Introduction An increasing number of monogenic auto-inflammatory conditions known as primary type I interferonopathies are being recognised. We present a case of the newly described condition, stimulator of interferon gene (STING) associated vasculopathy with onset in infancy (SAVI), with significant respiratory involvement.

Presentation A male infant, born to healthy non-consanguineous parents, presented to his local hospital with tachypnoea, intermittent fever and failure to thrive from 5 weeks of age. Over the following months, episodes of increased respiratory effort, cough and fever were attributed to infection. He subsequently developed a papular rash in discrete clusters over his back. At presentation to our centre at 7 months of age, he was tachypnoeic but not hypoxaemic. A chest radiograph showed extensive airspace shadowing with chest computed tomography demonstrating interstitial changes. Bronchoalveolar lavage was negative for infection. Laboratory tests revealed microcytic anaemia, raised inflammatory markers, positive anti-nuclear antibody, raised IgA and IgG and abnormal lymphocyte proliferation. A lung biopsy showed a mixed pattern of inflammation, with type 2 pneumocyte hyperplasia and endothelial tuboreticular inclusions on electron microscopy. Given the combination of interstitial lung disease, skin rash and likely vasculopathy, SAVI was suspected. This was confirmed on genetic testing with a heterozygous somatic mutation (c.463G >A, p. V155M) in exon 5 of the TMEM173, the gene encoding STING.

Treatment Treatment with pulsed methylprednisolone was commenced without improvement. He gained weight with supplemental feeding but had persistent tachynoea, subsequently becoming hypoxaemic requiring low flow oxygen therapy. He commenced on a trial of monthly intravenous immunoglobulin (IVIg) with evidence of clinical efficacy awaited. We are considering the use of the Janus kinase inhibitor, baricitinib, as a specific targeted therapy to block interferon signalling.

Conclusion SAVI is a recently described interferonopathy in which lung involvement is a major clinical feature with consequent significant morbidity and mortality. Twelve patients have been reported so far in the literature, with overall poor response to glucocorticoids and disease modifying anti-rheumatic drugs. In the context of failure to thrive, fevers, rash and interstitial lung disease in early life, we urge clinicians to consider SAVI as a differential diagnosis and to seek testing for TMEM173 mutations.