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Answer: Fibromatosis Colli.

Fibromatosis Colli

• Definition:
  
  • Non-neoplastic SCM muscle enlargement in early infancy.
• Imaging:
  • Best diagnostic clue: Non-tender sternocleidomastoid muscle enlargement without discrete mass in an infant
  • Mid to lower third of sternocleidomastoid muscle.
  • Right > left; rarely bilateral.
  • Oval or fusiform unilateral sternocleidomastoid muscle enlargement.
  • No discrete mass or adenopathy.
  • Best imaging tool: Ultrasound confirms clinical suspicion.
• Top Differential Diagnoses:
  • Lymphangioma.
  • Muscular Torticollis.
  • Myositis Related to Neck Infection.
  • Rhabdomyosarcoma.
  • Spinal Accessory Nodal Malignancy.
• Pathology:
  • Fibrocollagenous infiltration of SCM.
  • When necessary, diagnosis can be confirmed by thin needle aspiration cytology.
  • Possibly precipitated by in utero head position and SCM compartment syndrome.

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