Nosology of juvenile muscular atrophy of distal upper extremity: from monomelic amyotrophy to Hirayama disease--Indian perspective

Since its original description by Keizo Hirayama in 1959, "juvenile muscular atrophy of the unilateral upper extremity" has been described under many nomenclatures from the east. Hirayama disease (HD), also interchangeably referred to as monomelic amyotrophy, has been more frequently recognised in the west only in the last two decades. HD presents in adolescence and young adulthood with insidious onset unilateral or bilateral asymmetric atrophy of hand and forearm with sparing of brachioradialis giving the characteristic appearance of oblique amyotrophy. Symmetrically bilateral disease has also been recognized. Believed to be a cervical flexion myelopathy, HD differs from motor neuron diseases because of its nonprogressive course and pathologic findings of chronic microcirculatory changes in the lower cervical cord. Electromyography shows features of acute and/or chronic denervation in C7, C8, and T1 myotomes in clinically affected limb and sometimes also in clinically unaffected contralateral limb. Dynamic forward displacement of dura in flexion causes asymmetric flattening of lower cervical cord. While dynamic contrast magnetic resonance imaging is diagnostic, routine study has high predictive value. There is a need to lump all the nomenclatures under the rubric of HD as prognosis in this condition is benign and prompt diagnosis is important to institute early collar therapy.

Mutation in E1, the ubiquitin activating enzyme, reduces Drosophila lifespan and results in motor impairment

Neurodegenerative diseases cause tremendous suffering for those afflicted and their families. Many of these diseases involve accumulation of mis-folded or aggregated proteins thought to play a causal role in disease pathology. Ubiquitinated proteins are often found in these protein aggregates, and the aggregates themselves have been shown to inhibit the activity of the proteasome. These and other alterations in the Ubiquitin Pathway observed in neurodegenerative diseases have led to the question of whether impairment of the Ubiquitin Pathway on its own can increase mortality or if ongoing neurodegeneration alters Ubiquitin Pathway function as a side-effect. To address the role of the Ubiquitin Pathway in vivo, we studied loss-of-function mutations in the Drosophila Ubiquitin Activating Enzyme, Uba1 or E1, the most upstream enzyme in the Ubiquitin Pathway. Loss of only one functional copy of E1 caused a significant reduction in adult lifespan. Rare homozygous hypomorphic E1 mutants reached adulthood. These mutants exhibited further reduced lifespan and showed inappropriate Ras activation in the brain. Removing just one functional copy of Ras restored the lifespan of heterozygous E1 mutants to that of wild-type flies and increased the survival of homozygous E1 mutants. E1 homozygous mutants also showed severe motor impairment. Our findings suggest that processes that impair the Ubiquitin Pathway are sufficient to cause early mortality. Reduced lifespan and motor impairment are seen in the human disease X-linked Infantile Spinal Muscular Atrophy, which is associated with mutation in human E1 warranting further analysis of these mutants as a potential animal model for study of this disease.

Observational study of caloric and nutrient intake, bone density, and body composition in infants and children with spinal muscular atrophy type I

Clinical experience supports a critical role for nutrition in patients with spinal muscular atrophy (SMA). Three-day dietary intake records were analyzed for 156 visits in 47 SMA type I patients, 25 males and 22 females, ages 1month to 13years (median 9.8months) and compared to dietary reference intakes for gender and age along with anthropometric measures and dual-energy X-ray absorptiometry (DEXA) data. Using standardized growth curves, twelve patients met criteria for failure to thrive (FTT) with weight for age <3rd percentile; eight met criteria based on weight for height. Percentage of body fat mass was not correlated with weight for height and weight for age across percentile categories. DEXA analysis further demonstrated that SMA type I children have higher fat mass and lower fat free mass than healthy peers (p<0.001). DEXA and dietary analysis indicates a strong correlation with magnesium intake and bone mineral density (r=0.65, p<0.001). Average caloric intake for 1-3years old was 68.8±15.8kcal/kg - 67% of peers' recommended intake. Children with SMA type I may have lower caloric requirements than healthy age-matched peers, increasing risk for over and undernourished states and deficiencies of critical nutrients. Standardized growth charts may overestimate FTT status in SMA type I.

