Clinical diagnosis
Case 249
【Discussion】
When we breathe smoothly with inhalation and exhalation, intra-alveolar pressure in whole lung should change. The size of alveolar space varies between 200 and 500 μm (1, 2). Alveolar pressure is affected by respiration, surface pressure and diameter of alveolus. According to formula of LaPlace’s equation on balloon pressure, alveolar pressure is expressed as follows: Alveolar Pressure = 2 X surface tension / Alveolar Radius (3). This formula indicates the lesser size of alveolus induces the greater alveolar pressure. If surface tension is the same on the various sized alveoli, the smaller sized alveoli become collapsed due to its higher pressure when exhalation. The large sized alveoli become more inflated due to its lower pressure when inhalation. To avoid this situation, alveoli own the mechanism to lower surface tension, namely surfactant. Surfactant makes surface pressure lowered to preserve the same homogeneous alveolar pressure in whole lung. The smaller sizes alveoli are covered with more surfactant per area to lower surface pressure (4).
Pulmonary alveolar proteinosis (PAP) is visualized as melon skin appearance or crazy paving pattern at areas adjacent to pulmonary hilum and/or main pulmonary artery (5-7). Alveolar protein indicates surfactant secreted by Type II alveolar cells. Dysfunction of macrophages is reported to be the main cause of PAP. It is caused by lacking granulocyte macrophage colony stimulating factor (GM-CSF) due to the presence of GM-CSF antibody (8-10). In short, macrophages usually phagocyte waste surfactant, making alveolar space clean and open, and bring exchange of O2-Co2 smooth. However, dysfunction of phagocyte by macrophages induces storage of waste surfactant. Flow over surfactant is excreted as sputum via bronchus and trachea. The surfactant storage is usually found at areas near hilum rather than margin areas. This is probably because of the size of pulmonary alveoli. Alveolar sizes at hilum are smaller than those at margin because main pulmonary vessels and bronchi occupy the proximal area, implying the less functional pulmonary parenchyma area. Then, when surfactant storage occurs due to macrophages dysfunction, surfactant fulfill into alveoli at hilum more speedily than those at margin. As a result, butterfly pattern or melon skin sign at hilum is appeared on chest radiograph or CT.
Pulmonary proteinosis is caused most (90%) by autoimmune disease, followed by secondary disease such as myelodysplastic syndrome, and by congenital (8-10). GM-CSF antibody is positive on pulmonary proteinosis by autoimmune disease but not by secondary disease and by congenital. Symptoms are asymptomatic (30%) even with melon skin sign on chest radiograph, dry cough and dyspnea in case of advancing (8-10).
Management of symptomatic pulmonary proteinosis is correction for GM-CSF deficiency by inhaling GM-CSF and whole lung lavage (5-7).
In our case, her respiratory symptoms are least except feeling of salivary fluid attaching on throat. Then, follow-up observation was conducted without broncho-alveolar lavage and management.
【Summary】
We presented a seventy four-year-old female with feeling of salivary fluid attaching on throat. Chest radiograph showed fuzzy and ground glass opacity at bilateral areas adjacent to hilum. Chest CT showed melon skin appearance (crazy paving-pattern) adjacent to hilum. Although broncho-alveolar lavage was not done yet, she was under follow-up observation based on clinical diagnosis of possible pulmonary alveolar proteinosis. It is borne in mind that alveolar sizes vary between 200 and 500 μm, and LaPlace’s equation of Alveolar Pressure = 2 X surface tension / Alveolar Radius, indicates the lesser size of alveolus induces the greater alveolar pressure. To avoid this situation, alveolus owns surfactant to lower surface tension. The smaller alveolus at areas near hilum own more surfactant than those near margin. Pulmonary alveolar proteinosis arises from macrophages unable to make clearance of surfactant due to deficiency of granulocyte macrophage colony stimulating factor, inducing to accumulate surfactant more at areas near hilum than those near margin. The accumulated surfactant is visualized as melon skin sign or crazy paving pattern on chest CT.
【References】
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2021.11.11