Sternocleidomastoid tumor is characterized by a hard, immobile, fusiform swelling in the sternocleidomastoid muscle that is usually not present at birth but develops between 7 to 14 days of life. The mass increases in size for 2 to 4 weeks, remains stable for about 1 month (Figure 267.2[A]), and then decreases in size until it disappears completely by 5 to 8 months. Magnetic resonance imaging shows a significant mass (Figure 267.2[B]) that may enhance with contrast. The possibility of a malignant tumor should be considered but this possibility is very rare. Torticollis may be noted while the mass is present and often when the mass has disappeared. Craniofacial asymmetry may occur. Hip dislocation occurs in patients with more than 30 degrees of limitation in passive rotation of the neck. Treatment consists of passive exercise to stretch the sternocleidomastoid muscle and placing the infant in a position that encourages the face to move towards the side opposite to the facial deviation. Surgery is indicated in a small number of patients between 6 months and 2 years depending on the severity of the residual torticollis.

Figure 267.2.— [A] Sternocleidomastoid tumor of infancy. [B] MRI of the neck demonstrates a large mass in the left sternocleidomastoid muscle.