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Summary Table

Category	Details
Risk Factors	Breastfeeding woman (especially first 6 weeks postpartum), primiparity, history of mastitis, cracked or damaged nipples, incomplete breast emptying, poor latch technique, use of nipple shields, breast implants, immunosuppression
Etiology	Most commonly Staphylococcus aureus (including MRSA); less commonly Streptococcus species, Escherichia coli. Entry via cracked nipple or retrograde flow through milk ducts
Patient Presentation	Unilateral breast pain, swelling, warmth, and redness in a breastfeeding woman, often accompanied by fever (>38.5 C) and flu-like symptoms (myalgia, malaise, chills)
Classic Physical Exam	Wedge-shaped area of erythema and induration on one breast, tender to palpation, warm overlying skin; axillary lymphadenopathy may be present; if a fluctuant mass is palpated, consider abscess
Key Diagnostic Results	Clinical diagnosis; milk culture and sensitivity if refractory to initial antibiotics, MRSA suspected, or recurrent episodes; ultrasound if abscess suspected (hypoechoic fluid collection)
Management	Continue breastfeeding or pumping; first-line antibiotic is dicloxacillin or cephalexin for 10 to 14 days; if MRSA suspected, use trimethoprim-sulfamethoxazole (TMP-SMX) or clindamycin; incision and drainage or ultrasound-guided needle aspiration for abscess
Keywords	"Breastfeeding mother," "unilateral breast pain," "wedge-shaped erythema," "flu-like symptoms," "fever postpartum," "fluctuant mass" (abscess), "cracked nipple"

1. Pathophysiology

Mastitis is an inflammatory condition of the breast parenchyma that may or may not involve a bacterial infection. The most commonly tested scenario is lactational (puerperal) mastitis, occurring in the postpartum breastfeeding period.

The pathogenesis begins with milk stasis. When milk is not adequately drained from the breast, it accumulates within the ductal system. Stagnant milk serves as a medium for bacterial growth and also causes local ductal distension, leading to inflammation. The stasis alone can produce a clinical picture of engorgement and localized tenderness, which, if left unresolved, progresses to infectious mastitis.

Bacteria, most commonly Staphylococcus aureus, gain entry through micro-abrasions or fissures on the nipple, which are extremely common in early breastfeeding when the latch technique is still being established. The organisms travel in a retrograde fashion through the lactiferous ducts into the surrounding breast tissue. Once within the parenchyma, they trigger an acute inflammatory response: neutrophil recruitment, cytokine release, and tissue edema. This explains the characteristic wedge-shaped erythema that follows the anatomical distribution of a single ductal segment, as well as the systemic symptoms (fever, malaise, myalgia) driven by circulating inflammatory mediators such as IL-1 and TNF-alpha.

If the infection is not controlled, localized suppuration occurs, resulting in a walled-off collection of pus: a breast abscess. This represents a failure of host defenses to contain the infection and is the most important complication to recognize on examination questions. The transition from mastitis to abscess is signaled by the development of a fluctuant, palpable mass that does not improve with antibiotics alone.

A critical teaching point is that milk itself is not inherently infected in uncomplicated mastitis. The infection is in the surrounding periductal and interlobular connective tissue. This is why continued breastfeeding is both safe and therapeutic: it promotes drainage and reduces stasis, which is the underlying driver of the disease.

2. Classification and Clinical Manifestation

Type	Population	Presentation	Key Feature
Lactational (Puerperal) Mastitis	Breastfeeding women, peak incidence in the first 6 weeks postpartum	Unilateral breast pain, wedge-shaped erythema, fever, flu-like symptoms	Associated with milk stasis and nipple trauma; most commonly tested form
Non-lactational Mastitis (Periductal Mastitis)	Non-breastfeeding women, often smokers aged 30 to 60	Periareolar pain, erythema, possible nipple discharge or retraction	Squamous metaplasia of the lactiferous ducts due to smoking; can form subareolar abscess or fistula
Granulomatous Mastitis	Women of childbearing age, often with history of recent breastfeeding (within 5 years)	Firm, painful breast mass mimicking carcinoma	Idiopathic; diagnosis of exclusion after ruling out malignancy and tuberculosis; biopsy shows non-caseating granulomas
Breast Abscess	Complication of either lactational or non-lactational mastitis	Fluctuant, tender mass; persistent fever despite antibiotics for 48 to 72 hours	Requires drainage (needle aspiration or incision and drainage) in addition to antibiotics

