Hypotonic (Floppy) Infant

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Chapter 27 Hypotonic (Floppy) Infant

Floppy, or hypotonic, infant is a common scenario encountered in the clinical practice of child neurology. It can present significant challenges in terms of localization and is associated with an extensive differential diagnosis (Box 27.1). As with any clinical problem in neurology, attention to certain key aspects of the history and examination allows correct localization within the neuraxis and narrows the list of possible diagnoses. Further narrowing of the differential is achievable with selected testing based on the aforementioned findings. Understanding the anatomical and etiological aspects of hypotonia in infancy necessarily begins with an understanding of the concept of tone. Tone is the resistance of muscle to stretch. Categorization of tone differs among authors, but assessment is performed with the patient at rest and all parts of the body fully supported; examination involves tonic or phasic stretching of a muscle or the effect of gravity. Tone is an involuntary function and therefore separate and distinct from strength or power, which is the maximum force generated by voluntary contraction of a muscle. Function at every level of the neuraxis influences tone, and disease processes affecting any level of the neuraxis may reduce tone. Although a comprehensive review of conditions associated with hypotonia in infancy is beyond the scope of a single chapter, this chapter considers the basic approach to evaluating the floppy infant and considers several key disorders.

Approach to Diagnosis

Physical Examination

General Features of Hypotonia

Assessing tone in an infant involves both observation of the patient at rest and application of certain examination maneuvers designed to evaluate both axial and appendicular musculature. Beginning with observation, a normal infant lying supine on an examination table will demonstrate flexion of the hips and knees so that the lower extremities are clear of the examination table, flexion of the upper extremities at the elbows, and internal rotation at the shoulders (Fig. 27.1). A hypotonic infant lies with the lower extremities in external rotation, the lateral aspects of the thighs and knees touching the examination table, and the upper extremities either extended down by the sides of the trunk or abducted with slight flexion at the elbows, also lying against the examination table. Evaluation of the traction response is done with the infant in supine position; the hands are grasped and the infant pulled toward a sitting position. A normal response includes flexion at the elbows, knees, and ankles, and movement of the head in line with the trunk after no more than a brief head lag. The head should then remain erect in the midline for at least a few seconds. An infant with axial hypotonia demonstrates excessive head lag with this maneuver (Fig. 27.2), and once upright, the head may continue to lag or may fall forward relatively quickly. Absence of flexion of the limbs may also be seen and indicates either appendicular hypotonia or weakness. The traction response is normally present after 33 weeks postconceptional age. Vertical suspension is performed by placing hands under the infant’s axillae and lifting the infant without grasping the thorax. A normal infant has enough power in the shoulder muscles to remain suspended without falling through, with the head upright in the midline and the hips and knees flexed (Fig. 27.3, A). In contrast, a hypotonic infant held in this manner slips through the examiner’s hands, often with the head falling forward and the legs extended at the knees (see Fig. 27.3, B). Infants with axial hypotonia related to brain injury may also demonstrate crossing, or scissoring, of the legs in this position, which is an early manifestation of appendicular hypertonia. In horizontal suspension, the infant is held prone with the abdomen and chest against the palm of the examiner’s hand. A normal infant maintains the head above horizontal with the limbs flexed, while a hypotonic infant drapes over the examiner’s hand with the head and limbs hanging limply. Other examination findings in hypotonic infants include various deformities of the cranium, face, limbs, and thorax. Infants with reduced tone may develop occipital flattening, or positional plagiocephaly, as the result of prolonged periods of lying supine and motionless.

Localization

Once the presence of hypotonia in an infant is established, the next step in determining causation is localization of the abnormality to the brain, spinal cord, motor unit, or to multiple sites. A motor unit is a single spinal motor neuron and all the muscle fibers it innervates and includes the motor neuron with its cell body, axon, and myelin covering; the neuromuscular junction; and muscle. The major “branch point” at this stage of the assessment is whether the lesion is likely to be in the brain, at a more distal site, or at multiple sites.