[Questionnaire survey conducted on the parents of patients with spinal muscular atrophy type 1 in Japan regarding switch devices, language development, upper extremity function and QOL]

OBJECTIVE

As information required to rehabilitate, we aimed to clarify the use of switch devices, language development, upper extremity function, and quality of life (QOL) in patients with spinal muscular atrophy (SMA) type 1 in Japan.

METHODS

A questionnaire survey was conducted on the parents of these patients.

RESULTS

A total of 48 completed questionnaires were collected. The median age of patients with SMA type 1 was 8 years (range, 13 months to 27 years). The patients used a switch device were 46.5%. Their language development was delayed, and their voluntary movement in the distal upper extremities was relatively retained. In QOL, parents' labor and burden for the care of patients were high.

CONCLUSIONS

It is necessary to provide specialist support that contributes to the spread of communication devices, which improve their motor function, language development and overall QOL.

Prospective cohort study of spinal muscular atrophy types 2 and 3

OBJECTIVE

To characterize the natural history of spinal muscular atrophy type 2 and type 3 (SMA 2/3) beyond 1 year and to report data on clinical and biological outcomes for use in trial planning.

METHODS

We conducted a prospective observational cohort study of 79 children and young adults with SMA 2/3 who participated in evaluations for up to 48 months. Clinically, we evaluated motor and pulmonary function, quality of life, and muscle strength. We also measured SMN2 copy number, hematologic and biochemical profiles, muscle mass by dual x-ray absorptiometry (DXA), and the compound motor action potential (CMAP) in a hand muscle. Data were analyzed for associations between clinical and biological/laboratory characteristics cross-sectionally, and for change over time in outcomes using all available data.

RESULTS

In cross-sectional analyses, certain biological measures (specifically, CMAP, DXA fat-free mass index, and SMN2 copy number) and muscle strength measures were associated with motor function. Motor and pulmonary function declined over time, particularly at time points beyond 12 months of follow-up.

CONCLUSION

The intermediate and mild phenotypes of SMA show slow functional declines when observation periods exceed 1 year. Whole body muscle mass, hand muscle compound motor action potentials, and muscle strength are associated with clinical measures of motor function. The data from this study will be useful for clinical trial planning and suggest that CMAP and DXA warrant further evaluation as potential biomarkers.

[A case of spinal muscular atrophy type 0 in Japan]

The patient was a 2-month-old female infant born at 41 weeks and 2 days of gestation presenting multiple arthrogryposis, severe muscle hypotonia and respiratory distress with difficulty in feeding. She suffered from repeated complications with aspiration pneumonia. On admission to our hospital, she exhibited fasciculation and absence of deep tendon reflexes. Examination of the motor nerve conduction velocity (MCV) revealed no muscle contraction. Deletions of the SMN and NAIP genes were noted. Based on severe clinical course and disease development in utero, she was given a diagnosis of spinal muscular atrophy (SMA) type 0 (very severe type). Arthrogryposis and disappearance of MCV are exclusion criteria for SMA. However, the clinical course of the infant was very severe and included such exclusion items. Consequently, when an infant presents muscle hypotonia and respiratory distress, SMA must be considered as one of the differential diagnoses, even though arthrogryposis is an exclusion criterion for SMA. We discuss this case in relation to the few extant reports on SMA type 0 in Japanese infants in the literature.

Neuroelectrophysiological characteristics of Hirayama disease: report of 14 cases

BACKGROUND

Hirayama disease is a juvenile muscular atrophy of the distal upper extremities and affects mainly young males. The present study aimed to investigate the neuroelectrophysiological characteristics of Hirayama disease.

METHODS

We retrospectively analyzed the neural conduction velocity (NCV) parameters and needle-electrode electromyograms (EMG) of 14 patients with Hirayama disease. According to the clinical features of the patients, NCV was performed on affected upper-limb including median nerves and ulnar nerves, while EMG was selectively performed on upper and lower extremities, sternocleidomast and thoracic paraspinal muscles.

