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Congenital hypotonia is a relatively common, well-recognized entity that causes a diagnostic challenge for pediatricians due to a wide differential. Identification of the cause is crucial for determining the prognosis and possible early interventional options. Since the etiology is heterogeneous, a systematic approach to infants with hypotonia is necessary. In this chapter, we aim to give a general perspective of diseases that cause hypotonia and establish a diagnostic algorithm for infants who present with hypotonia.


Tone is the amount of resistance/tension to stretch in a relaxed muscle, and hypotonia is the condition of low muscle tone leading to decreased postural control and movement against gravity. Thus, hypotonic infants present with poor control of movement and delayed motor skills later in life. It is important to distinguish hypotonia from weakness. Weakness is decreased maximum strength that can be generated by a muscle. Weakness is always associated with hypotonia, but hypotonia may exist without weakness.

The maintenance of normal muscle tone requires intact coordination of the brain, spinal cord, nerves, neuromuscular junction, and muscles. The spindle apparatus is the cytoskeletal structure that senses the stretch and sends impulses through the sensory nerve to the anterior horn of spinal cord. This information is transferred to the alpha motor neuron and then to the muscle under the control of the central nervous system. Disruption at any level of this pathway can cause hypotonia (Table 175-1). If the hypotonia is caused by cerebral reasons (brain and spinal cord lesion before synapse at the anterior horn cells), the term “central hypotonia” is used. Damage between the spinal cord anterior horn and muscle is called “peripheral hypotonia.” A detailed neurologic exam can differentiate whether the hypotonia is of central origin, and thus localized to brain or spinal cord, versus that of peripheral origin and thus localized to spinal anterior horn disease, neuromuscular junction, and muscle.


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