Mycoplasma pneumonia overview

# Overview
Mycoplasma pneumonia is a common atypical pneumonia caused by the bacterium M. pneumoniae. In the USA, the incidence of Mycoplasma pneumonia is approximately 600 per 100,000 individuals. Mycoplasma pneumoniae is transmitted through airborne droplets from person to person. M. pneumoniae is primarily an extracellular pathogen that has evolved a specialized attachment organelle for close association with host cells. The organism's tropism for respiratory epithelial cells and its synthesis of hydrogen peroxide aid in the pathogenesis of Mycoplasma. Mycoplasma pneumonia must be differentiated from other causes of pneumonia, chest pain, and cough, such as other infectious causes, aspiration pneumonia, pneumonitis, lung abscess, empyema, COPD exacerbation, asthma, interstitial lung disease, cardiac diseases, and malignancies. Patients younger than 40 years of age (especially between 5 and 20 years of age) are more commonly affected with Mycoplasma pneumonia. In contrast, Mycoplasma pneumonia is less common (but still incident) among children  40-60 years of age. There is no gender predilection to the development of Mycoplasma pneumonia. The incidence of Mycoplasma pneumonia is higher in regions with temperate climate during the Summer and Fall. Additional risk factors include living in closed communities (healthcare settings, prisoners, military, college students), active lung disease, immunocompromised status, history of sickle cell disease, and active smoking. Following transmission, Mycoplasma incubation period is approximately 1 to 4 weeks. During this time, the patient remains asymptomatic. Onset of symptoms can be gradual and subacute. If left untreated, Mycoplasma pneumonia slowly progresses, and patients typically report high-grade fever, sore throat, headache, and a persistent dry cough. While the disease can persist for weeks or months, it is frequently mild and self-resolving.  In children < 5 years of age, the infection may run subclinical, mild, and non-pneumonia courses. Complications of Mycoplasma pneumonia include otitis, hemolytic anemia, asthma exacerbation, bronchiectasis, Swyer-James syndrome (post-infectious obliterative bronchiolitis), acute kidney injury, Guillain-Barre syndrome, Stevens-Johnson syndrome, pericarditis, myocarditis, transverse myelitis, or meningoencephalitis. Prognosis is usually good, and the majority of patients recover completely even without antimicrobial therapy. Elderly individuals and patients with sickle cell disease and other immunocompromised patients are at increased risk of developing complications of Mycoplasma pneumonia. Mycoplasma pneumonia is diagnosed by either culture, serology, or molecular methods. Antimicrobial therapy using either a macrolide, doxycycline, or a fluoroquinolone is the mainstay of therapy for atypical pneumonia caused by Mycoplasma. There is no vaccine against Mycoplasma, but avoiding infected individuals helps reduce risk of infection.

# Historical Perspective
Mycoplasma pneumoniae was first described by Hobart A. Reimann in 1938 in a patient with a self-limited atypical pneumonia.

# Pathophysiology
Mycoplasma pneumoniae is transmitted through airborne droplets from person to person. M. pneumoniae is primarily an extracellular pathogen that has evolved a specialized attachment organelle for close association with host cells. The organism's tropism for respiratory epithelial cells and its synthesis of hydrogen peroxide aid in the pathogenesis of Mycoplasma. Additionally, Mycoplasma produces community acquired respiratory distress syndrome (CARDS) toxin, a unique virulence factor which activates the host's inflammatory pathways and airway dysfunction.

# Causes
Mycoplasma pneumonia is caused by Mycoplasma pneumoniae, a very small bacterium that lacks cell wall and periplasmic space. On Gram-stain, Mycoplasma stains pink, i.e. it is Gram-negative by staining. However, it is structurally different from other Gram-negative organisms because it lacks a cell wall.

# Differential Diagnosis
Mycoplasma pneumonia must be differentiated from other causes of pneumonia, chest pain, and cough, such as other infectious causes, aspiration pneumonia, pneumonitis, lung abscess, empyema, COPD exacerbation, asthma, interstitial lung disease, cardiac diseases, and malignancies.

