Abstract Introduction The reverse halo sign (RHS) is a radiological finding characterized by central ground glass opacity (GGOs) surrounded by a rim of consolidation on chest CT. Owing to its resemblance to a coral atoll, it is also referred to as the “atoll sign.” Initially described in cases of cryptogenic organizing pneumonia (COP), RHS has since been recognized in a broad spectrum of both infectious and non-infectious conditions, including histoplasmosis, sarcoidosis, lymphomatoid granulomatosis, and granulomatosis with polyangiitis. Case Description A 60-year-old male, non-smoker with a medical history of hypertension and type II diabetes mellitus presented with confusion and polydipsia for one day duration. On arrival, heart rate was 139 bpm, oxygen saturation 91%, which improved with 2 L of oxygen via nasal cannula. Laboratory evaluation was significant for blood glucose 630 mg/dL with high anion gap metabolic acidosis, sodium 123 mmol/L, creatinine 5 mg/dL, mild transaminitis, beta-hydroxybutyrate 0.67 mmol/L, and wbc 25,000/uL with neutrophilic predominance. Chest X-ray was interpreted as normal, However, chest CT without contrast revealed bilateral GGOs with a peripheral rim of consolidation, consistent with the RHS (Figure 1). The patient was admitted to the ICU for hyperosmolar hyperglycemic state and community-acquired pneumonia. Treatment was initiated with insulin drip, ceftriaxone and azithromycin. A urine antigen test for Legionella pneumophila (L. pneumophila) returned positive. The patient's oxygen requirement improved with continued azithromycin monotherapy. Discussion Legionella pneumonia can resemble other types of pneumonia both clinically and radiologically. Radiological finding includes airspace consolidation, interstitial infiltrates, or a mixed pattern. The presence of gastrointestinal symptoms, hyponatremia, and transaminitis should prompt investigators to test for Legionella. Initial diagnostic evaluation typically includes the urine Legionella antigen due to high specificity and low cost. However, it has low sensitivity and can detect only L. Pneumophila serogroup 1. Polymerase chain reaction has high accuracy and can detect all serogroups making it a valuable tool for a comprehensive evaluation. The RHS is a non-specific radiological pattern; however, there is paucity of literature describing the presence of the RHS pattern with Legionella pneumonia. We write this atypical chest CT finding to sensitize the readers to consider Legionella pneumonia as a differential diagnosis when the RHS pattern is seen on chest CT. This abstract is funded by: None
Siew et al. (Fri,) studied this question.