Skeletal Muscle in Myotonic Dystrophy

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Signs and Symptoms 


There are three types of muscle in the body. Cardiac muscle is found in the heart. Smooth muscle is found in the lining of the intestine, the blood vessels, the bladder, and other internal organs. When we think of “muscle,” we are usually thinking of skeletal muscle. The muscles of the limbs, trunk, and head, over which we have voluntary control, are skeletal muscles. Skeletal muscle issues in DM include:

Myotonia: Myotonia is a term for slowing of muscle relaxation. Like nerves, muscles are operated by electrical controls. Normally a muscle will contract when it receives a signal from a nerve and it will stop contracting as soon as the nerve signal has ended. With myotonia, a muscle will start its contraction on cue, but it will continue to have electrical activity, and it will continue to contract, even after the nerve signal has ended. Thus, the muscle continues to work longer than the person intended, causing a stiffness or “locking up” of the muscle. The most common way in which this is noticeable is when people grip tightly with their fingers, causing the grip muscles to “lock up”. It may take 20 seconds or longer for the fingers to fully relax.

Myotonic dystrophy was named for the myotonia because this aspect of the condition is very distinctive and it distinguishes myotonic dystrophy from other forms of muscular dystrophy. The presence of myotonia is not the most disabling aspect of DM.  However, it is a hallmark of the condition – families and doctors often recognize that people have myotonic dystrophy because they have myotonia. It was partly for this reason that the Myotonic Dystrophy Foundation selected a drawing of a person’s hand grip as their logo.

The myotonia in DM1 tends to be noticeable in a few muscles of the body. It is most prominent in the forearm and finger muscles, causing grip lock. Sometimes it will also affect the muscles of the tongue and jaw, which can lead to difficulty with speech or chewing. Commonly it seems worse in cold weather. In DM2, the myotonia affects the finger grip muscles, but it also can be noticeable in leg muscles, especially the thigh muscles, and around the back and shoulders. A quick movement may trigger the muscle stiffness. For example, hitting a baseball and running to first base, or trying to sprint up stairs, may cause stiffness of the leg muscles in DM2.

Recently scientists have discovered that the myotonia in myotonic dystrophy is caused by a problem with the movement of chloride ions into and out of muscle cells. Ions are particles that carry a positive or negative electrical charge. Table salt is composed of sodium and chloride ions, and both of these ions are important for electrical control of muscle contraction and relaxation. The chloride ions enter and exit muscle cells through specialized tunnels, called ion channels. Most of the chloride ion channels are missing in myotonic dystrophy, and this is what causes the myotonia. Experiments in the lab have shown that when the chloride channels are restored to their normal place, the myotonia goes away.

Muscle weakness and atrophy. Atrophy or muscle wasting are terms that describe decreasing size of muscles. People with myotonic dystrophy can be outstanding athletes and quite strong when they are young, but over time there is loss of strength and atrophy of muscles. The severity of muscle atrophy and weakness is quite variable in different people with myotonic dystrophy, even when they are in the same family (see Patterns of Muscle Weakness below). For some people the weakness is very obvious in childhood, but for others it is very mild even after the age of 50. For most people the amount of weakness is somewhere between these two extremes.

The weakness in myotonic dystrophy tends to affect some muscles more than others. In fact, a common situation is that some muscles are severely weak when others have normal strength. Muscle weakness is the main cause of disability in myotonic dystrophy. For example, muscle weakness may affect mobility, hand dexterity, and lifting. Muscle weakness tends to worsen over time, but the rate of deterioration is slow.

For most people, the weakness increases noticeably year-by-year, or season-by-season. Weakness that increases week-by-week or month-by-month is not typical of myotonic dystrophy. If this occurs, other explanations should be considered, such as medication side effects, or another illness that is unrelated to myotonic dystrophy. Not uncommonly, people will experience extended periods of time when the condition seems to remain relatively stable. Serious problems in DM1, such as, difficulty with breathing or swallowing, are caused by weakness of the muscles in the throat and chest. 

