Developmental Disorders of Muscle

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Chapter 600 Developmental Disorders of Muscle

Harvey B. Sarnat

A heterogeneous group of congenital neuromuscular disorders is known as the congenital myopathies, but in some of these disorders, the assumption that the pathogenesis is primarily myopathic is unjustified. Most congenital myopathies are nonprogressive conditions, but some patients show slow clinical deterioration accompanied by additional changes in their muscle histology. Most of the diseases in the category of congenital myopathies are hereditary; others are sporadic. Although clinical features, including phenotype, can raise a strong suspicion of a congenital myopathy, the definitive diagnosis is determined by the histopathologic findings in the muscle biopsy specimen. In conditions for which the defective gene has been identified, the diagnosis may be established by the specific molecular analysis of the suspected gene expressed in lymphocytes. The morphologic and histochemical abnormalities differ considerably from those of the muscular dystrophies, spinal muscular atrophies, and neuropathies. Many are reminiscent of the embryologic development of muscle, thus suggesting possible defects in the genetic regulation of muscle development.

Myogenic Regulatory Genes and Genetic Loci of Inherited Diseases of Muscle

A family of four myogenic regulatory genes shares encoding transcription factors of “basic helix-loop-helix” (bHLH) proteins associated with common DNA nucleotide sequences (Table 600-1). These genes direct the differentiation of striated muscle from any undifferentiated mesodermal cell. The earliest bHLH gene to program the differentiation of myoblasts is myogenic factor 5 (Myf5). The second gene, myogenin, promotes fusion of myoblasts to form myotubes. Herculin (also known as MYF6) and MYOD1 are the other two myogenic genes. Myf5 cannot support myogenic differentiation without myogenin, MyoD, and MYF6. Each of these four genes can activate the expression of at least one other and, under certain circumstances, can autoactivate as well. The expression of MYF5 and of herculin is transient in early ontogenesis but returns later in fetal life and persists into adult life. The human locus of the MYOD1 gene is on chromosome 11, very near to the domain associated with embryonal rhabdomyosarcoma. The genes encoding Myf5 and herculin are on chromosome 12 and that for myogenin is on chromosome 1.

Table 600-1 INHERITANCE PATTERNS AND CHROMOSOMAL OR MITOCHONDRIAL LOCI OF NEUROMUSCULAR DISEASES AFFECTING THE PEDIATRIC AGE GROUP

DISEASE TRANSMISSION LOCUS
Duchenne and Becker muscular dystrophy XR Xp21.2
Emery-Dreifuss muscular dystrophy XR Xq28
Myotonic muscular dystrophy (Steinert) AD 19q13
Facioscapulohumeral muscular dystrophy AD 4q35
Limb-girdle muscular dystrophy AD 5q
Limb-girdle muscular dystrophy AR 15q
Congenital muscular dystrophy with merosin deficiency AR 6q2
Congenital muscular dystrophy (Fukuyama) AR 8q31-33
Myotubular myopathy XR Xq28
Myotubular myopathy AR Unknown
Nemaline rod myopathy (NEM1) AD 1q21-q23
Nemaline rod myopathy (NEM2) AR 2q21.2-q22
Nemaline rod myopathy (NEM3) AD, AR 1q42.1
Nemaline rod myopathy (NEM4) AD 9q13
Nemaline rod myopathy (NEM5) AR 19q13
Congenital muscle fiber-type disproportion AR, X-linked R 19p13.2, Xp23.12-p11.4, Xq13.1-q22.1; t(10; 17); sporadic
Central core disease AD 19q13.1
Myotonia congenita (Thomsen) AD 7q35
Myotonia congenita (Becker) AR 7q35
Paramyotonia congenita AD 17q13.1-13.3
Hyperkalemic periodic paralysis AD 17q13.1-13.3
Hyperkalemic periodic paralysis AD 1q31-q32
Glycogenosis II (Pompe; acid maltase deficiency) AR 17q23
Glycogenosis V (McArdle; myophosphorylase deficiency) AR 11q13
Glycogenosis VII (Tarui; phosphofructokinase deficiency) AR 1cenq32
Glycogenosis IX (phosphoglycerate kinase deficiency) XR Xq13
Glycogenosis X (phosphoglycerate mutase deficiency) AR 7p12-p13
Glycogenosis XI (lactate dehydrogenase deficiency) AR 11p15.4
Muscle carnitine deficiency AR Unknown
Muscle carnitine palmityltransferase deficiency 2 AR 1p32
Spinal muscular atrophy (Werdnig-Hoffmann; Kugelberg-Welander) AR 5q11-q13
Familial dysautonomia (Riley-Day) AR 9q31-33
Hereditary motor-sensory neuropathy (Charcot-Marie-Tooth; Dejerine-Sottas) AD 17p11.2
Hereditary motor-sensory neuropathy (axonal type) AD 1p35-p36
Hereditary motor-sensory neuropathy (Charcot-Marie-Tooth-X) XR Xq13.1
Mitochondrial myopathy (Kearns-Sayre) Maternal; sporadic Single large mtDNA deletion
Mitochondrial myopathy (MERRF) Maternal tRNA point mutation at position 8344
Mitochondrial myopathy (MELAS) Maternal tRNA point mutation at positions 3243 and 3271

