CHAPTER 128

Hypotonia

Hanalise V. Huff, MD, MPH, and Kenneth R. Huff, MD

Infants with decreased tone are referred to as hypotonic or “floppy.” Hypotonia is most simply defined as lower than normal resistance to passive motion across a joint but is also suggested by abnormal posture. Although the lack of resistance may have other components (eg, connective tissue abnormalities suggested by an unusual range of joint mobility and hyperelastic skin, features of Ehlers-Danlos syndrome, features of Down syndrome), muscle strength is a key component. Tone can be used as a surrogate indicator of strength in infants who cannot cooperate with resistance testing. Weakness can also be inferred from functional observations and the inability to sustain limbs against gravity or the lack of a withdrawal response of a limb to a painful stimulus. Identification of the affected central nervous system (CNS) level (eg, upper motor neurons, lower motor neurons, spinal cord, anterior horn cell, peripheral nerve components, myoneural junction, muscle fibers) is most important in determining the etiology of hypotonia in infants and children. Only after this localization is the likely pathology defined in most cases.

Epidemiology

Hypotonia is not unusual in neonates, and non-neuromuscular causes of hypotonia are more common than neuromuscular conditions. Non-neuromuscular causes include hypoxic-ischemic encephalopathy and brain lesions related to preterm birth, such as intraventricular hemorrhage and periventricular leukomalacia. Genetic abnormalities also are frequently present shortly after birth. Spinal muscular atrophy (SMA) is a common progressive genetic condition with a prevalence similar to that of cystic fibrosis (approximately 1 in 5,000). Duchenne muscular dystrophy (DMD), which manifests between ages 2 and 5 years, is the most common neuromuscular condition in childhood, with a prevalence of 1 in 1,700 to 1 in 3,500 male births. The DMD gene has been found to spontaneously mutate in approximately 50% of patients. Among non–genetically acquired disorders, the prevalence of acute postinfectious polyneuritis (ie, Guillain-Barré syndrome) is 2 to 8 per 100,000.

Clinical Presentation

Hypotonia in infants often presents with unusual posture, diminished resistance to passive movements of the limbs or trunk, or an excessive range of joint mobility. Infants with hypotonia have delayed motor milestones, decreased movements, and poor head and trunk control (Figure 128.1). Older children may have decreased ability to resist with strength testing of individual muscle groups as well as impaired functional strength in sitting, standing, walking, climbing, or running (Box 128.1). Swayback posture in standing may be indicative of hip girdle or proximal weakness. Pointed toes in the supine position may be indicative of an upper motor cause.

Figure 128.1. Traction maneuver. The arms are completely extended, head control is poor, and the legs are abducted at the hip.

Pathophysiology

Tone is a product of connective tissue structural elements, including ligaments, tendons, and joint capsules; muscle fiber number and integrity; and nerve fiber input to muscle. Nerve input to the muscle includes the number and myelination of axon fibers, trophic factors from the nerve, and frequency of action potentials depolarizing the muscle membrane. The control of tone through the anterior horn cells is complex and involves more than just the corticospinal tract (upper motor unit) but also other descending tract influences. Lesions of the neuromuscular apparatus (lower motor unit), which includes muscle, nerve, nerve sheaths, and anterior horn cells, can most directly decrease tone; however, lesions of any of the other structures at many levels of the nervous system also can affect tone. The final common pathway of upper or lower motor unit modification of tone is through the gamma loop, which is part of the fusimotor system. The fusimotor system consists of gamma motor neurons within the anterior horn of the spinal cord innervate the contractile muscle portions on each end of the intrafusal fiber and enhance the sensitivity of the sensory endings to stretch, which in turn transmit signals back to the alpha motoneurons in the anterior horn, thereby innervating the rest of the muscle. Different CNS levels (ie, motor cortex, thalamus, basal ganglia, vestibular nuclei, reticular formation, cerebellum) can modify tone through their effect on the gamma loop motor neuron.

The nature of the lesion at different levels of the nervous system may be quite variable. For example, ultrastructural abnormalities occur in congenital myopathies, including those related to respiratory chain defects; anterior horn cell apoptosis occurs in SMA and may also occur with ischemic insults; stripping of myelin by macrophages occurs with acute postinfectious polyneuritis; and muscle voltage-gated channels are involved in periodic paralyses.

