hypotonia refers to a pathologically decreased postural tone that involves
the four extremities, the trunk, and the neck. Facial involvement is not
a requisite for the diagnosis of generalized hypotonia.
In this book, postural tone
to: (1) the resistance that the striated muscles and the tendons offer
when stretched by a sustained low-intensity force either by an examiner
or by gravity; and (2) the recoil capability of the striated muscles and
the tendons once they are fully extended by a sustained force for a few
Postural tone is not the same as dynamic tone. Dynamic tone refers to
the resistance that the striated muscles and the tendons offer when stretched
by a brief high-intensity force by an examiner's maneuvers.
tone results from the spring-like properties of the striated muscle fibers
and the tendons. The spring-like properties of the striated muscle fibers
are a function of the intrinsic resistance of its contractile elements
(actin and myosin), intrinsic resistance of the collagen fibers in the
endo-, peri-, and epimysium, and the effects of the nervous system upon
the contractile elements.
The spring-like properties of the tendon are a function of the intrinsic
resistance of the collagen fibers.
Postural tone is determined
by observing the resistance muscles offer to the force of gravity, by
testing the resistance of the limbs to limb extension by slow motion maneuver
performed by the examiner, and by testing the passive recoil of the limbs
after such maneuvers.
Clinically, the resistance muscles offer to the force of gravity is evaluated
by observing the angle of the upper and lower limbs while the neonate
is resting during quiet wakefulness, the angle of the lower limb and shoulder
muscle resistance during vertical suspension, and the angle of the limbs,
and trunk and neck curvature during vertical suspension.Facial
tone is judged by facial expression.
low postural tone
hypotonia or pathologically low postural tone is a subjective diagnosis
given by an examiner based on experience. The diagnosis of generalized
hypotonia in neonates should take into account: (1) gestational age, and
(2) behavioral state. The diagnosis
of generalized hypotonia should be reserved for neonates whose postural
muscle tone is less than what is expected for their gestational age.
Normal premature neonates should not be
diagnosed with generalized hypotonia because their postural tone is not
that of full term-neonates.
The assessment of postural
tone during sleep is unreliable because muscle postural tone is physiologically
decreased during sleep and especially during active sleep.