Infantile and juvenile spinal muscular atrophy

Magnetic resonance reveals mitochondrial dysfunction and muscle remodelling in spinal muscular atrophy

Genetic therapy has changed the prognosis of hereditary proximal spinal muscular atrophy, although treatment efficacy has been variable. There is a clear need for deeper understanding of underlying causes of muscle weakness and exercise intolerance in patients with this disease to further optimize treatment strategies. Animal models suggest that in addition to motor neuron and associated musculature degeneration, intrinsic abnormalities of muscle itself including mitochondrial dysfunction contribute to the disease aetiology.

To test this hypothesis in patients, we conducted the first in vivo clinical investigation of muscle bioenergetics. We recruited 15 patients and 15 healthy age and gender-matched control subjects in this cross-sectional clinico-radiological study. MRI and ³¹P magnetic resonance spectroscopy, the modality of choice to interrogate muscle energetics and phenotypic fibre-type makeup, was performed of the proximal arm musculature in combination with fatiguing arm-cycling exercise and blood lactate testing. We derived bioenergetic parameter estimates including: blood lactate, intramuscular pH and inorganic phosphate accumulation during exercise, and muscle dynamic recovery constants. A linear correlation was used to test for associations between muscle morphological and bioenergetic parameters and clinico-functional measures of muscle weakness.

MRI showed significant atrophy of triceps but not biceps muscles in patients. Maximal voluntary contraction force normalized to muscle cross-sectional area for both arm muscles was 1.4-fold lower in patients than in controls, indicating altered intrinsic muscle properties other than atrophy contributed to muscle weakness in this cohort. In vivo³¹P magnetic resonance spectroscopy identified white-to-red remodelling of residual proximal arm musculature in patients on the basis of altered intramuscular inorganic phosphate accumulation during arm-cycling in red versus white and intermediate myofibres. Blood lactate rise during arm-cycling was blunted in patients and correlated with muscle weakness and phenotypic muscle makeup. Post-exercise metabolic recovery was slower in residual intramuscular white myofibres in patients demonstrating mitochondrial ATP synthetic dysfunction in this particular fibre type.

This study provides the first in vivo evidence in patients that degeneration of motor neurons and associated musculature causing atrophy and muscle weakness in 5q spinal muscular atrophy type 3 and 4 is aggravated by disproportionate depletion of myofibres that contract fastest and strongest. Our finding of decreased mitochondrial ATP synthetic function selectively in residual white myofibres provides both a possible clue to understanding the apparent vulnerability of this particular fibre type in 5q spinal muscular atrophy types 3 and 4 as well as a new biomarker and target for therapy.

Selective loss of alpha motor neurons with sparing of gamma motor neurons and spinal cord cholinergic neurons in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease characterised primarily by loss of lower motor neurons from the ventral grey horn of the spinal cord and proximal muscle atrophy. Recent experiments utilising mouse models of SMA have demonstrated that not all motor neurons are equally susceptible to the disease, revealing that other populations of neurons can also be affected. Here, we have extended investigations of selective vulnerability of neuronal populations in the spinal cord of SMA mice to include comparative assessments of alpha motor neuron (α-MN) and gamma motor neuron (γ-MN) pools, as well as other populations of cholinergic neurons. Immunohistochemical analyses of late-symptomatic SMA mouse spinal cord revealed that numbers of α-MNs were significantly reduced at all levels of the spinal cord compared with controls, whereas numbers of γ-MNs remained stable. Likewise, the average size of α-MN cell somata was decreased in SMA mice with no change occurring in γ-MNs. Evaluation of other pools of spinal cord cholinergic neurons revealed that pre-ganglionic sympathetic neurons, central canal cluster interneurons, partition interneurons and preganglionic autonomic dorsal commissural nucleus neuron numbers all remained unaffected in SMA mice. Taken together, these findings indicate that α-MNs are uniquely vulnerable among cholinergic neuron populations in the SMA mouse spinal cord, with γ-MNs and other cholinergic neuronal populations being largely spared.