RESULTS

The median nerves of all affected upper limbs of patients with Hirayama disease had normal conduction velocities and compound motor action potentials (CMAPs). The ulnar nerves of all affected upper limbs also had normal conduction velocities. Of the 16 measured ulnar nerves of the affected upper limbs, eight had normal CMAPS, while the other eight showed CMAPs below the normal value by < 20%. All patients had neurogenic injury on the affected side in muscles innervated by anterior horn cells at the lower cervical region (C7-8, T1). Four patients had unilateral upper-limb muscle neurogenic injury on the affected side. Seven patients had bilateral upper-limb muscle neurogenic injury, while only two patients experienced bilateral upper-limb muscle atrophy/weakness. The other three patients showed extensive neurogenic injury (unilateral upper-limb muscle atrophy/weakness in one patient, bilateral symptoms in the other two patients).

CONCLUSIONS

Electromyographic examination showed that the majority of Hirayama disease patients exhibited characteristic segmental injury in the anterior horn of the lower cervical region, while a few patients exhibited extensive neurogenic injury. These data suggest that the actual influence of Hirayama disease may be more extensive than indicated by the clinical presentations.

A clinical neurophysiology study of Hirayama disease

BACKGROUND

Hirayama disease is a rare disease characterized by juvenile-onset of asymmetric amyotrophy, of which etiology has not been clarified. The aim of our study was to investigate the clinical and neurophysiologic characteristics of Hirayama disease.

METHODS

Neurophysiological tests, including nerve conduction studies (NCS), F-wave and routine electromyography (EMG), were performed in seventy-three patients with Hirayama disease. EMG was selectively performed on upper and lower extremities, sternocleidomast and thoracic paravertebral muscles according to the clinical features of the patients.

RESULTS

Abnormal NCS parameters, including decreased compound muscle action potentials or delayed distal motor latency, were found in 34.2% (25/73) and 12.3% (9/73) of the patients, respectively. A total of 24.6% (18/73) of the patients showed decreased F-wave frequency. EMG demonstrated the presence of neurogenic lesions in all patients with spontaneous potentials, prolonged duration or augmentation of amplitude in motor unit potentials (MUPs), or a single pattern of MUP recruitment. About 17.8% (13/73) of the patients showed neurogenic lesions, mostly in the C7-8 level of the cervical cord, only in the upper extremity of affected side, whereas 35.6% (26/73) of the patients possessed lesions in the upper extremities bilaterally. A total of 46.6% (34/73) of patients exhibited abnormalities in the lower extremities, sterno- cleidomast or thoracic paravertebral muscle. Changes in motor NCS were significantly correlated with muscle strength.

CONCLUSIONS

EMG detects diffused subclinical neurogenic lesion in a high proportion of patients with Hirayama disease. Results of our study challenge the hypothesis that Hirayama disease is a type of cervical myelopathy.

[A study of motor unit number estimation by multiple point stimulation in patients with Hirayama disease]

OBJECTIVE

To explore the significance of motor unit number estimation (MUNE) by using multiple point stimulating technique to evaluate patients with Hirayama disease (HD).

METHODS

Multiple point stimulating technique was used to estimate the motor unit number of abductor pollicis brevis and abductor digiti minimi in 35 normal subjects [14 - 33 years old, mean (20.9 ± 4.0) years old, 33 men and 2 women] without nerve and muscle disease and 69 patients definitely diagnosed as HD [16 - 35 years old, mean (21.46 ± 6.61) years old, 67 men and 2 women]. The differences between the two groups were examined by Fisher's exact test and t test.

RESULTS

There were 42 patients with atrophy and 27 patients with normal clinical manifestation of left hand. For right hand there were 54 patients with atrophy and 15 normal. For controls, the MUNE value of left abductor pollicis brevis was 226.97 ± 30.59, while that of right side was 228.31 ± 25.35. The MUNE value of left abductor digiti minimi was 237.43 ± 30.78, while that of right side was 240.20 ± 37.73. For HD patients, the MUNE of left abductor pollicis brevis and abductor digiti minimi was 145.66 ± 126.10 (t = 5.07, P < 0.01) and 102.20 ± 112.67 (t = 9.31, P < 0.01) respectively, while those of right hand was 149.72 ± 117.80 (t = 5.31, P < 0.01) and 64.23 ± 69.27 (t = 16.76, P < 0.01) respectively. MUNE of left abductor digiti minimi in 17 patients that was below 200 among 27 patients with normal clinical manifestation (χ(2) = 9.57, P = 0.002). MUNE of right abductor digiti minimi in 12 patients that was below 200 among 15 patients with normal clinical manifestation (χ(2) = 4.64, P = 0.03).

