Rudolph's Pediatrics, 22nd Ed.

CHAPTER 569. The Hypotonic Infant

Basil T. Darras

Neonatal hypotonia, also known as floppy infant syndrome, is the main presenting clinical feature of most neuromuscular diseases of early life.1 However, disorders of the central nervous system may also manifest with hypotonia.

DEFINITION OF HYPOTONIA

Two types of muscle tone can be assessed clinically: postural and phasic. Postural (ie, antigravity) tone is a sustained, low-intensity muscle contraction in response to gravity and is mediated by both the gamma and alpha motor neuron systems in the spinal cord. It is assessed clinically by passive manipulation of the limbs. Phasic tone is a brief contraction in response to a high-intensity stretch and is mediated by the alpha motor neuron system only. It is examined clinically by eliciting the muscle stretch reflexes. Hypotonia is defined as reduction in postural tone, with or without a change in phasic tone. When postural tone is depressed, the trunk and limbs cannot counteract gravity, and the child appears hypotonic or floppy.

Sainte-Anne Dargassies2 described an approximate caudal-rostral progression in the development of muscle tone. At postconceptional age of 28 weeks, there is minimal resistance to passive manipulation in all limbs; by 32 weeks, flexor tone can be appreciated in the lower extremities; and by 36 weeks, flexor tone is also present in the upper limbs. By term, strong flexor tone can be demonstrated in all four limbs by passive manipulation.

PHYSICAL EXAMINATION AND ASSESSMENT OF A HYPOTONIC CHILD

Volpe1 describes the physical examination of a hypotonic infant in detail. Following a careful general physical examination, the neurologic assessment should include motor examination, evaluation of primary neonatal reflexes, and sensory examination (Table 569-1). A general physical examination may reveal organomegaly, cardiomyopathy, contractures, abnormalities of the genitalia, respiratory rate or pattern irregularities, or evidence of traumatic injury (eg, bruising, petechiae); the general examination may, however, be normal. The motor examination includes assessment of posture, muscle tone, motility and muscle power, muscle stretch reflexes, and primary neonatal reflexes. Abnormality of the primary neonatal reflexes lies in their persistence. In normal infants, the Moro reflex disappears by 6 months of age,3,4the palmar grasp becomes less obvious after 2 months of age, and the tonic neck response becomes less facile at 6 to 7 months of age.3-5 Sensation can be tested by withdrawal from a stimulus (eg, touching the infant with a small brush). Abnormalities in sensation may be present in some forms of congenital neuropathies, such as hereditary motor-sensory or sensory-autonomic neuropathies, but they are usually difficult to assess in infants.

When assessing muscle tone in young infants, the infant’s head should be placed in the midline to eliminate the effect of the tonic neck response. Minimal resistance to passive manipulation of arms or legs is an important clinical feature of hypotonia. Most hypotonic infants demonstrate a classical froglike posture: full abduction and external rotation of the legs as well as a flaccid extension or flexion of the arms. Congenital dislocation of the hips may be noted because poor muscle tone in utero has failed to maintain the femoral head in the acetabulum. Another sign of intrauterine hypotonia and limited fetal movements is arthrogryposis (ie, contractures of multiple joints). Weak cry, poor suck, and poor respiratory effort may be noticed in some clinically “alert” infants. Spontaneous antigravity movements of limbs may be absent or decreased. Pectus excavatum with a bell-shaped chest is sometimes seen, reflecting long-standing weakness of the chest wall muscles. Muscle stretch reflexes may be normal, brisk, or hypoactive (ie, absent or decreased). In a full-term newborn or older infant, passive movement of the infant’s elbow across the mid-line produces a positive scarf sign. Similarly, a positive heel-to-ear test is readily demonstrated by opposing the heel to the ear.

Tone can be evaluated further by performing the traction response, vertical suspension, and horizontal suspension maneuvers.6

 TRACTION RESPONSE

To elicit the traction response, the examiner grasps the infant’s hands and wrists and slowly raises the infant from supine to sitting. In the normal infant, no significant head lag is expected, and the head is maintained in the midline at least for a few seconds when the sitting position is reached. However, the hypotonic infant tends to have significant head lag when pulled up to sitting and does not maintain the head erect when sitting.

Table 569-1. Hypotonia: Physical Examination

General physical examination

Appearance/posture (flaccid)

Passive manipulation of the limbs

Motility and muscle power

Muscle stretch reflexes

Primary neonatal reflexes

Sensation

Traction response (head lag)

Vertical suspension (slips through)

Horizontal suspension (drapes over)

 VERTICAL SUSPENSION

The examiner places both hands beneath the infant’s armpits and lifts the infant straight up. In a normal infant, the shoulder muscles press down against the examiner’s hands and enable him/her to suspend vertically without falling. When the normal infant is in vertical suspension, the head is maintained in the midline and hips, knees, and ankles are in flexion. When this maneuver is performed in the hypotonic infant, the infant slips through the examiner’s hands with both legs usually extended.