Researchers still do not have a clear understanding of what causes the muscles to become weak and atrophic in myotonic dystrophy. Several groups of scientists are actively working on this important question. So far there is no treatment that can prevent muscle weakness, or slow it down, but there are many groups of researchers trying to develop treatments that can have these effects. As discussed below, assistive devices such as braces, canes, walkers, and wheelchairs can help to maintain independence and mobility. 

Muscle pain. Myotonic dystrophy can be associated with pain. In some cases the pain originates inside the muscles. In other cases, the pain originates in joints, ligaments, or the spine. Muscle weakness may predispose to arthritic changes or strain in these areas.

Patterns of Muscle Weakness in Myotonic Dystrophy: DM1 at different ages

DM1 is often divided into different subtypes, depending on the age of symptom onset.  Learn more about the disease types.

Muscle problems that are typical of different subtypes of DM1 are listed below. Note that different people in a family can have different subtypes of DM1.

Form
Sign and Symptoms

Congenital DM1

Newborn:

Muscle function in patients with congenital DM1 follows a predictable pattern: 

  • Severe muscle weakness in newborns

  • Substantial improvement in children who survive the first six months, often with delayed motor development in infancy and childhood.

  • Development of symptoms that mimic adult onset DM1 in the later years.

Prenatal:

  • Lower than normal fetal movement (fetal akinesia).

  • Accumulation of fluids (edema) in the tissues and organs of the fetus (hydrops fetalis).

  • Increased amniotic fluid in the mother (polyhydramnios). Breech presentation,

    placental abruption, and umbilical cord prolapse may result.

 Childhood/Adolescence:

  • Gradual improvement of newborn hypotonia and feeding issues (only rarely present at age 3-4 years).

  • Delayed gross motor skill development. Nearly all children learn to walk independently, although great variability exists as to when they achieve this milestone.

  • Delayed fine motor skill development: grasping a toy or finger; transferring a small object from hand to hand; pointing out objects; following movement with the eyes; self feeding.

  • Myotonia is typically not present at birth, but typically begins in adolescence or early twenties.

  • Weakness in muscles (including the hands, feet, and face) that may interfere with mobility and necessitate the use of assistive devices (such as ankle braces or canes.

  • Lack of facial expression due to weakness of facial muscles.

  • Muscle impairment in the mouth, palate and jaw that can delay speech development and inhibit proper pronunciation (may be exacerbated by hearing loss).

Adulthood:

  • Gradual worsening of symptoms; symptomatic progression similar to that seen in adult onset DM1.

Childhood Onset DM1

Childhood/Adolescence:

  • Normal or slightly delayed early motor development
  • Facial and neck muscle problems, typically without the facial appearance that is associated with the congenital form.
  • Foot drop (lower leg, foot and ankle dorsiflexor weakness) leading to a characteristic high-stepping, toe-dragging, or shuffling gait that may result in an increased number of falls.
  • Weakness in distal muscle (including hands, feet, and face) that may interfere with mobility and necessitate the use of assistive devices (such as ankle braces and canes).
  • Myotonia, particularly in hand intrinsic muscles (leading to difficulty relaxing grasp, especially in the cold) and the tongue (leading to slurred and slow speech, exacerbated by weakness of the facial muscles).
  • Additional symptoms of adult onset myotonic dystrophy DM1 will appear in later years.

Adulthood:

  • Gradual worsening of symptoms; symptomatic progression similar to that seen in adult onset myotonic dystrophy DM1.

