Neonatal hypotonia, or “floppy infant syndrome,” is a common condition encountered after delivery and in the neonatal intensive care unit (NICU). Early diagnosis and differentiating etiology of low tone are critical to help prevent further complications.

“Hypotonia can be a manifestation of systemic illness or dysfunction of any level of the nervous system,” said Amanda B. Rogers, M.D., pediatric neurologist with Norton Children’s Neuroscience Institute, affiliated with the UofL School of Medicine.

Due to a wide spectrum of underlying conditions associated with neonatal hypotonia, timely and accurate diagnosis is essential to improve patients’ long-term health outcomes. For health care providers, recognizing and assessing neonatal hypotonia requires a structured approach, with a thorough history and physical examination serving as critical first steps.

Assessing neonatal hypotonia

A thorough physical exam is essential when evaluating a hypotonic neonate. Recognizing abnormal postures, diminished resistance and excessive joint mobility is key. Understanding normal tone variations, based on gestational age and neonatal state of wakefulness versus sedation, helps to differentiate hypotonia.

Evaluating passive movements of the limbs helps assess tone abnormalities:

• Conduct the assessment when the infant is awake and calm — not crying or in deep sleep.
• Move the arms and legs passively, feeling for some resistance at the joints. Complete limb dangle is abnormal.
• Comparing findings with normal tone neonates can develop an accurate sense of abnormal tone.
• Observing the infant in various positions and maneuvers can indicate low muscle tone.

Supine positioning

A normal term newborn maintains flexion of the extremities, with limbs adducted toward the trunk. Also, observe spontaneous movements: Are they symmetrical and occurring against gravity?

Abnormal resting posture may indicate hypotonia. A hypotonic infant often assumes a frog-legged posture, with hips flexed, thighs externally rotated and legs resting flat. The arms may be extended flaccidly or rest in a “W” shape. The head may loll to one side rather than maintaining midline positioning.

Prone positioning

A term newborn should be able to lift the head slightly and move it side to side, especially during feedings. Hips should remain flexed, with the buttocks slightly elevated. If the arms are repositioned behind the infant, they should actively return to a flexed position.

Common clinical maneuvers for testing tone

Scarf sign
Gently move the neonate’s arm across the chest. In a term newborn, the elbow should reach mid-clavicle but not cross the sternum. A markedly hypermobile arm crossing the body suggests hypotonia.

Heel-to-ear
Bringing the neonate’s foot toward the chest should encounter some resistance. If the foot reaches the ear without resistance, hypotonia may be likely. Flexibility varies with age, so consider gestational maturity when interpreting results.

Exploring etiology
“Providers should always do a tone exam on any neonate,” said Erin McCoy, M.D., pediatric neurology resident with Norton Children’s Neuroscience Institute. “Afterward, when you’re ordering tests and trying to figure out the issue, focus on localizing it and differentiating central versus peripheral hypotonia.”

Central hypotonia accounts for most cases. Acute central nervous system issues, such as stroke or hypoxic-ischemic encephalopathy account for one-third of all cases. Other causes may involve genetic or chromosomal abnormalities (such as Down syndrome), brain structural defects or metabolic diseases. Peripheral causes are less common. (See Table 1: differentiating the features of a floppy neonate according to the site of involvement.) However, in approximately 13% of all cases, exact etiology is inconclusive.