What Is a Fibromatosis Colli?

Fibromatosis colli, also known as a sternomastoid tumor of infancy, is an overgrowth of cells that typically occurs in newborn babies. It represents the unregulated growth of fibroblasts, a type of cell important in maintaining the structural integrity of the body. Symptoms of the tumor include the appearance of a mass in the lateral neck region, and an inability to fully rotate the head. Diagnosis of fibromatosis colli relies on clinical history, radiographic imaging, and microscopic characteristics of a sample of the mass. Although typically no treatment is required for these tumors because they regress spontaneously, in some cases they do need to be surgically removed.

The growth, or tumor, known as fibromatosis colli is typically a harmless overgrowth of a type of cell called fibroblasts. Usually these cells are important for making collagen and other substances that support the structure of the body. In fibromatosis colli, these cells reproduce in an unregulated fashion, causing a mass to appear.

One of the most important symptoms of fibromatosis colli is the development of a mass in the neck region. This type of tumor typically appears in the lower part of a muscle called the sternocleidomastoid muscle, which helps to power the neck to rotate the head from side to side. As a result of the growth, some affected infants have a decreased ability to move their head, and might have their head permanently rotated in one direction or the other, a condition known as torticollis.

Diagnosis of fibromatosis colli is made based on the history of the development of the growth, the appearance of the tumor, and the characteristics of the tumor under the microscope. A classical history a parent might give of the tumor’s development is the appearance of a lump in the second or third week of life that grows rapidly over the next few months. Typically a dermatologist or other physician would take a sample of the growth in order to examine its characteristics under the microscope and to confirm that it doesn’t have any concerning features that might suggest it could develop into a cancer. Sometimes a radiologic technique known as ultrasonography, which uses sound waves to discern the characteristics of underlying structures, can help make the diagnosis of this tumor.

Treatment of fibromatosis colli typically focuses on watchful waiting. Most of these growths resolve on their own with time, and do not have to be surgically removed. In some cases, however, the microscopic characteristics are concerning, suggesting that the cells might have the ability grow and spread to other regions of the body. If a tumor with concerning features is found, it is typically surgically removed to prevent it from causing harm to the infant.