Maturational Assessment of Gestational Age - Neuromuscular Assessment

Neuromuscular Maturation and Neonatal Muscle Tone

The complexity of fetal brain development cannot be overstated. Even when one considers its unencumbered development, a myriad of untoward events could occur by chance alone. It is understandable then that any deviation from the normal intrauterine environment could influence the delicate balance between growth and differentiation of neural tissue and hence the rate of functional maturation of the developing fetal brain. When performing tests of neuromuscular maturation, we are making an indirect assessment of brain maturity, which in turn gives us an indirect measure of gestational age.

Neonatal Muscle Tone

Muscle tone may be defined as "the slight constant tension of healthy muscles which contribute a slight resistance to passive displacement of a limb." (ref 7)

The newborn's neuromuscular examination includes an assessment of both active and passive muscle tone. If all newborns were normal and healthy, both active and passive tone could be used routinely to assess neuromuscular maturation. Active muscle tone, (motility, activity, or efforts at righting oneself) is markedly affected by states of illness, recent maternal medications, acute perinatal compromise and level of alertness. Hence active muscle tone is not consistently useful in evaluating baseline neuromuscular maturity. Passive tone essentially is unscathed by those same factors that profoundly affect active tone. Hence passive tone is useful for evaluating maturational development of the neonatal brain, regardless of the infant's state of alertness or level of wellness.

Passive tone may be further subdivided into extensor and flexor tone. The human fetus, lying primarily with limbs extended in the very early phases of development, gradually assumes a progressively flexed attitude. This is true whether development occurs in utero or in the nursery, (ref 8) and thus reflects maturation of the central nervous system rather than extraneous compressive forces of the uterus. Passive flexor tone gradually overcomes passive extensor tone as maturation progresses.

Progression of neuromuscular tone development proceeds in a caudo-cephalad and centripetal direction; i.e., lower extremity passive flexor tone develops slightly ahead of upper extremity tone, and distal passive flexion precedes proximal passive flexion. (ref 8)

There are three possible methods of assessing passive flexor tone in the neonate. The first is extensor stretch or passive flexion, which may better be described as flexibility, and is used to evaluate the degree to which a limb can be flexed passively at the joint by the examiner. This maneuver requires no tone or extensor resistance on the part of the infant. We may be looking at mobility, flexibility or resistance to extensor stretch rather than at passive flexor tone.

The second method of assessing passive flexor tone is resistance to passive extension. These maneuvers require;

a) that the untested portion of the extremity be resting quietly on a supporting surface;

b) that the examiner be very sensitive to the infant's slight tendency to resist extension; and

c) that the examiner avoid placing pressure on flexors being tested, thereby interfering with their function.

The third method of testing passive flexor tone is measurement of angles of recoil to a previously flexed position. This maneuver requires that the examiner;

a) pre-set the extremity to a flexed position; and

b) avoid fatiguing the flexors by maintaining the extremity in the extended position for too long a period of time prior to releasing.

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References :
Ballard JL, Khoury JC, Wedig K, et al: New Ballard Score, expanded to include extremely premature
infants. J Pediatrics 1991; 119:417-423.