In a 31-year-old pregnant woman who wakes up with an isolated right peripheral facial nerve palsy and no limb weakness after visiting family in rural Wisconsin, what is the most likely diagnosis?

Most Likely Diagnosis: Lyme Disease-Associated Facial Palsy

Given the rural Wisconsin location (a Lyme-endemic area), this 31-year-old pregnant woman presenting with isolated unilateral facial droop most likely has Lyme disease-associated facial palsy (LDFP) rather than idiopathic Bell's palsy, and she requires immediate Lyme serology and antibiotic therapy. 1

Clinical Reasoning

Geographic Context is Critical

Rural Wisconsin is a Lyme-endemic region where Lyme disease can account for up to 25% of facial paralysis cases 1. The guidelines explicitly state that in endemic areas (or patients who have recently traveled to endemic areas), Lyme disease serology should be drawn, particularly when exposure history is suggestive 1.

Pregnancy as a Risk Factor

This patient has two overlapping risk factors:

• Pregnancy itself increases Bell's palsy risk 2-4 times compared to non-pregnant individuals 2, 3
• Pregnancy-associated Bell's palsy occurs more commonly in the third trimester with an incidence of 171.6 per 100,000 childbirths 4
• However, pregnancy does NOT increase Lyme disease risk—the geographic exposure remains the dominant concern here

Distinguishing Features to Assess

Immediately evaluate for these LDFP indicators (FACE DROPS criteria) 5:

• Fever (adds 8 points toward Lyme)
• Aches - arthralgia/myalgia (adds 6 points)
• Cephalalgia - headache (adds 3 points)
• Exhaustion - unusual fatigue (adds 4 points)
• Dermatomal/radicular pattern - transverse myelitis or radiculitis (adds 4 points)
• Otalgia/postauricular pain (subtracts 1 point—more typical of Bell's)
• Stiff neck - nuchal rigidity (adds 3 points)

FACE DROPS scores ≥7 predict LDFP with ≥96% accuracy 5

Key Differentiating Points

Bell's palsy characteristics 1:

• Rapid onset (<72 hours)
• Isolated facial weakness without limb involvement (✓ matches this case)
• Often postauricular pain
• Diagnosis of exclusion

Lyme disease facial palsy 1, 6:

• Can present identically to Bell's palsy
• May have systemic symptoms (fever, fatigue, arthralgias, headache)
• Geographic exposure is the key distinguishing factor
• Seventh nerve palsy can occur during early disseminated Lyme disease

Immediate Diagnostic Approach

Laboratory Testing

Order Lyme serology immediately 1:

• Two-step process: ELISA screening followed by Western blot if positive/borderline
• ELISA is more reliable than IFA
• Do NOT wait for results to treat if clinical suspicion is high

Routine laboratory testing is NOT recommended for typical Bell's palsy 1, but Lyme testing is specifically indicated given the endemic location.

Imaging

Do NOT obtain routine imaging 1. Imaging is only indicated if:

• Atypical features (bilateral palsy, isolated branch involvement, other cranial nerve involvement)
• No recovery after 3 months
• Progressive worsening

Treatment Algorithm

If LDFP is Suspected (High Clinical Suspicion + Endemic Area)

Start antibiotics immediately 6:

• Doxycycline is first-line for Lyme disease
• However, doxycycline is contraindicated in pregnancy
• Use amoxicillin or cefuroxime for pregnant patients with Lyme disease

Corticosteroid Consideration

This is the critical decision point:

For Bell's palsy: Oral steroids within 72 hours are strongly recommended for patients ≥16 years old 1

For LDFP: The role of adjuvant corticosteroids is unclear 5. The Lyme disease guideline does not specifically recommend steroids for LDFP 6.

Clinical decision:

• If FACE DROPS score ≥7 or strong Lyme suspicion → treat with antibiotics alone (pregnancy-safe regimen)
• If FACE DROPS score ≤4 and low Lyme suspicion → treat as Bell's palsy with steroids (prednisolone within 72 hours) 1
• If uncertain → treat with both antibiotics AND steroids given the time-sensitive nature and pregnancy implications

Eye Protection

Implement eye protection immediately if she has impaired eye closure 1:

• Artificial tears during the day
• Lubricating ointment at night
• Eye taping if needed
• This prevents corneal injury and is non-negotiable

Pregnancy-Specific Concerns

Prognosis Considerations

• Pregnancy-associated facial palsy has worse long-term outcomes than in non-pregnant individuals 7
• Delay in treatment is associated with persistent nerve palsy 7
• Treatment within 3 days of onset is associated with full recovery 7

Preeclampsia Monitoring

Monitor for preeclampsia development 3:

• Facial palsy in pregnancy can precede preeclampsia
• Check blood pressure and urine protein
• This patient needs close obstetric follow-up

Medication Safety

• Prednisone/prednisolone: Generally considered safe in pregnancy
• Doxycycline: Contraindicated in pregnancy
• Amoxicillin/cefuroxime: Safe alternatives for Lyme disease in pregnancy

Follow-Up Requirements

Reassess or refer to facial nerve specialist if 1:

1. New or worsening neurologic findings at any point
2. Ocular symptoms developing at any point
3. Incomplete facial recovery at 3 months

Common Pitfalls to Avoid

1. Dismissing geographic exposure: Rural Wisconsin = Lyme-endemic. This cannot be ignored.
2. Assuming all facial palsy in pregnancy is idiopathic: While pregnancy increases Bell's palsy risk, it doesn't negate other causes.
3. Waiting for Lyme serology before treating: If clinical suspicion is high, start antibiotics immediately.
4. Using doxycycline in pregnancy: Always use amoxicillin or cefuroxime for pregnant patients with suspected Lyme disease.
5. Neglecting eye care: Corneal injury can occur rapidly and cause permanent vision loss.