A 32-year-old multiparous woman is on postpartum day 8 following an uncomplicated vaginal delivery. She develops acute onset of severe left-sided breast pain, localized erythema over the upper outer quadrant, and a palpable firm wedge-shaped induration. Her temperature is 39.1°C, and she reports malaise and myalgias. She has been exclusively breastfeeding. Milk expressed from the affected breast appears normal. What is the most appropriate next step in management?

Explanation

## Clinical Diagnosis: Infectious Mastitis The clinical triad—**unilateral focal erythema, wedge-shaped induration, and systemic toxicity (fever 39.1°C, malaise, myalgias)**—on postpartum day 8 is pathognomonic for **infectious (bacterial) mastitis**. ### Diagnostic Criteria for Mastitis | Feature | Engorgement | Mastitis | |---|---|---| | **Laterality** | Bilateral, symmetric | Unilateral | | **Erythema** | Absent or diffuse | Focal, wedge-shaped | | **Induration** | Diffuse firmness | Localized, hard wedge | | **Fever** | Low-grade (< 38.5°C) or absent | High (> 38.5°C) | | **Systemic symptoms** | Minimal | Prominent (malaise, myalgias, chills) | | **Milk appearance** | Normal | May be normal, purulent, or blood-stained | | **Response to milk removal** | Rapid improvement | Requires antibiotics | **Key Point:** Mastitis is a **bacterial infection of breast parenchyma**, most commonly caused by *Staphylococcus aureus* (including MRSA), *Streptococcus agalactiae*, or *Escherichia coli*. It occurs when milk stasis and epithelial injury allow bacterial invasion. ### Pathogenesis ```mermaid flowchart TD A[Milk stasis<br/>Poor latch/infrequent feeding]:::outcome --> B[Epithelial injury<br/>Cracked nipples]:::outcome B --> C[Bacterial colonization<br/>S. aureus most common]:::outcome C --> D[Localized inflammation<br/>Erythema, induration, pain]:::outcome D --> E{Untreated?}:::decision E -->|Yes| F[Abscess formation<br/>Fluctuance, pus]:::urgent E -->|No| G[Resolution with antibiotics]:::action ``` ### Management of Infectious Mastitis **High-Yield:** The standard of care is: 1. **Obtain milk culture** (before antibiotics if possible, but do not delay treatment) - Gram stain and culture guide organism identification and susceptibility - Culture from affected breast only 2. **Start empirical oral antibiotics immediately** - **First-line:** Dicloxacillin 500 mg QID × 10–14 days OR Cephalexin 500 mg QID × 10–14 days - Both cover *S. aureus* (including β-lactamase producers) and are safe in breastfeeding - **Alternative (if penicillin allergy):** Clindamycin 300–450 mg TID–QID - **If MRSA suspected:** Trimethoprim-sulfamethoxazole DS BID or clindamycin 3. **Continue breastfeeding from both breasts** - Frequent emptying (8–12 times/24 hrs) prevents abscess formation - Milk from infected breast is safe for infant (antibodies present, bacteria killed by gastric acid) - Abrupt weaning increases abscess risk 4. **Supportive care** - Analgesics (ibuprofen 400–600 mg TID) - Warm compresses before feeding - Proper latch assessment and correction **Clinical Pearl:** Most cases resolve within 48–72 hours of appropriate antibiotic therapy. Failure to improve suggests abscess formation (see below). ### When to Suspect Abscess - Persistent fever and localized symptoms despite 48–72 hours of antibiotics - Fluctuance or pointing on examination - Ultrasound shows hypoechoic fluid collection - **Management:** Ultrasound-guided needle aspiration or incision and drainage (I&D); continue antibiotics; may need to temporarily express milk by hand while wound heals **Warning:** Do not delay antibiotics while awaiting culture results. Empirical therapy should start immediately based on clinical diagnosis. [cite:Williams Obstetrics 26e Ch 37; Cunningham & Leveno, Obstetrics 26e Ch 37]