AD, autosomal dominant; AR, autosomal recessive; MELAS, mitochondrial encephalopathy lactic acidosis, and stroke; MERRF, myoclonic epilepsy with ragged-red fibers; mtDNA, mitochondrial deoxyribonucleic acid; tRNA, transfer ribonucleic acid; XR, X-linked recessive.

The myogenic genes are activated during muscle regeneration, recapitulating the developmental process; MyoD in particular is required for myogenic stem cell (satellite cell) activation in adult muscle. PAX3 and PAX7 genes also play an important role in myogenesis and interact with each of the four basic genes mentioned above. Another gene, myostatin, is a negative regulator of muscle development by preventing myocytes from differentiating. The precise role of the myogenic genes in developmental myopathies is not yet fully defined.

Satellite cells in mature muscle that mediate regeneration have the same somitic origin as embryonic muscle progenitor cells, but the genes that regulate them differ. Pax3 and Pax7 mediate the migration of primitive myoblast progenitors from the myotomes of the somites to their peripheral muscle sites in the embryo, but only one of two Pax7 genes continues to act postnatally for satellite cell survival. Then it, too, no longer is required after the juvenile period for muscle satellite (i.e., stem) cells to become activated for muscle regeneration.

600.1 Myotubular Myopathy

Harvey B. Sarnat

The term myotubular myopathy implies a maturational arrest of fetal muscle during the myotubular stage of development at 8-15 wk of gestation. It is based on the morphologic appearance of myofibers: A row of central nuclei lies within a core of cytoplasm; contractile myofibrils form a cylinder around this core (Fig. 600-1). Many challenge this interpretation and use the more neutral term centronuclear myopathy when referring to this myopathy. This term is nonspecific because internal nuclei occur in many unrelated myopathies.

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Figure 600-1 Cross section of muscle from a 14 wk old human fetus (A), a normal full-term neonate (B), and a term neonate with X-linked recessive myotubular myopathy (C). Myofibers have large central nuclei in the fetus and in myotubular myopathy, and nuclei are at the periphery of the muscle fiber in the term neonate as in the adult (H&E, ×500).

Pathogenesis

The molecular mechanism appears similar in the X-linked recessive and autosomal recessive forms of myotubular myopathy. The common pathogenesis involves loss of myotubularin protein, leading to structural and functional abnormalities in the organization of T-tubules and sarcoplasmic reticulum and defective excitation-contraction coupling. This pathogenesis also provides a link to central core and multicore and minicore myopathies to at least partially explain the clinical and histopathologic similarities of these different congenital myopathies.

