What is the likely cause of an 18-month-old toddler's flu-like symptoms, croup cough, and fever of 102 degrees Fahrenheit, with occasional spasms when the fever spikes?

Likely Diagnosis: Febrile Seizures Associated with Influenza

The most likely cause of these spasms is febrile seizures triggered by influenza infection, which are significantly more common with influenza than with other febrile illnesses and occur in up to 20% of hospitalized children with influenza. 1

Understanding the Clinical Picture

Why Influenza-Associated Febrile Seizures Are Most Likely

• Influenza causes febrile seizures more frequently than other causes of fever, and these seizures are more likely to be recurrent compared to febrile seizures from other etiologies 1
• Up to 20% of children hospitalized with influenza experience febrile seizures, making this a well-recognized complication in this age group 1
• The combination of flu-like symptoms, croup cough, and fever of 102°F in an 18-month-old child strongly suggests influenza as the underlying viral illness 1
• Febrile convulsions with influenza generally occur with the onset of fever, which matches the pattern described where spasms occur when fever spikes 1

The Croup Component

• Influenza-associated croup follows a more severe clinical course than croup from parainfluenza virus and increases the likelihood that influenza is the causative pathogen 1
• Parainfluenza viruses are the most common cause of croup overall, but when croup occurs with influenza, it tends to be more severe and is more likely to be complicated by bacterial tracheitis 1, 2

Critical Red Flags to Monitor

When to Worry About Encephalopathy Instead of Simple Febrile Seizures

You must urgently evaluate for influenza-associated encephalopathy if any of the following develop:

• Altered level of consciousness, extreme irritability, lethargy, confusion, or personality changes persisting beyond 24 hours 1
• Seizures that become the first symptom of CNS involvement (as opposed to simple febrile seizures at fever onset) 1
• Neck pain or meningeal signs, which should raise immediate concern for encephalopathy or encephalitis 3
• Behavioral disturbances or neurological deficits 1

The Stakes Are High

• In a Japanese study of 202 children with influenza-associated encephalopathy, death occurred in 31%, residual neurological deficit in 26%, and full recovery in only 43% 1
• Encephalopathy typically follows a rapid and severe clinical course and is thought to be mediated by cytokine-induced brain edema rather than direct viral invasion 1

Immediate Management Approach

For Presumed Febrile Seizures

• Continue monitoring fever patterns and seizure characteristics closely 1
• Ensure adequate fever control with acetaminophen (never aspirin due to Reye's syndrome risk with influenza) 1
• Consider influenza testing to confirm the diagnosis, as this can guide antiviral treatment decisions 1, 4

For the Croup Component

• Administer a single dose of oral dexamethasone (0.15-0.60 mg/kg) for the croup, which is recommended for all children with croup regardless of severity 2, 5, 6
• Monitor for signs of moderate to severe croup that would require nebulized epinephrine 2, 5

Antiviral Consideration

• Oseltamivir may be considered if the child has been symptomatic for <48 hours (ideally) or up to 6 days in high-risk children, though evidence for benefit beyond 48 hours is limited 4
• Children under 5 years are at higher risk for severe influenza complications 4

When to Seek Immediate Hospital Evaluation

Admit or urgently evaluate if any of these develop:

• Signs of respiratory distress, cyanosis, or stridor at rest 4, 2
• Severe dehydration or inability to maintain oral intake 4
• Altered consciousness, extreme lethargy, or confusion 1, 4
• Signs of septicemia 4
• Seizures that are prolonged, focal, or occur without fever spike 1
• Fever persisting beyond 4 days, which may indicate bacterial superinfection 4

Additional Complications to Monitor

Bacterial Superinfection Risk

• Children with recent influenza-like illness are 12 times more likely to develop severe pneumococcal complications 1, 3
• Otitis media occurs in approximately 25% of children under 5 years with influenza 1
• If fever persists or worsens after initial improvement, consider bacterial pneumonia (S. pneumoniae, S. aureus) or bacterial tracheitis 1, 4

Other Rare but Serious Complications

• Myositis, myocarditis, pericarditis, and transverse myelitis have been associated with influenza in children 1
• Reye's syndrome can occur if aspirin is given during influenza illness 1