CONCLUSIONS

The differences of MUNE values by multiple point stimulating technique between the normal subjects and the HD patients is significant, which suggests this method is very useful to evaluate HD in the early state.

[Hammersmith functional rating scale for children with spinal muscular atrophy. Validation of the Spanish version]

AIMS

This study was conducted with the aim of developing the Spanish version of the Hammersmith functional rating scale for children with spinal muscular atrophy (SMA), to establish the interobserver reliability and to adapt it to Spanish children with SMA types II and III.

PATIENTS AND METHODS

We assessed 31 children with a mean age of 4.7 years (range: 2.5-9 years) and they were re-assessed at 3, 6 and 12 months. The children with SMA type II could not walk and the children with SMA type III were able to walk.

RESULTS

Interobserver reliability was 95%. The median of the score in the group with SMA type II was 17 (range: 0-34) and in the SMA type III group it was 39 (range: 37-40). The results of the group with SMA type II at 12 months were poorer, although not to a statistically significant extent, which suggested that the disease was getting worse.

CONCLUSIONS

The Spanish version presents a high degree of interobserver reliability, is easy to administer in clinical practice and is a good tool for assessing the severity of the disease in patients with SMA, above all in those who are unable to walk.

Benign monomelic amyotrophy in a 7-year-old girl with proximal upper limb involvement: case report

Monomelic amyotrophy (MMA) is a benign motor neuron disease characterized by neurogenic amyotrophy, which usually affects one of the upper or lower extremities. Progression is slow and symptoms are clinically stable. Symptoms are seen in the second or third decades of life. In this study, we present a seven-year-old girl who was diagnosed and directed to the Physiotherapy Department at the age of 5 years and had unilateral proximal upper limb involvement. Family history of the case was recorded. Neurologic evaluation was performed. Range of joint motion, muscle shortness and strength, posture, extremity lengths, gait, timed performance, arm function, and motor and mental maturation were assessed. The physiotherapy program was designed progressively as strengthening and resistive exercises. Motor and mental developmental milestones were normal. There was no limitation in active or passive motion of all joints. She had more flexible joints, scapula alata, asymmetry between shoulder levels, and weakness on proximal muscles of the right upper extremity. In the follow-up assessment at eight months, there was no asymmetry between shoulder levels and scapular symmetry began to improve. Female gender and involvement restricted to one proximal upper limb are rare in the literature. This patient demonstrates the positive effects of physical therapy with early diagnosis of MMA. The rapid recovery of muscle weakness shows the importance of strengthening and resistive exercises applied to specific muscles in the treatment.

Reliability of the Modified Hammersmith Functional Motor Scale in young children with spinal muscular atrophy

INTRODUCTION

The test-retest reliability of the Modified Hammersmith Functional Motor Scale (MHFMS) in children with spinal muscular atrophy (SMA) ≤30 months of age was assessed. The age at which typically developing children (TD) achieve maximum MHFMS scores was also studied.

METHODS

Twenty-two children with SMA type II [mean age (SD) = 20 (5) months, range 9-30 months) were tested twice using the MHFMS. Twenty-five TD children [mean age (SD) = 18 (7) months, range 9-30 months) were tested once.

RESULTS

The average difference between MHFMS scores for SMA children was 0.18 [first assessment: mean (SD) = 12.8 (9.8); second assessment: mean (SD) = 13.0 (8.8)]. Reliability was excellent (ICC(1,3) = 0.96, SEM 1.86). TD participants had MHFMS scores ranging from 36 to 40 [mean (SD) = 39.2 (1.2)] and achieved maximum test scores at 12 months of age.

