Myotonic dystrophy (DM) is a genetic disorder characterized by both progressive muscle wasting and stiffness, or an inability to relax muscles at will. Multi-system, it can affect the skeletal muscles, or those of the limbs and trunk; smooth muscles, or those found in the digestive system; and heart muscles.
There are two types of myotonic dystrophy, DM1 and DM2, based on the underlying genetic mutation — expansions of repeated areas of genes. DM1 is caused by an abnormal expansion of a repeated region in the DMPK gene, and DM2 in the CNBP gene.
The length of the expansion correlates with disease severity and age of onset. In other words, the longer the expansion, the more severe are the symptoms and the earlier they develop. Expanded regions can become even longer from one generation to the next, especially in DM1, meaning that a grandparent who is mildly affected can have more severely affected children and grandchildren who can develop the disease at birth (congenital DM).
Myotonic Dystrophy Type 1 (DM1)
DM1 is generally classified by its type: mild, classical or congenital. Patients with mild DM1 often have cataracts, or clouded eye lenses, and muscles that fail to relax after use (myotonia). Muscle weakness and wasting — muscle atrophy — are typical symptoms of classic DM1, as are cataracts that form before age 50, and an abnormal heart rate caused by unusual conduction of electrical impulses in the heart. Congenital DM1 is marked by general muscle weakness that also affects breathing, and intellectual disability.
Infants born with congenital-onset DM1 typically have significant cognitive as well as muscular disabilities. Whether these symptoms are exclusively related to the genetic mutation or are influenced by labor and delivery complications is a matter of debate. Mental disabilities typically do not worsen as children grow, but they also do not improve.
Babies with the disease generally have very weak muscles, causing them to appear “floppy.” A pointed, or dented, upper lip and inward rotation of one or both feet, known as clubfoot, are common physical signs of congenital DM1. Muscles that control breathing, swallowing, and sucking are affected. Infants with serious symptoms typically require neonatal intensive care.
Muscle tone usually improves during childhood, but progressive muscle wasting typically reappears later in life.
Unlike other forms of DM, initial signs of juvenile-onset DM1 typically involve social, cognitive and behavioral issues. Problems with visual-spatial processing and executive function, related to an inability to plan ahead and readily arrive at a decision, are commonly reported in children with this form of DM. Some display more severe cognitive disability than others, and may require special education. They may exhibit the communication problems typical of people with Asperger syndrome.
As these patients grow, they begin to show characteristic signs of muscle degeneration.
Adults with DM1 may experience symptoms of muscle wasting and muscle stiffness, or myotonia, in voluntary and involuntary muscles. Distal muscles, or those farthest from the center of the body, are usually most affected in DM1.
Myotonia in the hands and feet is usually the most noticeable and the first identifiable symptom of the disease. Stiff or contracted muscles can make it difficult for patients to open doors, write, and use tools, particularly in cold settings. Muscle weakness in the feet can cause patients to easily trip, due to incomplete flexion of lower leg muscles when walking. This is also known as foot drop.
Muscles in the head, neck, and face are also often affected, commonly resulting in limited facial expressions, droopy eyelids, and a hollow, thin facial structure. A patient may also have difficulty lifting the head, and stronger core muscles are often enlisted to assist in this task.
In some patients, weakness in muscles required for breathing and swallowing increases the risk of sleep apnea, choking, and aspiration (breathing in food or liquids).
Involuntary muscles, particularly those surrounding hollow organs, also tend to weaken with time, potentially causing digestive issues, difficulties in childbirth, and low blood pressure.
A person’s ability to maintain a healthy heart rate can also be affected, although this symptom generally does not develop until the later stages of the disease.
Adults with DM1 commonly develop cataracts, but the connection between this eye condition and DM is not well understood. Premature balding is frequent in men with the disease.
Myotonic Dystrophy Type 2 (DM2)
DM2 almost exclusively develops in adults and is generally the milder of the two forms. Proximal muscles, or those closer to the center of the body, tend to be more affected in DM2. Patients typically show initial muscle weakness in the upper leg, or thigh, but may also experience such weakness in hand and wrist muscles, similar to DM1 patients.
Involuntary muscles are less affected in DM2 patients, although the cardiac disorders affecting heart rate can be common in DM2 patients, as are cataracts.
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