Hypotonic Infant

E. Finanger , T.K. Koch , in Encyclopedia of the Neurological Sciences (Second Edition), 2014

Introduction

Hypotonia, one of the most common abnormalities of the motor system observed in infants, is distinct from muscle weakness. It refers to a decreased resting tone (tension) of the muscle and decreased resistance to passive movement. Weakness, however, refers to a reduction in the maximum power that can be generated against resistance or gravity. Because normal tone requires the integrity of both the central and peripheral nervous systems, hypotonia is a common sign of both central and/or peripheral neurological dysfunction in infants. The central motor system consists of the cerebral hemispheres, brainstem, cerebellum, and spinal cord, whereas the peripheral motor system consists of anterior horn cells, nerve fibers including the myelin sheath, neuromuscular junctions, and muscles ( Figure 1). Hypotonia caused by central nervous system disease or dysfunction is termed central hypotonia, whereas disorders of the peripheral nervous system cause peripheral hypotonia. Primary disorders affecting the anterior horn cell body are referred to as neuronopathies, disorders of the nerve are neuropathies, and muscle disorders are myopathies. Hypotonia is rarely seen in isolation without some degree of weakness. Comparing the severity of weakness to the severity of the hypotonia can be a helpful clinical tool in determining whether the etiology is central or peripheral.

Figure 1. Neuromuscular motor unit with anterior horn cell residing in the ventral gray matter of the spinal cord, nerve fiber with axon and surrounding myelin sheath, the neuromuscular junction, and muscle.

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Neonatal Hypotonia

Keung-kit Chan MBBS, MRCPCH , Basil T Darras MD , in Neonatology: Questions and Controversies Series: Neurology, 2008

DIFFERENTIAL ANATOMIC DIAGNOSIS OF HYPOTONIA

Hypotonia may be the manifestation of pathology involving the central nervous system (CNS), the peripheral nervous system (i.e., lower motor unit), or both ( Table 10-2). 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 (e.g., Moro or asymmetric tonic neck response) and the degree of weakness noted in the infant is usually less than the degree of hypotonia ("non-paralytic" hypotonia) (Table 10-3). 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 10-3). 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.

Because muscle tone is also determined by the viscoelastic properties of muscle and joints, connective tissue disorders such as Marfan, Ehlers–Danlos syndromes or osteogenesis imperfecta and also 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, some of the congenital muscular dystrophies, peroxisomal disorders, mitochondrial encephalomyopathies, neuroaxonal dystrophy, leukodystrophies (e.g., globoid cell leukodystrophy), familial dysautonomia, and asphyxia secondary to motor unit disease (Table 10-4). Further, hypotonia without significant weakness may be a feature of systemic diseases like sepsis, congenital heart disease, hypothyroidism, rickets, renal tubular acidosis, and others.

Neuromuscular diseases in infancy present primarily with hypotonia and weakness; however, infants with severe hypotonia but only marginal weakness usually do not have a disorder of the lower motor unit (anterior horn cell, peripheral and cranial nerves, neuromuscular junction, and muscle). These infants may have genetic conditions, metabolic disturbances, or systemic disorders (e.g., congenital heart disease, renal failure). Early on, neonates with central nervous system pathology may present with profound hypotonia, decreased reflexes, and moderate to severe but transient weakness; however, they also tend to have seizures, obtundation, cranial nerve signs, and/or history of perinatal asphyxia. With recovery, they gradually develop better strength, increased muscle stretch reflexes, and muscle tone distally first, in contrast to the asphyxiated infants with disorders of the lower motor unit, in whom the weakness, hypotonia, and hyporeflexia persist. Alternatively, profound weakness and hypotonia without signs of CNS involvement occur in newborn infants with isolated neuromuscular disease and no history of perinatal asphyxia. Muscle stretch reflexes vary depending on the anatomical level of pathology along the motor unit (i.e., 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 (8).

