Community acquired pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside of the hospital or less than 48 hours after hospital admission. CAP is classified into typical and atypical subtypes, differentiated by their presentation and causative pathogens. This illustration focuses on the classic features of typical CAP.
The most common cause of typical CAP is Streptococcus pneumoniae. It is an encapsulated, gram-positive, lancet-shaped diplococcus bacterium. Other common causative pathogens include Haemophilus influenzae, Moraxella catarrhalis, gram-negative bacilli (e.g. Klebsiella), and Staphylococcus aureus. Common viral agents include influenza viruses, respiratory syncytial virus (RSV), adenovirus, and parainfluenza viruses.
Typical CAP is characterized by the acute onset of fever, cough, sputum production, rigors, pleuritic chest pain, dyspnea, and tachycardia. Streptococcus pneumoniae infection is classically associated with the production of rust-colored sputum. Bronchial breath sounds and crackles may be heard on auscultation. Special findings due to lobar consolidation include egophony (E to A), whispered pectoriloquy, and increased tactile fremitus. CAP in the setting of a pleural effusion may demonstrate decreased tactile fremitus and dullness to percussion. Chest radiography is important in establishing the diagnosis, which may reveal lobar consolidation, patchy airspace opacities, or interstitial opacities. Treatment involves empiric antibiotics or organism-specific antibiotics if the pathogen is identified.