Sternocleidomastoid pseudotumor of infants and congenital muscular torticollis: fine-structure research.

Fifty cases, 16 with sternocleidomastoid pseudotumor of infants (SCMPOI) and 34 with congenital muscular torticollis (CMT), were investigated by light and electron microscopy. The ultrastructure of the pseudotumor revealed that there were myoblasts, fibroblasts, myofibroblasts, and mesenchyme-like cells, which consisted of fibrous tissue and were regarded as fibroblasts only in the literature. The myoblasts showed the various stages of differentiation and degeneration. For those cases without mass, the collagen fibrils and fibrocytes were often arranged in tight parallel bundles in which some muscles showed normal structure and some with decreased myofibrillae. The mesenchyme-like cells and myoblasts found in the pseudotumor may be the key point to its pathologic characteristics.