From the University of North Dakota School of Medicine & Health Sciences, Fargo, ND.
Abstract
Objective: To review the management of community-acquired pneumonia (CAP) in adults.
Methods: Review of the literature.
Results: Approximately 4 to 5 million cases of CAP are diagnosed in the United States annually, accounting for significant morbidity and mortality. While numerous studies have previously shown pneumococcus to be the most common causative pathogen, the 2015 EPIC study found that in nearly two-thirds of patients with CAP who required hospitalization, no pathogen was detected. Symptoms and signs of respiratory tract infection are useful in helping to diagnose pneumonia; however, they are less sensitive than chest imaging studies. Laboratory tests used in diagnosing pneumonia include sputum Gram stain and culture, blood culture, urinary antigen, polymerase chain reaction, and biologic markers. In empiric treatment of CAP, both the typical and atypical pathogens should be targeted. Influenza vaccine and pneumococcal polysaccharide and conjugate vaccines should be administered as recommended by the CDC to reduce risk of CAP.
Conclusion: CAP is a common illness with high rates of morbidity and mortality. Treatment is for the most part empirical; diagnostic testing can be used to identify the causative organism and guide pathogen-specific therapy.
Despite advances in medical science, pneumonia remains a major cause of morbidity and mortality. In 2014, 50,620 patients in the United States died from the disease [1]. Pneumonia can be classified as community-acquired, hospital-acquired, or ventilator-associated. Another category, healthcare-associated pneumonia, was included in an earlier American Thoracic Society (ATS) and Infectious Diseases Society of America (IDSA) guideline but was removed from the 2016 guideline because there was no clear evidence that patients diagnosed with healthcare-associated pneumonia were at higher risk for harboring multidrug-resistant pathogens [2]. In this article, we review the epidemiology, microbiology, predisposing factors, diagnosis, treatment, and prevention of community-acquired pneumonia (CAP).
Definition and Epidemiology
CAP is defined as an acute infection of the lungs that develops in patients who have not been hospitalized recently and have not had regular exposure to the health care system [3]. A previously ambulatory patient who is diagnosed with pneumonia within 48 hours after admission also meets the criteria for CAP. Approximately 4 to 5 million cases of CAP are diagnosed in the United States annually [4]. About 25% of CAP patients require hospitalization, and about 5% to 10% of these patients are admitted to the intensive care unit (ICU) [5]. In-hospital mortality is considerable (~10% in population-based studies) [6] and 30-day mortality was found to be as high as 23% in a review by File and Marrie [7]. CAP also confers a high risk of long-term morbidity and mortality compared with the general population who have never had CAP, irrespective of age [8].
Causative Organisms
Numerous microorganisms can cause CAP. Common causes and less common causes are delineated in Table 1.
Until recently, numerous studies had demonstrated that pneumococcus was the most common cause of CAP. However, the CDC Etiology of Pneumonia in the Community (EPIC) study team, in their 2015 prospective, multicenter, population-based study found that in the majority of patients diagnosed with CAP requiring hospitalization, no pathogen was detected. The most common pathogens they detected were rhinovirus (9%), followed by influenza virus (6%) and pneumococcus (5%) [9]. Factors considered to be contributing to the decrease in the percentage of pneumococcus in patients diagnosed with CAP are the widespread use of pneumococcal vaccine and reduced rates of smoking [10,11].
Predisposing Factors
Most people diagnosed with CAP have one or more predisposing factors [12,13] (Table 2).
These predisposing factors for development of pneumonia usually are working in a concerted manner than acting through a single factor. Aging, in combination with other risk factors, increases the susceptibility of a person to pneumonia.
Clinical Signs and Symptoms
Symptoms of CAP include fever, chills, rigors, fatigue, anorexia, diaphoresis, dyspnea, cough (with or without sputum production), and pleuritic chest pain. There is no individual symptom or cluster of symptoms that can absolutely differentiate pneumonia from other acute respiratory diseases, including upper and lower respiratory infections. However, if a patient presents with the constellation of symptoms of fever ≥ 1000F (37.80C), productive cough, and tachycardia, it is more suggestive of pneumonia [14]. Abnormal vital signs include fever, hypothermia, tachypnea, tachycardia, and oxygen desaturation. Auscultation of the chest reveals crackles or other adventitious breath sounds. Elderly patients with pneumonia report a significantly lower number of both respiratory and nonrespiratory symptoms compared with younger patients. Clinicians should be aware of this phenomenon so it does not lead to delayed diagnosis and treatment [15].
Imaging Evaluation
The presence of a pulmonary consolidation or an infiltrate on chest radiograph is required to diagnose CAP, and a chest radiograph should be obtained when CAP is suspected [16]. It should be noted that there is no pattern of radiographic abnormalities reliable enough to differentiate infectious pneumonia from noninfectious causes [17].
There are case reports and case series demonstrating false-negative plain chest radiographs existing in dehydrated patients [18] or in neutropenic state. However, animal studies have shown that dogs challenged with pneumococcus showed abnormal pulmonary shadow, suggestive of pneumonia, regardless of hydration status [19]. There is also no reliable scientific evidence to support the notion that severe neutropenia can cause false-negative radiographs because of the inability to develop an acute inflammatory reaction in the lungs [20].
A chest CT scan is more sensitive than a plain chest radiograph in detecting pneumonia. Therefore, a chest CT should be performed in a patient with negative plain chest radiograph when pneumonia is still highly suspected [21]. A chest CT scan is also more sensitive in detecting cavitation, adenopathy, interstitial disease and empyema. It also has the advantage of better defining anatomical changes than plain films [22].
Because improvement of pulmonary opacities in patients with CAP lags behind clinical improvement, repeating chest imaging studies is not recommended in patients who demonstrate clinical improvement. Sometimes clearing of pulmonary infiltrate or consolidation can take 6 weeks or longer [23].
