A rare complication of acute severe ulcerative colitis

A 21-year-old female with ulcerative colitis presented with abdominal pain and bloody diarrhea. Laboratory studies revealed markedly elevated C-reactive protein and thrombocytosis. Flexible sigmoidoscopy revealed severe endoscopic activity with ulceration and spontaneous bleeding along rectum and sigmoid colon. Ulcerative colitis had been diagnosed 3 years before, presenting as severe and extensive disease (pancolitis). She had previously failed therapy with infliximab and vedolizumab and had recently started induction therapy with golimumab. She responded well to intravenous corticosteroids but, when switched to oral corticosteroids, there was symptomatic recurrence. Intravenous corticosteroids were re-started and she was evaluated for surgery. At this time, she developed new-onset tachycardia. Electrocardiogram revealed sinus tachycardia with heart rate of approximately 120 bpm. Because tachycardia could result from worsening colitis with potential serious complications, abdominal computerized tomography scan was performed and demonstrated mild lumen dilation and wall thickening consistent with acute colitis without evidence of pneumoperitoneum or toxic megacolon. Surprisingly, thoracic planes revealed the presence of free air dissecting mediastinal space (Figure 1). What is your diagnosis?