DISCUSSION

MHFMS scores in young children with SMA type II showed excellent test-retest stability. This suggests that the MHFMS can be used reliably in this younger population for clinical trials and follow-up.

Physical medicine respiratory muscle aids to avert respiratory complications of pediatric chest wall and vertebral deformity and muscle dysfunction

The purpose of this article was to describe the use of noninvasive inspiratory and expiratory muscle aids to prevent chest wall deformities including pectus excavatum, to prevent respiratory complications of vertebral surgery, to prevent acute and long-term ventilatory insufficiency and failure in children with paralytic disorders who develop these deformities, and to permit the extubation and tracheostomy tube decanulation of "unweanable" patients. Noninvasive airway pressure aids can provide up to continuous ventilator support for patients with little or no vital capacity and can provide for effective cough flows for patients with severely dysfunctional expiratory muscles. An April 2010 consensus of clinicians from 20 centers in 14 countries reported over 1500 spinal muscular atrophy type 1 (SMA1), Duchenne muscular dystrophy (DMD), and amyotrophic lateral sclerosis (ALS) patients who survived using continuous ventilatory support without tracheostomy tubes. Four of the centers routinely extubated unweanable DMD patients so that none of their over 250 such patients has undergone tracheotomy.

Contractures of the upper extremities in spinal muscular atrophy type II. Descriptive clinical study with retrospective data collection

BACKGROUND

The most significant orthopaedic problem for patients with the intermediate form of spinal muscular atrophy, SMA type II, is the appearance of contractures in addition to progressive scoliosis and pelvic obliquity with increasing loss of sitting stability. This study deals with restrictions of the passive range of motion and the development of contractures in the joints of the upper extremities in these patients.

MATERIAL AND METHODS

We followed up 143 patients, 74 female and 69 male, with SMA type II for an average of 5.3 years (± 4.0, 0.2 - 18.7). Their average age at the first examination was 8.4 years (± 6.6, 0.1 - 34.1) and at the last examination 12.3 years (± 6.5, 0.7 - 37.1). The passive range of joint motion was determined using a goniometer. According to Johnson et al. (1992), we calculated the relative contracture index (CI).

RESULTS

The loss of range of motion and the contractures of the joints of the upper extremities (shoulder, elbow and wrist) increased progressively with age. The most marked restriction of motion was in the elbow joint with severe flexion contractures in some cases.

CONCLUSION

The findings of this study give us more information about the development of contractures of the upper extremities and aim to help to improve the quality of orthopaedic care of patients with SMA type II.

The Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND): test development and reliability

The motor skills of patients with spinal muscular atrophy, type I (SMA-I) are very limited. It is difficult to quantify the motor abilities of these patients and as a result there is currently no validated measure of motor function that can be utilized as an outcome measure in clinical trials of SMA-I. We have developed the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders ("CHOP INTEND") to evaluate the motor skills of patients with SMA-I. The test was developed following the evaluation of 26 infants with SMA-I mean age 11.5 months (1.4-37.9 months) with the Test of Infant Motor Performance and The Children's Hospital of Philadelphia Test of Strength in SMA, a newly devised motor assessment for SMA. Items for the CHOP INTEND were selected by an expert panel based on item mean and standard deviation, item frequency distribution, and Chronbach's alpha. Intra-rater reliability of the resulting test was established by test-retest of 9 infants with SMA-I over a 2 month period; Intraclass correlation coefficient (ICC) (3,1)=0.96. Interrater reliability was by video analysis of a mixed group of infants with neuromuscular disease by 4 evaluators; ICC (3,4)=0.98 and in a group of 8 typically developing infants by 5 evaluators ICC (3,5)=0.93. The face validity of the CHOP INTEND is supported by the use of an expert panel in item selection; however, further validation is needed. The CHOP INTEND is a reliable measure of motor skills in patients with SMA-I and neuromuscular disorders presenting in infancy.

Glossopharyngeal pistoning for lung insufflation in children with spinal muscular atrophy type II

AIM

To evaluate whether children with spinal muscular atrophy (SMA) type II were able to learn glossopharyngeal pistoning for lung insufflation (GI), and to evaluate the effects of GI on pulmonary function and chest expansion.

