Causes
The main cause of SMA is progressive damage and destruction of specific nerve cells in the spinal cord (motoneurons). These are responsible for the control and regulation of the musculature. This means that impulses from the brain can no longer be passed on to the corresponding muscles.
This leads to a weakening and regression (atrophy) of the affected muscles. The reason for the destruction of nerve cells is the change (mutation) of an important gene (SMN gene) in the genetic material (DNA). Under normal circumstances, this gene produces specific proteins that are supposed to protect the nerve cells from damage.
For the inheritance of the disease, it is decisive whether the altered gene is present on both carriers (chromosomes) or whether the presence of only one of the two carriers is sufficient to pass on the disease to the offspring. Many people carry a defective gene, but are not affected by the disease themselves. In this case, the gene defect is compensated by a second, intact gene copy on the carriers. The disease thus only comes to light when a child receives the altered gene from both parents.
Associated symptoms
In addition to weakening and breakdown of the muscles, accompanying symptoms include paralysis (paresis), reduced muscle tension and a decrease in muscle strength. If nerves in the area of the head (cranial nerves) are also affected, this leads to an obstruction in chewing, swallowing and speaking. Fasciculations, i.e. involuntary twitches of the muscles, can also occur.At the same time, pain can occur in the affected muscle groups.
If the respiratory muscles are damaged, respiratory distress occurs as an accompanying symptom. Problems with seeing or hearing as well as a decrease in mental abilities are not usually observed. Affected patients show a severe restriction of movement due to weakened leg, trunk and arm muscles. Changes in the shape of the spinal column also lead to limited mobility.
Diagnosis
The first priority is a thorough physical and neurological examination of the patient. Various tests should be carried out to determine muscle strength, paying attention to the regression of certain muscle groups and changes in muscle reflexes. Various tests are available to confirm the diagnosis.
The function of nerve cells can be checked by measuring the nerve conduction velocity. The determination of electrical muscle activity can provide information about the excitation state of the muscles. Specific blood tests can be used to detect genetic changes in the area of the genome. Muscle reflexes can be triggered and checked by stimulating the respective muscles. For this purpose, a reflex hammer is used to tap lightly on special tendons, for example in the lower area of the patella.