Persistently high fetal concentrations of vimentin and desmin are demonstrated in myofibers of infants with myotubular myopathy, although not reproduced in cultured myocytes of patients. These intermediate filament proteins serve as cytoskeletal elements in fetal myotubes, attaching nuclei and mitochondria to the sarcolemmal membranes to preserve their central positions. As intracellular organization changes with maturation, the nuclei move to the periphery and mitochondria are redistributed between myofibrils. At the same time, vimentin and desmin diminish. Vimentin disappears altogether by term, and desmin remains only in trace amounts. Persistent fetal vimentin and desmin in muscle fibers may be one mechanism of “maturational arrest.” A secondary myasthenia-like defect in neuromuscular transmission also occurs in some infants with myotubular myopathy. Myocytes of patients co-cultured with nerve in vitro develop normal innervation and mature normally, not reproducing the in vivo pathologic changes.

The defective gene of the X-linked form and 3 genes of the autosomal recessive form are known.

Clinical Manifestations

Fetal movements can decrease in late gestation. Polyhydramnios is a common complication because of pharyngeal weakness of the fetus and inability to swallow amniotic fluid.

At birth, affected infants have a thin muscle mass involving axial, limb girdle, and distal muscles; severe generalized hypotonia; and diffuse weakness. Respiratory efforts may be ineffective, requiring ventilatory support. Gavage feeding may be required because of weakness of the muscles of sucking and deglutition. The testes are often undescended. Facial muscles may be weak, but infants do not have the characteristic facies of myotonic dystrophy. Ptosis may be a prominent feature. Ophthalmoplegia is observed in a few cases. The palate may be high. The tongue is thin, but fasciculations are not seen. Tendon stretch reflexes are weak or absent.

Myotubular myopathy is not associated with cardiomyopathy (mature cardiac muscle fibers normally have central nuclei), but one report describes complete AV block without cardiomyopathy in a patient with confirmed X-linked myotubular myopathy. Congenital anomalies of the central nervous system or of other systems are not associated. A single patient with progressive dementia was reported, who had a mutation removing the start signal of exon 2.

Older children and adults can develop centronuclear myopathy with variable weakness. The relation of this disorder to the severe neonatal disease is uncertain.

Laboratory Findings

Serum levels of creatine kinase (CK) are normal. Electromyography (EMG) does not show evidence of denervation; results are usually normal or show minimal nonspecific myopathic features in early infancy. Nerve conduction velocity may be slow but is usually normal. The electrocardiogram (ECG) appears normal. Chest radiographs show no cardiomegaly; the ribs may be thin.

Diagnosis

The muscle biopsy findings are diagnostic at birth, even in premature infants. More than 90% of muscle fibers are small and have centrally placed, large vesicular nuclei in a single row. Spaces between nuclei are filled with sarcoplasm containing mitochondria. Histochemical stains for oxidative enzymatic activity and glycogen reveal a central distribution as in fetal myotubes. The cylinder of myofibrils shows mature histochemical differentiation with adenosine triphosphatase stains. The connective tissue of muscle spindles, blood vessels, intramuscular nerves, and motor end plates are mature. Ultrastructural features in neonatal myotubular myopathy, other than those that define the disease, are also mature. Vimentin and desmin show strong immunoreactivity in muscle fibers in myotubular myopathy and no demonstrable activity in normal term neonatal muscle. The molecular genetic marker in blood is available and is useful not only for confirming the diagnosis but also for early prenatal diagnosis. The differentiation from other forms of congenital myopathy is noted in Table 600-2.

Table 600-2 SPECIFIC CONGENITAL MYOPATHIES: DISTINGUISHING CLINICAL FEATURES

image

Genetics

X-linked recessive inheritance is the most common trait in this disease affecting boys. The mothers of affected infants are clinically asymptomatic, but their muscle biopsy specimens show minor alterations. Genetic linkage on the X chromosome has been localized to the Xq28 site, a locus different from the Xp21 gene of Duchenne and Becker muscular dystrophies. A deletion in the responsible MTM1 gene has been identified. It encodes a protein called myotubularin. This gene belongs to a family of similar genes encoding enzymatically active and inactive forms of phosphatidylinositol-3-phosphatases that form dimers. The pathogenesis is in the regulation of enzymatic activity and binding to other proteins induced by dimer interactions. Although only a single gene is involved, 5 distinct point mutations of the MTM1 gene out of 242 known mutations account for only 27% of cases; many different alleles can produce the same clinical disease. Other rarer centronuclear myopathies also are known, some being autosomal dominant or recessive and affecting both sexes and others being sporadic and of unknown genetics. The recessive forms are sometimes divided into an early-onset form with or without ophthalmoplegia and a late-onset form without ophthalmoplegia. Autosomal dominant forms are usually mild and might not manifest clinically until adult life as diffuse, slowly progressive weakness and generalized muscular pseudohypertrophy.

