Acute bronchitis is one of the most common diagnoses made by primary care physicians.^1–3 In the United States alone, the evaluation and treatment of this illness is estimated to cost $200 million to $300 million per year.⁴ Even though the diagnosis of acute bronchitis is frequently made, the definition of this illness lacks clarity and its pathophysiology is often misunderstood. Furthermore, acute bronchitis has traditionally been treated with antibiotics, although little evidence supports the effectiveness of antibiotic treatment in this illness.

This article examines the causes of acute bronchitis, as well as the physiologic responses to this illness. The value of antibiotics and other agents in the treatment of bronchitic cough is also appraised.

Definition and Epidemiology

One reason that acute bronchitis is such a common diagnosis in primary care practice is that physicians often lump various conditions together under the diagnosis of bronchitis. In the absence of clear diagnostic signs or laboratory tests, the diagnosis of acute bronchitis is purely clinical.⁵ Consequently, cough from upper respiratory tract infections, sinusitis or allergic syndromes (e.g., mild asthma or viral pneumonia) may be diagnosed as acute bronchitis.

True acute purulent bronchitis is characterized by infection of the bronchial tree with resultant bronchial edema and mucus formation.⁶ Because of these changes, patients develop a productive cough and signs of bronchial obstruction, such as wheezing or dyspnea on exertion. Unlike the chronic inflammatory changes of asthma, the inflammation in acute bronchitis is transient and usually resolves soon after the infection clears. In some patients, however, the inflammation can last several months.⁷ In rare cases, a postbronchitis cough can persist for up to six months.

Bronchitis can have causes other than infection. Bronchial wall inflammation can occur in asthma or can be secondary to mucosal injury in an acute event, such as smoke or chemical fume inhalation. This inflammation can also result from chronic toxic exposure, such as cigarette smoking. It is important to realize that when underlying inflammation is present, such as in asthmatics or smokers, infective agents are likely to cause more severe cough and wheezing.

Viruses are the most common cause of bronchial inflammation in otherwise healthy adults with acute bronchitis. Only a small portion of acute bronchitis infections are caused by nonviral agents, with the most common organism being Mycoplasma pneumoniae.^8–10 Study findings suggest that Chlamydia pneumoniae may be another nonviral cause of acute bronchitis.^11,12

The obstructive symptoms of acute bronchitis, as determined by spirometric studies, are very similar to those of mild asthma.¹³ In one study,¹³ forced expiratory volume in one second (FEV₁), mean forced expiratory flow during the middle of forced vital capacity (FEF_25–75%) and peak flow values declined to less than 80 percent of the predicted values in almost 60 percent of patients during episodes of acute bronchitis. In the five weeks following the infection, these values returned to normal. Another study¹⁴ found that patients with acute bronchitis were 6.5 times more likely to have been told they had asthma in the past and nine times more likely to be diagnosed with asthma in the future.

The findings of these studies^12,13 suggest that patients with acute bronchitis may have an underlying predisposition to bronchial reactivity during times of viral infection and that this reactivity may evolve into the more chronic bronchial inflammation which characterizes asthma. Recent epidemiologic findings of serologic evidence of C. pneumoniae infection in adults with new-onset asthma suggest that untreated chlamydial infections may have a role in the transition from the acute inflammation of bronchitis to the chronic inflammatory changes of asthma.^11,12

Diagnosis

Patients with acute bronchitis usually have a viral respiratory infection with transient inflammatory changes that produce sputum and symptoms of airway obstruction. The cough in acute bronchitis may produce either clear or purulent sputum. While this cough generally lasts seven to 10 days, it can persist. Approximately 50 percent of patients with acute bronchitis have a cough that lasts up to three weeks, and 25 percent of patients have a cough that persists for over a month.¹²

Physical Examination

While a lung examination may be useful in patients with acute bronchitis, it is not diagnostic. Wheezing, rhonchi, a prolonged expiratory phase or other obstructive signs may be present. However, some patients may exhibit no signs of bronchospasm. Patients should be asked about night coughing, and they should undergo forced expiration in the prone position to detect wheezing. A night cough or wheezing may be the only signs that bronchial obstruction is present.