Spectrum of neuropathophysiology in spinal muscular atrophy type I

Neuropathologic findings within the central and peripheral nervous systems in patients with spinal muscular atrophy type I (SMA-I) were examined in relation to genetic, clinical, and electrophysiologic features. Five infants representing the full clinical spectrum of SMA-I were examined clinically for compound motor action potential amplitude and SMN2 gene copy number; morphologic analyses of postmortem central nervous system, neuromuscular junction, and muscle tissue samples were performed and SMN protein was assessed in muscle samples. The 2 clinically most severely affected patients had a single copy of the SMN2 gene; in addition to anterior horn cells, dorsal root ganglia, and thalamus, neuronal degeneration in them was widespread in the cerebral cortex, basal ganglia, pigmented nuclei, brainstem, and cerebellum. Two typical SMA-I patients and a milder case each had 2 copies of the SMN2 gene and more restricted neuropathologic abnormalities. Maturation of acetylcholine receptor subunits was delayed and the neuromuscular junctions were abnormally formed in the SMA-I patients. Thus, the neuropathologic findings in human SMA-I are similar to many findings in animal models; factors other than SMN2 copy number modify disease severity. We present a pathophysiologic model for SMA-I as a protein deficiency disease affecting a neuronal network with variable clinical thresholds. Because new treatment strategies improve survival of infants with SMA-I, a better understanding of these factors will guide future treatments.

Spinal muscular atrophy: DNA fragmentation and immaturity of muscle fibers

The presence of apoptotic fibers and the embryonic proteins desmin and vimentin were investigated in muscle biopsy specimens from patients with spinal muscular atrophy (SMA). Apoptosis was studied in 24 cases of SMA by means of in situ end labeling of nuclear DNA fragmentation using TUNEL staining and immunohistochemistry. Apoptotic nuclei were observed in 54.1% of the cases, and desmin and vimentin positive fibers were found in the majority of cases. A significant negative correlation was observed between the number of apoptotic nuclei and the duration of the disease, as well as between the number of desmin and vimentin positive fibers and the age of onset. These findings indicate that apoptosis, although probably a secondary phenomenon following denervation, plays a role in the progress of spinal muscular atrophy.

Modern management of spinal muscular atrophy

Spinal muscular atrophy is an incurable disease with a frequency of 8 per 100,000 live births. The disease gene, survival motor neuron 1 (SMN1), was identified with a disease modifying gene, SMN2. There is a high mortality rate in infancy and severe morbidity in childhood. Management depends on treating or preventing complications of weakness and maintaining quality of life. Weakness may affect several organ systems: respiratory, due to restrictive lung disease; gastrointestinal, in terms of dysphagia and constipation; and orthopedic, with progressive deformities. This review focuses on management of restrictive lung disease, the most common and most serious complication. Three areas of recent development are noninvasive ventilation using new technology, new awareness of the importance of identifying sleep-disordered breathing, and a new multidisciplinary approach to standard of care. Noninvasive ventilation and improved airway clearance are helpful for preoperative and postoperative management. Standard of care requires a multidisciplinary approach.

Cognitive impairment in neuromuscular disorders

Several studies have suggested the presence of central nervous system involvement manifesting as cognitive impairment in diseases traditionally confined to the peripheral nervous system. The aim of this review is to highlight the character of clinical, genetic, neurofunctional, cognitive, and psychiatric deficits in neuromuscular disorders. A high correlation between cognitive features and cerebral protein expression or function is evident in Duchenne muscular dystrophy, myotonic dystrophy (Steinert disease), and mitochondrial encephalomyopathies; direct correlation between tissue-specific protein expression and cognitive deficits is still elusive in certain neuromuscular disorders presenting with or without a cerebral abnormality, such as congenital muscular dystrophies, congenital myopathies, amyotrophic lateral sclerosis, adult polyglucosan body disease, and limb-girdle muscular dystrophies. No clear cognitive deficits have been found in spinal muscular atrophy and facioscapulohumeral dystrophy.

Analysis of creatine kinase activity in 504 patients with proximal spinal muscular atrophy types I-III from the point of view of progression and severity