Treatment

Only supportive and palliative treatment is available. Progressive scoliosis may be treated by long posterior fusion.

Prognosis

About 75% of severely affected neonates die within the first few weeks or months of life. Survivors do not experience a progressive course but have major physical handicaps, rarely walk, and remain severely hypotonic.

Bibliography

Dowling JJ, Vreede AP, Low SE, et al. Loss of myotubularin function results in T-tubule disorganization in zebrafish and human myotubular myopathy. PLoS Genet. 2009;5:e1000372.

Lepper C, Conway SJ, Fan C-M. Adult satellite cells and embryonic muscle progenitors have distinct genetic requirements. Nature. 2009;460:627-631.

Pierson CR, Tomczak K, Agrawal P, et al. X-linked myotubular and centronuclear myopathies. J Neuropathol Exp Neurol. 2005;64:555-564.

600.2 Congenital Muscle Fiber-Type Disproportion

Harvey B. Sarnat

Congenital muscle fiber-type disproportion (CMFTD) occurs as an isolated congenital myopathy but also develops in association with various unrelated disorders that include nemaline rod disease, Krabbe disease (globoid cell leukodystrophy) early in the course before expression of the neuropathy, cerebellar hypoplasia and certain other brain malformations, fetal alcohol syndrome, some glycogenoses, multiple sulfatase deficiency, Lowe syndrome, rigid spine myopathy, and some infantile cases of myotonic muscular dystrophy. CMFTD should therefore be regarded as a syndrome, unless a specific genetic mutation is confirmed.

Pathogenesis

The association of CMFTD with cerebellar hypoplasia suggests that the pathogenesis may be an abnormal suprasegmental influence on the developing motor unit during the stage of histochemical differentiation of muscle between 20 and 28 wk of gestation. Muscle fiber types and growth are determined by innervation and are mutable even in adults. Although CMFTD does not actually correspond with any normal stage of development, it appears to be an embryologic disturbance of fiber type differentiation and growth.

Clinical Manifestations

As an isolated condition not associated with other diseases, CMFTD is a nonprogressive disorder present at birth. Patients have generalized hypotonia and weakness, but the weakness is usually not severe and respiratory distress and dysphagia are rare. Contractures are present at birth in 25% of patients. Poor head control and developmental delay for gross motor skills are common in infancy. Walking is usually delayed until 18-24 mo but is eventually achieved. Because of the hypotonia, subluxation of the hips can occur. Muscle bulk is reduced. The muscle wasting and hypotonia are proportionately greater than the weakness, and the child may be stronger than expected during examination. Cardiomyopathy is a rare complication.

The facies of children with CMFTD often raise suspicion, especially if the child is referred for assessment of developmental delay and hypotonia. The head is dolichocephalic, and facial weakness is present. The palate is usually high arched. Thin muscles of the trunk and extremities give a thin, wasted appearance. The phenotype is very similar to that of nemaline myopathy that also includes CMFTD as part of the pathological picture. Patients do not complain of myalgias. The clinical course is nonprogressive.

Laboratory Findings

Serum CK, ECG, EMG, and nerve conduction velocity results are normal in simple CMFTD. If other diseases are associated, laboratory investigation of those conditions discloses the specific features.

Diagnosis

CMFTD is diagnosed by muscle biopsy that shows disproportion in size and relative ratios of histochemical fiber types: Type I fibers are uniformly small, and type II fibers are hypertrophic; type I fibers are more numerous than type II fibers. Degeneration of myofibers and other primary myopathic features are absent. The biopsy is diagnostic at birth. Differentiating features from other congenital myopathies are noted in Table 600-2.

