Inherited neuromuscular disorders: Pathway to diagnosis

Authors

  • Manoj P Menezes,

    1. Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Sydney
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  • Kathryn N North

    Corresponding author
    1. Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Sydney
    2. Discipline of Paediatrics and Child Health, Faculty of Medicine, University of Sydney, New South Wales, Australia
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Professor Kathryn North, Institute for Neuroscience and Muscle Research, The Children's Hospital at Westmead, Sydney, NSW 2145, Australia. Fax: +61 2 9845 3389; email: kathryn@chw.edu.au

Abstract

Muscle weakness in childhood can be caused by a lesion at any point extending from the motor cortex, brainstem and spinal cord to the anterior horn cell, peripheral nerve, neuromuscular junction and muscle. A comprehensive history and physical examination is essential to aid classification of the neuromuscular disorder and direct gene testing. The more common disorders such as spinal muscular atrophy, Duchenne muscular dystrophy, myotonic dystrophy and facioscapulohumeral dystrophy may be diagnosed on direct gene testing based on the history and clinical examination. The congenital myopathies are classified based on structural abnormalities on muscle biopsy, while protein abnormalities on immunohistochemistry and immunoblotting aid classification of the muscular dystrophies. In this review, we provide an approach to diagnosis of a child with weakness, with a focus on the inherited neuromuscular disorders, and the features on history, examination and investigation that help to distinguish between them.

Muscle weakness in childhood can be caused by a lesion at any point extending from the motor cortex, brainstem and spinal cord to the anterior horn cell, peripheral nerve, neuromuscular junction and muscle. The clinical presentation can occur at any age; as a ‘floppy infant’, with delayed motor milestones in the first 2 years of life, or in childhood with abnormal gait, difficulty with running and climbing stairs and frequent falls. There is a very wide range of possible causes, both inherited and acquired. Acquired disorders can often be distinguished by acute or subacute onset in a child with normal developmental milestones and a history or associated features consistent with infectious, autoimmune or vascular pathology. In this review, we provide an approach to diagnosis of a child with weakness, with a focus on the inherited neuromuscular disorders, and the features on history, examination and investigation that help to distinguish between them.

Key Points

  • 1A comprehensive history and physical examination is essential to aid classification of the neuromuscular disorder and direct gene testing.
  • 2Most cases of spinal muscular atrophy, Duchenne muscular dystrophy, myotonic dystrophy and facioscapulohumeral dystrophy are diagnosed on gene testing based on a suggestive clinical phenotype without the need for invasive tests.
  • 3The congenital myopathies are classified based on structural abnormalities on muscle biopsy, while protein abnormalities on immunohistochemistry and immunoblotting aid classification of the muscular dystrophies.

Approach to Diagnosis of the Child with Weakness

Clinical history and physical examination

The clinical history (Table 1) helps identify the age of onset, distribution and progress of weakness, and mode of inheritance. Decreased fetal movements in utero may indicate a prenatal onset of weakness and a significantly affected fetus with lack of antigravity movement due to a neuromuscular disorder.1

Table 1.  Salient features in the history
• Pregnancy
 – Polyhydramnios, intrauterine movements, intrauterine lie
• Birth
 – Breech presentation, congenital hip dislocation, contractures, absent flexion creases
 – Medications given to mother (magnesium sulphate, opioids, anaesthetic agents)
• Neonatal period and infancy
 – Respiratory distress
 – Hypotonia and floppiness
 – Sucking and swallowing difficulties
• Distribution of the weakness
 – Proximal, distal or global
 – Facial or bulbar weakness
 – Ophthalmoplegia
• Family history
 – Consanguinity
 – Family history of a neuromuscular disorder and pedigree

On physical examination, the initial effort is to differentiate a disorder due to an upper motor neuron (UMN) lesion from one due to a lower motor neuron (LMN) lesion. Hypertonia, spasticity, hyperreflexia, clonus and the presence of a Babinski sign after 12 months of age indicate a UMN disorder, while hypotonia and diminished or absent reflexes are consistent with an LMN disorder. When asked to stand up, as quickly as possible, from a supine or seated position, children with a proximal weakness may turn on their abdomen, uses their hands to balance and push off from the floor and ‘climb’ up their thighs using their arms; the time taken to stand can provide an objective measure of the muscle weakness. This Gower's sign2 (Fig. 1) is best described in children with Duchenne muscular dystrophy (DMD) but can also be seen with other myopathies and dystrophies and in the milder forms of spinal muscular atrophy (SMA). Ptosis and ophthalmoplegia may be seen with congenital myopathies, myotonic dystrophy, myasthenic syndromes and with mitochondrial diseases.3 Facial weakness may be seen in facioscapulohumeral dystrophy (FSHD), myotonic dystrophy and in the congenital myopathies.4–6 Tongue fasciculations occur in SMA7 and other anterior horn cell disorders.

