as significant predictors of hypersensitivity pneumonitis and can help avoid misdiagnosis:
• Exposure to a known offending antigen.
• Recurrent episodes of symptoms.
• Symptoms four to eight hours after exposure.
• Inspiratory crackles.
• Weight loss.
• Positive precipitating antibodies (‘precipitins’).
If all six are present, the probability of hypersensitivity pneumonitis is 98%.8
It would be sensible to refer any patients in whom hypersensitivity pneumonitis is suspected to a respiratory physician, ideally one with an interest in interstitial lung disease. The key to management is to remove the trigger. For some patients, this may necessitate a change in employment, which can have significant implications. In acute and sub-acute forms, systemic steroids can improve symptoms, and although there is little evidence to support timing and dose of treatment, oral prednisolone 40-60mg is typically used and the dose then tapered.
Treatment is usually stopped once symptoms have resolved or no further changes are seen.6 In acute hypersensitivity pneumonitis, it is usually straightforward to identify the cause and, if removed, the prognosis is good.
Over 50% of cases of chronic hypersensitivity pneumonitis seem to be idiopathic and the prognosis is more variable.
There is no evidence of a beneficial effect of steroids, where established fibrosis is often present at presentation, nor of any other steroid-sparing agent. In patients with progressive disease refractory to treatment, lung transplantation may be considered.
Professor Paul Cullinan is professor in occupational and environmental respiratory disease at the National Heart and Lung Institute at Imperial College London, and honorary consultant physician in respiratory medicine at Royal Brompton Hospital, London.
Dr Johanna Feary is a respiratory physician and honorary clinical research fellow at Imperial College London and Royal Brompton and Harefield NHS Foundation Trust