Genetics

Many cases of simple CMFTD are sporadic, although autosomal recessive inheritance is well documented in some families and an autosomal dominant trait is suspected in others. The genetic basis is heterogeneous in hereditary forms; a mutation in the insulin receptor gene at 19p13.2 is reported. Translocation t(10;17) was seen in one family. X-linked transmission with linkage to Xp23.12-p11.4 and Xq13.1-q22.1 also is described. In 3 unrelated families with CMFTD, a heterozygous missense mutation of the skeletal muscle α-actin gene (ACTA1) was demonstrated, but this genetic defect represents a minority; mutations in TPM3 is a more common genetic mutation. Whether these genes affect striated muscle directly or they mediate their expression through motor neurons remains uncertain. In CMFTD associated with cerebellar hypoplasia, the genetic effect is on cerebellar development and the muscular expression is secondary.

Treatment

No drug therapy is available. Physiotherapy may be helpful for some patients in strengthening muscles that do not receive sufficient exercise in daily activities. Mild congenital contractures often respond well to gentle range-of-motion exercises and rarely require plaster casting or surgery.

Bibliography

Clarke NF, Kolski H, Dye D, et al. Mutations in TPM3 are a common cause of congenital fiber type disproportion. Ann Neurol. 2008;63:327-329.

Clarke NF, North KN. Congenital fiber type disproportion—30 years on. J Neuropathol Exp Neurol. 2003;62:977-989.

Laing NC, Clarke NF, Dye DE, et al. Actin mutations are one cause of congenital fibre type disproportion. Ann Neurol. 2004;56:689-694.

Schuelke M, Wagner KR, Stolz LE, et al. Myostatin mutation associated with gross muscle hypertrophy in a child. N Engl J Med. 2004;350:2682-2688.

Selcen D, Ohno K, Engel AG. Myofibrillar myopathy: Clinical, morphological and genetic studies in 63 patients. Brain. 2004;127:439-451.

600.3 Nemaline Rod Myopathy

Harvey B. Sarnat

Nemaline rods (derived from the Greek nema, meaning “thread”) are rod-shaped, inclusion-like abnormal structures within muscle fibers. They are difficult to demonstrate histologically with conventional hematoxylin-eosin stain but are easily seen with special stains. They are not foreign inclusion bodies but rather consist of excessive Z-band material with a similar ultrastructure (Fig. 600-2). Chemically, the rods are composed of actin, α-actinin, tropomyosin-3, and the protein nebulin. Nemaline rod formation may be an unusual reaction of muscle fibers to injury because these rod structures have rarely been found in other diseases. They are most abundant in the congenital myopathy known as nemaline rod disease. Most rods are within the myofibrils (cytoplasmic), but intranuclear rods are occasionally demonstrated by electron microscopy.

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Figure 600-2 Electron micrograph of the muscle from a patient shown in Figure 600-4. Nemaline rods (nr) are seen within many myofibrils. They are identical in composition to the normal Z bands (z) (×6,000).

Clinical Manifestations

Neonatal, infantile, and juvenile forms of the disease are known. The neonatal form is severe and usually fatal because of respiratory failure since birth. In the infantile form, generalized hypotonia and weakness, which can include bulbar-innervated and respiratory muscles, and a very thin muscle mass are characteristic (Fig. 600-3). The head is dolichocephalic, and the palate high arched or even cleft. Muscles of the jaw may be too weak to hold it closed (Fig. 600-4). Decreased fetal movements are reported by the mother, and neonates suffer from hypoxia and dysphagia; arthrogryposis may be present. Infants with severe neonatal and infantile nemaline myopathy have facies and phenotype that are nearly indistinguishable from those of neonatal myotonic dystrophy, but their mothers have normal facies. The juvenile form is the mildest and is not associated with respiratory failure, but the phenotype, including facial involvement, is similar.

image

Figure 600-3 Back of a 13 yr old girl with the juvenile form of nemaline rod disease. The paraspinal muscles are very thin, and winging of the scapulas is evident. The muscle mass of the extremities is also greatly reduced proximally and distally.

