SPINAL MUSCULAR ATROPHY
                     SPINAL MUSCULAR 
ATROPHY
What is spinal 
muscular atrophy?
• Spinal muscular atrophy (SMA) is a genetic 
disease affecting the central nervous system, 
peripheral nervous system, and voluntary 
muscle movement (skeletal muscle).
Source: www.mda.org 2
About spinal 
muscular atrophy
• Most of the nerve cells that control muscles are 
located in the spinal cord, which accounts for 
the word spinal in the name of the disease. 
SMA is muscular because its primary effect is 
on muscles, which don’t receive signals from 
these nerve cells. 
Source: www.mda.org 3
What causes SMA?
• Chromosome 5 SMA is caused by a deficiency 
of a motor neuron protein called SMN, for 
“survival of motor neuron.” 
• This protein, as its name implies, seems to be 
necessary for normal motor neuron function. 
SMN plays a pivotal role in gene expression in 
motor neurons.
• Its deficiency is caused by genetic flaws 
(mutations) on chromosome 5 in a gene called 
SMN1.
Source: www.mda.org 4
What are the 
symptoms of SMA?
• The primary symptom of chromosome 5-
related (SMN-related) SMA is weakness of the 
voluntary muscles. 
• The muscles most affected are those closest to 
the center of the body, such as those of the 
shoulders, hips, thighs, and upper back. 
• The lower limbs seem to be affected more than 
the upper limbs, and deep tendon reflexes are 
decreased.
Source: www.mda.org 5
Complications
• Special complications occur if the muscles used 
for breathing and swallowing are affected, 
resulting in abnormalities in these functions. If 
the muscles of the back weaken, spinal 
curvatures can develop.
Source: www.mda.org 6
What is the 
progression of 
SMA?
• In chromosome 5-related SMA, the later the 
symptoms begin and the more SMN protein 
there is, the milder the course of the disease is 
likely to be. 
• While in the past, infants with SMA typically did 
not survive more than two years, today most 
doctors now consider SMN-related SMA to be a 
continuum and prefer not to make rigid 
predictions about life expectancy or weakness 
based strictly on age of onset.
Source: www.mda.org 7
What is the status 
of research on 
SMA?
• Research has focused on strategies to increase 
the body's production of the SMN protein 
lacking in the chromosome 5-related forms of 
the disease. Approaches include methods to 
help motor neurons survive in adverse 
circumstances.
Source: www.mda.org 8
How common is 
spinal muscular 
atrophy?
• Approximately 10,000 to 25,000 children and 
adults are living with SMA in the United States. 
It’s a rare disease that affects one out of 6,000 
to 10,000 children.
Source: my.clevelandclinic.org 9
Types of spinal 
muscular atrophy
• Type 1 (severe): About 60% of people with SMA 
have type 1 , also called Werdnig-Hoffman disease.
• Type 2 (intermediate): Symptoms of type 2 SMA 
(also called Dubowitz disease) appear when a child 
is between six months and 18 months old.
• Type 3 (mild): Symptoms of type 3 SMA (also 
called Kugelbert-Welander or juvenile-onset SMA) 
appear after a child’s first 18 months of life. 
• Type 4 (adult): The rare adult form of SMA 
doesn’t typically appear until the mid-30s.
Source: my.clevelandclinic.org 1
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