Spinal Muscular Atrophy Type 3 (SMA3, Kugelberg-Welander Disease)

Definition - a genetic disease in which loss of nerve cells in the spinal cord called motor neurons affects the part of the nervous system that controls voluntary muscle movement.

Cause - A deficiency of a motor neuron protein called SMN, for survival of motor neurons, resulting from a faulty gene on chromosome 5.

Onset - after 18 months

Symptoms - Weakness is most severe in muscles closest to the center of the body, such as those of the shoulders, hips, thighs and upper back. Respiratory muscle weakness and spinal curvature sometimes develop.

Progression - Disease progresses slowly, with walking ability usually maintained until at least adolescence. Wheelchair often required later in life. Life span usually not affected.

Most doctors now consider SMN-related SMA to be a continuum of severity and prefer not to make rigid predictions about life expectancy or weakness. Some experts call the milder end of the SMA3 spectrum adult-onset SMA or SMA4.

Inheritance - autosomal recessive; a faulty gene must be inherited from each parent.

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