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Femur, hip and pelvis

Jane E Carreiro DO , in Pediatric Manual Medicine, 2009

Clinical Notes

Hypotonia in children may be due to primary muscle disease, peripheral neuropathy, endocrinopathy or neuromuscular junction anomalies. In cases of acute hypotonia electrolyte imbalance and anemia need to be considered. In many children with low muscle tone the etiology is unknown. These children may also have ligamentous laxity. Boys appear to be affected more than girls. Fortunately, most children with idiopathic hypotonia have moderate to mild disease.

Children with hypotonia create balance and stability mechanisms by locking weightbearing joints and shortening antigravity muscles. Many children with hypotonia often develop stiffness and myofascial restriction in the antigravity muscles. There is shortening of the adductor and quadriceps muscles, which produces an anterior tilt to the pelvis. Tibial rotation and torsion can be present, and depending on the severity of the hypotonia, femoral anteversion may persist. Children with hypotonia often have pes planus and genu valgus functional deformities. Recurrent ankle sprain is a hazard that can be helped by wearing high-top sneakers or ankle foot orthotics if necessary. Knee, hip and back pain are recurrent and frequent complaints owing to the postural mechanisms employed by the child. The cervical lordosis is often reduced, with a compensatory extension of the occiput on the atlas. This gives the child a head-forward posture. This shortening of the suboccipital muscles is typically the basis for the recurrent cervicogenic cephalgia from which many of these children suffer. Suboccipital muscle tightness may trigger cervicomandibular reflexes in the muscles of mastication, leading to bruxisms. (This same mechanism can also be a problem in children with spasticity.)

Isotonic eccentric and concentric techniques are useful in older children with hypotonia. Isotonic eccentric techniques can be used to address shortening of antigravity muscles. Concentric muscle energy techniques may help to strengthen and improve firing patterns in deconditioned flexors and long restrictors. Isometric techniques can be used to address articular dysfunctions that develop secondary to abnormal motor functioning. Facilitated positional release, balanced ligamentous technique and other indirect techniques are appropriate in children of all ages with hypotonia.

The child with hypotonia will benefit from activities or exercises that reinforce appropriate postural and balance strategies while strengthening muscles. Examples of these include horse riding, martial arts and some forms of dance. It goes without saying that the activity needs to be customized to the capabilities of the child and should foster self-confidence. Any activity that employs balance, form and movement can be used to reinforce the core stabilizing mechanisms while strengthening. The author had one 8-year-old patient with primary muscle disease who benefited from a love affair with duckpin bowling. He eventually learned to play ambidextrously, because we insisted that he had to practice rolling the ball with each hand.

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Neonatal Hypotonia and Neuromuscular Disorders

Jahannaz Dastgir DO , ... Basil T. Darras MD , in Neurology: Neonatology Questions and Controversies (Second Edition), 2012

Differential Anatomic Diagnosis of Hypotonia

Neonatal hypotonia may be the manifestation of pathology involving the central nervous system (CNS), the peripheral nervous system (i.e., lower motor unit), or both ( Box 14-2). In infants with cerebral or central hypotonia, nearly two thirds of cases, the perinatal or prenatal history may suggest a CNS insult. There may also be associated global (rather than an isolated gross motor) developmental delay, occasionally seizures, microcephaly, dysmorphic features, and/or malformation of the brain and/or other organs. Central hypotonia may be associated with brisk and/or persistent primitive reflexes and normal to brisk muscle stretch reflexes. The degree of weakness noted in infants with central hypotonia is usually less than the degree of hypotonia (nonparalytic hypotonia) (Table 14-1). In lower motor unit hypotonia or peripheral hypotonia, developmental delay is primarily gross motor and is associated with absent or depressed muscle stretch reflexes and/or muscle atrophy and fasciculations of the tongue. In general, antigravity limb movements are decreased and cannot be elicited via postural reflexes. In infants with this condition, the degree of weakness is proportional or in excess of the degree of hypotonia (paralytic hypotonia) (see Table 14-1). Trauma to the high cervical cord due to traction in breech or cervical presentation may also initially manifest as flaccid paralysis, which may be asymmetric, and absence of muscle stretch reflexes; later on, however, upper motor neuron signs develop.

