Paramyotonia congenita is a rare muscular disorder inherited as an autosomal dominant trait. This nonprogressive disorder is characterized by a condition in which the muscles do not relax after contracting (myotonia). Symptoms can be triggered by exposure to the cold. There are also intermittent periods of a type of paralysis in which there is no muscle tone (flaccid paresis). This condition does not necessarily coincide with exposure to cold temperatures or myotonia. There is no wasting (atrophy) or increase in bulk (hypertrophy) of muscles with this disorder.
Symptoms include muscle stiffness and weakness, mostly in the face, neck and upper extremities. The muscles are slow to relax after contracting (myotonia). This condition may become worse with exposure to cold.
Paramyotonia congenita is usually apparent during infancy and is not progressive. Individuals with this disorder do not have wasting of muscles (atrophy) or an increase of muscle bulk (hypertrophy).
This condition is transmitted as an autosomal dominant genetic trait. The malfunctioning gene has been tracked to the long arm of chromosome 17 (17q23.1-q25.3)
Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 17q23.1-q25.3” refers to a region on the long arm of chromosome 17 between bands 23.1 and 25.3. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.
All individuals carry a few abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.
Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary for the appearance of the disease. The abnormal gene can be inherited from either parent, or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy regardless of the sex of the resulting child.
Paramyotonia congenita is a very rare disorder that affects males and females in equal numbers. A detailed study conducted in Germany concluded that the incidence of PMC was between 1 in 180,000 and 1 in 350,000. They note that the distribution of this disorder is not uniform since they found a region of the country in which the incidence was 1 in 8000.
Three large families with multiple generations of affected members have accounted for at least 60 patients with paramyotonia congenita.
When paramyotonia congenita is suspected, a test is administered to test the capacity of muscles to conduct electricity (electromyography).
The aim of treatment is to reduce the intensity of acute symptoms and to prevent, as far as possible, further attacks. Some attacks are so mild that treatment is not necessary. However, in other instances drug therapy is required.
Some patients with paramyotonia congenita may benefit from acetazolamide or thiazide diuretic drugs to reduce the number of paralytic attacks. Treatment with the drug tocainide may help reduce the cold-induced symptoms in some patients.
Genetic counseling may be of benefit for patients and their families. Other treatment is symptomatic and supportive.
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