Figure 1.

Gower's sign in a boy with Duchenne muscular dystrophy. (Courtesy of Dr Richard S. Finkel, Children's Hospital of Philadelphia).

The floppy infant

Evaluation of the floppy neonate in the nursery, or the floppy infant who presents with delayed motor milestones, involves differentiating the infant with central hypotonia (‘floppy strong’) from one with a disorder of the LMN (‘floppy weak’). A guide to the physical examination of a floppy infant is listed in Table 2. A floppy infant will have prominent head lag on the ‘pull to sit’ manoeuvre, slip through on shoulder suspension, have an abnormal ‘scarf sign’ with the elbow crossing the midline when the hand is pulled towards the contralateral shoulder and be unable to maintain the head in the horizontal position on ventral suspension.8 If the deep tendon reflexes are easily elicited, then a primary neuromuscular disorder is less likely.

Table 2.  Physical examination in an infant with hypotonia
• Posture and activity at rest
 – ‘Frog-leg posture’ versus flexed posture
 – Presence of spontaneous and antigravity movement
 – Paradoxical respiration
 – Deep tendon reflexes
• Posture and activity with specific manoeuvres
 – Ventral suspension
 – Traction on hands in supine position –‘pull to sit’
 – Traction response of flexor muscles of arm and leg
 – ‘Scarf sign’
 – Posture of a passively elevated limb
 – Response to noxious stimuli
• Assessment of cranial nerve function
 – Ophthalmoplegia, facial or bulbar weakness
 – Fasciculation and atrophy of the tongue
• Other
 – Organomegaly (e.g. Pompe disease)
 – Bone or joint abnormalities (suggest in utero onset)
• Examination of the parents and siblings
 – Facial muscle weakness
 – Myotonia
 – Dysmorphic features
• Features suggesting a central hypotonia
 – Impaired alertness, seizures
 – Poor feeding and sucking
 – Microcephaly/macrocephaly
 – Dysmorphism
 – Nystagmus and retinal abnormalities
 – Hypotonia with normal strength
 – Reflexes normal or brisk
• Features suggesting a lower motor unit disorder
 – Alert baby who is ‘not moving’
 – Weakness with lack of antigravity movements
 – Diminished or absent reflexes

The most common cause of the floppy infant is birth asphyxia, and infants with weakness due to a primary neuromuscular disorder are more susceptible to difficulties at delivery and may present with a combination of hypoxic ischaemic encephalopathy and peripheral weakness. The list of causes for central hypotonia8 (syndromes like Down syndrome and Prader–Willi syndrome, structural brain abnormalities, delayed and abnormal myelination and spinal cord disorders) is long and varied and beyond the scope of this review. Initial evaluation for presumed central hypotonia usually includes thyroid function tests, urine metabolic screen, serum lactate, cranial and spinal imaging, karyotype and a comparative genomic hybridisation array.

The floppy infant with a primary neuromuscular disorder typically presents as an alert baby who has paucity of movement, particularly proximal antigravity movement, and diminished or absent reflexes. The floppy weak infant, and the weak older child, may have a lesion at the anterior horn cell, peripheral nerve, neuromuscular junction or muscle. The common diseases associated with each of the lesions are listed in Table 3.

