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Distal spinal muscular atrophy type 1

The meaning of «distal spinal muscular atrophy type 1»

Distal spinal muscular atrophy type 1 (DSMA1), also known as spinal muscular atrophy with respiratory distress type 1 (SMARD1), is a rare neuromuscular disorder involving death of motor neurons in the spinal cord which leads to a generalised progressive atrophy of body muscles.

The condition is caused by a genetic mutation in the IGHMBP2 gene[1][2] and is inherited in an autosomal recessive manner. There is no known cure to DSMA1, and research of the disorder is still in early stages due to low incidence and high mortality rates.[1]

Usually, the first respiratory symptoms are shortness of breath (dyspnea) and paradoxical respirations which then escalate within the first few months of life to diaphragmatic paralysis.[2] The symptoms of diaphragmatic paralysis come on very rapidly and without warning, and the patient is often rushed to a hospital where they are placed on a ventilator for respiratory support.[3] Due to the severe nature of diaphragmatic paralysis, the patient eventually needs continuous ventilation support to survive.[2] Continuous ventilation, however, may in itself cause damage to the anatomy of the lungs.[2]

In addition to diaphragmatic paralysis, other issues may arise: as the name suggests, the distal limbs are most affected with symptoms of weakness,[3] restricting mobility due to (near-)paralysis of the distal limbs as well as the head and neck.[3] Also, dysfunction of the peripheral nerves and the autonomic nervous system may occur.[1] Due to these dysfunctions, the patients have been shown to suffer from excessive sweating and irregular heartbeat.[2] The deep tendon reflex is also lost in patients with DSMA1.[3]

Uterine growth retardation and poor foetal movement have been observed in severe DSMA1 cases.[2]

DSMA1 is caused by a genetic mutation in the IGHMBP2 gene (located on chromosome 11, locus 11q13.3), which codes the immunoglobulin helicase μ-binding protein 2. The role of the IGHMBP2 protein is not fully understood, but it is known to affect mRNA processing.[1] The cellular mechanisms of the mutation, as well as the protein mechanisms disrupted by the mutation, are unknown.[3] IGHMBP2 mutations are usually random mutations which are normally not passed down through generations.[1]

The pathology underlying the observable characteristics of DSMA1 is cell body degeneration of motor nerves. Specifically, the anterior horn α-motorneurons degenerate within the first six months of life leading to a variety of symptoms. Muscle deterioration increases at around 1–2 years of age, resulting in reduced motor function. The most severely affected muscles include facial muscles and the tongue (which may develop a twitch due to hypoglossal nerve paralysis). Reduced pain sensation and excessive sweating are sometimes observed. Non-ambulant patients may develop pressure ulcers, severe constipation, urinary incontinence, and (rarely) reflux nephropathy in the advanced stages of the disease.[3]

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