Because muscle tone is also determined by the viscoelastic properties of muscles and joints, connective tissue disorders such as Marfan's and Ehlers-Danlos syndromes, osteogenesis imperfecta, and also benign laxity of the ligaments can manifest as hypotonia. In addition, there is combined cerebral and lower motor unit hypotonia seen in infants and older children with congenital myotonic dystrophy, some congenital muscular dystrophies, peroxisomal disorders, mitochondrial encephalomyopathies, neuroaxonal dystrophy, leukodystrophies (e.g., globoid cell leukodystrophy), familial dysautonomia, and asphyxia secondary to motor unit disease (Box 14-3). Further, hypotonia without significant weakness may be a feature of systemic diseases such as sepsis, congenital heart disease, hypothyroidism, rickets, and renal tubular acidosis.

Neuromuscular diseases in infancy manifest primarily with hypotonia and weakness; however, infants with severe hypotonia, and only marginal weakness, usually do not have a disorder of the lower motor unit (anterior horn cell, peripheral and cranial nerves, neuromuscular junction, and muscle). These infants may have genetic conditions, metabolic disturbances, or systemic disorders (e.g., congenital heart disease, renal failure). Early on, neonates with CNS pathology may present with profound hypotonia, decreased reflexes, and moderate to severe but transient weakness; however, they also tend to have seizures, obtundation, cranial nerve abnormalities, and/or a history of perinatal asphyxia. With recovery, they gradually develop better strength, increased muscle stretch reflexes, and muscle tone often distal to proximal. This picture is in contrast to that of asphyxiated infants with disorders of the lower motor unit in whom the weakness, hypotonia, and hyporeflexia persist. Alternatively, profound weakness and hypotonia without signs of CNS involvement occur in newborn infants with isolated neuromuscular disease and no history of perinatal asphyxia. Muscle stretch reflexes vary depending on the anatomic level of pathology along the motor unit (i.e., prominent hyporeflexia or total areflexia in anterior horn cell disorders and neuropathies, reduced reflexes in proportion to the degree of weakness in myopathies, and often normal reflexes in disorders of the neuromuscular junction). Again, approximately two thirds of patients with neonatal hypotonia have cerebral etiologies, and one third have lower motor unit diseases. 8

Box 14-4 lists the most common causes of cerebral (central) hypotonia. Prasad and Prasad 9 review the metabolic and genetic disorders manifesting with hypotonia and suggest a diagnostic algorithm.

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Hypotonia, Arthrogryposis, and Rigidity

Gerald M. Fenichel , in Neonatal Neurology (Fourth Edition), 2007

Cerebral Hypotonia with Acute Encephalopathy

Hypotonia is a constant feature of acute encephalopathies in all age groups. The differential diagnosis in the newborn is summarized in Table 3-1 and is detailed in other chapters. All acute encephalopathies cause generalized cerebral dysfunction because of edema, inflammation, or hemorrhage. States of decreased consciousness are invariably associated with the hypotonia, and seizures may be present as well. During the acute phase, the tendon reflexes are unobtainable, and the Moro reflex is depressed. As the encephalopathy clears, postural tone remains depressed, but tendon reflexes are obtainable and ankle clonus is common. The Moro reflex is re-established, and jitteriness observed.

The outcome for the child depends directly on the underlying process. In general, muscle tone tends to improve with time. The diagnostic category used to describe infants who were hypotonic at birth and later recovered normal tone is benign congenital hypotonia. The term is retrospective and not applicable to the newborn. In many such infants, the cause of the original hypotonia was an acute encephalopathy that later resolved. Despite the recovery of normal muscle tone, an increased incidence of mental retardation, learning disabilities, and other sequelae of cerebral stress are evident later in life. In others, the cause either is a mild myopathy or not established.

