Mastitis

01Pathophysiology

Mastitis is an infection of the breast parenchyma that occurs almost exclusively in lactating women during the first six weeks postpartum. The process begins with milk stasis, which happens when a duct becomes obstructed or the breast is not adequately emptied. Stagnant milk serves as a nutrient-rich culture medium for bacteria. Organisms, most commonly Staphylococcus aureus colonizing the infant's oropharynx, gain entry through microscopic cracks or fissures in the nipple. Once inside, they ascend retrograde through the lactiferous ducts into the surrounding breast tissue.

The bacterial invasion triggers a localized inflammatory response in the periductal and interlobular connective tissue. This explains the classic wedge-shaped distribution of erythema, which corresponds to the drainage territory of an individual ductal segment. Cytokine release and neutrophil recruitment produce the hallmark signs of inflammation: erythema, warmth, swelling, and tenderness. Because the infection is parenchymal (not superficial), patients develop systemic signs including high fever, rigors, and myalgia, which distinguishes mastitis from a simple blocked duct.

If the infection is left untreated or inadequately treated, neutrophilic infiltration leads to tissue necrosis and liquefaction, forming a breast abscess. This is a walled-off collection of pus within the breast tissue, and its presence changes management entirely from antibiotics alone to drainage.

It is critical to understand the distinction between milk stasis (plugged duct) and true mastitis. A plugged duct causes focal tenderness and a palpable lump but without fever or systemic symptoms. Mastitis adds infection on top of stasis: the moment you see fever and constitutional symptoms in a lactating woman with a painful, erythematous breast, the diagnosis shifts from mechanical obstruction to infection.

02Classification and Clinical Manifestation

03Diagnostic Workup

Mastitis is a clinical diagnosis. A breastfeeding woman presenting with unilateral breast erythema, tenderness, and fever does not need imaging or cultures to initiate treatment. The best initial step is to start empiric antibiotics immediately.

Breast milk culture becomes important in two scenarios: first, when the patient fails to improve after 48 hours of first-line antibiotics, suggesting a resistant organism such as MRSA; second, in nosocomial infections where the bacterial flora may differ from community-acquired pathogens.

Ultrasound is the imaging modality of choice when you suspect progression to abscess. The key finding is a hypoechoic or anechoic fluid collection with posterior acoustic enhancement. Ultrasound also differentiates abscess from phlegmon (non-drainable inflammatory tissue), which matters because phlegmon is managed with antibiotics alone while abscess requires procedural drainage.

A critical point for exam purposes: if a non-lactating woman presents with signs of mastitis, you must have a much lower threshold for biopsy. Inflammatory breast cancer is the diagnosis you cannot afford to miss, and it mimics mastitis closely. The absence of fever and the failure to improve with antibiotics should prompt immediate tissue sampling.

04Management and Treatment

The single most important management principle on the exam is: do not stop breastfeeding. Continued breastfeeding (or pumping if the infant cannot latch) is therapeutic because it prevents further milk stasis and does not transmit infection to the infant. The only exception is when the abscess has been drained and the incision is close to the areola, in which case pumping on the affected side with discarding of milk is preferred until the wound heals.

First-line empiric therapy targets S. aureus with an anti-staphylococcal penicillin (dicloxacillin) or a first-generation cephalosporin (cephalexin). Both are safe during breastfeeding. The choice between them is largely interchangeable, though dicloxacillin is traditionally listed as first-line in most guidelines.

If MRSA is suspected (risk factors include prior MRSA infection, healthcare workers, known community MRSA prevalence, or failure of first-line therapy), switch to TMP-SMX or clindamycin. Note the contraindication for TMP-SMX: it should be avoided if the infant is premature, under 30 days old, jaundiced, or glucose-6-phosphate dehydrogenase (G6PD) deficient, because sulfonamides displace bilirubin from albumin and may worsen neonatal hyperbilirubinemia. In these cases, clindamycin is the safer alternative.

Abscess management represents a shift from medical to procedural therapy. The next best step when you identify a fluctuant mass or ultrasound-confirmed collection is drainage, not stronger antibiotics. Ultrasound-guided needle aspiration has become the preferred initial approach for abscesses smaller than 3 cm because it is less invasive and yields comparable cure rates. Larger, multiloculated, or recurrent abscesses may require formal incision and drainage.

Supportive measures include NSAIDs (ibuprofen 400 mg every 6 hours) for analgesia and anti-inflammatory effect, adequate oral hydration, and rest.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on mastitis questions is by selecting "stop breastfeeding" as part of the management. This is a classic distractor. Continued breastfeeding is safe and therapeutic; stopping it worsens milk stasis and increases the risk of abscess formation. The only reason to temporarily halt direct breastfeeding on the affected side is if there is an open drainage wound near the areola.

A second frequent trap involves the non-lactating woman with a "mastitis-like" presentation. If the vignette describes erythema, skin thickening, and breast enlargement in a non-breastfeeding patient, especially one without fever, the answer is not mastitis. The test is checking whether you can recognize inflammatory breast cancer, and the next step is biopsy, not antibiotics.

Third, students often confuse mastitis with breast abscess and select antibiotics alone when drainage is required. The key trigger is a fluctuant mass or failure to improve after 48 to 72 hours of antibiotics. When either of these is present, the next best step is always drainage, not escalation of antibiotics.

Finally, know the TMP-SMX contraindication in neonates. If the vignette mentions a newborn under 30 days or a jaundiced infant, TMP-SMX is wrong even if MRSA is suspected. The correct alternative is clindamycin. This tests your ability to factor in the breastfeeding infant's safety when choosing maternal antibiotics.

The core competency being tested across mastitis questions is clinical staging (plugged duct vs. mastitis vs. abscess), appropriate empiric antibiotic selection with attention to the nursing infant, and the ability to recognize inflammatory breast cancer as the dangerous mimic that demands tissue diagnosis rather than empiric treatment.