DESCRIPTION

• “Floppy infant” refers to the newborn or infant presenting with hypotonia, a symptom of diminished tone of skeletal muscles associated with decreased resistance of muscles to passive movement.
• Hypotonia may or may not be associated with weakness (i.e., decreased muscle power).
• Hypotonia can be caused by neurologic disorders (primary hypotonia) and systemic diseases (secondary hypotonia).
• Primary hypotonia might be a manifestation of central nervous system (CNS) abnormalities (central hypotonia), peripheral neuromuscular system abnormalities (peripheral hypotonia), or a combined abnormality involving both (combined hypotonia).

EPIDEMIOLOGY

• Central hypotonia is more common (60–80%) than peripheral hypotonia (20–40%).
• The leading genetic causes for central hypotonia are Down syndrome (1 in every 700 infants born in the United States), followed by Prader Willi syndrome (prevalence ~1 in 25,000).
• The leading genetic causes for peripheral hypotonia are spinal muscle atrophy (incidence of ~1 in 25,000 live births), congenital myopathies (~1 per 26,000), and congenital muscular dystrophy (~1 per 100,000).

ETIOLOGY

• Causes of primary hypotonia may be divided into three major subcategories:
  • Central: hypotonia predominantly affecting postural muscles of the trunk, shoulder, and pelvic girdles; may co-occur with decreased alertness, developmental delay, and minimal or no weakness; caused by disorders of the CNS, including corticospinal tracts, basal ganglia, and/or cerebellum; creatine kinase (CK) is normal.
  • Peripheral: hypotonia predominantly affecting the extremities; with associated weakness, paucity of antigravity movements, decreased or absent deep tendon reflexes (DTRs); preserved alertness; caused by disorders of the motor unit: anterior horn cell, peripheral nerve, neuromuscular junction, or skeletal muscle; CK is normal or increased.
  • Combined: central and peripheral features
• Secondary causes of hypotonia, in an ill-appearing infant, are common and are routinely diagnosed during an initial evaluation and in the context of additional clinical features. They include systemic infection, hypoglycemia, metabolic derangements, hypoxic-ischemic encephalopathy, and heart failure, among others.

RISK FACTORS

COMMONLY ASSOCIATED CONDITIONS

• With central hypotonia: hypersomnolence, cognitive/developmental delay, seizures, dysmorphism, multiple congenital abnormalities
• With peripheral hypotonia: respiratory insufficiency, feeding or swallowing difficulties, myopathic facies, hip dislocation, contractures, joint hyperlaxity