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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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Table 569-1. Hypotonia: Physical Examination 
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When assessing muscle tone in young infants, the infant’s head should be placed in the midline ...

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