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Hiroaki Hagiwara,1 Yoshinori Kinno,1 Tadashi Ikegami2 1Department of Radiology, Yokohama Minamikyosai Hospital, Yokohama, Japan; 2Department of Diagnostic Imaging, Kanagawa Dental University, Yokosuka, JapanCorrespondence: Hiroaki Hagiwara, Department of Radiology, Yokohama Minamikyosai Hospital, 1-21-1 Mutsuura-higashi Kanazawa-ku, Yokohama, 236-0037, Japan, Tel +81-45-782-2101, Fax +81-45-701-9159, Email hhagi1936@gmail.comPurpose: Spontaneous pneumomediastinum, supposedly attributed to air leakage from the respiratory tract, is a common complication of interstitial lung disease often resulting in mediastinal widening. However, several cases of pneumomediastinum without mediastinal widening have been observed. This study aimed to investigate the cause of pneumomediastinum in patients without mediastinal widening.Patients and Methods: This study included 41 patients diagnosed with pneumomediastinum using computed tomography (CT) between July 2011 and September 2021 at Yokohama Minamikyosai Hospital; patients had undergone previous CT showing no gas density. Based on a comparison with previous CT images, the patients were classified into two groups: without mediastinal widening and with mediastinal widening.Results: Of the 41 patients, 13 and 28 had pneumomediastinum without and with mediastinal widening, respectively. There were no significant differences in the sex, age, body mass index, or pneumomediastinum distribution between the groups. However, the rate of weight loss per month was significantly greater in the group without mediastinal widening than in that with mediastinal widening. No significant differences were observed in the respiratory function test results between the two groups; that said, 10 of the 13 patients without mediastinal widening had restrictive disorders. Pulmonary disease in this group included idiopathic pulmonary fibrosis (n = 6), interstitial lung disease with collagen disease (n = 4), and other disease (n = 1). Pneumomediastinum occurred during periods of weight loss in all patients without widening, excluding two patients without data.Conclusion: Pneumomediastinum without mediastinal widening occurs during rapid weight loss and is often associated with restrictive lung disorders. The negative pressure attributed to the decreased plasticity of the lungs, which complements the space where the mediastinal fat has disappeared, is presumably the cause of pneumomediastinum. This pathophysiology is different from that of conventional pneumomediastinum attributed to increased intrapleural space pressure; thus, we propose to name the abovementioned pathophysiology “negative pressure pneumomediastinum”.Graphical Abstract: Keywords: interstitial lung disease, idiopathic lung fibrosis, restrictive lung disorder

Hiroaki Hagiwara,1 Ikuo Torii2 1Department of Radiology, Yokohama Minamikyosai Hospital, 2Department of Radiology, Yokohama Municipal Citizen's Hospital, Yokohama, Japan Abstract: We report a case involving an 83-year-old man with interstitial lung disease who developed atypical pneumomediastinum caused by gas replacement of diminished fat. The patient presented with a complaint of worsening symptoms of respiratory difficulty since a diagnosis of chronic eosinophilic pneumonia 5 months back. He had been under observation with no particular treatment for 5 months. Computed tomography performed on admission revealed pneumomediastinum. When the current scan was compared with that obtained 5 months ago, it was evident that the fat surrounding the mediastinum had been replaced by gas density. There was no mediastinal enlargement, pneumothorax, or pneumopericardium. Because the patient was elderly, home oxygen therapy was initiated for the interstitial pneumonia with no steroid therapy. Computed tomography performed 10 months after discharge showed the reappearance of mediastinal fat and no evidence of gas density. This case is unique because the pneumomediastinum was distinct from spontaneous pneumomediastinum caused by alveolar air leaks and resembled the vacuum phenomenon caused by intervertebral disc degeneration. Keywords: spontaneous pneumomediastinum, vacuum phenomenon, interstitial lung disease, anorexia nervosa