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Figure 600-4 Infantile form of nemaline rod disease in a 6 yr old boy. Facial weakness and generalized muscle wasting are severe. The head is dolichocephalic. The mouth is usually open because the masseters are too weak to lift the mandible against gravity for more than a few seconds.

Laboratory Findings

Serum CK level is normal or mildly elevated. The muscle biopsy is diagnostic. In addition to the characteristic nemaline rods, it also shows CMFTD or at least fiber type I predominance. In some patients, uniform type I fibers are seen with few or no type II fibers. Focal myofibrillar degeneration and an increase in lysosomal enzymes have been found in a few severe cases associated with progressive symptoms. Intranuclear nemaline rods correlate with the most severe clinical manifestations. They are demonstrated by electron microscopy. Because nemaline bodies can occur in other myopathies, their presence in the muscle biopsy is not pathognomonic in the absence of the supportive clinical manifestations. Adult-onset cases may be associated with monoclonal gammopathy.

Genetics

Autosomal dominant and autosomal recessive forms of nemaline rod disease occur, and an X-linked dominant form in girls also can occur. Five genes are associated with this condition. One autosomal dominant nemaline rod myopathy (NEM1) has been mapped to the 1q21-23 locus; the responsible TPM3 gene produces defective α-tropomyosin. Another genetic mutation (NEM2) at the 2q21.2-q22 locus produces nebulin, a large molecule also needed for Z-band integrity, and is transmitted as an autosomal recessive trait, particularly in Ashkenazi Jews. Nebulin defects account for half the total cases of nemaline myopathy. NEM3 is due to an α-actin defect and both autosomal dominant and recessive varieties occur at the same 1q42.1 locus. NEM4 is an autosomal dominantly inherited defect of β-tropomyosin at 9q13. The α- and β-tropomyosin defects are rare and account for only 3% of patients with nemaline myopathy. NEM5 is an autosomal recessive troponin-T defect at 19q13, but it has been found only in the Amish population; in the Amish community the incidence of nemaline myopathy is as high as 1:500, whereas in Australia it is estimated at 1:500,000. The myopathy occurs in all ethnic groups.

Treatment and Prognosis

Therapy is supportive. Survivors are confined to an electric wheelchair and are usually unable to overcome gravity. Both proximal and distal muscles are involved. Congenital arthrogryposis can occur and predicts a poor prognosis. Gastrostomy may be needed for chronic dysphagia. In the juvenile form, patients are ambulatory and are able to perform most tasks of daily living. Weakness is not usually progressive, but some patients have more difficulty over time or enter a phase of progressive weakness. Cardiomyopathy is an uncommon complication. Death usually results from respiratory insufficiency, with or without superimposed pneumonia.

Bibliography

Wallgren-Pattersson C, Pelin K, Nowak KH, et al. Genotype-phenotype correlations in nemaline myopathy caused by mutations in the genes for nebulin and skeletal muscle α-actin. Neuromuscul Disord. 2004;14:461-470.

600.4 Central Core, Minicore, and Multicore Myopathies

Harvey B. Sarnat

Central core myopathies are transmitted as either an autosomal dominant or recessive trait and are caused by the same abnormal gene at the 19q13.1 locus. The gene programs the ryanodine receptor (RYR1), a tetramere receptor to a non–voltage-gated calcium channel in the sarcoplasmic reticulum. Mutations in this gene are also the cause of malignant hyperthermia. Infantile hypotonia, proximal weakness, muscle wasting, and involvement of facial muscles and neck flexors are the typical features in both the dominant and recessive forms. Contractures of the knees, hips, and other joints are common, and kyphoscoliosis and pes cavus often develop, even without much axial or distal muscle weakness. There is a high incidence of cardiac abnormalities. The course is not progressive, except for the contractures.