The key features of disorders of cerebral function, particularly the cerebral cortex, are encephalopathy and seizures. Encephalopathy manifesting as decreased level of consciousness may be difficult to ascertain, given the large proportion of time normal infants spend sleeping. However, full-term or near-term infants with normal brain function spend at least some portion of the day awake with eyes open, particularly with feeding. Encephalopathy also manifests with excessive irritability or poor feeding, although the latter problem is rarely the sole feature of cerebral hemispheric dysfunction and may occur with disorders at more distal sites. Infants with centrally mediated hypotonia of many different etiologies frequently have relatively normal power despite a hypotonic appearance. Power may not be observable under normal circumstances because of a paucity of spontaneous movement, but it may be observable with application of a noxious stimulus such as a blood draw or placement of a peripheral intravenous catheter. Other indicators of central rather than peripheral dysfunction include fisting (trapping of the thumbs in closed hands), normal or brisk tendon reflexes, and normal or exaggerated primitive reflexes. Tendon reflexes should be tested with the infant’s head in the midline and the limbs symmetrically positioned; deviations from this technique often result in spuriously asymmetrical reflexes. Primitive reflexes are involuntary responses to certain stimuli that normally appear in late fetal development and are supplanted within the first few months of life by voluntary movements. Abnormalities of these reflexes include absent or asymmetrical responses, obligatory responses (persistence of the reflex with continued application of the stimulus), or persistence of the reflexes beyond the normal age range. Two of the most sensitive primitive reflexes are the Moro and asymmetrical tonic neck reflexes. The Moro reflex is a startle response present from 28 weeks after conception to 6 months postnatal age (Gingold et al., 1998). Quickly dropping the infant’s head below the level of the body while holding the infant supine with the head supported in one hand and the body supported in the other readily elicits this reflex. The normal response consists of initial abduction and extension of the arms with opening of the hands, followed quickly by adduction and flexion with closure of the hands. The tonic neck reflex is a vestibular response and is present from term until approximately 3 months of age. The response is elicited by rotating the head to one side while the infant is lying supine. The normal response is extension of the ipsilateral limbs while the contralateral limbs remain flexed. Central disorders resulting in hypotonia may also be associated with dysmorphism of the face or limbs, or malformations of other organs. Various defects in O-linked glycosylation of α-dystroglycan, a protein associated with the dystrophin glycoprotein complex that stabilizes the sarcolemma, result in structural defects of the brain, eye, and skeletal muscle.

Disorders of the spinal cord leading to neonatal hypotonia are usually secondary to perinatal injury. Spinal cord injury may occur in the setting of a prolonged, difficult vaginal delivery with breech presentation, resulting in trauma to the spinal cord, or may result from hypoxic-ischemic injury to the cord concurrently with encephalopathy. In the latter case, hypotonia may initially be attributable to the encephalopathy. In cases of hypotonia resulting from spinal cord injury, diminished responsiveness to painful stimuli, sphincter dysfunction with continuous leakage of urine and abdominal distension, and priapism may provide clues to localization of the lesion.

The hallmark of disorders of the motor unit is weakness. Tendon reflexes are absent or reduced. Tendon reflexes reduced out of proportion to weakness usually indicate a neuropathy, often a demyelinating neuropathy, whereas tendon reflexes reduced in proportion to weakness are more likely to result from myopathy or axonal neuropathy. The motor unit is the final common pathway for all reflexes, and for this reason, primitive reflexes are depressed or absent in motor unit disorders. This phenomenon may hinder detection of central nervous system (CNS) abnormalities when lesions at both levels coexist. Other abnormalities related to motor unit disorders in infants include underdevelopment of the jaw (micrognathia), a high arched palate, and chest wall deformities, in particular pectus excavatum. Muscle atrophy may also occur but also occurs in cerebral disorders. Sensory function is not assessable in detail in a neonate or young infant, particularly in the presence of encephalopathy, although reduced responsiveness to pinprick may provide clues to the presence of a polyneuropathy or spinal cord lesion in the setting of normal mental status. Some motor unit disorders may result in perinatal distress due to weakness and may result in a superimposed encephalopathy that confounds the localization of hypotonia.

Hypotonic infants may have reduced movement during fetal development, leading to fibrosis of muscles or of structures associated with joints, as well as foreshortening of ligaments. This results in restricted joint range of motion, or contractures. The term arthrogryposis refers to joint contractures that develop prenatally. The most common form of arthrogryposis is unilateral or bilateral clubfoot. The most severe end of this clinical spectrum is arthrogryposis multiplex congenita, or multiple joint contractures. The causes of this condition may be abnormalities of the intrauterine environment, motor unit disorders, or disorders of the CNS. Hypotonia in utero may also result in congenital hip dysplasia.

Diagnostic Studies

Selective laboratory testing allows confirmation of the clinical localization of hypotonia, and in many cases leads to identification of a specific diagnosis. In all cases, ancillary testing guided by historical features and examination findings has the greatest chance of yielding a diagnosis. Available modalities include various forms of neuroimaging; electrophysiological techniques including electroencephalography (EEG), nerve conduction studies (NCV), electromyography (EMG), and repetitive nerve stimulation; muscle and nerve biopsy; and other laboratory studies such as serum creatine kinase (CK), metabolic studies, and genetic studies.

Nerve Conduction Studies and Electromyography

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