Mild to moderately elevated creatine kinase (CK) activity is a frequent biochemical finding in proximal spinal muscular atrophy (SMA). In a collaborative study on all types of childhood and juvenile onset SMA, we analysed the CK activity of 504 SMA patients (138 type I, 127 type II, 144 type IIIa, and 95 type IIIb patients). Under the assumption of a lognormal distribution of CK activity as the most appropriate statistical model, CK levels were transformed into logarithms and compared by standard deviation scores = CK-SDS (log). CK activity was statistically different between early and later onset SMA: in SMA I and II, about one-third of patients showed CK-SDS (log) >2 SD, the analysis of the means did not show significant differences. In SMA III, CK-SDS (log) was significantly higher (p < 0.01) than in the two other groups, which was most pronounced in SMA IIIb. More than 90% of SMA IIIb patients showed CK-SDS (log) values >2 vs. 57% in SMA IIIa. As similar values were obtained for a subgroup of 100 patients in whom the diagnosis of autosomal recessive SMA was confirmed by a deletion of the telomeric copy of the survival motor neuron gene, our results can be considered representative for SMA I-III. There was no correlation between CK level and disease duration. The fact that patients were ambulatory or chair-bound had no influence on CK activity in type III SMA. There was no sex influence in SMA I, II and IIIa. The observed higher male values in the group SMA IIIb are most likely the result of a lack of female patients with onset after puberty.

A collaborative study on the natural history of childhood and juvenile onset proximal spinal muscular atrophy (type II and III SMA): 569 patients

We analyzed clinical data of 569 patients in two combined series with childhood and juvenile proximal SMA. This cohort included only patients who had achieved the ability to sit unaided (type II and III SMA). The survival rate among 240 type II patients (who sat but never walked) was 98.5% at 5 years and 68.5% at 25 years. SMA III (n = 329) (those who walked and had symptoms before age 30 years) was subdivided into those with an onset before and after age 3 years (type IIIa, n = 195; SMA IIIb, n = 134). In patients with SMA III, life expectancy is not significantly less than a normal population. The probabilities of being able to walk at 10 years after onset was 70.3%, and at 40 years, 22.0% in SMA IIa. For SMA IIIb, 96.7% were walking 10 years after onset and 58.7% at 40 years. The subdivision of type III SMA was justified by the probability of being ambulatory depending on age at onset; the prognosis differed for those with onset before or after age 3 years. The data provide a reliable basis of the natural history of proximal SMA and support a classification system that is based primarily on age at onset and the achievement of motor milestones.

Spinal muscular atrophy in childhood

Diagnosis and classification of spinal muscular atrophy (SMA) in childhood are based on clinical, electrophysiological, and histological studies. The concept of maturational arrest of motoneurons and their targets (muscle cells in SMA type I) is documented by ultrastructural and immunohistochemical data. The prolongated or markedly delayed process of muscle cell and motoneuron elimination by apoptosis seen in SMA type I is discussed according to the new finding of a gene for a neuronal apoptosis inhibitory protein that is partially deleted in children with spinal muscular atrophy.

Fatty acid oxidation abnormalities in childhood-onset spinal muscular atrophy: primary or secondary defect(s)?

The purpose of this study was to further identify and quantify the fatty acid oxidation abnormalities in spinal muscular atrophy, correlate these with disease severity, and identify specific underlying defect(s). Fifteen children with spinal muscular atrophy (3 type I, 8 type II, 4 type III) were studied. Serum carnitine total/free ratios demonstrated a tendency toward an increased esterified fraction ranging 35-58% of total carnitine (normal: 25-30% of total) in younger children with types I and II. The remaining type II and III patients, older than 23 months of age at sampling, had normal esterified carnitine levels. Urinary organic acid analysis demonstrated mild to moderate medium-chain dicarboxylic aciduria in type I patients and normal, mild, or moderate increases in short-chain and medium-chain organic acids in type II patients. In the type III group, the organic acids were normal except for one patient with mild medium-chain dicarboxylic aciduria. Muscle intramitochondrial beta-oxidation was measured in 5 children (2 type I, 2 type II, and 1 type III) and a significant reduction in the activities of short-chain L-3-hydroxyacyl-CoA dehydrogenase, long-chain L-3-hydroxyacyl-CoA dehydrogenase, acetoacetyl-CoA thiolase, and 3-ketoacyl-CoA thiolase were found; however, normal crotonase activity was documented. Most strikingly, there was a marked increase (3- to 5-fold) in the activity ratios of crotonase to L-3-hydroxyacyl-CoA dehydrogenase and thiolase activities with both short- and long-chain substrates. The combined abnormalities suggest a defect in a mitochondrial multifunctional enzyme complex, distinct from the trifunctional enzyme. These abnormalities may be either primary or secondary and may respond to dietary measures to reduce the dependence on fatty acid oxidation.

