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Neonatal hypotonia, also known as floppy infant syndrome,
is the main presenting clinical feature of most neuromuscular diseases
of early life.1 However, disorders of the central nervous
system may also manifest with hypotonia.
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Two types of muscle tone can be assessed clinically: postural
and phasic. Postural (ie, antigravity) tone is
a sustained, low-intensity muscle contraction in response to gravity
and is mediated by both the gamma and alpha motor neuron systems
in the spinal cord. It is assessed clinically by passive manipulation
of the limbs. Phasic tone is a brief contraction
in response to a high-intensity stretch and is mediated by the alpha
motor neuron system only. It is examined clinically by eliciting
the muscle stretch reflexes. Hypotonia is defined
as reduction in postural tone, with or without a change in phasic
tone. When postural tone is depressed, the trunk and limbs cannot
counteract gravity, and the child appears hypotonic or floppy.
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Sainte-Anne Dargassies2 described an approximate
caudal-rostral progression in the development of muscle tone. At
postconceptional age of 28 weeks, there is minimal resistance to
passive manipulation in all limbs; by 32 weeks, flexor tone can
be appreciated in the lower extremities; and by 36 weeks, flexor
tone is also present in the upper limbs. By term, strong flexor
tone can be demonstrated in all four limbs by passive manipulation.
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Volpe1 describes the physical examination of a hypotonic
infant in detail. Following a careful general physical examination,
the neurologic assessment should include motor examination, evaluation
of primary neonatal reflexes, and sensory examination (Table
569-1). A general physical examination may reveal organomegaly,
cardiomyopathy, contractures, abnormalities of the genitalia, respiratory
rate or pattern irregularities, or evidence of traumatic injury
(eg, bruising, petechiae); the general examination may, however,
be normal. The motor examination includes assessment of posture,
muscle tone, motility and muscle power, muscle stretch reflexes,
and primary neonatal reflexes. Abnormality of the primary neonatal
reflexes lies in their persistence. In normal infants, the Moro
reflex disappears by 6 months of age,3,4 the palmar
grasp becomes less obvious after 2 months of age, and the tonic
neck response becomes less facile at 6 to 7 months of age.3-5 Sensation can
be tested by withdrawal from a stimulus (eg, touching the infant with
a small brush). Abnormalities in sensation may be present in some
forms of congenital neuropathies, such as hereditary motor-sensory
or sensory-autonomic neuropathies, but they are usually difficult
to assess in infants.
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When assessing muscle tone in young infants, the infant’s
head should be placed in the midline to eliminate the ...