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.
• Corynebacterium sp. 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 the previous 5 years.
    • New cases have been reported in male-to-female transgender patients in setting of exogenous progesterone and estrogen treatment.
  • 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. Occasionally, secondary monilial infection in the face of recurrent mastitis or diabetes. Seeding from mother to neonate in cyclical fashion may occur.
• 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

• 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.
• 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