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Skip Navigation LinksHome Page    >    The various genetic diseases    >    Spinal muscular atrophy (SMA)
 
 
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Spinal muscular atrophy (SMA)

Clinical types

There are 3 main types differentiated mainly by severity and age of onset:

Type 1- onset from birth- a very severe condition causing death at a young age due to respiratory failure.

Type 2- onset from early infancy, a severe type.

Type 3- relatively late onset, moderately severe.

Clinical signs

The disease is marked by significant muscle laxity and weakness. There are no tendon reflexes. The EMG and muscle biopsy show a typical picture. There are fine involuntary movements of the muscle, known as fasciculations, which can be seen in the tongue and in other muscles. The defect that causes this disease is in the motor nuclei of the anterior spinal cord.

In the first type, the disease can be detected in the uterus by a lack of fetal movements as felt by the mother.

Immediately after birth, up to the age of a few months, signs of muscle laxity and weakness appear, which develop into marked breathing difficulty. The disease progresses rapidly, and the infants usually die within 1- 2 years of diagnosis.

In the second type, the muscle weakness appears between 24 and 64 months of age. The disease progresses rapidly, and these children only live for a few years.

In the third type, the signs usually appear after the age of one year. The lifespan is probably normal, as is the child's intelligence. However, the muscle weakness is marked, manifesting as a characteristic disability. These patients usually need a wheelchair.

Inheritance pattern

Autosomal recessive

Penetrance

Full

Associated features that can be demonstrated by imagery

Usually there are no defects that can be identified by ultrasound examination during pregnancy.

What is the risk of recurrence in a subsequent pregnancy?

Parents who have one affected child have a 25% risk for recurrence in every subsequent pregnancy. More distant relatives also have a risk of having an affected child, but this is relatively low. Healthy siblings of the patient have a 1 in 300 chance of having an affected child, and the risk is even lower for more distant relatives.

Molecular genetic information

The gene for the disease

SMN

Location

Chromosome 5q.

Genetic testing

Diagnostic testing

A direct test to identify the mutation in the gene can be performed - this is usually a characteristic deletion that can be demonstrated in patients using a relatively simple method. See: Finding the defect (mutation) by establishing the gene sequence - autosomal recessive diseases.

Carrier testing

A direct test can be performed on carriers, but it is technically more difficult compared to that which can be carried out on patients.

For identifying carriers among the patient's relatives

The direct or indirect tests specified below in the section on fetal testing can be employed in order to identify carriers in the families. See: Indirect testing for genetic markers in a family that has a one or more patients - when there is only one gene that can cause the disease- autosomal recessive diseases.

Carrier testing for partners of carriers in the general population

A direct test can be performed for carrier detection, but, as mentioned, it is technically more difficult. See: Finding the defect (mutation) by establishing the gene sequence- autosomal recessive diseases.

Fetal testing

The method described for diagnosis is also used here. See above under diagnostic testing. See: Finding the defect (mutation) by establishing the gene sequence- autosomal recessive diseases.

A diagnosis can also be made by linkage analysis. See: Indirect testing for genetic markers in a family that has a one or more patients- when there is only one gene that can cause the disease - autosomal recessive diseases.
 
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