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Neonatal Hypotonia
Introduction
Neonatal hypotonia, manifested by the clinical appearance of a “floppy infant” ( Figure 6.1 ) and by diminished resistance to passive movement, is the principal presenting feature of most neuromuscular disorders of the newborn. Thus, the disorders responsible for the hypotonia are discussed throughout this book. This chapter reviews the major features of the examination of a newborn with neonatal hypotonia and a suspected neuromuscular disorder, and then discusses the distinguishing features of the major categories of neuromuscular disorders. Such important laboratory studies as assessment of cerebrospinal fluid and serum muscle enzyme levels; electromyography; determination of nerve conduction velocity; muscle biopsy and genetic studies; and imaging and other assessments of the central nervous system, are discussed elsewhere in this book in relation to the specific entities.
History and General Examination
As with other assessments of an infant with a neurological disorder, the evaluation begins with a careful history and general examination. The neonatal neuromuscular examination, the focus of this chapter, then follows.
History
The importance of acquiring a careful history frequently is overlooked in the evaluation of an infant with a motor disorder. The pertinent historical features will become apparent in subsequent discussions of the various disorders elsewhere in the book. However, it should be emphasized here that certain findings that initially might not be considered relevant to a motor abnormality, such as polyhydramnios, may prove to be a valuable clue in diagnosis (e.g. myotonic dystrophy). Moreover, the family history should be supplemented by examination of the infant’s parents. The myotonia and facial weakness of myotonic dystrophy or the pes cavus and leg weakness of familial polyneuropathy are easily overlooked in many affected adults.
The peripartum and early neonatal history may provide evidence for an asphyxial insult, e.g. fetal heart rate abnormalities, intrapartum “sentinel event” (e.g. cord prolapse, uterine rupture, placental abruption), depressed Apgar scores, fetal acidosis, signs of encephalopathy. Hypoxic–ischemic encephalopathy is a very common cause of neonatal hypotonia. The early neonatal history may provide a clue for a serious metabolic disorder (e.g. metabolic acidosis, hyperammonemia), in which hypotonia is a common feature. The gestational age of the infant is important because prematurity is associated with a wide variety of cerebral and cerebellar disorders that result in hypotonia. Evidence for intrauterine growth retardation may suggest a scenario that includes hypoglycemia, a common cause of neonatal hypotonia. Finally, detection of seizures, which often are subtle in the newborn, will suggest structural or metabolic involvement of the central nervous system, a frequent anatomical substrate for neonatal hypotonia.
Physical Examination
The physical examination must be performed carefully and completely. As will become apparent later, dysmorphic features, cardiac abnormalities, respiratory insufficiency, hepatomegaly, and the like may be features of certain disorders of the motor system. Congenital hip dislocation and other joint contractures are particularly common features in neonatal motor disorders; these and related joint abnormalities are discussed in more detail later. (see “Arthrogryposis Multiplex Congenita”).
The neurological evaluation of the motor system is discussed in detail next. It need only be emphasized here, as explained later, that the anatomical site of the disorder of the motor system is determined best by a careful determination of muscle bulk, power, tone, tendon reflexes, primary neonatal reflexes, and the presence or absence of myotonia, myasthenia, and fasciculations. Other neurological features, such as abnormalities of cranial nerve function, sensory discrimination, or the occurrence of seizures, provide important supplementary information in selected instances.
Neonatal Neuromuscular Examination
Examination of a floppy infant of course should emphasize the motor examination, the evaluation of the primary neonatal reflexes, and the sensory examination. Careful attention to central nervous system signs (seizures, focal motor deficits, impaired level of alertness, spinal cord signs, etc.) is also important and is discussed in detail elsewhere.
Motor Examination
The major features of the motor examination in the neonatal period are muscle tone and posture of limbs, motility and muscle power, muscle stretch reflexes, and the plantar response. The infant’s postnatal age and level of alertness have an important bearing on essentially all these features. Most of the observations described are applicable to infants more than 24 hours old with an optimal level of alertness, unless otherwise indicated.
Tone and Posture
Muscle tone is assessed best by passive manipulation of the limbs with the head placed in the midline. Because the tone of various muscles partly determines the posture of the limbs at rest, careful observation of posture is valuable for the proper evaluation of tone. Some investigators have devised various maneuvers for the passive manipulation of limbs (e.g. approximation of heel to ear, hand to opposite ear [scarf sign], measurement of such joint angles as the popliteal angle) to quantitate tone. I have not found these maneuvers particularly useful and do not discuss them in detail here.