Table 3.  Inherited disorders of the lower motor neuron
Level of lesionCommon disorders
Anterior horn cellSpinal muscular atrophy
Peripheral nerveCharcot-Marie-Tooth disease
Neuromuscular junction‘Transient’ neonatal myasthenia
Congenital myasthenic syndromes
Juvenile myasthenia gravis
MuscleDuchenne and Becker muscular dystrophy
Other limb-girdle dystrophies
Congenital myopathies
Congenital muscular dystrophies
Myotonic dystrophy

Investigations

The challenge for the clinician is first to determine the level of the lesion and then to proceed to definitive genetic diagnosis. This is usually achieved by a combination of clinical assessment and relevant investigations. The serum creatine kinase (CK) may be normal to moderately raised (5× normal) in SMA and the congenital myopathies, with a more significant rise (often greater than 10× normal) in the muscular dystrophies.9 Electromyography (EMG) and nerve conduction studies help to differentiate neurogenic from myopathic weakness and help classify the neuropathy as demyelinating, axonal or intermediate and as sensorimotor or primarily motor or sensory neuropathy.10 The presence of a decrement on repetitive nerve stimulation at low rates (2–5 Hz) is useful in identifying those with congenital or acquired myasthenia. Prolonged runs of motor unit potentials with a waxing and waning frequency and amplitude, described as resembling the sound of a motorcycle or chainsaw, may be heard on insertion of the needle electrode on EMG in older patients with myotonic dystrophy.11

The muscle biopsy (Fig. 2) may show evidence of chronic denervation, dystrophic change or structural abnormalities. The presence of small angular fibres, fibre-type grouping and type 2 fibre atrophy indicates chronic denervation as seen in SMA and Charcot-Marie-Tooth disease (CMT).12,13 In the congenital and limb-girdle dystrophies, the muscle biopsy is characterised by diffuse variation in fibre size, necrotic and regenerating fibres and fibrosis.14 The nerve biopsy sections and teased fibre preparations are useful in the evaluation of neuropathies that remain genetically unclassified and may show evidence of axonal degeneration or demyelination, helping to target genetic studies more precisely.15,16

Figure 2.

Sections stained with haematoxylin and eosin showing (a) normal muscle biopsy, (b) dystrophic muscle biopsy and (c) muscle biopsy with neuropathic change with normal (N) and atrophied fascicles (A).

Increasingly, for the common disorders (SMA, DMD) and disorders such as myotonic dystrophy and FSHD, where genetic testing is readily available, the experienced clinician will make a provisional diagnosis based on history (including family history) and examination and go straight to DNA diagnosis, bypassing other investigations including muscle or nerve biopsy. In the next section, we outline the clinical clues to diagnosis for these common disorders and the pathway to diagnosis of the rarer inherited myopathies and muscular dystrophies.

Spinal Muscular Atrophy

SMA is an anterior horn cell disorder characterised by progressive proximal weakness, hypotonia, absent tendon reflexes, postural tremor of fingers and tongue fasciculations. Infants with the disease have a bright and alert appearance with normal facial expression. The breathing pattern is ‘paradoxical’, with the chest wall collapsing in during inspiration due to intercostal paralysis and the diaphragm being spared early in the disease. The clinical phenotype is differentiated into three major categories: SMA I (Werdnig–Hoffmann disease) where children never achieve the ability to sit independently, SMA II where affected children achieve the ability to sit but not stand and SMA III (Kugelberg–Welander disease) where children are able to stand and walk.7 While EMG and muscle biopsy show evidence of chronic denervation, these tests have been largely replaced by direct genetic testing. About 94% of cases are due to homozygous deletions of exons 7 and 8 of survival of motor neuron 1 (SMN1), while a deletion on one allele and a mutation on the other is seen in the remaining.17

Charcot-Marie-Tooth Disease

CMT is a genetically heterogeneous group of inherited neuropathies characterised by progressive distal weakness and wasting, a high stepping gait, foot deformities, absent deep tendon reflexes and distal sensory loss.15 Dyck et al. classified CMT into two main types: primarily demyelinating (CMT1) with severely reduced motor nerve conduction velocities (MNCV < 38 m/s), and primarily axonal (CMT2) with decreased compound muscle action potential (CMAP) amplitudes and normal or slightly reduced MNCV (MNCV > 38 m/s).18 The number of genes shown to cause the CMT phenotype is rapidly increasing (http://www.molgen.ua.ac.be/CMTMutations/), with classification done on the basis of nerve conduction test results and pattern of inheritance.