Adult Onset DM1 

Classical Form

This type of DM1 commonly starts in the teens, twenties, or thirties. The earliest symptom is often myotonia of the hand grip. Later, a person notices weakness of gripping or pinching with the fingers, or moving the ankles. Examination at this point usually shows weakness and wasting in the long finger flexors and weakness of the facial and neck flexor muscles. Typical ways in which adult-onset myotonic dystrophy DM1 can affect a person include the following:

  • Weakness and atrophy of the jaw (masseter and temporalis) and facial muscles leads to thinning of the facial contour and reduced facial expression.
  • Indistinct speech and problems with articulation due to weakness of facial, tongue, and palatal muscles, and myotonia of the tongue.
  • Drooping of the eyelids (ptosis) due to weakness of muscles that hold up the eyelids (levator palpebrae and Mueller muscle); in severe cases requiring splints or surgery. Weakness of the other ocular muscles can lead to limitation of lateral and vertical eye movements.
  • Weakness in distal muscles (hand and ankle movements) that interferes with dexterity, handwriting and mobility. The combination of finger weakness and myotonia is particularly challenging for jobs that require rapid, repeated, or forceful finger movements. Weakness of lifting the toes and foot, known as “foot drop”, can lead to a characteristic high-stepping, toe-dragging, or shuffling gait. Weakness of the calf muscles can lead to difficulty jumping or rising up on the toes. Combined with foot drop, this can lead to instability of the ankles, difficultly standing still, and frequent falls. This aspect of DM1 often is markedly improved by use of light weight braces called ankle-foot orthoses (AFOs).
  • Neck flexor weakness can make it difficult to lift the head off the pillow. Weakness in the neck extensor muscles can make it difficult to hold the head upright, leading to a dropped head posture. 
  • Weakness of the breathing muscle can lead to shortness of breath. When this occurs, it may be noticeable when a person tries to exercise. However, breathing problems can occur mainly during sleep, and a person may not be aware that this is happening. During sleep, people tend to rely more on their diaphragm as the main breathing muscle. DM1 can lead to marked weakness of the diaphragm. This can be assessed by checking people's breathing capacity when they are sitting up and lying down, or by checking oxygen levels overnight while they sleep.
  • For anyone with DM1 who is contemplating surgery, it is important to know whether there is weakness of the diaphragm or other breathing muscles.
  • Muscle stretch reflexes are often reduced
  • Myotonia tends to decline over time as muscle weakness increases.

Adult Onset DM1

Mild Form

Some people with DM1 have minor weakness and only mild myotonia begins later in life (in the fifties, sixties, or seventies). This can explain how a person can have DM1 even if neither parent was known to be affected: one of the parents did have the condition but never sought medical attention because it was so mild.

 

Patterns of muscle weakness in DM2

Of the types of myotonic dystrophy (click here to learn of the disease types), congenital and childhood-onset disease probably does not occur in DM2. Muscle symptoms in DM2 may begin in the teenage years, but more commonly these symptoms develop in the twenties, thirties, forties, or fifties. The initial symptoms may relate to grip myotonia. Alternatively, myotonia may be inconspicuous, and the initial symptoms may relate to weakness of muscles around the hips or shoulders. For example, common symptoms are difficulty standing up from a low chair, rising from the ground or a squatting position, or climbing stairs. Reaching up or working with the arms overhead also may be difficult. People with DM2 often experience unusual fatigue with exercise. Muscle atrophy also occurs, but it is less noticeable than in DM1, and tends to occur in later years. Muscle pain in the neck, back, shoulders, hip flexors, and upper legs may be a prominent symptom. The severity of pain can fluctuate from day to day.

Diagnosis

Neuromuscular assessment: The most important procedure to diagnose DM is a careful neurological, and sometimes ophthalmological, examination. When the characteristic changes of myotonia and muscle weakness have developed, the examination can provide strong evidence for DM1, so that a physician is reasonably confident about the diagnosis. Checking for myotonia is not routine for most general physicians, but neuromuscular specialists will usually check for this either by having a person make a tight grip or using a percussion hammer (a small rubber hammer) to tap on the muscles in the hand or forearm. Delay in reaching a diagnosis is quite common, because people with DM1 may not recognize the exact nature of their symptoms, and physicians in several different specialties may be consulted before the diagnosis of DM1 is even considered. Congenital DM1 is more difficult to recognize because there are many different causes of weakness and hypotonia in newborn babies. DM2 can be difficult to differentiate from other types of late-onset muscular dystrophy, especially when the myotonia is not very apparent and cataracts are not recognized. Other diagnostic procedures are helpful in establishing a definitive diagnosis.

