Positional plagiocephaly is associated with sternocleidomastoid muscle activation in healthy term infants.

PURPOSE: To explore the relationship between sternocleidomastoid activation and positional plagiocephaly in healthy full term infants.

METHODS: Participants were 82 infants from a regionally based-longitudinal study of infant development. Sternocleidomastoid (SCM) activation was assessed using active head-righting responses of body-on-head with and against gravity and head-on-body against gravity at 3, 6 and 9 weeks. Plagiocephaly was assessed using the Modified Cranial Vault Asymmetry Index (mCVAI) at 9 weeks.

RESULTS: More severe plagiocephaly was associated with more severe asymmetry in active head-righting responses at all ages (p < 0.001). Greater right-sided occipital flatness was related to stronger contralateral/left SCM activation at 3 and at 9 weeks (p = 0.008). Greater left-sided occipital flatness was related to stronger contralateral/right SCM activation at 3 weeks (p = 0.004). In infants with any right-sided occipital flatness, the mCVAI was greater in infants with asymmetrical gravity assisted body-on-head responses at 3 weeks (mCVAI = 4.31 (2.01)%, 95% CI 2.87-5.75) compared to those with symmetrical responses (mCVAI = 2.64 (1.66)%, 95% CI 2.06-3.22) (p = 0.011).

CONCLUSIONS: Sternocleidomastoid activation asymmetry is a significant contributor to plagiocephaly development by 9 weeks of age due to stronger contralateral SCM activation. Active head-righting responses are appropriate to assess sternocleidomastoid activation in infants under 2 months of age.