Description

• Mastitis is an inflammation of the breast parenchyma and possibly associated tissues (areola, nipple, subcutaneous [SC] fat).
• Usually associated with bacterial infection (and milk stasis in the postpartum mother)
• Can be lactational or nonlactational
• Usually an acute condition but can become chronic cystic mastitis

Epidemiology

• Predominantly affects females
• Mostly in the puerperium; epidemic form rare in the age of reduced hospital stays for mothers and newborns
• Neonatal form
• Posttraumatic: ornamental nipple piercing increases risk of transmission of bacteria to deeper breast structures; Staphylococcus aureus is the predominant organism.

Incidence

• 3–20% of breastfeeding mothers develop nonepidemic mastitis, with greatest incidence among breastfeeding mothers 2 to 6 weeks postpartum.
• Neonatal form occurs at 1 to 5 weeks of age, with equal gender risk and unilateral presentation.
• Pediatric form occurs at or around or after puberty, with 82% of cases in girls.

Etiology and Pathophysiology

• Microabscesses along milk ducts and surrounding tissues
• Inflammatory cell infiltration of breast parenchyma and surrounding tissues
• Nonpuerperal (infectious) S. aureus (including methicillin-resistant S. aureus [MRSA]), Bacteroides spp., Peptostreptococcus, Staphylococcus (coagulase negative), Enterococcus faecalis, Histoplasma capsulatum, Salmonella enterica, rare case of Actinomyces europaeus
• Puerperal (infectious) S. aureus (including MRSA), Streptococcus pyogenes (group A or B), Enterobacteriaceae, Corynebacterium spp., Bacteroides spp., Staphylococcus (coagulase negative), Escherichia coli, Salmonella spp. (1)
• Rare secondary site for tuberculosis in endemic areas (1% of mastitis cases in these areas): single breast nodule with mastalgia
• Tuberculosis mastitis in nonendemic areas has also been reported in patients with exposure to TNF-α inhibitors and other immunomodulating compounds (2)[C].
• Corynebacterium spp. associated with greater risk for development of chronic cystic mastitis
• Granulomatous mastitis
  • Idiopathic
    • Predilection for Asian and Hispanic women
    • Association with α₁-antitrypsin deficiency, hyperprolactinemia with galactorrhea, oral contraceptive use, Corynebacterium spp. infection, and breast trauma
    • Most women have a history of lactation in previous 5 years.
    • New cases have been reported in male-to-female transgender patients in setting of exogenous progesterone and estrogen treatment (3)[B].
  • Lupus; autoimmune
• Puerperal
  • Retrograde migration of surface bacteria up milk ducts
  • Bacterial trapping behind plugged milk in the ductal outflow tracts
  • Bacterial migration from nipple fissures to breast lymphatics
  • Secondary monilial infection in the face of recurrent mastitis or diabetes
  • Seeding from mother to neonate in cyclical fashion
• Nonpuerperal—a variety of causes including: ductal ectasia, breast carcinoma, inflammatory cysts, chronic recurring SC or subareolar infections, parasitic infections (Echinococcus, filariasis, guinea worm in endemic areas), herpes simplex, cat-scratch disease, and, in older patients, smoking. Lupus is a rare cause.

Risk Factors

• Breastfeeding
• Milk stasis: inadequate emptying of breast (scarring due to previous breast surgery [breast reduction, biopsy, or partial mastectomy], scarring of breast due to prior mastitis), breast engorgement: interruption of breastfeeding, milk oversupply, plugged ducts
• Nipple trauma increases risk of transmission of bacteria to deeper breast structures: S. aureus predominant organism.
• Neonatal colonization with epidemic Staphylococcus
• Neonatal—occurs more commonly in bottle-fed babies; may be related to manual expression of “witch’s milk” and can lead to lethal necrotizing fasciitis
• Maternal diabetes
• Maternal HIV
• Smoking

General Prevention

Regular emptying of both breasts and nipple care to prevent fissures when breastfeeding; also good hygiene including hand washing and washing breast pumps after each use

Commonly Associated Conditions

Breast abscess