Differential Diagnosis: Painful Left Breast Lump in Pregnancy, Aggravated by Breastfeeding, Radiating to Left Arm

Most Likely Diagnoses

1. 🥇 Lactational Mastitis ± Breast Abscess

• Presents as a red, tender, hot, swollen wedge-shaped area corresponding to a lobe
• Caused by milk stasis → bacterial entry through nipple (most commonly S. aureus, E. coli, rarely Streptococcus)
• Pain characteristically worsens during breastfeeding (milk let-down increases pressure)
• Axillary lymphadenopathy can refer pain along the medial arm (T2 dermatome) → explains left arm radiation
• Systemic flu-like symptoms (fever, malaise) typically accompany
• Can progress to abscess: a well-defined, fluctuant, inflamed fluid collection
• Organisms: S. aureus (including MRSA), mixed anaerobes

"Mastitis is an infectious process in the breast producing localized tenderness, redness, and heat, together with systemic symptoms of a flulike illness... It can evolve into formation of a phlegmon and/or an abscess." — Creasy & Resnik's Maternal-Fetal Medicine

2. Plugged (Blocked) Lactiferous Duct

• Tender lumps caused by obstruction/stasis in a collecting duct
• Aggravated by feeding (increased intraductal pressure)
• Usually localized, no systemic symptoms
• May precede mastitis if not relieved

3. Galactocele (Milk Retention Cyst)

• Round, smooth, nontender cyst (though can become tender if infected)
• Located centrally or under nipple
• Associated with lactation; forms from inspissated milk plugging a duct
• Aspiration yields thick, creamy material — diagnostic and therapeutic
• Generally nontender unless secondarily infected; less likely to cause arm pain

"Milk-retention cysts are uncommon and are usually associated with lactation. The swelling is smooth, rounded, and nontender." — Creasy & Resnik's, p. 251

4. Lactating Adenoma

• Benign mass, most common benign tumor specific to pregnancy/lactation
• Rubbery/firm, fairly well-circumscribed
• Usually painless, upper outer quadrant
• Presumed hormonal origin; often resolves post-lactation
• Does NOT typically cause arm radiation; less likely in this scenario

Important Diagnoses Not to Miss

5. ⚠️ Gestational (Pregnancy-Associated) Breast Cancer

• Defined as cancer diagnosed during pregnancy, postpartum year, or during lactation
• Typically presents as a painless lump, but can be painful
• The "milk rejection sign" (infant refuses to nurse from affected breast) may be a clue
• Axillary lymph node metastasis → lymphedema/pain radiating to arm is a red flag
• Physiologic engorgement and nodularity cause diagnostic delays of ≥2 months
• 80% of biopsies in pregnant women are benign, but any lump present for 2–4 weeks must be biopsied
• Imaging: Ultrasound (safe, distinguishes solid vs. cystic in 97%); mammography is safe in pregnancy

"Any discrete lump felt in the breast should be investigated further by a specialist breast team." — Creasy & Resnik's, p. 1357

6. Mondor's Disease (Superficial Thrombophlebitis)

• Underdiagnosed cause of breast pain radiating to arm/chest wall
• Thrombophlebitis of the lateral thoracic, thoracoepigastric, or superior epigastric vein
• Presents as a tender, cord-like structure / palpable cord over breast/chest
• Associated with pregnancy (hypercoagulable state)
• May radiate along the thoracoepigastric vein toward the axilla and arm
• Treatment: NSAIDs + warm compresses (supportive)

"Mondor's disease is a variant of thrombophlebitis that involves the superficial veins of the anterior chest wall and breast... presenting as a tender, cord-like structure." — Schwartz's Principles of Surgery

7. Accessory Breast Tissue

• Common in axilla, bilateral in ~1/3 of cases
• Noticed/enlarges most often during pregnancy
• Axillary accessory breast tissue classically causes a lump in the axilla/upper outer breast that becomes tender during lactation
• Due to its axillary location, discomfort radiates down the medial arm
• Distinguishing feature: bilateral, axillary location, no fever

Less Common but Relevant

Summary Table: Key Differentiating Features

Recommended Workup

1. Breast ultrasound — first-line; safe in pregnancy; distinguishes solid vs. cystic, detects abscess
2. Axillary ultrasound — if lymph nodes palpable (staging, or to confirm accessory breast tissue)
3. Mammography — safe in pregnancy (fetal dose 0.0004 cGy — negligible); use if malignancy suspected
4. Core needle biopsy — mandatory for any lump persisting >2–4 weeks
5. Blood cultures + milk culture — if mastitis suspected with no antibiotic response in 48h

• Creasy & Resnik's Maternal-Fetal Medicine, pp. 249–252, 1357–1358
• Sabiston Textbook of Surgery, pp. 1385–1386
• Berek & Novak's Gynecology, pp. 988–989
• Schwartz's Principles of Surgery, 11th ed.
• Andrews' Diseases of the Skin