81. Neonatal Hypotonia

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Neonatal Hypotonia / 531TABLE 81-3.HypotoniaClinical Features of Cerebral and PeripheralCerebralPeripheralClinical Feature Hypotonia HypotoniaOther abnormal brain function +++ ± (variable)Malformation of other organs + –Tendon reflexes Brisk or normal Decreased or absentPostural reflexes Preserved AbsentFisting of hands + –Fasciculations – ±Facial weakness – VariableLimb weakness ± +++– = feature may be absent; ± = feature variable (may be present or absent); + =feature is present; +++ = major feature (strongly present).A battery of laboratory investigations (eg, serum creatinekinase [CK], cerebrospinal fluid [CSF] analysis, nerveconduction studies and electromyography, neostigmine oredrophonium test, and muscle or sural nerve biopsy) maybe performed to determine the precise level of the pathologicalprocess affecting the motor unit. The greatestadvances in recent years have been made in moleculargenetic testing, which in many instances have obviatedmore invasive investigations.Specific aspects of these investigations relevant to newbornsrequire clarification to avoid pitfalls in the interpretationof results. Thus, elevated levels of serum concentrationsof CK generally imply skeletal or cardiac musclenecrosis, as may occur in rapidly progressive muscular dystrophiesand metabolic myopathies. The blood for CKmeasurement should be obtained before performing theelectromyography or muscle biopsy, as these proceduresmay cause false elevation of CK levels. The CK and isoenzymelevels may be increased 10-fold for up to 1 week followingnormal vaginal delivery (presumably due to muscletrauma). The CK may be even higher in the context ofacidosis (eg, severely asphyxiated newborns). Other biochemicalinvestigations (eg, serum lactate and carnitinelevels) may be required in specific circumstances.A chest radiograph may demonstrate enlarged cardiacsilhouette signifying cardiomyopathy or thin ribs, presumablyrelated to diminished fetal respiratory movements. Thelatter observation is a useful clue to antenatal onset of neuromusculardisease. Markedly increased protein concentrationin CSF may indicate peripheral neuropathy or specificdegenerative conditions (eg, Krabbe’s disease).Nerve conduction studies are consistent and reliableafter 32 weeks of gestation. Nerve conduction velocities,which are slower in normal newborns than in older individuals,may be very slow or unrecordable in the context ofcongenital peripheral neuropathies. The technical performanceand interpretation of the electromyogram (EMG)may be problematic in the newborn. For example, myotonicdischarges, which occur in older individuals with myotonicdystrophy, are rarely elicited in the newborn. Muscle biopsy(either by open surgical technique or needle biopsy) is technicallyfeasible in the newborn. Early biopsy may berequired if there is severe respiratory muscle involvementto provide definitive diagnosis and prognosis to assist managementdecisions. Otherwise, muscle biopsy is often postponedfor several months. In some instances (eg, congenitalmuscular dystrophy), repeated muscle biopsies may beuseful for establishing an accurate diagnosis.Major Disorders Associated withCerebral HypotoniaHypotonia is a feature of almost every cerebral disorder innewborns and older infants. In many instances, other presentingsymptoms or signs (eg, seizures, intracranial hemorrhage,trauma, and decreased level of consciousness associatedwith moderate or severe hypoxic-ischemicencephalopathy) suggest the most probable underlying diagnosis.However, other disorders may have few clinical featuresother than hypotonia, which may lead to a mistakendiagnosis of disease involving the motor unit. The mostcommon of these latter conditions are listed in Table 81-4and will be discussed briefly as follows.Brain and Spinal Cord Injuryor TraumaModerate or severe acute hypoxic-ischemic encephalopathyis usually suspected on the basis of clinical indicatorsof intrapartum hypoxic-ischemic insult in concert withpostnatal features of encephalopathy (eg, decreased level ofconsciousness or seizures). Intracranial hemorrhage maybe associated with seizures, anemia, and raised intracranialpressure. Cervical spinal cord injury, although uncommon,may be related to traction forces during vaginal delivery,especially in the context of difficult forceps delivery orbreech presentation. In such circumstances, the newborninfant is often flaccid, with absent spontaneous respirations.When infants are conscious, there may be evidenceof preserved cranial nerve function. Tendon reflexes maybe absent initially but become exaggerated later. The bladdermay be distended with overflow incontinence.Genetic and Chromosomal DisordersDown syndrome, the most common chromosomal disorderassociated with neonatal hypotonia, should be easily recognizable.Prader-Willi syndrome is associated with deletionsor translocations of the chromosome 15q11-13 region, andmost commonly, the abnormal chromosome is contributedby the father, although inheritance of both copies of chro-Current Management in Child Neurology, Third Edition© 2005 Bernard L. Maria, All Rights Reserved Neonatal HypotoniaBC Decker Inc Pages 528–534