Diagnostic Studies

The appearance of sputum is not predictive of whether a bacterial infection is present. Purulent sputum is most often caused by viral infections.^5,15 Microscopic examination or culture of sputum in the healthy adult with acute bronchitis generally is not helpful. Since most cases of acute bronchitis are caused by viruses, cultures are usually negative or exhibit normal respiratory flora.

When M. pneumoniae infection is present, routine sputum cultures are still negative. Rapid tests for the identification of Mycoplasma organisms have been developed. However, these tests are not routinely available, and they are unlikely to be cost-effective studies in the acute care setting.¹⁶

No available test can provide a definitive diagnosis of acute bronchitis. While decreases in pulmonary function have been demonstrated in patients with acute bronchitis, diagnostic pulmonary function testing should not be performed in previously healthy patients. When underlying asthma is suspected, pulmonary function testing should be considered.

It must be kept in mind that acute bronchitis can cause transient pulmonary function abnormalities. Therefore, to diagnose asthma, the physician must find changes that persist after the acute phase of the illness. When pneumonia is suspected, chest radiographs and pulse oximetry may be helpful in making the diagnosis.

Differential Diagnosis

Many conditions other than acute bronchitis present with cough (Table 1). Acute bronchitis or pneumonia can present with fever, constitutional symptoms and a productive cough. While patients with pneumonia often have rales, this finding is neither sensitive nor specific for this illness. When pneumonia is suspected on the basis of the presence of a high fever, constitutional symptoms, severe dyspnea and certain physical findings or risk factors, a chest radiograph should be obtained to confirm the diagnosis.

Asthma and allergic bronchospastic disorders, such as allergic aspergillosis or bronchospasm due to other environmental and occupational exposures, can mimic the productive cough of acute bronchitis. When obstructive symptoms are not obvious, mild asthma may be diagnosed as acute bronchitis. Furthermore, since respiratory infections can trigger bronchospasm in asthma, patients with asthma that occurs only in the presence of respiratory infections resemble patients with acute bronchitis.

Asthma should be considered in patients with repetitive episodes of acute bronchitis. Patients who repeatedly present with cough and wheezing can be given full spirometric testing with bronchodilation or provocative testing with a methacholine challenge test to help differentiate asthma from recurrent bronchitis. Those with findings suggestive of the chronic inflammatory changes that occur in asthma may benefit from chronic anti-inflammatory therapy.

Upper respiratory tract infection and sinusitis can also be confused with acute bronchitis. In all three of these illnesses, patients may have a productive cough. However, the material produced from the cough in an upper respiratory infection or sinusitis is from the deep pharynx and has accumulated from postnasal drainage.

The differential diagnosis should also include nonpulmonary causes of cough and shortness of breath. In older patients, congestive heart failure may cause cough, shortness of breath and wheezing. Symptoms are often worse at night. Reflux esophagitis with chronic aspiration can cause bronchial inflammation with cough and wheezing.¹⁷ Finally, bronchogenic tumors may produce a cough and obstructive symptoms.⁴

Treatment

Antibiotics

Although many authorities have argued that antibiotics have no role in the treatment of acute bronchitis, these agents remain the predominant therapy offered to patients. Primary care physicians in the United States have treated acute bronchitis with a wide range of antibiotics,^18,19 even though scant evidence exists that antibiotics offer any significant advantage over placebo (Table 2).²⁰ Placebo-controlled studies using doxycycline,^7,15 erythromycin^21,22 and trimethoprim-sulfamethoxazole²³ have failed to show consistent significant benefit for antibiotic therapy in acute bronchitis. Even when patients with M. pneumoniae infection can be identified using a rapid identification system, treatment with erythromycin has provided only limited benefit.²⁴

Frequently, antibiotics are prescribed primarily to meet patient expectations.²⁵ While physicians may understand that antibiotics are not effective for acute bronchitis, they prescribe them anyway, fearing that failure to do so will leave patients less satisfied. However, one study²⁶ found that patient satisfaction with care did not depend on the receipt of an antibiotic prescription. As long as physicians explained the rationale for treatment, patients who expected antibiotics and did not get them were just as satisfied as those who were given antibiotics.