METHODS

Eleven children with SMA type II were recruited. They performed 10 cycles of GI, four times per week, for 8 weeks. Lung function and chest expansion were measured before and after the 8-week period.

RESULTS

Five of the 11 children learned the technique. The median GI volume was 0.28 (range 0.15-0.98) L. Four of the children who completed the study showed a mean increase in inspiratory vital capacity (IVC) of 0.13 L (95% confidence interval (CI) 0.03-0.23) and peak expiratory flow (PEF) of 116 L/min (95% CI 60-173). They also had an increased chest expansion with GI at the level of the xiphoid process of 1.50 cm (95% CI 0.16-2.84) and at the level of the fourth costa of 1.79 cm (95% CI 0.85-2.73). The children reported temporary symptoms of dizziness and tension in the chest.

CONCLUSION

Five of the 11 children were able to learn the technique of GI and for the four who fulfilled the training, it had positive effects on IVC, PEF and chest expansion. GI did not cause major discomfort.

[A clinical research of hirayama disease]

OBJECTIVE

To summarize and analyze the clinical features of Hirayama disease.

METHODS

The authors summarized the clinical data of 29 cases of Hirayama disease in recent 9 years.

RESULTS

All of the 29 cases were male and the age of onset was 12 - 25 years. 22 of the 29 patients only had the symptoms of one upper limb and 7 of them had symptoms of both upper limbs. Most of them had symptoms on the right or the symptoms were on more severe in the right than left, while only one case had symmetric symptoms of both upper limbs. The arm appeared as a slope, this was due to partial muscular atrophy. EMG indicated that the impairment of spinal anterior cells were limited to the arm relevant segments and both sides were involved. Cervical MRI scan suggested spinal atrophy at the level of C(5 - 7) vertebral bodies, being asymmetric in most of the cases. The atrophy was more obvious on the side with more severe symptoms. There was one patient whose MRI showed high T(2) signal in the spinal anterior cells of the C(5 - 6) segments. All the cases used cervical supporter and were followed up for 1 - 5 years. All of them had good prognosis.

CONCLUSIONS

Hirayama disease is more common in men with hidden onset before 20 years of age. Most of the patients noted the symptoms of hands gradually about 2 years after a period of rapid growth in height in adolescence. The predominant clinical manifestations are as follows: atrophy of lower cervical cord in cervical MRI, asymmetric spinal atrophy in horizontal impaction and the degree of atrophy consistent with the symptoms of limbs. EMG could indicate bilateral spinal anterior impairment in lower cervical cord. Rare case may present the abnormal signal in the spinal cord, but pyramid signs were absent.

Spinal muscular atrophy

Spinal muscular atrophy is an autosomal recessive neurodegenerative disease characterised by degeneration of spinal cord motor neurons, atrophy of skeletal muscles, and generalised weakness. It is caused by homozygous disruption of the survival motor neuron 1 (SMN1) gene by deletion, conversion, or mutation. Although no medical treatment is available, investigations have elucidated possible mechanisms underlying the molecular pathogenesis of the disease. Treatment strategies have been developed to use the unique genomic structure of the SMN1 gene region. Several candidate treatment agents have been identified and are in various stages of development. These and other advances in medical technology have changed the standard of care for patients with spinal muscular atrophy. In this Seminar, we provide a comprehensive review that integrates clinical manifestations, molecular pathogenesis, diagnostic strategy, therapeutic development, and evidence from clinical trials.

Physician attitudes towards ventilatory support for spinal muscular atrophy type 1 in Australasia

BACKGROUND

Without ventilatory support, premature death from respiratory insufficiency is virtually universal in infants with spinal muscular atrophy type 1 (SMA1). With mechanical ventilation, however, long-term survival has been reported from numerous international centres. We aimed to characterize physician attitudes to the various forms of ventilatory support for children with SMA1.

METHODS

We surveyed neurologists, respiratory physicians, clinical geneticists and intensivists from all major paediatric hospitals in Australia and New Zealand regarding their views on ventilatory management of SMA1.

