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Chapter 175: Congenital Hypotonia

Davut Pehlivan; Fernando Scaglia

INTRODUCTION

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.

PATHOGENESIS AND EPIDEMIOLOGY

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.

TABLE 175-1DIFFERENTIAL DIAGNOSIS OF CONGENITAL HYPOTONIA
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TABLE 175-1DIFFERENTIAL DIAGNOSIS OF CONGENITAL HYPOTONIA

  • Central hypotonia

    1. Cerebral

      1. Benign congenital hypotonia

      2. Static encephalopathies due to injuries pre-/perinatal

        1. Hypoxic ischemic encephalopathy (HIE)

        2. Intracranial hemorrhage

        3. Congenital infections

      3. Neurogenetic disorders

        1. Chromosomal disorders

        2. Single-gene disorders causing brain anomalies

        3. Neurometabolic disorders (eg, mitochondrial disorders, Zellweger syndrome, phenylketonuria, galactosemia, propionic acidemia)

        4. Methylation anomalies (Prader-Willi and Angelman syndromes)

      4. Maternal drug effect

    2. Spinal cord

      1. Spinal cord injuries (birth trauma)

      2. Spinal cord anomalies (syringomyelia, neural tube defects)

  • Peripheral hypotonia

    1. Anterior horn cell disorder (spinal muscular atrophy [SMA])

    2. Neuromuscular junction disorders

      1. Congenital myasthenic syndrome (CMS 1A-12)

      2. Transient acquired neonatal myasthenia

      3. Infantile botulism

      4. Drug toxicity (magnesium and aminoglycosides)

    3. Peripheral neuropathies

      1. Congenital hypomyelinating neuropathy

      2. Dejerine-Sottas syndrome

      3. Giant axonal neuropathy

    4. Muscle disorders

      1. Congenital muscular dystrophies (CMD)

        1. Extracellular matrix protein abnormalities

          1. Merosin-deficient congenital muscular dystrophy (MDC1A)

          2. Collagen VI–deficient CMD

        2. Collagen VI–deficient congenital muscular dystrophies

          1. Ullrich CMD

          2. Bethlem myopathy

        3. Defects of glycosylation

          1. Dystroglycanopathies

          2. Fukuyama CMD

          3. Muscle-eye-brain disease

          4. Walker-Warburg syndrome

          5. Merosin-deficient CMD ...

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