Prenatal diagnosis of spinal muscular atrophy in Russia

Ninety-two families with spinal muscular atrophy (SMA) applied for genetic counselling and further prenatal diagnosis. To minimize expenses, only one tightly linked informative marker was determined in the course of preliminary examination, and non-radioactive allele detection was preferably used. Four prenatal diagnoses of SMA type I, four of SMA type II, and one of SMA type III were made. This trial programme shows the considerable requirements, importance, and potential effectiveness of prenatal prediction of SMA in Russia.

Biochemical and immunocytochemical analysis in chronic proximal spinal muscular atrophy

Immunocytochemical and biochemical analyses were carried out on patients affected by chronic SMA. Three groups of patients were identified. In group I, the muscle presented a fascicular atrophy; a high percentage of atrophic type II fibers; and fibers expressing fast, slow, embryonic, and fetal myosin isoforms. In group II, the muscle was characterized by atrophic fibers and normal/hypertrophic fibers expressing only slow myosin isoforms. In group III, the muscle was characterized by fiber type grouping and fibers coexpressing fast and slow myosin isoforms but never embryonic or fetal MHC isoforms. The muscles of groups I and III contained both fast and slow myosins whereas group II muscles were predominantly slow by immunocytochemical analysis or only slow by biochemical analysis. In view of these results, immunocytochemical and histochemical analyses could help to classify chronic SMA and help to understand the different pathogenic processes which seem to be related to the maturational stage of the muscle at the age of onset of the disease.

Chronic childhood spinal muscular atrophy in Germany (West-Thüringen)--an epidemiological study

This study presents the most extensive epidemiological data on chronic forms of spinal muscular atrophy in childhood (CSMA) in West-Thüringen in Germany. The incidence of CSMA was calculated to be 1 in 9,420 live births. The prevalence was 1.624 in 100,000 of the general population (as of 31 December 1980).

Epidemiological data on Werdnig-Hoffmann disease in Germany (West-Thüringen)

This study contains the largest body of epidemiological data on Werdnig-Hoffmann disease (acute infantile spinal muscular atrophy; ASMA) in West-Thüringen in Germany. The incidence of ASMA was calculated to be 1 in 10,202 live births. The prevalence was 1 in 595,362 of the general population (as of 31 December 1987). The study gives an unexpectedly high incidence rate confirming the suggestion that ASMA in Central and Eastern Europe might be more frequent than in Western Europe. However, we consider that this high incidence rate in West-Thüringen is a result of the almost complete ascertainment made possible because of the well-organised and centralised health system existing in Thüringen over the last few decades.

Muscle biopsy and the clinical course of infantile spinal muscular atrophy

Eight infants with severe early infantile spinal muscular atrophy diagnosed by clinical presentation and muscle biopsy were studied. The extent of alterations in muscle histology, histochemistry, and ultrastructure did not reflect the relative severity of the clinical presentation or the course of the illness. In seven biopsies, ultrastructural studies demonstrated empty sleeves of basal lamina projecting from the surface of small myofibers. We conclude that severe infantile spinal muscular atrophy often results in myofiber atrophy similar to that found in other motor neuron diseases, and it is not solely a hypotrophic process. Muscle biopsy findings are important because they help to establish the diagnosis, but they do not help predict the severity of disease among infants with this condition.

Chronic progressive and relapsing neuromyopathy with massive dilatations of endoplasmic reticulum in muscle fibers

Large intracytoplasmic inclusions arising from the endoplasmic reticulum and nuclear envelope were found in the muscle fibers of a 32-year-old individual with a life-long, chronic and progressive sensory-motor neuropathy. The morphological features of the inclusions were similar to that occasionally reported in the striated muscles in several unrelated conditions as well as to "hyaline" or "colloid" inclusions commonly seen in motor neurons of the brain stem and spinal cord. The chemical nature of the inclusions is not known. Their occurrence in the muscle fibers is probably secondary to chronic denervation.

Population data on acute infantile and chronic childhood spinal muscular atrophy in Warsaw

This study provides epidemiological data on acute infantile (ASMA) and chronic childhood spinal (CSMA) muscular atrophy in Warsaw for the period 1976-1985. All calculations are based on the assumption that ASMA and CSMA result from mutations at two different gene loci. The incidence of ASMA and CSMA was 1 in 19474 live births with a corresponding gene and carrier frequency of 714 x 10(-5) and 1 in 70, respectively. The prevalence of CSMA for the year 1985 was 1.26 x 10(-5). These figures are higher than in similar studies in other countries. This fact might be connected with the careful ascertainment in this study.

