What are the clinical features of lower respiratory tract infection caused by Mycoplasma pneumoniae?

Clinical Features of Lower Respiratory Tract Infection Due to Mycoplasma pneumoniae

Classic Presentation

Mycoplasma pneumoniae LRTI characteristically presents as a slowly progressive illness developing over 3-5 days with malaise, sore throat, low-grade fever, and cough—particularly in school-aged children and young adults. 1

Cardinal Symptoms

• Fever is typically present and generally exceeds 38.5°C 2, 3
• Cough begins as non-productive, irritating, and intermittent, often lasting an average of 23 days 1, 2
• Headache is a frequent and characteristic symptom that helps distinguish this from typical bacterial pneumonia 2, 3
• Arthralgias (joint pains) are commonly reported and represent a distinguishing feature 2, 3
• Sore throat and pharyngitis frequently accompany the respiratory symptoms 2

Physical Examination Findings

• Crackles on auscultation are the most characteristic physical finding 2, 3
• Wheezing occurs in approximately 30% of cases, more frequently in older children, which can lead to diagnostic confusion with asthma 2, 3
• Tachypnea is observed with frequency varying by age 2
• Chest pain can occur, particularly in older children, sometimes manifesting as referred abdominal pain reflecting diaphragmatic pleural irritation 2

Age-Related Patterns

School-Aged Children and Adolescents (Primary Age Group)

• This is the most commonly affected population, with M. pneumoniae identified in 3-23% of pediatric CAP cases 1
• The classic constellation of fever, arthralgias, headache, cough, and crackles is most reliably seen in this age group 2, 3
• Older children (10-16 years) show a much lower percentage of viral co-pathogens compared to younger children 1

Preschool Children and Infants

• M. pneumoniae infection is more common in children under 5 years than traditionally recognized, including infants as young as 8 months 4
• Younger children more frequently present with tachypnea, upper respiratory tract involvement, diarrhea, and vomiting compared to older age groups 4
• The infection may be more severe in children under 5 years of age 5

Laboratory Findings

• White blood cell count is typically lower in M. pneumoniae LRTI (median 9.9 × 10⁹/L) compared to bacterial pneumonia 6
• Lymphocytosis is more common in preschool-aged children 4
• Thrombocytosis occurs in approximately 35.7% of cases, more frequently in younger children 4

Radiographic Features

• Unilateral infiltrates are more frequently observed in M. pneumoniae LRTI (57.5% of cases) 6
• Consolidations are the most common radiographic finding (present in approximately 76% of cases with abnormal chest X-rays) 4
• Radiographic patterns are not sufficiently distinctive to establish diagnosis on imaging alone 1
• Radiographic changes caused by atypical pathogens clear more quickly than those from bacterial pneumonia 1

Temporal Characteristics

• Symptoms progress slowly over 3-5 days, contrasting with the abrupt onset typical of pneumococcal pneumonia 1
• Duration of symptoms at presentation is longer (median 7 days) compared to typical bacterial pneumonia 6
• The cough can persist for weeks, with M. pneumoniae accounting for 26% of coughs lasting 7-100 days in children 2
• Carrier state may persist for several months following acute infection 7

Critical Diagnostic Pitfalls

The "Atypical Pneumonia" Misnomer

The term "atypical pneumonia" should be abandoned as it incorrectly implies a characteristic clinical presentation. 1 Clinical features cannot reliably distinguish M. pneumoniae from other etiologies:

• No clinical or radiographic pattern is sufficiently specific to diagnose M. pneumoniae infection without laboratory confirmation 1, 2
• M. pneumoniae can mimic viral respiratory syndromes including bronchitis, bronchiolitis, pharyngitis, and croup 7
• Wheezing in the presence of crackles makes primary bacterial pneumonia unlikely and suggests viral or M. pneumoniae infection 8

Diagnostic Limitations

• Diagnosis is often made retrospectively by serology, limiting early clinical decision-making 2
• No constant sign on physical examination, laboratory tests, or radiology specifically indicates M. pneumoniae 2
• The infection is underestimated in clinical practice due to lack of specific features and need for PCR or serology 5

Extra-Pulmonary Manifestations

• Dermatological complications including Stevens-Johnson syndrome can occur 2
• Neurological complications such as Bell's palsy are possible 2
• Cardiac, renal, and other systemic complications have been documented, though frequency data are limited 7
• Patients with humoral immunodeficiency may develop severe infections with joint involvement and persistent carrier states 7

Clinical Decision-Making Algorithm

When evaluating a child with suspected LRTI:

1. Consider M. pneumoniae if: school-aged child (5-15 years) with gradual onset over 3-5 days of fever >38.5°C, headache, arthralgias, and persistent cough with crackles 2, 3

2. Suspect M. pneumoniae over bacterial pneumonia if: wheezing accompanies crackles, WBC is relatively low, and symptoms have been present for ≥7 days 8, 6

3. Do not rule out M. pneumoniae based on age alone: it occurs in preschool children and infants, though presentation may differ with more GI symptoms and tachypnea 4

4. Recognize that clinical diagnosis alone is unreliable: a combination of age, symptom duration, and unilateral infiltrates on chest X-ray provides the highest predictive value, but laboratory confirmation is needed for definitive diagnosis 6