Mycoplasma are pleomorphic, filamentous organisms. They lack a cell wall, are of a smaller size, and possess different genetic features than bacteria. Of all the Mycoplasma spp isolated from the respiratory tract, Mycoplasma pneumoniae is the most frequent cause of disease. Mycoplasma pneumonia is considered one of the "atypical pneumonias."
This organism is a common cause of pneumonia and may be responsible for up to 20% of all pneumonias in the general population, and 30%-50% of pneumonias in specific closed populations such as military recruits and college students. Infection is most common in those younger than 20, although it is seen in all age groups. The disease is seen throughout the year with a slight increase in incidence in the fall and winter.
Patients typically present with a dry, sometimes paroxysmal, cough, fever, headache, and malaise. Patients appear generally well or mildly ill, not toxic appearing. Up to 50% of patients will also present with upper respiratory involvement with a sore throat and earache. Hemorrhagic or bullous myringitis may be seen in those presenting with an earache. Extrapulmonary findings are not uncommon and may include meningoencephalitis, meningitis, cranial nerve palsies, transverse myelitis, hemolysis related to cold agglutinins, myocarditis, pericarditis, hepatitis, gastroenteritis, pancreatitis, glomerulonephritis, arthralgias, and erythema multiforme. These extrapulmonary findings may present up to 3 weeks after the respiratory symptoms.
Transmission is usually from person to person by droplet inhalation. There is typically a 2- to 3-week incubation period. The majority of disease is mild, resolving in 7-10 days. Rarely, a more severe illness can occur that results in respiratory failure, acute respiratory distress syndrome, or necrotizing pneumonia.
Viral pneumonias – The diagnosis of viral pneumonia is usually one of exclusion. Lack of sputum production, inability to culture bacteria, typically benign clinical findings, and normal or minimally elevated WBC count as well as lack of response to antibiotic therapy all point to a diagnosis of a viral pneumonia.
Legionella pneumonia – Features that suggest legionellosis over M pneumoniae include diarrhea preceding pneumonia, mental confusion, hyponatremia, relative bradycardia, and abnormal liver function tests. These findings are not always present with legionellosis but, when present, are useful in the differentiation from the other atypical pneumonias.
Psittacosis – A macular, blanching erythematous rash can sometimes be seen. Diagnosis is usually based on a contact history. Diagnosis can also be confirmed serologically.
Q fever – Chest x-ray may help distinguish this entity from M pneumoniae, as the chest x-ray will typically demonstrate air space opacities and not interstitial opacities.
Tularemia – Chest x-ray may demonstrate bilateral hilar lymphadenopathy and cavitation, findings not typical of M pneumoniae. In tularemia, patients can develop peripheral cutaneous ulcers and typhoid-like symptoms.
Chlamydia pneumoniae pneumonia – Pharyngitis and laryngitis are commonly present, whereas, laryngitis is not commonly present with M pneumoniae.
Frontal chest x-ray demonstrating bilateral hilar lymphadenopathy, (long arrows), as well as fine reticular opacities in the bilateral lower lobes (right greater than left), (short arrow).
