[1] Untreated FMC tumors may also progress to facial asymmetry, plagiocephaly (i.e. flattened head), permanent loss of neck mobility, scoliosis (i.e. sidewise curvature of the spine), or other structural disfigurements that result from compensatory mechanisms.
[6] FMC tumors most commonly present as slow-growing, firm, mobile, nontender, spindle-shaped masses[1] in the lower two-thirds of the sternocleidomastoid muscle[7] of infants within 8 weeks (average: 24 days) of delivery.
[3] Untreated, the masses may continue to grow for weeks after birth[7] but then stabilize, start regressing after 4–8 months of life,[4] and over the ensuing 1–2 years typically fully resolve.
However, there are no features of hematomas (e.g. hemosiderin released by dying red blood cells) in theses lesions at diagnosis and no evidence of trauma (e.g. overlying skin changes or discolorations) at birth in infants who later present with FMC tumors.
[4] It has also been suggested that venous outflow obstruction occurring in the fetus while in the uterus or during delivery leads to degeneration of sternocleidomastoid muscle fibers and subsequent fibrosis of the damaged areas.
[7] This method commonly reveals diagnostic (up to 100% sensitivity rates[3]) findings of diffuse or focal enlargement of sternocleidomastoid muscle that has no cysts or other ultrasonography-detected abnormalities.
The recommended treatment for infants who show no improvements in mobility after one year of physical therapy or who initially present at >12 months of age is surgical tenotomy (i.e. cutting) of one of the tendons on the involved sternocleidomastoid muscle.