Muscle tone is the resistance of muscle to passive stretch. Hypotonia is a decreased tone in the limbs, trunk, or other skeletal muscles. With hypotonia, there is decreased resistance to passive movement across a joint.
Hypotonia in a newborn may be caused by a wide variety of conditions. It can occur if there are abnormalities in the peripheral neuromuscular system, the central nervous system (CNS), or both. The severity may vary widely depending on the underlying cause.
A careful history of the pregnancy, labor, and delivery is important in determining possible congenital conditions or antenatal injury. The onset and the quality of fetal movements should be noted. Lack of fetal movements may suggest a myopathy. A history of unusual hyperextension of the neck in utero or breach delivery may suggest a spinal cord injury. Polyhydramnios suggests a fetal swallowing dysfunction, which may be an indicator of brainstem dysgenesis.
A family history of genetic or neuromuscular disorders and mental retardation is critical. A family history of disorders such as myasthenia gravis, muscular dystrophies, or myotonic dystrophies can be useful in determining the etiology of hypotonia in the newborn infant.
The delivery method and course are also important to explore for possible injury to the brain and spinal cord or for hypoxic-ischemic brain injury. Hypoxic-ischemic encephalopathy is 1 of the most common causes of hypotonia in the neonatal intensive care unit intensive care unit.
The gestational age of the infant at delivery is important because most premature infants will display hypotonia. Postterm infants may also have various reasons for being hypotonic.
The neonatal course is also important for determining the temporal nature of the disorder and determining if there are any acquired conditions. Transient hypotonia is common in disorders such as hypermagnesemia (caused by maternal administration of intravenous magnesium) and transient myasthenia of newborns. Neuromuscular disorders and genetic syndromes will have effects on tone that are more lasting.
A careful general and neurologic examination of the newborn may provide clues to the etiology. The general examination should focus on dysmorphic features that could be associated with genetic syndromes (eg, Down syndrome, Prader-Willi syndrome); size and shape of the head; neck and back examination (for any evidence of spinal dysraphisms); extremities (for limb dysgenesis and joint contractures); and organomegaly for storage diseases (eg, Zellweger syndrome).
Hypotonia is assessed by passive range of motion of muscles across joints and functional tests such as the “traction response” test. The infant is pulled by arms from a supine position to a “sitting” position while assessing tone in the shoulder muscle and neck. If the infant has significant hypotonia, there will be a head lag (extension) and poor resistance in the upper limbs (Figure ...