Review possible etiologies of neonatal hypotonia, an approach to the diagnostic evaluation, and options for clinical management.
Neonatal hypotonia can result from a variety of underlying disease conditions. In contrast to weakness, which is a reduction in maximum voluntary power of the muscles, hypotonia is defined by decreased resistance to passive range of motion or loss of postural control against gravity. Here we consider conditions characterized by hypotonia, which may or may not be accompanied by weakness.
Conditions causing hypotonia
Hypotonia in infants may result from central or peripheral causes (Figure 54.1). Central nervous system disorders represent the underlying etiology for primary hypotonia in 60–80% cases, whereas neuromuscular disorders (involving either peripheral nerve, neuromuscular junction, or muscle) are present in 15–30% (Figure 54.1).
Central Nervous System Disorders
These conditions affect the structure or function of the brain or spinal cord. They are often associated with axial hypotonia, normal or exaggerated deep tendon reflexes (DTRs), and relative preservation of strength with or without altered consciousness, seizures, feeding difficulties, abnormal brainstem reflexes, and abnormal extraocular movements. Included in this category would be encephalopathy, such as hypoxic-ischemic encephalopathy or acute infectious encephalopathy, intracranial hemorrhage, cerebral malformations (such as schizencephaly, lissencephaly, or holoprosencephaly), trauma, and malformations including the spinal cord, such as a Chiari malformation or syringomyelia. Systemic conditions causing secondary hypotonia, such as sepsis or certain inborn errors of metabolism, often present with central hypotonia. Also in this category are infants with delayed myelination of the brain seen on MRI, and those with no abnormalities found on imaging or other evaluation (benign congenital hypotonia). These infants often normalize over time and may become indistinguishable from their peers, particularly if only gross motor delays are present.
Peripheral Nerve Disorders. These are characterized by normal alertness, absent DTRs, and profound weakness. This includes anterior horn cell disease such as spinal muscular atrophy (SMA), caused by a loss of motor neurons in the spinal cord and characterized by proximal weakness, absent DTRs, facial sparing, paradoxical breathing, and tongue fasciculation. Peripheral neuropathies, characterized by predominantly distal symptoms, absent DTRs, and pes cavus, such as congenital motor sensory neuropathy (Charcot-Marie-Tooth disease) and hereditary sensory and autonomic neuropathy, would also be included in this category.
Neuromuscular Junction Disorders. These may present with preserved DTRs and include transient acquired neonatal myasthenia (placental transfer of maternal antibodies against acetylcholine receptor), congenital myasthenia gravis (notable for including bulbar weakness, can be rapidly progressive), magnesium or aminoglycoside toxicity, or infantile botulism.