Q1. C - The levels of neutrophils in BAL decline with time after antigen exposure in HP.
Q2. B - Only a few cases have been reported to be caused by native, uncaged birds such as owls.
Q3. D - This structure represents a feather barbule which is a lateral branch of a feather barb.

BAL lymphocytosis with the presence of feather barbules and atypical imaging of hypersensitivity pneumonitis.

The HRCT of the patient revealed pleural thickening and fibrotic lesions with mixed subpleural patterns and traction bronchiectasis predominantly at the bilaterally level of the upper and basal lobes, without pathological pleuro-pericardial collections (Fig.1.). BAL fluid analysis was remarkable for a high percentage of 40% of lymphocytes (Fig.2.), high numbers of lipid-laden alveolar macrophages (Fig.3.), and a few multinucleated giant cells (Fig.4.). Additionally, upon revision of the slides, feather barbules were identified with hooklets approximately 100 μm apart (Fig.5.). Following a multidisciplinary discussion, a diagnosis of hypersensitivity pneumonitis was rendered. The patient was interviewed for possible exposure to feathers and admitted that he made his own pillows from the geese he grew at his country cottage. In hypersensitivity pneumonitis (HP) the upper lungs may exhibit diffuse centrilobular nodules of ground-glass attenuation, which can be seen bilaterally. In the lower lungs, there can be a combination of ground-glass opacities, a mosaic pattern, and normal lung tissue. This combination is often referred to as the "head-cheese sign". In our case the HRCT pattern was atypical and the use of BAL fluid cytology was diagnostic. BAL fluid cytology in HP can often be impressive for the high count of lymphocytes especially if the BAL is performed after acute exposure to the antigenic stimulus. A high percentage of lymphocytosis in BAL fluid, more than 50% is diagnostic for HP if coupled with typical clinical and image presentation on HRCT. Here, the patient’s BAL lymphocytosis of 40%, with signs of mucostasis (presence of a high number of lipid-laden alveolar macrophages), and the presence of feather barbules, and the clinical interview of exposure was sufficient for a diagnosis of HP.

Take home message for trainees: BAL fluid cytology is important, especially in cases where HRCT patterns are atypical. Always view the entire slide and always integrate your microscopic findings with the clinical information.

Costabel U, Miyazaki Y, Pardo A, et al. hypersensitivity pneumonitis. Nature Reviews Disease Primers 2020;Aug 6;6:65.

Davidson K, Ha D, Schwarz M, et al. bronchoalveolar lavage as a diagnostic procedure: a review of known cellular and molecular findings in various lung diseases. Journal of Thoracic Disease 2020;Sep;12:4991.

Patolia S, Tamae Kakazu M, Chami HA, et al. bronchoalveolar lavage lymphocytes in the diagnosis of hypersensitivity pneumonitis among patients with interstitial lung disease. Annals of the American Thoracic Society 2020;Nov;17:1455-67.

Gheorghe-Emilian Olteanu MD PhD
Thoracic pathologist
Spitalul Clinic de Boli Infecțioase și Pneumoftiziologie Dr. Victor Babeș
Timișoara, Romania.