532 / The Hospitalized ChildTABLE 81-4.NewbornMajor Causes of Central Hypotonia in theI. Hypotonia is present at birthBrain and spinal cord injury or trauma: hypoxic-ischemic encephalopathyand intracranial hemorrhageGenetic and chromosomal: Down syndrome and Prader-Willi syndromeCerebral dysgenesis and other structural cerebral abnormalitiesPeroxisomal disorders: cerebrohepatorenal syndrome (Zellwegersyndrome) and neonatal adrenoleukodystrophyOther rare metabolic or genetic conditions: generalized GM1, gangliosidosis,familial dysautonomia, Cohen syndrome, and oculocerebrorenalsyndrome (Lowe syndrome)Benign congenital hypotoniaII. Hypotonia usually develops shortly after birthSepsis, including meningitis and encephalitisMetabolic derangements or disordersEndocrine: hypothyroidismDrug intoxicationmosome 15 from the mother (maternal uniparental disomy)may also occur, often in the context of advancedmaternal age. In addition to profound hypotonia andhypogonadism, affected infants have severe feeding difficultiesthat may necessitate prolonged nasogastric tube feeding.Normal respiratory function is helpful for distinguishingthis condition from other disorders of the motor unit. Astime progresses, the muscle tone becomes normal and insatiablehunger, obesity, and mental retardation become evident.Diagnosis is confirmed by chromosome analysis withprophase banding. Specific treatment is not available.Other genetic conditions that present with neonatalhypotonia and feeding difficulties include familial dysautonomia,Cohen syndrome, and oculocerebrorenal syndrome(Lowe syndrome).Peroxisomal DisordersThese neurodegenerative disorders are caused by dysfunctionof peroxisomes, which are subcellular organelles thatassist in the catabolism of very-long-chain fatty acids,pipecolic acid, dicarboxylic acid, and phytanic acid and inthe synthesis of plasmalogens. The most reliable indicatorof peroxisomal disorders is an elevation of very-long-chainfatty acids in blood. Other biochemical abnormalities arehyperpipecolic acidemia and aciduria. Mutations of thePEX1 gene, which are responsible for more than 50% ofcases, may be analyzed in fibroblast cultures and may permitprenatal diagnosis by chorionic villus sampling.Zellweger syndrome (cerebrohepatorenal syndrome) ischaracterized by profound hypotonia, seizures, arthrogryposiswith camptodactyly (limited finger extension) andflexion deformities of knees and ankles, diminished tendonreflexes, and craniofacial dysmorphic features (eg, highforehead, hypertelorism, and unusual fullness of cheeks). Inaddition, there are abnormalities of other organs, includingbiliary cirrhosis, renal cysts, epiphyseal stippling, and retinaldegeneration. Neonatal adrenoleukodystrophy is anautosomal recessive peroxisomal disorder with hypotonia,failure to thrive, hepatomegaly, retinitis pigmentosa, andprogressive neurologic deterioration. Adrenal glands arehypoplastic but adrenal failure does not occur. There is nospecific treatment for peroxisomal disorders and deathoften occurs in early infancy.Benign Congenital HypotoniaThe term “benign congenital hypotonia” has been used todescribe infants who are hypotonic at birth but without aspecific underlying diagnosis and whose tone normalizesduring early childhood. Most probably the term encompassesmany different pathologic entities and affectedinfants have an increased incidence of other features ofcerebral dysfunction (eg, learning difficulties). In these situations,ongoing effort should be directed toward establishmentof a more specific diagnosis.Peripheral Causes of HypotoniaAbnormalities at any level of the motor unit may presentwith hypotonia in association with weakness and diminishedtendon reflexes. The major disorders are listed inTable 81-5.Anterior Horn Cell DiseasesThese disorders, specifically spinal muscular atrophy type 1(Werdnig-Hoffmann disease) account for approximatelyTABLE 81-5.Major Peripheral Causes of HypotoniaLevel of anterior horn cell (lower motor neuron)Spinal muscular atrophy type I (Werdnig-Hoffman disease)Glycogen storage disease type II (Pompe’s disease)Hypoxic-ischemic injuryNeonatal poliomyelitisLevel of peripheral nerveDemyelinating neuropathies: hereditary or chronic inflammatoryNeuroaxonal dystrophy (rare)MitochondrialLysosomal (Krabbe’s disease)Level of neuromuscular junctionMyasthenia: neonatal transient or congenital myasthenic syndromeToxic-metabolic: hypermagnesemia and aminoglycosidesInfant botulismLevel of the muscleMuscular dystrophy: congenital myotonic and congenital muscular dystrophyMetabolic myopathies: mitochondrial, glycogen disorder, and lipid disordersCongenital myopathies: nemaline (rod body), myotubular (centronuclear),and otherCurrent Management in Child Neurology, Third Edition© 2005 Bernard L. Maria, All Rights Reserved Neonatal HypotoniaBC Decker Inc Pages 528–534
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81. Neonatal Hypotonia

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