Sarcoidosis with the three-density pattern mimicking hypersensitivity pneumonitis

Dear Editor, A 56-year-old woman presented to our hospital with a 2-month history of progressive dyspnoea, asthenia and weight loss. Laboratory test findings were unremarkable. CT of the chest revealed findings consistent with the three-density pattern [TDP; Figure 1]. Despite the lack of an identifiable exposure, hypersensitivity pneumonitis was suspected based on the tomographic findings.Figure 1: Chest computed tomography images (current period) with lung window with axial (a) and sagittal (b) reconstruction demonstrating multiple lobules with three different densities [ground-glass (G), normal (N), and low (L) attenuation] and well-defined marginationA subsequent search of the patient’s medical records revealed that she had been evaluated for respiratory symptoms approximately 10 years previously. CT scan performed at that time showed small nodules with perilymphatic distribution and peribronchovascular nodular thickening, in addition to hilar and mediastinal lymph node enlargement, suggestive of sarcoidosis [Figure 2]. A transbronchial biopsy performed at this time confirmed the diagnosis.Figure 2: (a and b) Axial chest computed tomography images (performed 10 years previously), with lung window showing small nodules with perilymphatic distribution and peribronchovascular nodular thickening, and hilar and mediastinal lymph node enlargementThe clinical presentation suggested that the most likely diagnosis for the current condition was recurrence of sarcoidosis, although a new pulmonary disease as hypersensitivity pneumonitis could not be ruled out. A new clinical evaluation was performed and confirmed that there was no history of exposure to any potential antigens. Bronchoscopy with bronchoalveolar lavage was negative for infectious agents. A new transbronchial biopsy was performed, which was compatible with sarcoidosis. The patient was treated with corticosteroids and had a good clinical and radiological response. The TDP on chest CT, previously known as the “headcheese” sign, is a distinct pattern of mosaic attenuation characterized by the juxtaposition of low-, normal-, and high-attenuation lobules. The term “three-density pattern” was coined by a committee of experts from the American Thoracic Society, Japanese Respiratory Society, and Latin American Thoracic Association.[1] It was unanimously determined to be preferred over the term “headcheese sign,” as headcheese is a regional meat dish that is unknown in most of the world. With the TDP, three sharply demarcated densities are detected, with abrupt transitions corresponding to the margins of secondary pulmonary lobules. The low-attenuation lobules almost always reflect air trapping, which is accentuated on expiration, and the areas of high attenuation, which correspond to ground-glass opacities or fibrosis, are mixed with normally aerated lobules.[2-4] The TDP is indicative of a mixed infiltrative and obstructive process, usually associated with bronchiolitis. The ground-glass opacity component represents the infiltrative portion of the underlying disease, and the low-attenuation lobules reflect obstructive small-airway disease with resultant air trapping and vasoconstriction due to localized hypoxia. This pattern is considered to be highly specific for hypersensitivity pneumonitis, and particularly for the distinction of the disease from idiopathic pulmonary fibrosis.[1,3-5] Although Chang et al.[3] mentioned other possible causes of TDP, including sarcoidosis, an exhaustive review of the literature revealed no reported case of a disease other than hypersensitivity pneumonitis showing the pattern. In conclusion, although TDP is highly suggestive of hypersensitivity pneumonitis, it is not pathognomonic. Sarcoidosis may also present with the TDP, with lobules affected by parenchymal infiltration associated with lobules with an obstructive pattern, basically due to bronchiolar compression by granulomas scattered among normal density lobules. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.