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General introduction to paediatric neurology

C deSousa , H Rattue , in Physical Management in Neurological Rehabilitation (Second Edition), 2004

Hypotonia

Hypotonia in infancy may be due to neuromuscular disorders or to central causes (including neurological disorders, chromosomal abnormalities and metabolic disorders). There are some children who are hypotonic in infancy in the absence of discernible underlying disease and who can be markedly delayed in their early motor development, but with normal acquisition of other developmental skills. After infancy many such children often have little residual abnormality, if any. The term 'benign congenital hypotonia' has been applied to this group, which probably includes more than one cause for this striking variation in early development. A similar group of children often have a familial tendency to joint hypermobility, in the absence of a definable disorder of connective tissue, and many of them are also often delayed in their early gross motor development. Hypotonia and other abnormalities of tone and movement are discussed in Chapters 4 and 25.

The role of physiotherapy includes the assessment of such children and the differentiation of patterns which are normal variants from those due to underlying disease. This requires familiarity with the range of normal development (see Ch. 17).

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Membrane transporters and the diseases corresponding to functional defects

Yurong Lai , in Transporters in Drug Discovery and Development, 2013

SLC3A1 and SLC7A9: cystinuria, cystinuria-lysinuria and hypotonia-cystinuria syndrome

Hypotonia-cystinuria syndrome (HCS), formerly known as homozygous 2p16 deletion syndrome, is a recessive contiguous gene syndrome. The disorder is characterized by neonatal and infantile hypotonia and failure to thrive, cystinuria (type 1) and nephrolithiasis, growth hormone deficiency leading to growth retardation, and minor facial dysmorphism ( Jaeken et al., 2006). Two recessive contiguous gene deletions that differ in size are associated with HCS: one is a microdeletion on chromosome 2p21 that disrupts two genes, SLC3A1 and PREPL; the other is a larger homozygous deletion that disrupts two other genes C2orf34 and PPM1B (Chabrol et al., 2008). The SLC3A1 gene localized at 2p16.3 encodes one part of an amino acid transporter protein (Pras et al., 1996; Bisceglia et al., 2007). This subunit, along with another protein encoded by SLC7A9, forms a functional complex, the heterodimeric amino-acid transporter, which absorbs neutral/dibasic amino acids (Fernandez et al., 2002). Mutations of SLC3A1 cause abnormal amino acid reabsorption in the kidneys resulting in certain amino acids becoming concentrated in the urine leading to HCS (Font-Llitjos et al., 2005; Martens et al., 2007).

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The Floppy Infant

Graeme A.M. Nimmo , Ronald D. Cohn , in Swaiman's Pediatric Neurology (Sixth Edition), 2017

Central Hypotonia

Hypotonia in the absence of weakness is suggestive of disorders affecting the central nervous system. This suspicion is often further supported with findings of altered level of consciousness, dysmorphic features, major congenital anomalies, and/or the presence of seizures. The creatine kinase (CK) is normal.

The approach to an infant with central hypotonia should, as in all areas of medicine, be directed at identifying the most life-threatening and actionable diagnoses first with consideration of the most common etiologies next. It is therefore imperative that the general health of the infant be a part of the initial assessment. Both sepsis and severe congenital heart disease can cause a picture of central hypotonia. Hypoxic-ischemic injury accounts for approximately one-third of infants with hypotonia. Intraventricular hemorrhage accounts for a significant proportion of preterm neonates with hypotonia.

All infants presenting with hypotonia should have a CK level measured. Creatine kinase is not raised with CNS disease, but care must be taken in interpreting this in the early neonatal period as CK may be normally raised in the first few days of life. Magnetic resonance imaging of the brain should be obtained to identify structural abnormalities. Importantly, a cranial ultrasound or MRI may also identify abnormalities characteristic of HIE or IVH as reviewed in Chapter 19. Additional investigations based on the history and examination should be considered in order to identify and characterize any congenital abnormalities that may be a part of a particular syndrome. Further testing may include echocardiogram, abdominal ultrasound, ophthalmology assessment, and/or hearing testing. Biochemical testing for inborn errors of metabolism should be considered and include plasma amino acids, urine organic acids, acylcarnitine profile, glycosylation patterns, and lactate/pyruvate. Additional biochemical testing may be warranted with specific clinical findings suggestive of peroxisomal disorders, Smith-Lemli-Opitz syndrome, or in the presence of seizures as outlined in Figure 138-1.