Another explanation for the frequent prescription of antibiotics is the lack of distinction between acute and chronic bronchitis. Chronic bronchitis is characterized by persistent and irreversible inflammatory changes in the bronchial tree, with these changes resulting in chronic cough, daily sputum production and shortness of breath. Patients with underlying chronic bronchitis may periodically become infected with a wide variety of organisms that produce changes in their usual respiratory symptoms. In such cases, the evidence regarding the effectiveness of antibiotic therapy is variable.

Although some studies show that antibiotic therapy is beneficial in patients with exacerbations of chronic bronchitis,^27,28 other studies are less convincing.^29,30 Antibiotic effectiveness trials that do not differentiate between acute bronchitis and exacerbations of chronic bronchitis add to the confusion. Misconceptions about the role of bacteria in acute bronchitis and the widespread practice of treating this illness with antibiotics may stem from studies that included patients with chronic bronchitis who were mislabeled as having acute bronchitis.³¹

Bronchodilators

The pulmonary function findings in mild asthma and acute bronchitis are similar. Thus, it has been hypothesized that bronchodilating agents may offer symptomatic relief to patients with bronchitis.

Three studies^32–34 have evaluated the effectiveness of bronchodilators in the treatment of acute bronchitis (Table 3). These studies all demonstrated significant relief of symptoms in patients with bronchitis who received oral albuterol (4 mg four times daily),³³ inhaled albuterol (two puffs four times daily)³⁴ or fenoterol (not available in the United States).³² Compared with patients who received placebo, those who were treated with albuterol were more likely to have stopped coughing within a week of the initiation of therapy.^33,34 The patients who were treated with inhaled albuterol also returned to work sooner.³³ The effects of combining albuterol with an antibiotic have also been assessed. In one of these studies,³² no benefit was shown from adding erythromycin to the treatment regimen of patients who were already receiving albuterol.

Another study³⁵ in patients with undifferentiated cough found no beneficial effect from albuterol therapy. The investigators who conducted this study did not attempt to select patients with productive cough and/or obstructive symptoms. Because of the multiple causes of cough, the study population most likely included patients with acute bronchitis as well as many other conditions, including sinusitis, upper respiratory infection and asthma. The results of this study demonstrate that bronchodilators are likely to be effective only when bronchial inflammatory changes are present.

Until better clinical data are available, physicians must rely on an accurate history and the clinical picture of productive cough and wheezing to guide them in the use of bronchodilator therapy.

Possible Complication of Bronchitis: Adult-Onset Asthma

Serologic evidence of previous infection with C. pneumoniae has been found in some adults with new-onset asthma.³⁶ Consequently, considerable attention has been focused on whether adult-onset asthma is frequently preceded by a chlamydial respiratory infection.^11,12 Both Chlamydia trachomatis³⁷ and C. pneumoniae³⁸ have been cultured from the sputum of children with asthma. However, no prevalence studies have assessed the frequency with which patients who have respiratory illnesses such as bronchitis are infected with Chlamydia species and the percentage of these patients who progress to asthma.

The reversibility of bronchial inflammation when chlamydial infections are treated is uncertain. In one small open-label study of patients in a single practice,³⁹ bronchial obstruction was reversed in about one half of the patients who received antibiotic therapy. The patients who responded to antibiotics tended to have serologic evidence of acute infection or infection for a shorter period of time compared with the patients who did not respond to antibiotic therapy. While this evidence is still preliminary, it suggests that early treatment of persistent wheezing with agents effective against Chlamydia species may prevent the development of asthmatic symptoms in adults. Further studies that confirm the effectiveness of antichlamydial therapy would be useful in guiding treatment decisions.