The disease is characterized pathologically by central cores within muscle fibers in which only amorphous, granular cytoplasm is found with an absence of myofibrils and organelles. Histochemical stains show a lack of enzymatic activities of all types within these cores. The serum CK value is normal in central core disease except during crises of malignant hyperthermia (Chapter 603.2). Central core disease is consistently associated with malignant hyperthermia, which can precede the diagnosis of central core disease. All patients should have special precautions with pretreatment by dantrolene before an anesthetic agent is administered.

Variants of central cores, called minicores and multicores, are described in some families, but minicore myopathy is a different genetic disease without gender bias. Cases with a similar mutation in the RYR1 gene are reported; others have a defective selenoprotein-N (SEPN1) gene, the latter also implicated in rigid spine myopathy, but together these 2 genes account for only half of patients genetically tested. Children with this disorder are hypotonic in early infancy and have a benign course but often develop progressive kyphoscoliosis or a rigid spine in adolescence. Distal joint hypermobility is another finding, particularly in ryanodine-mediated minicore myopathy. In one variant, external ophthalmoplegia also is present. Rare cases of minicore myopathy also show hypertrophic cardiomyopathy associated with short chain acyl-CoA dehydrogenase deficiency.

Preliminary trials of salbutamol therapy for central core and minicore myopathy suggest a potentially effective treatment.

Bibliography

Quinlivan RM, Muller CR, Davis M, et al. Central core disease: clinical, pathological, and genetic features. Arch Dis Child. 2003;88:1051-1055.

600.5 Myofibrillar Myopathies

Harvey B. Sarnat

Most myofibrillar myopathies are not symptomatic in childhood, but occasionally older children and adolescents show early symptoms of nonspecific proximal and distal weakness. An infantile form also occurs and can cause mild neonatal hypotonia and weakness with disproportionately severe dysphagia and respiratory insufficiency, at times leading to early death. It is not progressive, however, and some patients show improvement in later infancy and early childhood, acquiring the ability to swallow by 3 yr of age. Cardiomyopathy is a complication in a minority. The diagnosis is by muscle biopsy: some sarcomeres of myofibers have disorganization or dissolution of myofibrils adjacent to other areas of normal sarcomeres within the same fiber. These zones are associated with streaming of the Z-bands and focally increased desmin intermediate filaments, myotilin, and αB-crystallin. Immunocytochemical and ultrastructural study of the muscle biopsy tissue is required. Mutation in the desmin gene is implicated as the etiology. An associated mitochondrial defect is detected in some patients.

A unique autosomal recessive myopathy in Cree native infants is characterized by severe generalized muscular hypertonia that is not relieved by neuromuscular blockade, hence is myopathic in origin. Most die in infancy of respiratory insufficiency due to diaphragmatic involvement. The muscle biopsy shows findings similar to many other myofibrillar myopathies (Fig. 600-5).

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Figure 600-5 Electron micrograph (EM) of quadriceps femoris muscle biopsy of a 1-month-old native girl with Cree myofibrillar myopathy. Within the same myofiber, some sarcomeres are well formed and others exhibit disarray of the thick and thin myofilaments and fragmentation of Z-bands. Mitochondria appear normal (×21,400).

600.6 Brain Malformations and Muscle Development

Harvey B. Sarnat

Infants with cerebellar hypoplasia are hypotonic and developmentally delayed, especially in gross motor skills. Muscle biopsy is sometimes performed to exclude a congenital myopathy. A biopsy specimen can show delayed maturation of muscle, fiber-type predominance, or CMFTD. Other malformations of the brain may also be associated with abnormal histochemical patterns, but supratentorial lesions are less likely than brainstem or cerebellar lesions to alter muscle development. Abnormal descending impulses along bulbospinal pathways probably alter discharge patterns of lower motor neurons that determine the histochemical differentiation of muscle at 20-28 wk of gestation. The corticospinal tract does not participate because it is not yet functional during this period of fetal life. In several congenital muscular dystrophies, including the Walker-Warburg syndrome, Fukuyama disease, and muscle-eye-brain disease of Santavuori, major cerebral malformations, such as pachygyria and lissencephaly, are present.

Bibliography

Sarnat HB. Cerebral dysgeneses and their influence on fetal muscle development. Brain Dev. 1986;8:495-499.

