What differentiates upper from lower respiratory infections?

Differentiating Upper from Lower Respiratory Tract Infections

Upper respiratory tract infections (URTIs) occur above the vocal cords, while lower respiratory tract infections (LRTIs) involve the airways below the vocal cords and are distinguished by specific clinical features, anatomical location, and causative pathogens.

Anatomical Distinction

• Upper respiratory tract: Includes nose, sinuses, pharynx, larynx
• Lower respiratory tract: Includes trachea, bronchi, bronchioles, and lung parenchyma

Key Clinical Differentiating Features

Signs and Symptoms of URTI 1:

• Rhinitis (nasal congestion, rhinorrhea)
• Sore throat
• Post-nasal drip
• Mild or absent fever
• Normal pulmonary auscultation
• Retrosternal burning sensation
• Symptoms often preceded by common cold symptoms

Signs and Symptoms of LRTI 1:

• Cough (productive or non-productive)
• At least one functional/physical sign of lower respiratory involvement:
  • Dyspnea
  • Chest pain
  • Wheezing
  • Diffuse or focal signs on auscultation
• At least one general sign suggesting infection:
  • Fever
  • Sweating
  • Headache
  • Joint pain

Specific Features Suggestive of Pneumonia 1:

• Fever >37.8°C
• Tachycardia >100 bpm
• Tachypnea >25/min
• Chest pain
• No infection of the upper respiratory tract
• Overall impression of severity
• Focal signs on auscultation (crackles, rales)
• Chest X-ray confirms the diagnosis

Diagnostic Approach

Clinical Assessment

1. Location of symptoms: Upper vs. lower respiratory tract involvement
2. Auscultation findings: Normal in URTI; abnormal (focal or diffuse) in LRTI
3. Severity indicators: High fever, tachypnea, tachycardia suggest LRTI, particularly pneumonia

Laboratory Tests

• C-reactive protein (CRP) 1:
  • CRP <20 mg/L with symptoms >24 hours makes pneumonia highly unlikely
  • CRP >100 mg/L makes pneumonia likely

Imaging

• Chest X-ray 1:
  • Gold standard for diagnosing pneumonia
  • Should be performed when pneumonia is suspected based on clinical features
  • Not necessary for all LRTIs due to cost and limited availability

Common Pathogens

URTI Pathogens 2:

• Streptococcus pyogenes (pharyngitis/tonsillitis)
• Streptococcus pneumoniae
• Haemophilus influenzae
• Moraxella catarrhalis
• Viruses (rhinovirus, coronavirus, adenovirus)

LRTI Pathogens 1, 2:

• Streptococcus pneumoniae (most common bacterial cause)
• Haemophilus influenzae
• Mycoplasma pneumoniae
• Chlamydia pneumoniae
• Respiratory viruses

Clinical Pitfalls to Avoid

1. Assuming all cough is URTI: Cough receptors exist in both upper and lower respiratory tract 1
2. Missing pneumonia: Pneumonia has higher mortality and requires specific management 3
3. Over-reliance on single symptoms: Combinations of symptoms are more reliable for differentiation 4
4. Failure to consider chronic conditions: Asthma or COPD exacerbations can present similarly to infections 1

Special Considerations

When to Suspect Chronic Airway Disease 1:

Consider in patients with persistent cough and at least two of:

• Wheezing
• Previous consultations for wheezing/cough
• Dyspnea
• Prolonged expiration
• Smoking history
• Symptoms of allergy

When to Suspect Pneumonia 1:

A patient should be suspected of having pneumonia when one of the following is present:

• New focal chest signs
• Dyspnea
• Tachypnea
• Pulse rate >100
• Fever >4 days

Treatment Implications

The distinction between URTI and LRTI is crucial for treatment decisions:

• URTIs are predominantly viral and rarely require antibiotics
• LRTIs, particularly pneumonia, often require antibiotics with coverage for likely pathogens
• Bronchitis in healthy adults rarely requires antibiotics despite being classified as LRTI 1

Remember that accurate differentiation between URTI and LRTI directly impacts patient outcomes by ensuring appropriate treatment and reducing unnecessary antibiotic use.