Fibromatosis colli (FC), also known as 'sternocleidomastoid tumour of infancy', is a benign spindle cell lesion of the sternocleidomastoid muscle.

It is often associated with torticollis due to contraction of the sternocleidomastoid muscle.

Although the pathogenesis of this lesion is not clear, it is probably related to birth trauma (breech or forceps delivery). Injury leads to secondary pressure necrosis and fibrosis within the muscle. In the early stage the mass may grow rapidly. Later the growth slows and ultimately ceases.  After a period of 1 to 2 years it  begins to regress . About  2/3rd of the cases resolve spontaneously.

Age:  Most cases show no abnormality at birth but manifest between the 2nd and 4th week of life.  Fibromatosis colli affects male patients slightly more often than female patients.

Site:  The lesion presents as a firm soft-tissue mass in the lower one-third of the sternocleidomastoid muscle.

The disease is usually unilateral  (slightly more common on the right side) and affects both the sternal and clavicular heads of the muscle. Bilateral involvement is rare.  

Gross:  The mass may be excised at an early stage. Macroscopically it is a small firm mass (2-3 cm in diameter), with a greyish white, glistening cut surface . The lesion blends with the surrounding smooth muscle.

Microscopic features: IMAGE1 ;    IMAGE2

1. There is replacement of muscle fibres by hyalinized fibrous tissue.
2. Throughout the lesion there are  scattered residual muscle fibres which shows prominent degenerative and reactive changes (atrophy, swelling, loss of cross striations). There is little cellular pleomorphism or mitotic activty.

Differential diagnosis: Myositis ossificans progressiva ; Desmoid type fibromatosis: (Unlike fibromatosis colli damaged muscle fibres are not present in fibromatosis.)