What is the cause of a subcutaneous lump on a baby's thigh that is tethered to the skin, causing skin puckering when squeezed, and is non-tender?

Subcutaneous Lipoma - Most Likely Diagnosis

The most likely cause is a subcutaneous lipoma, which is a benign fatty tumor that commonly presents as a non-tender subcutaneous mass in infants and children, and the skin tethering with puckering is a characteristic feature of these lesions. 1

Clinical Reasoning

Key Diagnostic Features Supporting Lipoma

• Subcutaneous location with skin tethering: The description of a mass "tethered to skin" with "skin puckers at site when squeezed" is pathognomonic for a subcutaneous lipoma, as these lesions characteristically attach to overlying skin and cause dimpling when compressed 1

• Non-tender nature: Lipomas are typically painless, benign lesions, which matches this presentation 2

• Location away from immunization sites: This effectively excludes post-vaccination granulomas or nodules, which would be expected at injection sites 1

Important Differential Considerations

While lipoma is most likely, the location on the thigh requires careful evaluation to exclude spinal dysraphism-associated lesions, particularly if this were located in the lumbosacral region rather than the thigh 1, 3:

• Subcutaneous lipomas are classified as HIGH-RISK cutaneous markers when located in the lumbosacral midline, as they are associated with underlying spinal cord malformations in up to 70% of cases 1, 3

• However, thigh location (away from midline spine) significantly reduces concern for occult spinal dysraphism 1

Recommended Evaluation Approach

Initial Assessment

• Examine for additional cutaneous markers: Look specifically for hypertrichosis (focal hair tuft), capillary malformations, dermal sinus tracts, or other skin abnormalities that would elevate concern for spinal dysraphism 1, 3

• Assess exact anatomic location: Confirm the lesion is truly on the lateral/anterior thigh and not in the lumbosacral midline region 1

• Neurological examination: Evaluate lower extremity strength, tone, reflexes, and gait (if age-appropriate) to exclude signs of tethered cord 3

Imaging Recommendations

For a simple thigh lipoma without concerning features, ultrasound is the appropriate initial imaging modality 1, 2:

• Ultrasound can confirm the diagnosis of lipoma and assess depth and size 1, 2

• MRI is NOT routinely indicated for peripheral extremity lipomas unless:

  • The lesion is deep (>5 cm) 2
  • There is concern for liposarcoma (rapid growth, size >5 cm, deep location) 2
  • The lesion is actually in the lumbosacral region with high-risk cutaneous markers 3

If this lesion were lumbosacral/midline, MRI without and with IV contrast would be mandatory to evaluate for occult spinal dysraphism, as this is the imaging modality of choice per ACR guidelines 3

Management Algorithm

For Confirmed Simple Thigh Lipoma:

1. Observation is appropriate for small (<5 cm), asymptomatic lesions 2

2. Surgical excision is indicated if:

   • Size >5 cm 2
   • Compressing adjacent structures 2
   • Cosmetically concerning to family 2
   • Any suspicion for malignancy 2

3. Serial monitoring for growth or development of symptoms 2

Critical Pitfalls to Avoid

• Do not dismiss lumbosacral subcutaneous masses as "just lipomas" - these require MRI evaluation for spinal dysraphism even if asymptomatic 1, 3

• Do not perform needle aspiration or biopsy of cystic thigh lesions without imaging - in endemic areas, hydatid cysts can present as thigh masses and puncture risks anaphylaxis and dissemination 4

• Do not assume all subcutaneous masses are benign - lipoblastoma, fibrosarcoma, and other rare tumors can present similarly in infants, though these typically show rapid growth 5, 6

• Recognize that skin tethering is the key distinguishing feature - this finding strongly suggests lipoma rather than other differential diagnoses like ganglion cysts or epidermal inclusion cysts 1, 2