600.7 Amyoplasia

Harvey B. Sarnat

Congenital absence of individual muscles is common and is often asymmetric. A common aplasia is the palmaris longus muscle of the ventral forearm, which is absent in 30% of normal subjects and is fully compensated by other flexors of the wrist. Unilateral absence of a sternocleidomastoid muscle is one cause of congenital torticollis. Absence of one pectoralis major muscle is part of the Poland anomalad.

When innervation does not develop, as in the lower limbs in severe cases of myelomeningocele, muscles can fail to develop. In sacral agenesis, the abnormal somites that fail to form bony vertebrae can also fail to form muscles from the same defective mesodermal plate, a disorder of induction resulting in segmental amyoplasia. Skeletal muscles of the extremities fail to differentiate from embryonic myomeres if the long bones do not form. Absence of 1 long bone, such as the radius, is associated with variable aplasia or hypoplasia of associated muscles, such as the flexor carpi radialis. End-stage neurogenic atrophy of muscle is sometimes called amyoplasia, but this use is semantically incorrect.

Generalized amyoplasia usually results in fetal death, and liveborn neonates rarely survive. A mutation in 1 of the myogenic genes is the suspected etiology because of genetic knockout studies in mice, but it has not been proven in humans.

600.8 Muscular Dysgenesis (Proteus Syndrome Myopathy)

Harvey B. Sarnat

Proteus syndrome is a disturbance of cellular growth involving ectodermal and mesodermal tissues. The cause is unknown, but it is not a mendelian trait. It manifests as asymmetric overgrowth of the extremities, verrucous cutaneous lesions, angiomas of various types, thickening of bones, syndromic hemimegalencephaly, and excessive growth of muscles without weakness. Severe seizures, beginning in neonates, are uncommon. Histologically, the muscle demonstrates a unique muscular dysgenesis. Abnormal zones are adjacent to zones of normal muscle formation and do not follow anatomic boundaries.

600.9 Benign Congenital Hypotonia

Harvey B. Sarnat

Benign congenital hypotonia is not a disease, but it is a descriptive term for infants or children with nonprogressive hypotonia of unknown origin. The hypotonia is not usually associated with weakness or developmental delay, although some children acquire gross motor skills more slowly than normal. Tendon stretch reflexes are normal or hypoactive. There are no cranial nerve abnormalities, and intelligence is normal.

The diagnosis is one of exclusion after results of laboratory studies, including muscle biopsy and imaging of the brain with special attention to the cerebellum, are normal (see Table 599-2). No known molecular genetic basis for this syndrome has been identified. The differential diagnosis is noted in Table 599-3.

The prognosis is generally good; no specific therapy is required. Contractures do not develop. Physical therapy might help achieve motor milestones (walking) sooner than expected. Hypotonia persists into adult life. The disorder is not always as “benign” as its name implies because a common complication is recurrent dislocation of joints, especially the shoulders. Excessive motility of the spine can result in stretch injury, compression, or vascular compromise of nerve roots or of the spinal cord. These are particular hazards for patients who perform gymnastics or who become circus performers because of agility of joints without weakness or pain.

Bibliography

Laugel V, Cossee M, Matis J, et al. Diagnostic approach to neonatal hypotonia: retrospective study on 144 neonates. Eur J Pediatr. 2008;167:517-523.

Mintz-Itkin R, Lerman-Sagie T, Zuk L, et al. Does physical therapy improve outcome in infants with joint hypermobility and benign hypotonia? J Child Neurol. 2009;24:714-719.

600.10 Arthrogryposis

Harvey B. Sarnat

Arthrogryposis multiplex congenita is not a disease but is a descriptive term that signifies numerous congenital contractures (Chapter 674).

Bibliography

Compton AG, Cooper ST, Hill PM, et al. The syntrophin-dysbrevin subcomplex in human neuromuscular disorders. J Neuropathol Exp Neurol. 2005;64:350-361.

Goebel HH. Congenital myopathies in the new millennium. J Child Neurol. 2005;20:94-101.

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