Mystery Quiz- The Answer

July 30, 2008

Posted by Nishay Chitkara MD, Instructor of Clinical Medicine, Division of Pulmonary and Critical Care Medicine, Vivian Hayashi MD, Instructor of Clinical Medicine, Division of General Internal Medicine and Robert Smith MD, Associate Professor of Medicine, Division Pulmonary and Critical Care Medicine

The answer to last week’s mystery quiz is bacterial pneumonia. A number of possible diagnoses may be considered for the left lower lobe mass-like density refractory to antibiotic therapy. A bronchogenic cyst can occur as a mediastinal or intrapulmonary mass. The mass in this case however does not have the characteristic smooth, sharply-outlined border of a bronchogenic cyst. Bronchopulmonary sequestration is a congenital malformation of nonfunctioning lung tissue in the lower respiratory tract. The lower lobes are a common place of occurrence for pulmonary sequestration which has a feeding branch from the aorta. The lesion depicted here lacks an arterial branch from the aorta. Lipoid pneumonia can be ruled out by measuring the attenuation of the mass on CT scan. The mass lesion of lipoid pneumonia has fat density due to aspirated oil. The density of fat is lower (i.e., darker appearing) than the soft tissue density depicted here.

Important considerations in this case also include atypical pneumonia, mycobacterial disease (the patient had a history of MAC colonization), and malignancy such as lymphoma or bronchogenic carcinoma. However, the subsequent hospital course makes these considerations less likely.

When the patient’s antibiotic regimen was changed to linezolid, he improved dramatically within a few days. Some forms of community acquired MRSA have been known to produce a pore-forming toxin named Panton-Valentine Leukocidin (PVL), which is strongly associated with necrotizing skin and soft tissue infections, as well as necrotizing forms of pneumonia. The pneumonia in our case has the appearance of necrosis (the center of the lesion is less dense). Because linezolid inhibits protein synthesis at the 50S ribosome subunit, it can have enhanced efficacy against toxin-producing strains of MRSA. Furthermore, linezolid has excellent bioavailability while vancomycin may require high trough concentrations in order to adequately treat pneumonia. Some retrospective data suggest that linezolid is superior to vancomycin in the treatment of MRSA pneumonia and complicated skin and soft tissue infections.

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