RESULTS

Ninety-two of the 157 (59%) physicians surveyed replied. Respondents included 16 clinical geneticists, 19 intensive care physicians, 28 neurologists and 29 respiratory physicians. Almost half (47%) opposed invasive ventilation of children with SMA1 and respiratory failure precipitated by intercurrent illness. The majority (76%) opposed invasive ventilatory support for chronic respiratory failure in SMA1. In contrast, non-invasive ventilation was felt by 85% to be appropriate for acute respiratory deteriorations, with 49% supporting long-term non-invasive ventilatory support. Most physicians felt that decisions regarding ventilation should be made jointly by parents and doctors, and that hospital Clinical Ethics Committees should be involved in the event of discordant opinion regarding further management. A majority felt that a defined hospital policy would be valuable in guiding management of SMA1.

CONCLUSIONS

Respiratory support in SMA1 is an important issue with significant ethical, financial and resource management implications. Most physicians in Australian and New Zealand oppose invasive ventilatory support for chronic respiratory failure in SMA1. Non-invasive ventilation is an accepted intervention for acute respiratory decompensation and may have a role in the long-term management of SMA1. Clinical Ethics Committees and institutional policies have a place in guiding physicians and parents in the management of children with SMA1.

Molecular functions of the SMN complex

The SMN complex is essential for the biogenesis of spliceosomal small nuclear ribonucleoproteins and likely functions in the assembly, metabolism, and transport of a diverse number of other ribonucleoproteins. Specifically, the SMN complex assembles 7 Sm proteins into a core structure around a highly conserved sequence of ribonucleic acid (RNA) found in small nuclear RNAs. The complex recognizes specific sequences and structural features of small nuclear RNAs and Sm proteins and assembles small nuclear ribonucleoproteins in a stepwise fashion. In addition to the SMN protein, the SMN complex contains 7 additional proteins known as Gemin2-8, each likely to play a role in ribonucleoprotein biogenesis. This review focuses on the current understanding of the mechanism of the role of the SMN complex in small nuclear ribonucleoprotein assembly and considers the relationship of this function to spinal muscular atrophy.

Molecular mechanisms of spinal muscular atrophy

Significant strides have been made during the past decade in the understanding of the molecular mechanisms that lead to the autosomal recessive motor neuron disease spinal muscular atrophy. Genetic studies revealed that spinal muscular atrophy is caused by mutation of the telomeric copy of the survival motor neuron gene ( SMN1), with all patients retaining at least one copy of the centromeric form of the gene, SMN2. SMN2 produces reduced amounts of full-length SMN messenger ribonucleic acid because of alterative splicing of SMN2 -derived transcripts, a process that is governed by specific cisand trans-acting factors. The resulting insufficient expression level of full-length SMN protein likely causes the disease manifestations of spinal muscular atrophy; however, the mechanism for the selective vulnerability of the motor unit to deficiency of this ubiquitously expressed protein remains unknown. It also remains unclear specifically when and where in the motor unit SMN is required. Despite the remaining questions, progress has been made in developing therapeutic strategies targeted to specific points along the pathogenetic pathway of spinal muscular atrophy. Histone deacetylase inhibitors will be discussed as an example.

Spinal muscular atrophy diagnostics

Spinal muscular atrophy is a common autosomal recessive neuromuscular disorder caused by mutations in the survival motor neuron gene (SMN), which exists in 2 nearly identical copies (SMN1 and SMN2). Exon 7 of SMN1 is homozygously absent in about 95% of spinal muscular atrophy patients, whereas the loss of SMN2 does not cause spinal muscular atrophy. Small mutations are found in the other 5% of affected patients, and these mutations cluster in the 3' end of SMN1, a region important for protein oligomerization. SMN1 dosage testing can be used to determine the SMN1 copy number and to detect spinal muscular atrophy carriers and affected compound heterozygotes. Dosage testing is compromised by the presence of 2 SMN1 copies per chromosome, which occurs in about 2% of carriers. Finally, although SMN2 produces less full-length transcript than SMN1, the number of SMN2 copies modulates the phenotype.

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.