# Epidemiology and Demographics
In the USA, the incidence of Mycoplasma pneumonia is approximately 600 per 100,000 individuals.  Patients younger than 40 years of age (especially between 5 and 20 years of age) are more commonly affected with Mycoplasma pneumonia. In contrast, Mycoplasma pneumonia is less common (but still incident) among children  40-60 years of age. There is no gender predilection to the development of Mycoplasma pneumonia. The incidence of Mycoplasma pneumonia is higher in regions with temperate climate during the Summer and Fall.

# Risk Factors
Risk factors in the development of Mycoplasma pneumonia include age between 5-40 years, living in closed communities (healthcare settings, prisoners, military, college students), active lung disease, immunocompromised status, history of sickle cell disease, active smoking, and living in regions with temperate climates.

# Natural History, Complications and Prognosis
Following transmission, Mycoplasma incubation period is approximately 1 to 4 weeks. During this time, the patient remains asymptomatic. Onset of symptoms can be gradual and subacute. If left untreated, Mycoplasma pneumonia slowly progresses, and patients typically report high-grade fever and a persistent cough. While the disease can persist for weeks or months, it is frequently mild and self-resolving.  In children < 5 years of age, the infection may run subclinical, mild, and non-pneumonia courses. Complications of Mycoplasma pneumonia include otitis, hemolytic anemia, asthma exacerbation, bronchiectasis, Swyer-James syndrome (post-infectious obliterative bronchiolitis), acute kidney injury, Guillain-Barre syndrome, Stevens-Johnson syndrome, pericarditis, myocarditis, transverse myelitis, or meningoencephalitis. Prognosis is usually good, and the majority of patients recover completely even without antimicrobial therapy. Elderly individuals and patients with sickle cell disease and other immunocompromised patients are at increased risk of developing complications of Mycoplasma pneumonia.

# Diagnosis

## History and Symptoms
Common symptoms of Mycoplasma pneumonia include malaise,  fever, dry cough, chest pain, night sweats, sore throat, and headache. Less common symptoms include eye pain, arthralgia, myalgia, dyspnea, rash, vomiting, and diarrhea.

# Physical Examination
Patients with Mycoplasma pneumonia generally appear well. Common physical examination findings include fever, decreased oxygen saturation on room air, tachycardia, tachypnea, non-exudative pharyngitis, and decreased breath sounds with crackles and increased tactile fremitus on lung auscultation. Other physical examination findings include rash, pallor, sinus tenderness, cervical lymphadenopathy, and possibly bullous myringitis.

## Laboratory Findings
Mycoplasma pneumonia is diagnosed by either culture, serology, or molecular methods. Other laboratory findings include elevated CRP and/or ESR, leucocytosis, or hemolytic anemia.

## Chest X Ray
Findings of Mycoplasma pneumonia on chest x ray include either interstitial pneumonia or bronchopneumonia with evidence of patchy consolidation that may be unilateral (more common) or bilateral. Involvement of the lower lobes is common.

## Chest CT
On chest CT scan, findings of Mycoplasma pneumonia include areas of ground-glass attenuation, air-space consolidation, and formation of intra-pulmonary nodules.

# Treatment

## Medical Therapy
Antimicrobial therapy is the mainstay of therapy for atypical pneumonia caused by Mycoplasma. Pharmacologic therapies for Mycoplasma pneumonia include either a macrolide (e.g. azithromycin) for 2-5 days, doxycycline for 14 days, or a fluoroquinolone for 14 days.  Supportive therapy includes rest, adequate fluid intake, and administration of either non-steroidal anti-inflammatory drugs (NSAIDS) or acetaminophen if needed.  Aspirin should be avoided among children.

## Prevention
There is no vaccine against Mycoplasma. There are no preventive measures against Mycoplasma pneumonia. However, avoiding infected individuals helps reduce risk of infection.