Croup and bronchiolitis are both common respiratory conditions in children, but they differ in their causes, affected anatomical structures, clinical presentation, and treatment. Here is a detailed comparison:

Croup

Definition
Croup, also called laryngotracheobronchitis, is an upper respiratory tract infection that causes inflammation of the larynx and trachea, leading to airway obstruction.

Cause

Most common: Parainfluenza virus (types 1, 2, and 3).
Others: Respiratory syncytial virus (RSV), influenza, and adenovirus.
Age group affected
Typically affects children aged 6 months to 3 years.

Clinical features

Characteristic "barking" cough (like a seal).
Inspiratory stridor (due to narrowing of the upper airway).
Hoarseness of voice.
Fever and upper respiratory tract symptoms.
Symptoms often worse at night.

Diagnosis

Clinical diagnosis based on symptoms.
Chest X-ray may show the steeple sign (subglottic narrowing), though not routinely needed.
Treatment
Mild croup (no stridor at rest):

Supportive care: Humidified air, adequate hydration.
Single dose of oral dexamethasone (0.15–0.6 mg/kg).
Moderate to severe croup (stridor at rest, significant respiratory distress):

Nebulized epinephrine (adrenaline) for rapid symptom relief.
Dexamethasone (as above).
Oxygen if hypoxic.
Admit if symptoms persist or worsen.

Bronchiolitis

Definition
Bronchiolitis is a lower respiratory tract infection causing inflammation of the bronchioles, leading to obstruction due to mucus and swelling.

Cause

Most common: Respiratory syncytial virus (RSV).
Others: Rhinovirus, human metapneumovirus, and adenovirus.
Age group affected
Typically affects infants and young children, especially those under 2 years, with a peak incidence in the first year of life.

Clinical features

Initial cold-like symptoms: Runny nose, mild cough, and fever.

Progression to respiratory distress:

Tachypnoea.
Wheezing or crackles.
Nasal flaring, chest retractions, and cyanosis (in severe cases).
Poor feeding and irritability in infants.

Diagnosis

Clinical diagnosis based on history and examination.
Testing for RSV or other viruses may be done in hospitalized patients but is not routinely needed.

Treatment

Supportive care (mainstay of treatment):

Hydration (oral or intravenous if severe).
Oxygen if oxygen saturation <92%.
Suctioning of nasal secretions to ease breathing.
Medications:

Bronchodilators (e.g., salbutamol) are not routinely recommended but may be tried if wheezing is significant.
Corticosteroids are generally not effective in bronchiolitis.
Hospitalization:

Needed for severe cases with respiratory distress, apnoea's, or poor feeding.

References

NICE Clinical Guidelines - Bronchiolitis in children: Diagnosis and management (NG9).
UpToDate - Croup: Clinical features, evaluation, and diagnosis.
WHO. Croup and bronchiolitis: Differences in respiratory infections in children.