Diaphragmatic paralysis due to spinal muscular atrophy. An unrecognised cause of respiratory failure in infancy?

An unusual form of spinal muscular atrophy presenting with respiratory failure was observed in four infants from two families. In one, whose death was attributed to pneumonia, the diagnosis was inferred retrospectively after two siblings died from an identical illness and were shown to have diaphragmatic paralysis and the typical electrophysiological and histological features of spinal muscular atrophy. Other signs of skeletal muscular weakness were absent or inconspicuous. The fourth, unrelated infant presented in an identical way but has survived for over a year on a ventilator. Two months after the onset of respiratory paralysis, more extensive skeletal muscular weakness was seen. Other infants, dying of unexplained respiratory illness, may have this disorder and some may be included in the miscellany of disorders that constitute the sudden infant death syndrome.

A clinico-pathological study of chronic hereditary motor neuropathy

Forty cases of chronic hereditary motor neuropathy (CHMN) were divided into five categories according to the distribution of muscle atrophy; they were proximal, facioscapulohumeral, bulbospinal, distal and scapuloperoneal forms. Their clinical features and laboratory data were analysed, and muscle biopsies from 32 of them were studied by histological, histochemical and electron microscopical methods. An attempt at quantitative assessment of the histological changes was also made. All muscle biopsies showed a mixture of neurogenic and 'myopathic' changes in varying proportions. They showed more 'myopathic' changes than Werdnig-Hoffmann's disease, amyotrophic lateral sclerosis and other neurogenic atrophies except Charcot-Marie-Tooth disease. There was marked variation in the average number of atrophied muscle fibers contained in grouped atrophy. Frequently, deranged internal structure of the muscle fibres was revealed both by histochemical and electron microscopical methods. Based on the evidence of heterogeneity of CHMN in respect of genetics, clinical features and histological changes, it was speculated that CHMN represents a group of diseases which involves primarily different parts of the motor units.

Classification of spinal muscular atrophies

Clinical heterogeneity within the spinal muscular atrophies (SMA) has long been a source of confusion for questions of prognosis and genetic counselling. Comprehensive clinical and genetic analyses of 240 consecutive index cases from two English centres (The English SMA Study) have enabled some nosological questions to be resolved. The different SMA syndromes can be discriminated by (a) age at the first clinical signs of the disease, (b) pattern of muscle involvement, (c) age at death of other patients within an affected kindred, and (d) genetic evidence. Seven different SMA syndromes can be defined clinically and genetically; thirteen mutant genes are incriminated. Prevalence and incidence figures are presented. SMA type I (Werdnig-Hoffman disease) and chronic childhood SMA together comprise 74% of all SMA cases. The classification of the spinal muscular atrophies presented also provides the differential diagnosis for newly presenting cases.

Spinal muscular atrophy and hypertrophy of the calves

In 100 patients (59 males and 41 females) suffering from proximal spinal muscular atrophy, 23 presented with hypertrophy of the calves. These were all males. They all showed an elevation of serum CK-activity. The combination of spinal muscular atrophy, hypertrophy of the calves and elevated serum CK activity was not found in females.

Congenital hypotonia revisited

The definition of neuromuscular diseases affecting infants has depended on factors as various as the rate of progression of the illness, the clinical picture, and, recently, the morphologic peculiarities in the muscle biopsy. A review of the literature suggests that there are discrepancies in the classification of such illnesses, no matter what system is used. In some instances, a single diagnosis seems to include patients with quite separate illnesses, whereas other patients with seemingly identical diseases have been given different diagnoses.

Intelligence level of patients with the Duchenne type of progressive muscular dystrophy (pmd-d)

The I.Q. of 129 patients with PMD-D and 27 patients suffering from Werdnig-Hoffmann disease were estimated. Among the patients with PMD-D there was one group without any complicating factors and 3 other groups with additional factors that might influence the intelligence level. Comparing mean values and distribution of I.Q. for all these groups, one can conclude that, besides additional unfavourable pathological and environmental factors in all cases, PMD-D itself causes a small decrease of the I.Q. by about 1 SD. The frequent changes of the EEG record in these patients could reflect involvement of the CNS by the pathological process.

