Case Report: Acute interstitial pneumonia as the initial presentation of Anti-EJ antisynthetase syndrome: a case of severe respiratory failure with rapid response to cyclophosphamide.

BACKGROUND: Antisynthetase syndrome (ASyS) associated with anti-EJ antibodies (directed against glycyl-tRNA synthetase) is increasingly recognized as an ILD-dominant phenotype with a propensity for rapidly progressive ILD (RP-ILD). Acute interstitial pneumonia (AIP)-like presentations may mimic severe infection, posing significant diagnostic challenges. CASE PRESENTATION: A 55-year-old man with hypertension and a 20-pack-year smoking history presented with a fever (39°C) and rapidly progressive dyspnea. Laboratory findings revealed markedly elevated inflammatory markers. CT pulmonary angiogram demonstrated diffuse bilateral ground-glass opacities, consolidations, and interlobular septal thickening consistent with AIP-pattern. Despite broad-spectrum antibiotics, the patient developed refractory respiratory failure requiring mechanical ventilation, hemodynamic instability requiring vasopressors, and acute kidney injury. Bronchoscopy with transbronchial biopsy confirmed acute interstitial inflammation. After more than one week in the intensive care unit (ICU) without improvement, rheumatologic evaluation revealed positive for anti-EJ and anti-Th/To antibodies, supporting ASyS. High-dose of immunosuppressive drug were initiated, resulting in marked improvement in oxygenation, hemodynamics, and muscle strength within 48 hours. Follow-up CT at three weeks demonstrated significant regression of ground-glass opacities with residual findings consistent with nonspecific interstitial pneumonia/usual interstitial pneumonia (NSIP/UIP) overlap. CONCLUSION: ASyS should be considered in patients with severe pneumonia-like illness who fail to respond to appropriate antibiotic therapy. Anti-EJ antibodies are associated with AIP-like patterns and fulminant respiratory failure. Early serologic testing and prompt immunosuppression can be life-saving. The recently published 2024 Classification Criteria for Anti-Synthetase Syndrome (CLASS) provide a contemporary framework for the classification of ASyS, particularly in ILD-dominant presentations.