Answers

Short answers

1 The FEV₁ to FVC ratio is 0.7 (0.84 in this case) and both FEV₁ and FVC are lower than the predicted values; this means that the patient has a restrictive ventilatory defect.

2 The clinical features and restrictive ventilatory defect suggest a diffuse parenchymal lung disease. The results in this patient are not in keeping with asthma, where spirometry would be normal or show airflow obstruction (FEV₁/FVC <0.7).

3 The next two most important investigations include high resolution computerised tomography of the chest and specific IgG antibodies to budgie feathers or droppings.

4 The most likely diagnosis is chronic extrinsic allergic alveolitis (hypersensitivity pneumonitis). To try to avoid irreversible and progressive lung damage, the patient should be advised to relocate her budgies and be given a course of steroids.

Long answers
1 Spirometry results
In cases of difficult to treat "asthma," it is important to check whether the patient actually has asthma, and—if the diagnosis is correct—look for the presence of aggravating factors, such as a concomitant medical condition or lack of adherence to prescribed drugs.^{1 2} In our case, the clinical features, spirometry results, and changes on chest radiography were not in keeping with a diagnosis of asthma, and an alternative cause of cough and breathlessness was considered—diffuse parenchymal lung disease.

2 Differential diagnosis
The most common causes of restrictive ventilatory defects are diffuse parenchymal lung diseases, neuromuscular disorders such as motor neurone disease, chest wall deformities such as kyphoscoliosis, large pleural effusions, and morbid obesity.

The diffuse parenchymal lung diseases are classified into:

• Diffuse parenchymal lung disease of known cause (drugs such as amiodarone, connective tissue disorders such as rheumatoid arthritis, extrinsic allergic alveolitis, and occupational dusts such as asbestos)
  
• Diffuse parenchymal lung disease of unknown cause (such as idiopathic interstitial pneumonias, which have a variety of subtypes)
  
• Other distinctive forms such as sarcoidosis, histiocytosis X, lymphangioleiomyomatosis, and eosinophilic pneumonia.^{3 4}
  

Idiopathic pulmonary fibrosis is the most common form of diffuse parenchymal lung disease. Around 2000 new cases of this disease are diagnosed each year in England and Wales, which gives a prevalence of 15-18 per 100 000 person years.⁴ Reports on the prevalence of extrinsic allergic alveolitis vary because of the lack of a standard screening tool to establish the diagnosis. Bird fanciers lung disease is the most common type of extrinsic allergic alveolitis in the United Kingdom, but it is less prevalent than idiopathic pulmonary fibrosis or sarcoidosis.⁴

3 Investigations
Chest radiography may show diffuse ground glass changes with micronodules in acute extrinsic allergic alveolitis, whereas chronic extrinsic allergic alveolitis results in reticular shadowing or honeycombing, often with a predilection for the upper zones. High resolution computed tomography often helps define the distribution and extent of the interstitial process, which in some instances may not be obvious on a plain film. It usually shows ground glass changes, with fibrosis and honeycombing in advanced disease. Pleural involvement is uncommon.⁵

Serum precipitins can help identify the antigen responsible for the disease. IgG antibodies in the blood suggest exposure rather than disease. However, in people who keep budgies, only a small number of healthy people develop precipitins, which makes this test useful in the diagnosis of extrinsic allergic alveolitis. Many antigens are known to cause extrinsic allergic alveolitis—for example, mouldy hay, which causes farmers’ lung; mushroom compost, which causes mushroom workers’ lung; feather bedding, which causes feather duvet lung; mouldy cheese, which causes cheese workers’ lung; and mouldy grapes, which cause wine makers’ lung.

In addition to a restrictive ventilatory defect, pulmonary function testing may also show impaired gas transfer. Patients with extrinsic allergic alveolitis can develop type 1 respiratory failure and may significantly desaturate during a six minute walk.⁵

Lung biopsy (usually video assisted) may be considered when there is discordance between the history, clinical findings, and radiology. Infrequently, radiology may suggest an alternative diagnosis to that suspected clinically. Histological examination is needed in these cases to establish the diagnosis, exclude atypical infections, provide prognostic information, and guide treatment.^{4 5 6}

4 Likely diagnosis and treatment
All patients with extrinsic allergic alveolitis should be advised to avoid further antigen exposure. In patients where complete avoidance is difficult—for example, farmers who react to mouldy hay—protective masks with filters are necessary. Oral steroids are needed in people with a combination of functional, radiological, and physiological impairment. A high dose of steroids is given for the first month; this is then gradually reduced over two to three months to a maintenance dose and subsequently discontinued. Disease activity is monitored according to symptoms, changes on chest radiography, and lung function.^{4 5}

Patient’s outcome
Precipitins to budgie antigens were strongly positive, and high resolution computed tomography showed extensive alveolar shadowing throughout both lungs. The patient was given high dose oral steroids for a month, and the dose was slowly reduced over the next few months. The budgies were relocated and all inhalers were discontinued. At clinic follow-up, the patient’s symptoms had resolved completely, and the results of spirometry and chest radiograph had returned to normal.

Cite this as: BMJ 2009;338:b1862