Staging of Lactational Breast Inflammation

Stage	Description	Management Implication
Milk Stasis / Engorgement	Painful, swollen breast without systemic symptoms; no erythema	Conservative: frequent feeding, warm compresses, massage
Non-infectious Mastitis	Localized erythema and pain, no or low-grade fever; symptoms present for less than 24 hours	Conservative measures continued; antibiotics not yet indicated; reassess in 12 to 24 hours
Infectious Mastitis	Wedge-shaped erythema, significant tenderness, fever >38.5 C, systemic symptoms	Antibiotics required in addition to continued breastfeeding/pumping
Breast Abscess	Fluctuant mass, failure to improve after 48 to 72 hours of antibiotics	Drainage procedure required; antibiotics continued

3. Diagnostic Workup

Test	Role	When to Order	Expected Finding
Clinical Diagnosis	Best initial approach	All cases	Unilateral breast erythema, warmth, tenderness, and fever in a breastfeeding woman
Breast Milk Culture and Sensitivity	Guide antibiotic therapy	Refractory cases, MRSA suspected, recurrent mastitis, nosocomial infection, severe or atypical presentation	Identifies organism and resistance pattern; >$10^3$ CFU/mL of a pathogenic organism is considered significant
Breast Ultrasound	Best initial imaging test for suspected abscess	Failure to improve after 48 to 72 hours of appropriate antibiotics, palpable fluctuant mass	Hypoechoic or anechoic fluid collection with internal echoes and surrounding edema; guides needle aspiration
Complete Blood Count (CBC)	Supportive	Systemically ill patients	Leukocytosis with neutrophilic predominance
Blood Cultures	Rule out bacteremia/sepsis	Patients with high fever, rigors, hemodynamic instability	Positive in severe cases; guides IV antibiotic selection

Mastitis is fundamentally a clinical diagnosis. The combination of unilateral breast pain, localized erythema (classically wedge-shaped), warmth, and systemic symptoms (fever, chills, myalgia) in a breastfeeding woman is sufficient to initiate treatment. No laboratory or imaging studies are needed to make the initial diagnosis in a straightforward presentation.

Milk culture is not routinely indicated for a first episode of uncomplicated mastitis. However, it becomes important in several situations: when the patient fails to respond to first-line antibiotics within 48 hours, when there is a concern for MRSA (prior history, community prevalence, healthcare worker), when the episode is recurrent, or when the patient acquired the infection in a hospital setting. The technique involves collecting a mid-stream clean-catch milk sample after discarding the first few drops to reduce skin flora contamination.

Breast ultrasound is the best initial imaging modality when an abscess is suspected. The clinical clue that should prompt imaging is a failure to improve after 48 to 72 hours of appropriate antibiotics or the presence of a fluctuant, palpable mass. On ultrasound, an abscess appears as a well-defined hypoechoic or anechoic collection, often with internal debris and posterior acoustic enhancement. Ultrasound also serves a therapeutic role by guiding percutaneous needle aspiration.

Mammography is not the initial test for suspected infectious mastitis in a young breastfeeding woman. However, in women over 40 or in cases where the inflammation does not resolve completely after treatment, follow-up imaging (mammography or ultrasound) should be considered to exclude inflammatory breast carcinoma, which can mimic mastitis.

4. Management and Treatment

Clinical Scenario	Treatment	Details
Milk Stasis / Engorgement	Conservative measures only	Frequent breastfeeding or pumping (every 2 to 3 hours), warm compresses before feeding, cold compresses after feeding, gentle massage toward the nipple, adequate hydration, analgesics (ibuprofen preferred)
Infectious Mastitis (outpatient, non-MRSA)	Dicloxacillin 500 mg PO QID for 10 to 14 days	First-line; covers methicillin-sensitive S. aureus. Alternative: cephalexin 500 mg PO QID for 10 to 14 days
Infectious Mastitis (penicillin allergy)	Cephalexin 500 mg PO QID (if non-anaphylactic) or clindamycin 300 mg PO QID (if anaphylactic)	Clindamycin also provides MRSA coverage
Infectious Mastitis (MRSA suspected)	TMP-SMX DS (160/800 mg) PO BID for 10 to 14 days	Safe during breastfeeding for infants older than 1 month and without G6PD deficiency. Alternative: clindamycin 300 mg PO QID
Severe / Hospitalized Mastitis	Vancomycin 15 to 20 mg/kg IV every 8 to 12 hours	For systemically ill patients, sepsis, or failure of oral therapy; adjust based on renal function and trough levels
Breast Abscess	Drainage + antibiotics	Ultrasound-guided needle aspiration (preferred for abscesses <3 cm) or incision and drainage (for larger or multiloculated collections); send aspirate for culture; continue antibiotics for 10 to 14 days

Step 1: Do not stop breastfeeding. This is one of the most frequently tested points. Continued breastfeeding or pumping from the affected breast is essential because it promotes drainage of the infected ductal system and prevents worsening stasis. The milk is safe for the infant unless there is purulent drainage from the nipple or the mother is on a contraindicated medication. If the mother cannot tolerate feeding from the affected side, she should pump to maintain flow.

