The influence of gestational age and birth weight in the clinical assessment of the muscle tone of healthy term and preterm newborns.

OBJECTIVE To evaluate the influence of gestational age (GA) and birth weight (BW) in the clinical assessment of the muscle tone of healthy term and preterm newborns. METHOD Cross sectional study. The muscle tone of healthy 42 preterm and 47 term newborns was quantified and measured with a goniometer (an instrument for measuring angles) respectively between 7th-14th day of life and 24-48 hours of life. Newborns were grouped according to GA and BW and evaluated at fixed time intervals by one examiner. Preterm newborns were matched to term at 40 weeks postconceptional age (PCA). RESULTS The evolution of muscle tone in the preterm occurred gradually, following PCA, independent of birth weight. Preterm newborns had lower scores in all muscle tone indicators when compared to term at the first assessment. Differences were observed among preterm small for GA and adequate to GA for the indicator heel to ear (p<0.001). When compared at 40 weeks PCA, except for posture, all other indicators were significantly different (p< 0.001) among groups. CONCLUSION Prematurity and intrauterine malnutrition are influential factors in some indicators of the newborn muscle tone state. Muscle tone assessment of preterm infants does not seem to be influenced by birth weight, however evolution is clearly related to postconceptional age. When we compared term newborns AGA and SGA it seems to have a clear influence of the birth weight on some indicators of the muscle tone. The use of devices such as the goniometer allows the performance of a more objective assessment of muscle tone and helps to quantify findings.