CMT1A accounts for around 50% of patients with the CMT phenotype,19 and occurs due to duplication of chromosome 17p11.2-p12, which contains the peripheral myelin protein 22 (PMP22) gene. CMT1A is characterised by a childhood onset and a mild disease course. Patients have a typical CMT phenotype but could also have nerve hypertrophy, hearing loss, scoliosis or hip dysplasia.20–22 Nerve conduction studies show marked slowing of the motor and sensory nerve conduction velocity, reduction in CMAP amplitude and reduction or absence of the sensory nerve action potentials.

Duchenne Muscular Dystrophy

DMD is an X-linked disorder due to mutations in the dystrophin gene. Affected children usually present in their pre-school years with delayed gross motor milestones and inability to keep up with their peers in running and jumping; serum CK is usually markedly raised (10–100× normal). The disease is characterised by progressive proximal weakness resulting in the Gower's sign and, when untreated, leads to loss of independent ambulation by 13 years of age. Scoliosis and contractures, respiratory muscle weakness, cardiac conduction defects and cardiomyopathy are associated and need to be part of the surveillance protocol. Children may occasionally present with a raised CK and deranged transaminases without obvious weakness.23,24 Reduction in the mean IQ scores and specific learning difficulties are often associated.25,26 Becker muscular dystrophy is an allelic disorder (i.e. also due to mutations in the dystrophin gene), but patients have later onset and more variable progression and severity.27

Diagnosis is confirmed by identifying a deletion or duplication on multiplex polymerase chain reaction or multiplex ligation-dependent probe amplification. If this is negative, sequencing of the dystrophin gene may identify point mutations or small deletion/insertions. A muscle biopsy is required if genetic testing is negative despite a suggestive phenotype and may show absence or reduction in dystrophin on immunohistochemistry or immunoblotting23 (Fig. 3). Consensus recommendations for diagnosis and management have recently been published.23,28

Figure 3.

Absence of dystrophin in a patient (P) with Duchenne muscular dystrophy, when compared with control (C), on immunohistochemistry (Panel a) and western blot (Panel b).

Other Forms of Muscular Dystrophy

There are numerous other genetic forms of muscular dystrophy, with X-linked, autosomal dominant and autosomal recessive inheritance. They are classified according to the age of presentation and pattern of weakness and have in common histological evidence of degeneration and regeneration on muscle biopsy associated with weakness and a raised serum CK. When children present at birth with hypotonia and weakness with a dystrophic picture on muscle biopsy, they are classified as having a congenital muscular dystrophy. Common genetic causes include mutations in the genes encoding the extracellular matrix proteins collagen VI, merosin (laminin-α2) and proteins that glycosylate α-dystroglycan. Childhood- or adult-onset muscular dystrophies are classified as a limb-girdle muscular dystrophy (LGMD). DMD can be considered as a severe childhood onset form of X-linked LGMD. Common genetic causes of other LGMDs include mutations in the genes encoding dystrophin-associated proteins at the muscle membrane such as the sarcoglycans, dysferlin, caveolin-3 and calpain.9

Specific protein analysis of the patient muscle biopsy by immunohistochemistry and western blot plays an important role in distinguishing between the different forms of muscular dystrophy and in directing genetic testing. In the X-linked and autosomal recessive forms of muscular dystrophy, an absence or reduction of the disease-associated protein may be seen on immunohistochemistry, while reduction or abnormal protein may be identified on western blot.14 In autosomal dominant disorders, protein studies are usually less helpful since expression of protein from the normal copy of the gene may conceal the effects of the mutant copy.

Occasionally, clinical findings may help to direct gene sequencing in combination with protein findings on muscle biopsy. For example, calf hypertrophy is seen with Duchenne and Becker muscular dystrophy, sarcoglycanopathies, calpainopathy and the α-dystroglycanopathies, while selective calf atrophy is seen with LGMD due to mutations in the gene encoding dysferlin.9 The phenotype with collagen VI-deficient myopathy includes distal laxity and hyperextensibility, long finger flexion contractures, follicular hyperkeratosis on the extensor surfaces of the limbs and prominent calcanei.29 An extensive list of the monogenic neuromuscular disorders and their causative mutation is available at http://www.musclegenetable.org