  • Electromyography (EMG). A needle that is connected to a wire is inserted into the muscle so that the electrical activity in the muscle can be recorded. This procedure shows myotonia in a high proportion of people with DM1 or DM2. This test shows an extended burst of electrical discharges in a saw tooth-like pattern. These are the abnormal electrical signals that are associated with slowing of muscle relaxation.
  • Muscle biopsy. Results of a muscle biopsy can point strongly in the direction of DM, but they are not definitive in making the diagnosis. [Note, however, that research methods for analyzing the biopsy sample, which are becoming more common in non-research pathology laboratories, are very accurate for identifying myotonic dystrophy.] If DM1 is suspected, a biopsy is usually not performed, now that genetic testing is available. Recognition of DM2 can be a greater challenge, and sometimes the first test result to suggest the possibility of DM2 is muscle biopsy.
  • Routine blood tests. An enzyme in muscle, called creatine kinase (CK), will leak into the blood when muscle tissue is damaged. The level of CK in blood can be elevated in myotonic dystrophy, but often it remains in the normal range. Some other enzymes, call ALT or AST, can also leak into the blood when there is muscle damage. These tests are commonly ordered during routine annual physicals, because physicians generally consider these as indicators of liver health. If the possibility of a muscle condition is not considered, the blood test abnormalities may be taken as evidence for liver problems, when in fact the ALT or AST has come from the muscle tissue. Confusion about these blood tests is not uncommon in people with DM2.
  • Genetic testing. Definitive confirmation of DM1 or DM2 can come through genetic testing click here to learn more. When performed properly, the genetic testing for DM1 and DM2 is definitive. In other words, a negative result means that DM is not present, and a positive result means that a person has DM or is at risk for developing it.

Treatment of skeletal muscle problems in DM

Weakness: There are currently no medications available that address myotonic dystrophy weakness. Symptomatic treatments include:

  • Occupational therapy and physiotherapy
  • Molded ankle supports and leg braces to reduce foot-drop and enhance gait stability
  • Fitted collar to reduce the effects of neck muscle weakness
  • Low-intensity exercise strength training, to the extent that individuals are capable and without undue physical or cardiac stress (see precautionary note in Systemic Characteristics: Cardiovascular System)
    Pain: Conventional pain medications may be useful in treating the painful aspects of myotonic dystrophy.

Myotonia:

  • Using gloves or heated gloves to keep the hands warm in cool weather
  • Anti-myotonic medications. Studies show that drugs affecting ion channels, such as, mexilitene, can improve myotonia. Additional testing of these medications is needed. However, for some people with moderate to severe myotonia, that interferes with daily life and justifies taking medications every day, it may be reasonable to consider use of this type of medication. Potential side effects need to be carefully considered.

Future developments:

A major focus of current research effort, including research supported by the Myotonic Dystrophy Foundation, is to clarify exactly why muscles become weak, and find treatments that can prevent this from happening, or restore strength even if the muscles have already become weak. Progress in solving this problem and working towards treatment was slow for the first 90 years of research on DM, but since the year 2000 the pace of research progress has been much faster. Researchers in universities and in pharmaceutical or biotechnology companies have now begun to focus, for the first time, on correcting the chemical problems that exist in muscle cells of people with myotonic dystrophy. Very encouraging progress has been made in laboratory testing, but it is hard to predict how quickly this can progress to treatment of people. The MDF will make every effort to encourage these efforts and track their progress.

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