sidewise curvature of the spine), or other structural disfigurements that result from compensatory mechanisms. Presentation
FMC tumors most commonly present as slow-growing, firm, mobile, nontender, spindle-shaped masses in the lower two-thirds of the sternocleidomastoid muscle of infants within 8 weeks (average: 24 days) of delivery. At diagnosis, these infants heads may tilt toward the side with the mass while their chins tilt to the opposite, uninvolved side. This tilting is due to sternocleidomastoid muscle contracture. FMC tumors are more common in males and the right sternocleidomastoid muscle  although very rare cases present with bilateral tumors. Untreated, the masses may continue to grow for weeks after birth but then stabilize, start regressing after 4–8 months of life, and over the ensuing 1–2 years typically fully resolve. In three studies, >25%, >50% and >80% of these infants have had difficult deliveries such as a breech birth, delivery requiring forceps, primigravida birth (i.e. mothers first child), prolonged, difficult labor, or delivery by Caesarean section. From 6-20% of newborns with fibromatosis colli also present with other congenital lesions such as hip dysplasia or, less commonly, facial asymmetry. Pathology
Microscopic histopathological analyses of biopsied FMC tumor tissues typically find benign-appearing, spindle-shaped fibroblasts, decomposing skeletal muscle fibers, and, in some cases, regenerating skeletal muscle fibers in a collagen fiber-containing background. If necessary, these tumors are typically diagnosed by microscopic examination of fine-needle aspiration samples rather than the more invasive approach of tumor biopsy sampling. The asperates show scant to moderately cellular, scattered, oval-shaped to spindle-shaped fibroblasts, naked nuclei (i.e.
Fibromatosis colli (FMC), also termed sternocleidomastoid tumor of infancy, pseudotumor of infancy, and infancy sternocleidomastoid pseudotumor, is an uncommon (incidence: 0.4%-1.3% of live births), congenital tumor in one of the two sternocleidomastoid neck muscles although rare cases have presented with a FMC tumor in both sternocleidomastoid muscles. A tumor is here defined as a growth of tissue that is not coordinated with the normal surrounding tissue and persists in growing even if the original trigger for its growth is removed. FMC tumors are benign growths that may cause disfigurements but are not cancers and do not metastasize (i.e. spread) to distant tissues.As judged by microscopic cytology analyses, fibromatosis colli tumors consist of spindle-shaped fibroblasts (i.e. the most common cell type in connective tissue) located in a background of collagen fibers, decomposing skeletal muscle fibers, and, in some cases, regenerating skeletal muscle fibers. The fibroblasts have a completely normal appearance with no evidence suggesting that they are malignant. The World Health Organization, 2020, classified fibromatosis colli as in the category of benign fibroblastic and myofibroblastic tumors.In the majority of cases, FMC tumors decrease in size and completely resolve by the newborns second year. If left untreated, however, a significant percentage of cases progress to, and are the most frequent cause of, congenital muscular torticollis, i.e. an abnormal, asymmetrical head or neck position commonly called wry neck. Untreated FMC tumors may also progress to facial asymmetry, plagiocephaly (i.e. flattened head), permanent loss of neck mobility, scoliosis (i.e.