Genetic investigations on chronic forms of infantile and juvenile spinal muscular atrophy

A material of 247 cases selected from 260 cases of spinal muscular atrophy in the Warsaw Department of Neurology in 1960-1974 was analyzed. The size of sibships was established and calculations were made of the mean distribution of the age at onset, also according to sex, for the different clinical forms, genetical proportions by the method of siblings and of probands, and coefficient of sib-sib correlation for the material as a whole and separately for males, females and male-female pairs. The analysis shows the course of the disease to differ between the sexes and to be mild in males more often than in females, as is particularly noticeable in the higher age groups. Cases of Kugelberg-Welander's disease are predominantly male. The hypothesis is advanced that a proportion of male patients have a sex-linked modifying gene of a fairly high frequency (possibly of the range of 1 in 5 males, and 1 in 25, in the homozygous state, in females). Although it would not disprove conclusively the nosological distinctness of different forms of infantile and juvenile spinal muscular atrophy, the existence of the modifying gene, if proved, would tend rather to add to the likelihood of their constituting a single recessive autosomal disease.

Cardiac involvement in the Kugelbert-Welander syndrome

Two cases of the Kugelberg-Welander syndrome (juvenile form of progressive spinal muscular atrophy) associated with cardiomyopathy and cardiomegaly are presented. The first patient, a 24 year old man, had atrial flutter with complete atrioventricular (A-V) block due to A-H block. Echocardiography revealed an increase in the left atrial and right ventricular dimensions. The second patient was a 26 year old man whose electrocardiogram revealed an A-V junctional rhythm, deep Q wave in leads I, aVL and V5 to V6 and an RS pattern in lead V1. Histologic examination of the myocardium in Case 2 showed slight interstitial fibrosis. Review of previously reported cases shows that (1) the atrium, the ventricular myocardium and A-V conducting tissue may be involved, and (2) atrial arrhythmias, A-V conduction disturbances and congestive heart failure may occur in the Kugelberg-Welander syndrome.

Morphological differences between the atrophied small muscle fibres in amyotrophic lateral sclerosis and Werdnig-Hoffmann disease

Muscle biopsies from 5 cases of Werdnig-Hoffmann disease and 4 cases of ALS were investigated by histology and electronmicroscopy. The differences in morphology found in the atrophic muscle fibers in ALS and in W.H. disease consisted mainly in the shape and structure of their cells and the number of the nuclei. Atrophic muscle fibres irregular in shape and size with degenerative changes and accumulated nuclei observed in ALS were markedly different from the picture of the fibres in W.H. disease. The presence of small muscle cells uniform diameter with well preserved architecture and numerous myotube-like cells seem to indicate a foetal defect in children with W.H. disease.

Ultrastructural aspects of muscle and nerve in Werdnig-Hoffmann disease

The authors describe in biopsies from 6 cases of Werdnig-Hoffmann disease, including 2 of the more benign type, the ultrastructural typical aspects of denervation. They compare their findings with those of other workers. The striking points are the great variation in the diameter of the muscle fibres and the myofibrils, the disorganisation of the myofibrils, the sarcomeres and the filaments, with persistance of the relations between thick and thin filaments at various levels, the modifications of the Z-band and the triads in chains. The folds and the basement membrane are examined. Centrioles are present in a muscle fibre and in a satellite. Glycogen is very abundant. The nerves seem normal but some Schwann cells contain pi granules which are not observed usually at the age of the patient. The end plates and a muscle spindle are normal.

Electromyographic studies in parents of children with spinal muscular atrophy

Electromyographic studies have been carried out on the quadriceps and deltoid muscles of a number of healthy parents of children with infantile spinal muscular atrophy (types I and II) and the results compared with those obtained in healthy controls of the same sex. The results indicated an increase in mean amplitude of action potentials in quadriceps and deltoid muscles in both fathers and mothers (statistically significant in the quadriceps in the latter) of children with type I spinal muscular atrophy (Werdnig-Hoffmann disease). Increase in the mean amplitude in the quadriceps and deltoid muscles in mothers of children with type II spinal muscular atrophy and the mean duration of action potentials in the quadriceps muscle in mothers of children with type I and type II spinal muscular atrophy were also observed, but the differences from controls were not significant.

The nosology of the spinal muscular atrophies

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