. The Hypotonic Infant

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,3,4 the 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


Image 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



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Jan 7, 2017 | Posted by in PEDIATRICS | Comments Off on . The Hypotonic Infant

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