Hypersensitivity Pneumonitis
DIAGNOSIS
DIAGNOSTIC APPROACH2
| Thorough Exposure History | HRCT | Serum Ab Against Suspected Antigens | Lung Biopsy | BAL |
| Most common causes are exposure to birds or fungi at home or work. | In fibrotic HP, see reticulation, traction bronchiectasis and volume loss, with or without honeycombing. | Positive precipitins are considered supportive of diagnosis. | In fibrotic HP, fibrotic changes of variable degrees similar to those of NSIP or UIP. | Lymphocytosis > 50% |
Symptoms9
- Fever
- Dyspnea on exertion
- Cough
- Fatigue
- Malaise
- Anorexia
- Weight loss
High Resolution Computed Tomography (HRCT)2
- Multiple chest HRCT scan features are possible:
- Typical HP
- Compatible with HP
- Indeterminate for HP
HRCT is an essential component of the diagnostic work-up of a patient with fibrotic lung disease.
In nonfibrotic HP, ground-glass opacities and mosaic attenuation are seen alongside poorly-defined centrilobular nodules.
In fibrotic HP, reticulation, traction bronchiectasis and volume loss, with or without evidence of honeycombing, are seen.
- Mid lung zone or random distribution of fibrosis is often noted with relative basal sparing
- The “three-density pattern”, consisting of ground glass opacity, lobules of decreased attenuation and vascularity and normal-appearing lung, is highly specific to fibrotic HP
Serum Antibody (Ab) Against Suspected Antigens2
- Sensitivity (83%) and specificity (68%) of testing is suboptimal
- A positive test warrants consideration for further exploration of home/work/hobby exposures
Surgical Lung Biopsy
- Warranted in patients with newly identified ILD and all other diagnostic tests have failed to result in a definitive diagnosis2
- Multidisciplinary discussion recommended prior to biopsy2
A surgical lung biopsy is needed in cases with no identifiable exposure or HRCT patterns such as UIP which are less suggestive of HP.1
Histopathological features of fibrotic HP include variable degrees of fibrotic changes similar to those of NSIP or UIP in up to 88% of cases.15 There are factors that can help distinguish HP from other patterns.15 They include bronchiolocentric distribution and the presence of granulomatous inflammation (eg, giant cells and granulomas).15
The presence of fibrosis is an independent predictor of morbidity and mortality.15
BAL with Lymphocyte Celluar Analysis2
- Can distinguish fibrotic HP patients from IPF and sarcoidosis
- Can distinguish nonfibrotic HP patients from sarcoidosis
