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Second most common lethal recessive disease in the white population. Neurological disorder (five types) that leads to severe amyotrophy with respiratory insufficiency has been described. Clinical evolution according to the type involved.

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  • Spinal Muscular Atrophy Type I (Werdnig-Hoffman Disease; SMA Infantile Acute form; Infantile SMA)
  • Spinal Muscular Atrophy Type II (Dubowitz Disease; Intermediate type of SMA; Infantile Chronic form of SMA)
  • Spinal Muscular Atrophy Type III (Juvenile Muscular Atrophy; Childhood-Onset Proximal SMA; Kugelberg Welander Syndrome; Childhood Isolated SMA; Wohlfart-Kugelberg-Welander Syndrome)
  • Spinal Muscular Atrophy Type IV (Distal Hereditary Motor Neuropathy; Adult Spinal Muscular Atrophy)
  • Spinal Muscular Atrophy Type V or Adult Onset X-Linked SMA (Kennedy Disease; Bulbospinal Muscular Atrophy)

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1:6000 live births.

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Autosomal recessive. However, some forms have been suggested to be inherited as autosomal dominant and occasionally X-linked. In adult onset (type IV), the causal gene is located on chromosome 12 (12q24).

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The disorder is characterized by degeneration and loss of anterior horn cells in the spinal cord and sometimes also in the brainstem, leading to symmetrical muscle weakness and wasting of voluntary muscles. There is evidence that at least two identifiable genes are associated with the disease: SMN1 (survival motor neurone) and NAIP (neuronal apoptosis inhibitory protein), and possibly a third gene, BTF2p44 (basal transcription factor 2H, subunit p44). The SMN gene is missing in the majority of SMA patients and small, intragenic mutations have also been associated with SMA. Approximately half of the severely affected patients are also missing the NAIP gene, which may affect the severity of the disease.

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Clinical features. CT scan and MRI are used to exclude other pathologies. Electromyography usually shows neurogenic and occasional myogenic pattern. Nerve conduction study is normal. Serum creatine kinase is normal. Muscle biopsy shows a large number of round atrophic fibers and clumps of hypertrophic fibers that are type I by ATPase reaction. DNA testing for SMN and NAIP genes is reliable and confirmatory for the phenotype. Prenatal DNA testing is less reliable because a small percentage of population lack an SMN gene but are not clinically affected.

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Spinal Muscular Atrophy is the second most common lethal autosomal recessive disease in the white population.

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Type I (Werdnig-Hoffman Disease): Early onset and diagnosis (before 3 months), with severe intrauterine growth retardation, polyhydramnios, and tongue fasciculations. Child never sits or walks. There is difficulty with swallowing and feeding. Respiratory function is always severely impaired: diaphragmatic breathing, respiratory infection, and distress. Absent deep tendon reflexes, hypotonia, and weakness, but normal intelligence and no sensory loss are observed. Restricted joint mobility and kyphoscoliosis are also frequent. Death before age of 2 years old is common but does not always occur.

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Type II (Dubowitz Disease or Chronic form): Intermediate SMA with onset between the age of 3 and 18 months and survival beyond 4 years (usually ...

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