In a one‑month‑old infant with a unilateral firm 1‑inch (2.5 cm) mass near the clavicle, what are the likely diagnoses and recommended evaluation?

Unilateral Firm Mass Near Clavicle in 1-Month-Old Infant

The most likely diagnosis is sternocleidomastoid pseudotumor (fibromatosis colli), and ultrasound of the neck is the appropriate initial imaging study to confirm the diagnosis and differentiate it from other congenital or vascular lesions. 1

Most Likely Diagnoses by Location and Age

Primary Differential: Sternocleidomastoid Pseudotumor (Fibromatosis Colli)

• This is the most common cause of a firm unilateral neck mass near the clavicle in a 1-month-old infant, typically presenting as a firm, non-tender mass within the sternocleidomastoid muscle 2
• The mass is usually discovered between 2-4 weeks of age and may be associated with torticollis 2
• Natural history: spontaneous resolution occurs in 70-90% of cases by 4-8 months of age without intervention 2

Alternative Diagnoses to Consider

Infantile Hemangioma:

• Would typically present as a bright red, raised "strawberry" lesion that appears within the first weeks of life, not as a firm, flesh-colored mass 3, 2
• Most rapid growth occurs between 1-3 months of age, with growth usually completed by 5 months 3, 4
• Location near clavicle is not a high-risk location (unlike facial, periorbital, airway, or lumbosacral sites) 3, 2

Infantile Myofibromatosis:

• Rare mesenchymal tumor presenting as firm, nodular mass in soft tissue or bone 5, 6, 7
• Can be solitary or multicentric; approximately one-third involve head and neck 6
• Spontaneous regression may occur, but visceral involvement carries unfavorable prognosis 7

Congenital Abnormalities:

• Thyroglossal duct cyst, branchial cleft cyst, or lymphatic malformation should be considered 1
• These are typically cystic rather than firm on palpation 1

Recommended Initial Evaluation

First-Line Imaging: Ultrasound with Doppler

Ultrasound is the appropriate initial imaging modality for this presentation because:

• It differentiates solid from cystic neck lesions 1
• Color-flow Doppler distinguishes high-flow from low-flow vascular malformations 1
• No sedation required and no radiation exposure 2
• Highly sensitive for characterizing vascular flow in solid lesions 1

When to Escalate Imaging

CT or MRI with contrast is reserved for:

• Suspected malignancy or deep neck infection requiring surgery 1
• Atypical features or uncertain diagnosis after ultrasound 2
• Not indicated as routine initial imaging for this presentation 1

Clinical Assessment Priorities

Key Physical Examination Features

• Assess for torticollis (head tilt toward affected side with chin rotation away), which strongly suggests sternocleidomastoid pseudotumor 2
• Evaluate overlying skin: erythema suggests infection; bright red color suggests hemangioma 1, 3
• Palpate for pulsatility (would indicate vascular malformation) 1
• Document exact location, size, consistency, mobility, and tenderness 1

Red Flags Requiring Urgent Specialist Referral

• Rapid growth over days to weeks 3, 2
• Respiratory symptoms or stridor (airway involvement) 3, 2
• Multiple cutaneous lesions (≥5 hemangiomas mandate hepatic screening) 3, 2
• Segmental distribution suggesting PHACES syndrome 3

Management Algorithm

If Sternocleidomastoid Pseudotumor Confirmed by Ultrasound:

• Observation with serial physical examinations every 4-6 weeks 2
• Physical therapy for associated torticollis if present 2
• Reassure parents that spontaneous resolution occurs in 70-90% of cases by 4-8 months 2
• Surgical release of sternocleidomastoid muscle reserved only for persistent torticollis after 12 months of age 2

If Infantile Hemangioma Confirmed:

• Observation alone is appropriate for this non-problematic location (near clavicle, not involving face, airway, or periorbital region) 3, 4
• Serial monitoring during proliferative phase (first 5 months) to detect complications 3, 4
• Oral propranolol 2 mg/kg/day in 3 divided doses is reserved for high-risk locations or complications (ulceration, functional impairment, rapid growth threatening disfigurement) 3, 2, 4

If Infantile Myofibromatosis Suspected:

• True-cut biopsy required for definitive diagnosis 5, 6
• Imaging to assess for multicentric disease (skeletal survey, abdominal ultrasound) 6, 7
• Watchful waiting is appropriate given potential for spontaneous regression, unless visceral involvement present 7

Common Pitfalls to Avoid

• Do not assume all neck masses in infants are lymph nodes—congenital etiologies are more common in this age group 1
• Do not order CT as initial imaging—ultrasound is first-line and avoids radiation exposure 1, 2
• Do not biopsy sternocleidomastoid pseudotumor—diagnosis is clinical and confirmed by ultrasound; biopsy risks muscle damage 2
• Do not surgically excise infantile hemangiomas during proliferative phase—medical management with propranolol is first-line when treatment indicated 3, 2
• Do not delay specialist referral if the mass is rapidly growing, involves high-risk locations, or has atypical features 3, 2