 HORIZONTAL SUSPENSION

The examiner uses one hand to support the infant’s trunk in a prone position and observes the resulting posture. A normal infant flexes or fully extends the limbs, straightens the back, and maintains the head in the midline for at least a few seconds. The hypotonic infant’s head and limbs hang loosely, and the trunk drapes over the examiner’s hand.

DIFFERENTIAL ANATOMIC DIAGNOSIS OF HYPOTONIA

Hypotonia may be the manifestation of pathology involving the central nervous system (CNS), the peripheral nervous system (ie, lower motor unit), or both. The differential diagnosis includes pathologies affecting brain, spinal cord, anterior horn cell, peripheral nerve, neuromuscular junction, and muscle. In infants with cerebral or central hypotonia, who constitute about two thirds of the cases, the perinatal or prenatal history may be consistent with a CNS insult; there is usually global (rather than an isolated gross motor) developmental delay, sometimes seizures, microcephaly, dysmorphic features, and malformation of the brain and/or other organs. Infantile reflexes may be brisk and/or persistent, and muscle stretch reflexes are normal or brisk. Movement can be triggered via postural reflexes (eg, Moro or asymmetric tonic neck response), and the degree of weakness noted in the infant is usually less than the degree of hypotonia (nonparalytic hypotonia; Table 569-2). Common causes of central hypotonia include chromosomal disorders (trisomy 21); other genetic defects, such as Prader-Willi syndrome; acute hemorrhagic or other brain injury; hypoxic-ischemic encephalopathy; chronic nonprogressive encephalopathies; peroxisomal disorders such as Zellweger syndrome, neonatal adrenoleukodystrophy, and others; and metabolic defects. In lower motor unit hypotonia or peripheral hypotonia, the developmental delay is primarily gross motor and is associated with absent or depressed muscle stretch reflexes and, in some cases, with muscle atrophy and fasciculations of the tongue. In general, antigravity limb movements are decreased, and movements cannot be elicited via postural reflexes; notably, the weakness is proportional or in excess to the degree of hypotonia (paralytic hypotonia; Table 569-2). Trauma to the high cervical cord due to traction in breech or cervical presentation may initially manifest itself as flaccid paralysis, which may be asymmetric, and absent muscle stretch reflexes; later on, however, upper motor neuron signs develop.

Table 569-2. Cerebral (Central) versus Lower Motor Unit (Peripheral) Hypotonia

Because muscle tone is also determined by the viscoelastic properties of muscle and joints, connective tissue disorders such as Marfan and Ehlers-Danlos syndromes or osteogenesis imperfecta, as well as benign laxity of the ligaments, can present with hypotonia. In addition, there is combined cerebral and lower motor unit hypotonia seen in infants and older children with congenital myotonic dystrophy, as well as some of the congenital muscular dystrophies, peroxisomal disorders, mitochondrial encephalomyopathies, neuroaxonal dystrophy, leukodystrophies (eg, globoid cell leukodystrophy), familial dysautonomia, and asphyxia secondary to motor unit disease. Furthermore, hypotonia without significant weakness may be a feature of systemic diseases such as sepsis, congenital heart disease, hypothyroidism, rickets, renal tubular acidosis, and others. Hypotonia present during the neonatal period, which reverses to normal tone within months to a few years, is known as benign congenital hypotonia. Considering the high incidence of global developmental delay and learning disabilities in this group of patients, cerebral etiologies must be common.

Table 569-3. Neuromuscular Diseases in the Hypotonic Infant and Child

Neuromuscular diseases in infancy present primarily with hypotonia and weakness. Muscle stretch reflexes vary depending on the anatomic level of pathology along the motor unit (ie, prominent hyporeflexia or total areflexia in anterior horn cell disorders and neuropathies, reduced reflexes in proportion to the degree of weakness in myopathies, and almost normal reflexes in neuromuscular junction defects). Again, approximately two thirds of patients with neonatal hypotonia have cerebral etiologies, and one third have lower motor unit diseases.7

COMMON NEUROMUSCULAR DISORDERS PRESENTING PRINCIPALLY WITH HYPOTONIA

Table 569-3 shows the most frequent genetic and acquired disorders of the lower motor unit. Most of these conditions present with hypotonia.