X-linked spinal muscular atrophy type 2

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X-linked spinal muscular atrophy type 2 (SMAX2, XLSMA), also known as arthrogryposis multiplex congenita X-linked type 1 (AMCX1), is a rare neurological disorder involving death of motor neurons in the anterior horn of spinal cord resulting in generalised muscle wasting (atrophy). The disease is caused by a mutation in UBA1 gene and is passed in a X-linked recessive manner by carrier mothers to affected sons.^[1]^[2]

Affected babies have general muscle weakness, weak cry and floppy limbs; consequently, the condition is usually apparent at or even before birth. Symptoms resemble the more severe forms of the more common spinal muscular atrophy (SMA); however, SMAX2 is caused by a different genetic defect and only genetic testing can correctly identify the disease.^{[citation needed]}

The disorder is usually fatal in infancy or early childhood due to progressive respiratory failure, although survival into teenage years have been reported.^[3] As with many genetic disorders, there is no known cure to SMAX2. Appropriate palliative care may be able to increase quality of life and extend lifespan.^{[citation needed]}

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Classification	ICD-10: G12.1 OMIM: 301830 MeSH: C535380
External resources	Orphanet: 1145

Diseases of the nervous system, primarily CNS (G04–G47, 323–349)

Inflammation

Brain	Encephalitis Viral encephalitis Herpesviral encephalitis Limbic encephalitis Encephalitis lethargica Cavernous sinus thrombosis Brain abscess Amoebic
Spinal cord	Myelitis: Poliomyelitis Demyelinating disease Transverse myelitis Tropical spastic paraparesis Epidural abscess
Both/either	Encephalomyelitis Acute disseminated Myalgic Meningoencephalitis

Brain/
encephalopathy

Degenerative

Extrapyramidal and movement disorders	Basal ganglia disease Parkinsonism PD Postencephalitic NMS PKAN Tauopathy PSP Striatonigral degeneration Hemiballismus HD OA Dyskinesia Dystonia Status dystonicus Spasmodic torticollis Meige's Blepharospasm Athetosis Chorea Choreoathetosis Myoclonus Myoclonic epilepsy Akathisia Tremor Essential tremor Intention tremor Restless legs Stiff-person
Dementia	Tauopathy Alzheimer's Early-onset Primary progressive aphasia Frontotemporal dementia/Frontotemporal lobar degeneration Pick's Dementia with Lewy bodies Posterior cortical atrophy Vascular dementia
Mitochondrial disease	Leigh syndrome

Demyelinating

Episodic/
paroxysmal

Seizure/epilepsy	Focal Generalised Status epilepticus Myoclonic epilepsy
Headache	Migraine Familial hemiplegic Cluster Tension
Cerebrovascular	TIA Amaurosis fugax Transient global amnesia Acute aphasia Stroke MCA ACA PCA Foville's Millard–Gubler Lateral medullary Medial medullary Weber's Lacunar stroke
Sleep disorders	Insomnia Hypersomnia Sleep apnea Obstructive Congenital central hypoventilation syndrome Narcolepsy Cataplexy Kleine–Levin Circadian rhythm sleep disorder Advanced sleep phase disorder Delayed sleep phase disorder Non-24-hour sleep–wake disorder Jet lag

CSF

Other

Spinal cord/
myelopathy

Both/either

Degenerative

SA	Friedreich's ataxia Ataxia–telangiectasia
MND	UMN only: Primary lateral sclerosis Pseudobulbar palsy Hereditary spastic paraplegia LMN only: Distal hereditary motor neuronopathies Spinal muscular atrophies SMA SMAX1 SMAX2 DSMA1 Congenital DSMA SMA-PCH SMA-LED SMA-PME Progressive muscular atrophy Progressive bulbar palsy Fazio–Londe Infantile progressive bulbar palsy both: Amyotrophic lateral sclerosis

X-linked disorders

X-linked recessive
Immune	Chronic granulomatous disease (CYBB) Wiskott–Aldrich syndrome X-linked severe combined immunodeficiency X-linked agammaglobulinemia Hyper-IgM syndrome type 1 IPEX X-linked lymphoproliferative disease Properdin deficiency
Hematologic	Haemophilia A Haemophilia B X-linked sideroblastic anemia
Endocrine	Androgen insensitivity syndrome/Spinal and bulbar muscular atrophy KAL1 Kallmann syndrome X-linked adrenal hypoplasia congenita
Metabolic	Amino acid: Ornithine transcarbamylase deficiency Oculocerebrorenal syndrome Dyslipidemia: Adrenoleukodystrophy Carbohydrate metabolism: Glucose-6-phosphate dehydrogenase deficiency Pyruvate dehydrogenase deficiency Danon disease/glycogen storage disease Type IIb Lipid storage disorder: Fabry's disease Mucopolysaccharidosis: Hunter syndrome Purine–pyrimidine metabolism: Lesch–Nyhan syndrome Mineral: Menkes disease/Occipital horn syndrome
Nervous system	X-linked intellectual disability: Coffin–Lowry syndrome MASA syndrome Alpha-thalassemia mental retardation syndrome Siderius X-linked mental retardation syndrome Eye disorders: Color blindness (red and green, but not blue) Ocular albinism (1) Norrie disease Choroideremia Other: Charcot–Marie–Tooth disease (CMTX2-3) Pelizaeus–Merzbacher disease SMAX2
Skin and related tissue	Dyskeratosis congenita Hypohidrotic ectodermal dysplasia (EDA) X-linked ichthyosis X-linked endothelial corneal dystrophy
Neuromuscular	Becker's muscular dystrophy/Duchenne Centronuclear myopathy (MTM1) Conradi–Hünermann syndrome Emery–Dreifuss muscular dystrophy 1
Urologic	Alport syndrome Dent's disease X-linked nephrogenic diabetes insipidus
Bone/tooth	AMELX Amelogenesis imperfecta
No primary system	Barth syndrome McLeod syndrome Smith–Fineman–Myers syndrome Simpson–Golabi–Behmel syndrome Mohr–Tranebjærg syndrome Nasodigitoacoustic syndrome

X-linked dominant
X-linked hypophosphatemia Focal dermal hypoplasia Fragile X syndrome Aicardi syndrome Incontinentia pigmenti Rett syndrome CHILD syndrome Lujan–Fryns syndrome Orofaciodigital syndrome 1 Craniofrontonasal dysplasia

Disorders of translation and posttranslational modification

Translation

Initiation factor: Leukoencephalopathy with vanishing white matter

snRNP: Retinitis pigmentosa 33

Posttranslational modification

Protein folding	Alzheimer's disease Huntington's disease Creutzfeldt–Jakob disease chaperonins: 3-Methylglutaconic aciduria 5
Protein targeting	I-cell disease
Ubiquitin	E1: X-linked spinal muscular atrophy 2 E3: Johanson–Blizzard syndrome Von Hippel–Lindau disease 3-M syndrome Angelman syndrome Deubiquitinating enzyme: Machado–Joseph disease Aneurysmal bone cyst Multiple familial trichoepithelioma 1
SUMO	OFC10
Other	Multiple sulfatase deficiency Hyperproinsulinemia Ehlers–Danlos syndrome 6

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X-linked spinal muscular atrophy type 2

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