The Association of American Family Physicians states that family doctors commonly find sternocleidomastoid enlargement (neck masses) in adults. While variations in neck anatomy do occur, these are usually diagnosed during childhood. Variations in neck muscles caused by disease can often be identified and diagnosed by a family practitioner.
For doctors, the patient's age is an important factor for diagnosis. Sternocleidomastoid enlargement in young adults is often caused by inflammatory or infectious disease. In older adults, sternocleidomastoid enlargement can be a symptom of both benign and malignant tumours.
In newborns, sternocleidomastoid enlargement is associated with torticollis, or "twisted neck." In newborns, a mass or enlargement of the sternocleidomastoid muscle may at first grow rapidly. However, these masses often stabilise and regress over time. In many cases, no sternocleidomastoid enlargement is detectable after age two. Enlargements may be due to birthing trauma or fetal position.
Sternocleidomastoid muscles are attached to the jugular lymph nodes. These lymph nodes are located both above and below the SCMs, on both sides of the neck. Lymphatic draining in this area can cause sternocleidomastoid enlargement and a tender neck. Enlarged jugular lymph nodes are commonly caused by pharyngitis and rubella.