The findings of multiple congenital abnormalities, dysmorphic facial features, or delay in cognitive milestones should prompt consideration of a syndromic diagnosis or chromosomal abnormalities, and involvement of a geneticist may be valuable. Depending on the clinical context, genetic tests may be indicated and include karyotype, microarray, fluorescence in situ hybridization (FISH), and/or methylation analysis (Watson et al., 2014). The more common diagnoses, including Down syndrome, Prader-Willi syndrome, and William syndrome, are listed in Table 138-2 with their distinguishing features.

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Down Syndrome: From Understanding the Neurobiology to Therapy

Ira T. Lott , in Progress in Brain Research, 2012

Hypotonia

Hypotonia is ubiquitous in infants with DS and is defined as decreased resistance to passive muscle stretch ( Fig. 2). The ligamentous laxity resulting from hypotonia is associated with a delay in motor development (Carr, 1970; Melyn and White, 1973). Infants with DS show a sequence of motor development similar to typically developing toddlers but a lower rate of motor milestone acquisition (Agiovlasitis et al., 2009). The hypotonia induces difficulty in postural control such that an individual with DS is focused on overcoming a lack of equilibrium compared to controls and this necessitates a different strategy in motor development (Rigoldi et al., 2011). The instability in trunk control may be linked to atypical finger grasping patterns (Jover et al., 2010). In a quantitative movement protocol measuring functional mobility, individuals with DS appeared to show longer durations of execution across all of the standardized tasks (Galli et al., 2010). As a consequence of hypotonia, individuals with DS have inherent joint laxity resulting in reduced gait stability and increased energetic costs for physical exertion (Agiovlasitis et al., 2009). These atypical gait patterns include longer stance time, decreased hip extension, and increased hip abduction during the swing phase. Hypotonia in DS is often associated with low levels of physical activity with the result of decreased bone mass accrual and predisposition to fractures (Hawli et al., 2009). The low levels of physical activity in DS result in a higher body mass index, lower levels of lean mass, and reduced bone mass-related parameters, which, in turn, may affect cardiovascular strength capacities (Gonzalez-Aguero et al., 2010).

Fig. 2. Hypotonia in Down syndrome. Note the head lag upon pull to sitting and the inability to support posture in ventral suspension.

Ligamentous laxity in DS provokes instability of vertebral movement at the atlanto-axial junction. The occiput, the atlas (C1), and the axis (C2) normally form a functional unit which assures a high degree of mobility of the upper cervical spine providing that strong ligaments keep these structures in place. In individuals with DS, the excessive laxity of the posterior transverse ligament which attaches the odontoid bone to C1 appears to be the most common cause of atlanto-axial subluxation although there is no universal agreement on this point (Merrick et al., 2000). In addition, C-1 hypoplasia and occult spinal canal stenosis have been linked to the increased risk of compressive cervical myelopathy in individuals with DS.

The anatomic locus for hypotonia in DS appears to be the cerebellum, a structure which shows hypocellularity due to impairment in the granular and periventricular zones during fetal development (Guidi et al., 2011). Additionally, the cerebellum plays a major role in the regulation of proprioceptive-motor control and motor learning. Beyond its effect on motor functioning, disorganized cerebellar output may impair higher order functioning such as emotion and cognition (Schmahmann, 2004; Teipel et al., 2004).

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Neonatal Hypotonia

Joseph J. Volpe , in Neuromuscular Disorders of Infancy, Childhood, and Adolescence (Second Edition), 2015

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. 1 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.

Figure 6.1. Major features of neonatal hypotonia ("floppy infant"). With vertical suspension (A), note the dangling lower limbs with lack of hip flexion, tendency of upper limbs to slip through the examiner's hands, and lack of neck flexion with resulting head lag. When subject is supine, note the lack of traction response (B) and the lag of head (C) with attempts by examiner to pull infant to sitting position. With horizontal suspension (not shown), the child drapes over the examiner's hand.

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