Determining the cause of hypotonia in newborns and infants can be challenging. All providers caring for these patients should have a basic understanding of how to approach such a diagnostic dilemma.
Hypotonia is decreased resistance to passive muscle elongation or to muscle stretch. In the case of a floppy infant, it is the inability of postural muscles to sustain low-intensity muscle contractions against gravity. Tone involves interactions between the corticospinal tracts, cerebellum, alpha and gamma motor neurons, interneurons, neuromuscular junctions, muscle spindles, and Golgi tendon organs. It is beyond the scope of this discussion to describe all of these connections, but it serves to show the complexity of maintaining normal tone. Abnormalities in any of these locations can cause loss of muscle tone. It is important to note that muscle weakness is often associated with hypotonia, but not always.
Historical information such as previous fetal demise, decreased fetal movements, and breech presentation or dislocated hip may suggest issues with hypotonia/weakness in utero. A history of encephalopathy suggests cortical involvement. A thorough history of medications administered to the mother or infant is important, as some may cause hypotonia. Obtaining a history of other family members with infantile hypotonia or with unexplained death in infancy is important. Examination of the parents can be helpful, even if they claim to be asymptomatic, as may be the case in myotonic dystrophy. Knowing the infant’s gestational and chronological age is critical. Furthermore, it is helpful to be aware of major motor milestones and of muscle tone at differing ages, as these characteristics change with maturation in a normal infant. For example, head lag is expected in the term newborn, but not after 4 months of age.
Once an infant is determined to be hypotonic, the next step is to localize the problem or problems causing hypotonia. Dysmorphic features of the face may suggest a disorder such as a myopathy (long face, ptosis) or a genetic syndrome (long philtrum, upturned nose, and flat nasal bridge). Poorly formed palmar creases may suggest onset of hypotonia or muscle weakness prior to delivery. Brain, spinal cord, peripheral nerve, neuromuscular junction, and muscle disorders can cause hypotonia. Nonneurologic disorders such as skeletal and connective tissue disorders can also cause hypotonia. This chapter will focus on the major neurologic disorders causing hypotonia in infancy.
Central hypotonia goes by many other names: cerebral hypotonia, upper motor neuron hypotonia, central nervous system (CNS) hypotonia, and supranuclear hypotonia. Central hypotonia typically refers to low tone due to disorders proximal to the motor neuron (anterior horn cell) in the spinal cord. Central hypotonia is not an isolated finding in most infants. Symptoms and signs related to the brain stem or brain disorder causing hypotonia include facial weakness; abnormalities ...