Differential Diagnosis

Distinguishing clinical features in determining the level of the lesion in the nervous system includes pattern of weakness, activity of deep tendon reflexes, presence of fasciculations or sensory loss, cerebrospinal fluid findings, serum muscle enzyme levels, electromyography (EMG) pattern, nerve conduction studies, and histologic appearance or assays of the muscle or nerve biopsy. When assessing the child with hypotonia it is useful to separate first upper motor neuron or brain causes from neuromuscular causes (Table 128.1). The former includes perinatal hypoxia-ischemia, intracranial hemorrhage, and cerebral dysgenesis. These problems may present with hypotonia in infancy but later are evident as a static encephalopathy. A child with a CNS cause may have a below-normal level or range of attention and may lack age-appropriate social skills. Seizures or hemiparesis also signify a likely CNS cause. In an older child, fine motor coordination, quality and repertoire of movements, and language may be affected. The deep tendon reflexes may be brisk or easily elicited. The Babinski reflex may be dorsiflexor. In infants, fisting of the hands, scissoring of the extended legs on vertical suspension, and movement through postural reflexes, such as the asymmetric tonic neck reflex, are clues to an upper motor neuron process. In a patient with an upper motor neuron lesion, serum muscle enzymes, EMG, nerve conduction studies, and muscle biopsy are all normal. However, the child with 1 of the neuromuscular causes for hypotonia, including congenital myopathy, SMA, muscular dystrophy, and acute postinfectious polyneuritis, that does not also involve brain or facial nerves or muscles may have an interested and visually attentive facial appearance. This may be the case even in the presence of severe weakness that allows only sparse or nearly absent limb movements. With many of these neuromuscular lesions, deep tendon reflexes may be difficult to elicit or may be absent.

Older children may display a different pattern of weakness that is suggestive of the level of involvement. If the weakness is preferentially in the upper extremity extensor and lower extremity knee flexor, ankle dorsiflexor, and ankle evertor muscle groups (ie, anti-gravity muscles), the lesion likely involves the upper motor neuron. Static bilateral hemiparesis is suggestive of severe hypoxemicischemic insult, and in the child who was born preterm, diplegia is suggestive of periventricular leukomalacia. A differential weakness in opposing muscle groups resulting from CNS causes may be present and contribute to hypotonia for a long period before spasticity intervenes and increases tone. If the weakness involves agonist and antagonist muscles equally across a limb joint, however, it probably represents a neuromuscular process. It is also important to remember that hypotonia can result from disorders with combined lesions in levels above the lower motor neuron and in the motor unit. Examples include Krabbe disease, glycogen storage disease type 2 (ie, Pompe disease), congenital myotonic dystrophy, mitochondrial and peroxisomal disorders, and hypoxic-ischemic insults involving the upper motor neuron and anterior horn cell.

Evaluation

A methodical assessment using clinical and laboratory features in children to localize the lesion to a particular nervous system level can help anatomically narrow a wide differential diagnosis (Table 128.2).

History

Particular aspects of the history are especially important (Box 128.2). Strength of fetal movements relative to other pregnancies and ischemic, toxic, metabolic, or infectious fetal exposures should be ascertained. Relatively decreased fetal movements may signify an early degenerative condition. Polyhydramnios can signal prenatal weakness in swallowing. Birth events should be investigated for sources of possible insults to the neonatal nervous system, such as preterm status or birth asphyxia. The physician should determine whether any of the child’s developmental skills have been lost. Any associated loss of tone or strength could signify a progressive condition rather than a static problem, such as would occur with a birth injury. The acuity of the developing weakness is another important clue in the differential diagnosis. Rapid-onset hypotonia accompanied by constipation, poor feeding, and other bulbar involvement may be suggestive of botulism. Acute losses of tone and strength may also occur with enteroviral poliomyelitis, Guillain-Barré syndrome, myasthenia, and myositis. Relapsing-remitting courses of hypotonia may occur with myasthenia; metabolic myopathies, including mitochondrial dis-orders; and periodic paralysis in the older child. The child who has recently been hiking should be evaluated for tick paralysis. A parallel deterioration in intellectual functions or seizures may be indicative of leukodystrophy, storage disorder, or another degenerative disorder, including 1 of the several types of Leigh disease, particularly if ataxia, brain stem symptoms, and respiratory dysfunction subsequently occur.

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