Etiology

The etiology of CMT is unknown, but several theories have been proposed.

Some authors have found increased incidence in breech deliveries (13% to 17%) and cesarean section (16% to 22%) to support the theory of intrauterine malposition or crowding.[8][10][16] There is an increased incidence of plagiocephaly in twin births, and it is thought that the lower in utero twin A is, the more twin A is at risk is for development of plagiocephaly due to the more restrictive intrauterine environment.[17][18]

Complicated deliveries, such as those requiring use of forceps or vacuum, have been associated with CMT in 22% to 29% of cases.[8][10][16] Birth trauma theory proposes that a congenitally shortened sternocleidomastoid (SCM) muscle is torn at birth, with the formation of a hematoma and subsequent development of fibrous contracture.[19] However, hemorrhagic inflammatory reaction and myofiber disruption are not often found histologically in the SCM mass.[20]

The ischemic hypothesis suggests that venous occlusion causes ischemic changes in the SCM, resulting in a compartment-type syndrome.[19]

Plagiocephaly can develop at birth resulting from in utero or intrapartum molding, or postnatally, resulting from lack of varied supine positioning. Although these deformities usually resolve spontaneously, they can be perpetuated in the supine position as gravity will force the head to turn to the side of the flattened occiput. Associated torticollis can then result from this persistent unidirectional positioning.

Pathophysiology

CMT consists of a unilateral fibrous contracture of the SCM. A mass can be felt in the involved SCM muscle in up to 55% of cases, and the size of the mass ranges from 8 mm to 3 cm.[2][19] Pathophysiology is not clear; however, in the evaluation of pathologic changes in the SCM muscle of patients with suspected CMT, ultrasound evidence showed that, in the group studied, 15% had a fibrotic mass in the SCM, 77% had diffuse fibrosis mixing with normal muscle, 5% had normal muscle, and 3% had a fibrotic cord.[20] The SCM mass is benign and self-limited, with spontaneous recession and disappearance within weeks to months. A study on surgical specimens of SCM masses using light and electron microscopy showed that myoblasts in various stages of differentiation and degeneration are found in the interstitium of the mass.[21] It is thought that passive stretching of the involved muscle provides an adaptable stimulation and favors the normal myogenesis of the mass.[21]

Classification

Classification of CMT

Several studies divide patients with CMT into clinical groups, but these groups are not uniformly used in the literature nor formally recognized.[1][2] These may include:

  • SCM tumor group: patients with a palpable tumor (mass) in the SCM

  • Muscular torticollis group: patients with tightness of the SCM but no clinical tumor (mass)

  • Postural torticollis group: patients with postural head tilt but without tumor (mass) or tightness in the SCM.

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