Congenital Myopathies

The congenital myopathies usually present at birth or childhood with hypotonia and generalised weakness and a static or slowly progressive course, and are classified into subgroups based on the presence of distinct structural abnormalities in the muscle biopsy. These include distinctive protein accumulations in the form of rods or nemaline bodies (nemaline myopathy), ‘cores’ devoid of oxidative activity (central core and multi-minicore disease), centrally, rather than peripherally, placed nuclei (centronuclear myopathies) and selective atrophy of type 1 (slow) fibres (congenital fibre-type disproportion).30,31 Genes that cause congenital myopathies often encode protein components of the muscle contractile apparatus (the sarcomere) and proteins involved in Ca2+ handling,32 resulting in inefficient muscle contraction. CK is usually normal or only mildly elevated. The presence of ophthalmoplegia, facial and bulbar involvement may help differentiate congenital myopathies from congenital muscular dystrophies, anterior horn cell disorders and early-onset peripheral neuropathies.32

Other Inherited Disorders Involving Muscle

Myotonic dystrophy is an autosomal dominant disorder characterised by hypotonia, weakness that may be proximally or distally predominant, bilateral facial weakness, learning difficulties and myotonia on clinical examination and EMG. There are two genetic subtypes: myotonic dystrophy type 1 (DM1) is caused by an expanded CTG repeat in the dystrophia myotonica-protein kinase gene,33 while type 2 (DM2) is caused by a CCTG expansion in the zinc finger protein 9 gene.34 There is a wide range of clinical severity and age on onset that roughly correlates with the size of the repeat.

Clinical variability between parent and offspring and among family members may be due to an increase in the size of the expansion; this most often occurs when the mutant gene is inherited from an affected mother. Congenital myotonic dystrophy only occurs in DM1 and is associated with maternal transmission of an expanded trinucleotide repeat (usually with >1500 repeats). Patients present in the newborn period with hypotonia, talipes, bilateral facial weakness with a tented upper lip, open mouth and high arched palate. There may be a history of prematurity, reduced fetal movements and polyhydramnios. A weak cry and poor suck are often present, and respiratory insufficiency may be prominent and require mechanical ventilation. Reflexes are absent. The clinical course is characterised by motor and speech delay and a variable degree of learning difficulty. Neither clinical nor electrical myotonia is common in newborns and infants with myotonic dystrophy, and clinical and EMG analysis of the mother may be more useful. Cognitive difficulties are the presenting feature in those with a childhood onset in whom distal limb weakness, facial weakness and clinical myotonia may be present, but respiratory difficulties are not prominent.5 Cardiac arrhythmias and cardiomyopathy may occur in the second decade in both the congenital and childhood forms.35,36 DM2 differs from DM1 in having proximal weakness and absence of a congenital onset.

FSHD has a characteristic distribution of muscle involvement that often leads to targeted genetic testing without the need for a muscle biopsy. Clinical presentation is mostly in the second decade, with facial weakness, weakness of the scapular fixators leading to scapular winging and biceps and triceps wasting with sparing of the deltoid. Muscle involvement is typically asymmetric and progresses to involve the trunk muscles and then the hip girdle. High-frequency hearing loss and retinal telangiectasia are common, and cardiac arrhythmias are rarely associated.6 In the majority of patients with typical FSHD (>90%), there is a detectable deletion involving the subtelomeric region of chromosome 4q35.37 The specific genes involved have not yet been determined.

Newer Diagnostic Modalities in Neuromuscular Disorders

Two exciting new modalities, microarray or ‘chip’ technology and exome sequencing, promise to greatly improve the rate of positive gene diagnosis in neuromuscular disorders. Microarray technology involves the use of new generation sequencers to sequence for all known myopathy/muscular dystrophy/neuropathy genes in a single reaction, significantly reducing time and cost.9 While this technology is useful for identifying mutations in known genes, targeted sequencing of all protein-coding regions (‘exomes’) holds promise for identifying yet unknown genes that cause muscle diseases, especially in small kindreds or among sporadic unrelated individuals.38 False-positive and false-negative results remain critical issues with these emerging technologies, increasing the importance of accurately establishing the phenotype through history and clinical examination.

