What is the initial step in evaluating an 18-month-old baby girl with asymmetrical breast enlargement and otherwise normal examination findings?

Evaluation of Asymmetrical Breast Enlargement in an 18-Month-Old Girl

Pelvic ultrasound (US) is the most appropriate initial imaging study for an 18-month-old girl with asymmetrical breast enlargement and otherwise normal examination to rule out peripheral precocious puberty.

Rationale for Pelvic Ultrasound

Asymmetrical breast enlargement in an infant or toddler raises concerns about precocious puberty, which can be either central (originating from the hypothalamic-pituitary axis) or peripheral (from other sources of estrogen). When evaluating this condition:

• Ultrasound is the preferred initial imaging modality for pediatric patients due to its lack of radiation exposure (RRL rating: B - no radiation) 1
• Pelvic ultrasound specifically can identify potential sources of peripheral precocious puberty such as ovarian tumors (including Leydig-Sertoli cell tumors) that may secrete estrogen 2
• Breast ultrasound alone would only characterize the breast tissue but not identify the underlying cause of asymmetrical development

Clinical Considerations

Differential Diagnosis

1. Peripheral precocious puberty

   • Ovarian tumors (including Leydig-Sertoli cell tumors)
   • Adrenal pathology
   • Exogenous estrogen exposure

2. Central precocious puberty

   • Hypothalamic-pituitary axis activation
   • May require brain MRI if suspected

3. Normal variant/asymmetric thelarche

   • Premature thelarche may be unilateral 3
   • Usually self-limiting

4. Other breast conditions

   • Cyst
   • Fibroadenoma (rare in this age group)
   • Infection/inflammation

Key Assessment Points

• Unilateral breast enlargement is particularly concerning for pathologic causes rather than normal development
• The otherwise normal examination is reassuring but does not rule out peripheral causes of precocious puberty
• Age of onset (18 months) is extremely early for normal thelarche, increasing suspicion for pathology

Imaging Algorithm

1. First step: Pelvic ultrasound

   • Evaluates ovaries for tumors or cysts
   • Assesses uterus for signs of estrogen effect (endometrial thickening)
   • Non-invasive and radiation-free 1, 2

2. If pelvic US is negative:

   • Consider breast ultrasound to characterize the breast tissue
   • Ultrasound is the ideal imaging modality to evaluate breast lesions in children 3, 4

3. If peripheral precocious puberty is still suspected:

   • Consider abdominal CT to evaluate adrenal glands
   • Only if pelvic US is negative and clinical suspicion remains high

4. If central precocious puberty is suspected:

   • Brain MRI would be indicated to evaluate the hypothalamic-pituitary region
   • Not first-line without other neurological symptoms

Important Considerations

• Avoid unnecessary radiation: CT scans should not be the initial imaging study in pediatric patients due to radiation exposure concerns 1
• Avoid breast biopsy: Most breast lesions in children are benign, and surgery/biopsy should be avoided to prevent later deformity 3, 4
• Follow-up: If initial imaging is negative but asymmetry persists, clinical follow-up in 3-6 months is appropriate to assess for changes

Pitfalls to Avoid

1. Assuming benign etiology: While most pediatric breast masses are benign, unilateral breast enlargement at this young age warrants investigation
2. Excessive imaging: Starting with the least invasive, non-radiating modality (ultrasound) is appropriate before considering CT or MRI
3. Delayed diagnosis: Peripheral precocious puberty from ovarian tumors requires timely diagnosis and treatment
4. Unnecessary breast intervention: Avoid direct breast biopsy or intervention as the initial approach in pediatric patients

By following this approach, clinicians can appropriately evaluate asymmetrical breast enlargement in this 18-month-old girl while minimizing radiation exposure and unnecessary procedures.