Developmental Aspects. Saint-Anne Dargassies described an approximate caudal-rostral progression in the development of tone, particularly flexor tone, with maturation. Thus, at 28 weeks' gestational age, there is minimal resistance to passive manipulation in all limbs, and indeed, premature infants of ≤28 weeks' gestational age appear clinically like “floppy infants.” By 32 weeks, distinct flexor tone becomes apparent in the lower extremities. By 36 weeks, flexor tone is prominent in the lower extremities and palpable in the upper extremities. By term, passive manipulation affords an appreciation of strong flexor tone in all extremities.
The posture of an infant in repose reflects these changes in tone, to some extent. In my experience, these postures are apparent principally in the slightly drowsy state. An alert infant at these various gestational ages is more active and motile, and fixed postures or so-called preference postures are difficult to define. This fact has been documented by Prechtl and coworkers and by others. Nevertheless, a very quiet infant at 28 weeks' gestational age often lies with minimally flexed limbs, whereas by 32 weeks there is distinct flexion of the lower extremities at the knees and hips. By 36 weeks, flexor tone in the lower extremities results in a popliteal angle of 90 degrees, and there is consistent and frequent flexion at the elbows. By term, the infant assumes a flexed posture of all limbs. The evolution of hip (and knee) flexor tone with maturation is reflected in the developmental increase in pelvic elevation when the infant is in the prone position.
Preference of Head Position. A consistent and interesting aspect of posture in newborn infants is a preference for position of the head toward the right. Prechtl and coworkers demonstrated head position toward the right 79% of the time, versus 19% toward the left, and only 2% toward the midline. The head orientation preference may be less prominent in the first 24 hours of life. This preference is not attributable to differences in lighting, nursing practices, or other factors; it appears to reflect a normal asymmetry of cerebral function at this age.
Motility and Power
The quantity, quality, and symmetry of motility and muscle power are the parameters of interest. Hadders-Algra and Prechtl combined videotape and electrophysiologic methods to describe the postnatal development of motor activity in term infants. In the first 4 weeks, movements with a writhing quality predominate; in the period from 4 to 12 weeks, “fidgety” movements are prominent; and after 8 to 12 weeks, rapid large-amplitude “swipes” and “swats” are the predominant movements. In general, preterm infants exhibited similar patterns of motor development when they attained comparable postmenstrual ages. Prechtl and others have emphasized that the quality of spontaneous movements in preterm and term infants is of major importance in evaluating the status of the central nervous system.
Saint-Anne Dargassies, using less sophisticated techniques, also noted a writhing quality of the initial movements of preterm infants. By 32 weeks of gestation, movements are predominantly flexor, especially at the hips and knees, often occurring in unison. Although head turning is present, neck flexor and extensor power is negligible, as judged by complete head lag on pull-to-sit maneuvers or when the infant is held in the sitting position. By 36 weeks, the active flexor movements of the lower extremities are stronger and often occur in an alternating rather than symmetrical fashion. Flexor movements of the upper extremities are prominent. For the first time, definite neck extensor power can be observed. When the infant is supported in the sitting position, the head is lifted off the chest and remains upright for several seconds. By term, the awake infant is particularly active if stimulated with a gentle shake. Limbs move in an alternating manner, and neck extensor power is better. Neck flexor power also becomes apparent; when the infant is pulled to a sitting position with a firm grasp of the proximal upper limbs, the head is held in the same plane as the rest of the body for several seconds.
The importance of a fixed program in motor development is suggested by the similarities among (at the same postmenstrual age) the fetus, the premature infant, and the term infant. These similarities markedly outweigh the rather small differences when one compares similar age infants.
Muscle Stretch Reflexes
The muscle stretch reflexes readily elicited in term newborns are the pectoralis, biceps, brachioradialis, knee, and ankle jerks. I have considerable difficulty obtaining triceps jerks in term infants. Most of these reflexes can be elicited but are less active in preterm infants. The knee jerk is often accompanied by crossed adductor responses, which should be considered a normal finding in the first several months of life (<10% of normal infants demonstrate crossed adductor responses after 8 months of age).
Ankle clonus of 5 to 10 beats should also be accepted as a normal finding in newborn infants if no other abnormal neurologic signs are present and the clonus is not distinctly asymmetrical. Ankle clonus usually disappears rapidly, and more than a few beats beyond 3 months of age is abnormal.
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