Step 2: Supportive care. Ibuprofen (400 mg every 6 to 8 hours) is preferred over acetaminophen because it provides both analgesia and anti-inflammatory effects. Warm compresses applied before feeding facilitate milk let-down, while cold compresses after feeding reduce swelling.

Step 3: Start empiric antibiotics. The "next best step" after confirming clinical mastitis with systemic symptoms is to initiate an antibiotic that covers Staphylococcus aureus. Dicloxacillin is the classic first-line drug. The full course is 10 to 14 days; shorter courses are associated with higher recurrence rates and should be avoided.

Step 4: Reassess at 48 to 72 hours. If the patient is not improving, consider MRSA (switch to TMP-SMX or clindamycin), obtain a milk culture, and perform breast ultrasound to evaluate for abscess formation.

Step 5: Drain an abscess if present. Antibiotics alone will not resolve a walled-off collection. Ultrasound-guided needle aspiration is the preferred initial approach for small (less than 3 cm), unilocular abscesses. Larger or multiloculated abscesses typically require formal incision and drainage. Breastfeeding can continue even after drainage, provided the incision site is distant from the areola.

Special Considerations

TMP-SMX and breastfeeding: This drug is generally compatible with breastfeeding but should be avoided in mothers of infants younger than 1 month or infants with known G6PD deficiency due to the theoretical risk of kernicterus and hemolytic anemia.

Fluoroquinolones: These are not recommended during breastfeeding due to concerns about cartilage toxicity in the nursing infant.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Breast Engorgement	Bilateral or unilateral breast fullness, tenderness, warmth in a postpartum woman	Bilateral involvement, no fever, no erythema beyond mild fullness; resolves with effective breastfeeding and does not require antibiotics
Inflammatory Breast Carcinoma	Breast erythema, warmth, edema (peau d'orange), and skin thickening mimicking infection	Does not respond to antibiotics; no fever or systemic infectious symptoms initially; peau d'orange skin changes; nipple retraction; diagnosed by skin punch biopsy showing dermal lymphatic invasion by tumor cells; occurs in older or non-lactating women
Breast Abscess	Localized breast pain, erythema, and fever; evolved from mastitis	Fluctuant mass on exam; failure to improve with 48 to 72 hours of antibiotics; requires drainage in addition to antibiotics; not a differential but rather a complication
Plugged Milk Duct	Localized, firm, tender lump in a breastfeeding woman	No fever, no erythema, no systemic symptoms; resolves with massage, warm compresses, and continued feeding; a precursor to mastitis if untreated
Paget Disease of the Nipple	Eczematous, erythematous changes of the nipple and areola	Unilateral nipple scaling, crusting, or erosion that does not respond to topical treatment; often painless initially; biopsy shows Paget cells; associated with underlying ductal carcinoma
Mondor Disease	Breast pain with a visible or palpable cord on the breast surface	Thrombophlebitis of the superficial breast veins; presents as a linear, tender, cord-like structure; no fever, no erythema of the parenchyma; self-limited
Cellulitis of the Breast	Erythema, warmth, and tenderness of the breast skin	Involves the skin and subcutaneous tissue, not the breast parenchyma; not associated with lactation; no wedge-shaped pattern; may follow trauma or insect bite

6. Traps and High-Yield Pearls

The single most common way students miss mastitis questions is by selecting "discontinue breastfeeding" as part of the management. This is almost always a distractor answer. Continued breastfeeding is the standard of care because the milk itself is not dangerous to the infant, and cessation of feeding worsens the milk stasis that drove the infection in the first place. The only scenario where breastfeeding from the affected breast is temporarily halted is when there is purulent drainage directly from the nipple.

A second common trap involves the "mastitis that does not resolve." When a vignette describes persistent erythema and skin changes despite a full course of antibiotics, especially in an older woman who is not breastfeeding, the answer is not a second course of antibiotics. The answer is to investigate for inflammatory breast carcinoma with a skin punch biopsy. This is a classic exam pivot designed to test whether the student recognizes when an infectious diagnosis has been effectively ruled out by treatment failure.

Third, students frequently confuse milk stasis/engorgement with infectious mastitis. The distinguishing feature is systemic symptoms: fever above 38.5 C, chills, and myalgia point toward infection and justify antibiotics. Localized tenderness and fullness without systemic features should be managed conservatively first.

Finally, when the question stem mentions a "fluctuant mass," the expected answer is drainage, not "change antibiotics." Abscess formation is a mechanical problem that requires a mechanical solution. Switching antibiotic classes without draining the collection is a predictable wrong answer.

The core competency being tested across all mastitis questions is the ability to stage the severity of breast inflammation (stasis vs. non-infectious mastitis vs. infectious mastitis vs. abscess) and match each stage to its correct management, while understanding that breastfeeding continues throughout.

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