What are the potential causes and treatments for sudden loss of smile in a 6-month-old infant after a viral illness?

Sudden Loss of Smile in a 6-Month-Old After Viral Illness

Immediate Differential Diagnosis

The sudden loss of smile in a 6-month-old infant following a viral illness most likely represents either post-viral facial nerve palsy (Bell's palsy) or, less commonly, a neurological complication such as viral encephalitis or Guillain-Barré syndrome. 1

Critical Red Flags Requiring Urgent Evaluation

• Altered consciousness, lethargy, or excessive drowsiness - suggests possible viral encephalitis requiring immediate lumbar puncture and neuroimaging 1, 2
• Bulging fontanelle with listlessness - concerning for bacterial meningitis, which can follow viral illness 2
• Seizures or focal neurological deficits - may indicate encephalitis, particularly with influenza, enterovirus, or herpes simplex virus 1
• Bilateral facial weakness with ataxia or limb weakness - raises concern for Guillain-Barré syndrome or brainstem encephalitis 1
• Fever with progressive neurological deterioration - requires immediate evaluation for acute necrotizing encephalopathy, particularly if recent influenza infection 1

Specific Viral Etiologies to Consider

Post-Infectious Cerebellitis (VZV-Associated)

• Varicella zoster virus commonly causes post-infectious cerebellitis in young children, presenting with ataxia, nystagmus, and unsteadiness, which is usually self-limiting but can cause severe symptoms 1
• Children may present after the rash has cleared, with a mean delay of 3 months (range 1 week to 48 months) 1

Enterovirus Infection (Especially EV-71)

• Enteroviral encephalitis can produce brainstem syndrome with lower cranial nerve involvement, affecting facial expression and smile 1
• Children under 5 years are more commonly affected, with highest mortality in those aged 6-12 months 1
• May present with papular lesions on hands, feet, and mouth, though neurological complications can develop rapidly 1

Influenza-Associated Encephalopathy

• Influenza (particularly influenza B) can cause acute necrotizing encephalopathy with focal neurological signs and altered consciousness 1
• Recent evidence suggests H1N1 strain may cause more neurological manifestations than seasonal flu, including encephalopathy, focal signs, and aphasia 1

Herpes Simplex Virus Encephalitis

• HSV encephalitis in infants can present with fever, seizures, and focal neurological deficits, though labial herpes may be noted in primary infections 1
• Mild HSV encephalitis has been reported in young children with less severe symptoms 1

Diagnostic Workup

Immediate Assessment

• Perform thorough neurological examination focusing on cranial nerve function, particularly facial nerve (CN VII) symmetry, eye movements, and other brainstem signs 1
• Assess for meningeal signs including neck stiffness, altered mental status, and irritability 3, 2
• Document fever pattern and temperature - current fever suggests active infection requiring more aggressive evaluation 1, 3

Laboratory and Imaging Studies

• Lumbar puncture is mandatory if any meningeal signs, altered consciousness, or concern for encephalitis exists 1, 2
• CSF PCR for HSV, enterovirus, and other viral pathogens should be obtained, as CSF PCR for HSV remains positive for 7-10 days even after aciclovir initiation 1
• MRI brain with gadolinium to evaluate for brainstem involvement, encephalitis, or acute disseminated encephalomyelitis 1
• Blood cultures and complete blood count - 71% of bacterial meningitis cases have positive blood cultures 2

Specific Diagnostic Considerations

• EEG should be obtained if seizures occurred or encephalopathy is present 1
• Urinalysis to rule out urinary tract infection, which is the most common serious bacterial infection in febrile children (5-7% prevalence) 3

Treatment Approach

Empiric Antiviral Therapy

• Initiate intravenous aciclovir immediately if viral encephalitis is suspected, without waiting for diagnostic confirmation 1
• Dosing for 3 months to 12 years: 500mg/m² every 8 hours 1
• Continue for minimum 14-21 days for proven HSV encephalitis, with consideration for repeat lumbar puncture to confirm CSF negativity 1
• In children aged 3 months-12 years, give minimum 21 days before repeating LP 1

Supportive Care

• Maintain adequate hydration and monitor for complications 3
• Administer paracetamol for fever control for comfort, though this does not prevent complications 3

Antibiotic Coverage

• If bacterial meningitis cannot be excluded, initiate empiric antibiotics immediately after obtaining cultures 2
• Most common pathogens in this age group are E. coli (43.7%) and Group B Streptococcus 2

Prognosis and Follow-Up

Post-Viral Facial Palsy

• If isolated facial nerve palsy without other neurological signs, prognosis is generally excellent with spontaneous recovery in most cases 1

Viral Encephalitis

• Outcome depends on early treatment initiation - delays beyond 48 hours from hospital admission worsen prognosis 1
• Serial audiological monitoring every 4-6 months through at least age 2 years is required if congenital CMV is diagnosed 4
• Comprehensive multidisciplinary follow-up including infectious disease, neurology, audiology, and ophthalmology 4

Critical Pitfalls to Avoid

• Do not delay lumbar puncture in well-appearing infants aged 29-90 days with fever, as there are no adequate predictors to identify which infants require CSF evaluation 2
• Do not assume mild symptoms exclude serious infection - 58% of febrile infants with bacteremia or bacterial meningitis appear clinically well 1
• Do not delay aciclovir while awaiting diagnostic confirmation if encephalitis is suspected, as early treatment is critical 1
• Do not miss congenital CMV - testing must be performed within first 21 days of life to distinguish congenital from acquired infection 4
• Recognize that viral infections can coexist with bacterial infections - presence of viral illness does not exclude bacterial meningitis 1