Multiple Choice Questions

  • 1A 4-year-old boy is seen in the outpatients department after having been noticed in pre-school having difficulty in getting off the floor and falling more frequently than his peers. On examination, he has prominent calves, a Gower's sign while rising from the floor, normal facial strength and extra-ocular movements and a persistently raised creatine kinase (CK) >13 000 U/L. There is no family history of a neuromuscular disorder.
  • The most appropriate next diagnostic test would be:

    • A Nerve conduction tests/electromyogram (EMG).
    • B Deletions/duplication analysis of the dystrophin gene.
    • C Testing for size of CTG repeats in DMPK gene.
    • D Targeted mutational analysis of the SMN1 gene.
    • E Muscle biopsy.
  • 2An 8-month-old girl presents to the outpatients department with delayed motor milestones, not having learned to roll or sit yet. On examination, she is not dysmorphic and has a bright and alert appearance with normal facial expression. She is hypotonic with no proximal antigravity movement and absent reflexes in the upper and lower limbs. She has a paradoxical breathing pattern and tongue fasciculations. Her CK is 300 U/L (normal 0–180 U/L).
  • The test most likely to yield a positive diagnosis is:

    • A Magnetic resonance imaging (MRI) of the brain.
    • B Karyotype.
    • C FISH for Prader–Willi syndrome.
    • D Testing for size of CTG repeats in DMPK gene.
    • E Targeted mutational analysis of the SMN1 gene.
  • 3A child with an early onset proximal weakness, with ophthalmoplegia, facial weakness and a static course with a normal CK would be classified as having a:
    • A Congenital myopathy.
    • B Congenital muscular dystrophy.
    • C Limb-girdle muscular dystrophy.
    • D Peripheral neuropathy.
    • E Anterior horn cell disorder.

Answers

1 B The presentation with proximal weakness in childhood, with a significantly raised CK is consistent with a limb-girdle muscular dystrophy, and Duchenne muscular dystrophy (DMD) would be the most common cause in a male child. Calf hypertrophy is often seen in DMD. A family history would not be present in the case of a de novo mutation. A multiplex ligation-dependent probe amplification (MLPA) or multiplex polymerase chain reaction would identify deletions and duplications, present in approximately 70% of cases of DMD, and sequencing of the dystrophin gene identifies small deletions/insertions/mutations in the remaining.

  • A The significantly raised CK and proximal weakness would suggest a muscular dystrophy. Nerve conduction tests would be normal and EMG may show only non-specific myopathic changes.
  • C Children with a childhood onset of myotonic dystrophy (testing for size of CTG repeats in DMPK gene) usually present with cognitive difficulties and have facial and distal limb weakness. CK is normal or only mildly (×5 normal) raised.
  • D While the child with SMA3 (targeted mutational analysis of the SMN1 gene) may present with a proximal weakness and Gower's sign, calf hypertrophy is not seen and the CK is only mildy (×5 normal) raised.
  • E A muscle biopsy would be invasive, but may need to be performed if MLPA and sequencing of the dystrophin gene do not show a mutation.

2 E The hypotonia, weakness and areflexia would place this child into the ‘floppy weak’ category. The proximal weakness, areflexia and tongue fasciculations would be consistent with an anterior horn cell disorder, of which spinal muscular atrophy (targeted mutational analysis of the SMN1 gene) would be the most common.

A–C MRI Brain, karyotype and FISH for Prader–Willi syndrome would be more appropriate for a child with central hypotonia (floppy strong).

D Generalised muscle weakness and facial weakness would be more likely with congenital myotonic dystrophy (testing for size of CTG repeats in DMPK gene).

3 A The congenital myopathies usually present at birth or childhood with hypotonia and weakness and a static or slowly progressive course. CK is usually normal or only mildly elevated. The presence of ophthalmoplegia, facial and bulbar involvement may help differentiate congenital myopathies from congenital muscular dystrophies, anterior horn cell disorders and early onset peripheral neuropathies.

B, C Muscular dystrophies are usually associated with a raised serum creatine kinase. Ophthalmoplegia and facial weakness are not associated. The course may be progressive with deterioration in strength or respiratory function.

D A child with an inherited peripheral neuropathy would have distal weakness and no ophthalmoplegia or facial weakness.

E Anterior horn cell disorders, characterised by SMA, have a progressive course and ophthalmoplegia and facial weakness are not associated, while tongue fasciculations may be seen.

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