Skeletal muscle weakness is a major feature of DM1.

The weakness progresses at the relatively slow rate of 1 to 3 percent per year. With time, it impedes mobility and activities of daily living.

In general, flexors weaken more than extensors, and distal muscles weaken before proximal muscles.

Bone abnormalities of the skull create elongated facial features and other impacts including jaw and palate abnormalities. Some may require surgical intervention.

Myotonia – sustained muscle contraction and difficulty relaxing muscles – is a hallmark of DM1 and is an aspect of the disease that distinguishes it from other forms of muscular dystrophy. It affects nearly 100 percent of adult-onset DM1.

Myotonia can contribute to muscle stiffness, pain, prolonged hand grip, speech and swallowing difficulties, and GI issues.

Typical effects of adult-onset DM1 on skeletal muscle include the following:

• Weakness and atrophy of the jaw and facial muscles, leading to thinning of the facial contour and reduced facial expression.

• Weakness of the facial, tongue and palatal muscles, leading to indistinct speech and chewing and swallowing difficulties.

• Weakness of the eyelid muscles, leading to drooping of the eyelids.

• Neck flexor weakness, causing difficulty raising the head from a surface.

• Neck extensor weakness, leading to a dropped head posture and difficulty holding the head up.

• Abdominal and spine erectors weakness.

• Weakness of the diaphragm and other breathing muscles, causing respiratory symptoms.

• Distal upper limb muscle weakness, interfering with dexterity, handwriting and activities of daily living.

• Weakness of the foot dorsiflexor muscles, leading to ankle foot drop and subsequent difficulties of balance and walking.

• Calf muscle weakness, causing difficulty with jumping or rising up on toes and running.

• Impacts to employment and activities of daily living due to loss of ambulation The combination of weak calf muscles and foot drop can lead to instability of the ankles, difficulty standing still, frequent falls and difficulty with walking and stair climbing. As proximal knee and hip muscles are affected, greater difficulty rising from a seated position is experienced.

Discuss the following tests with your doctor:

• Assessment of difficulty related to activities of daily life.

• Grip myotonia test.

• Percussion myotonia test.

• Needle electromyogram (EMG).

Assistive devices or modifications in the home, school or workplace.

Evaluate annually through the primary care provider or appropriate specialists, including physical therapists/physiotherapists, occupational therapists, speech/language pathologists, dietitians/nutritionists, social workers, nurses/nurse practitioners, physiatrists and orthopedists, to monitor the above.

Moderate- or low-intensity aerobic and resistance exercise, minimizing sedentary activities, if possible.

Consider a cardiac evaluation prior to starting a new exercise routine.

Assistive and adaptive devices such as orthoses, braces, canes, walkers, hand-splints, etc.

Home and environmental modifications as necessary.

Mexiletine is often recommended for the treatment of myotonia.

Skeletal muscle weakness and myalgia are major features of DM2.

The weakness is bilateral and progresses at the relatively slow rate of 1 to 3 percent per year. Involvement of distal and facial muscles is usually absent. Initial weakness is in proximal hip girdle and neck (flexors > extensors) muscles. Axial muscle weakness is frequent in DM2 and may result in lower back pain.

Myotonia – sustained muscle contraction and difficulty relaxing muscles may be absent. Even if it is not the most disabling aspect of the disease, myotonia can contribute to muscle stiffness, pain, prolonged hand grip, speech and swallowing difficulties, and GI issues, and may be associated with hand tremor.

Myalgic pains, which can be the most prominent clinical feature in the early stages and may severely affect occupational performance.

Neck flexor weakness, causing difficulty raising the head from a surface.

Impacts to employment and activities of daily living due to proximal and axial muscle weakness (e.g. climbing stairs, standing up from the floor, etc.).

Difficulty with myalgia, mobility, balance and falls.

Need for assistive devices or modifications in the home, school or workplace.

Discuss the following tests with your doctor:

• Needle electromyogram (EMG) in proximal muscles.

Moderate- or low-intensity aerobic and resistance exercise minimizing sedentary activities, if possible.

Obtain a cardiac evaluation prior to starting a new exercise routine.

Orthoses, braces.

Walking aids such as a walking cane or walker.

Home modifications as necessary.

Evaluate annually through the primary care provider or by appropriate specialists, including physical therapists/physiotherapists, occupational therapists, speech/language pathologists, dietitians/nutritionists, social workers, nurses/nurse practitioners, physiatrists and orthopedists.

Mexiletine as an option for myotonia, if myotonia is present and is distressing. As mexiletine is an antiarrhythmic, obtain a electrocardiogram (ECG) prior to use.

Drugs affecting ion channels can improve myotonia, although their potential for causing cardiac arrhythmias must be weighed against their possible benefits.

Skeletal muscle weakness is not usually a major feature of childhood-onset DM1 but can be functionally limiting in those with CDM.

Children with CDM have delayed gross motor skills, but almost all obtain independent ambulation.

Children with CDM have improving motor function during the first few years of life, even those with profound hypotonia at birth. They benefit from promotion of motor function through therapy and other activities.

Myotonia can become problematic in late childhood or adolescence. Myotonia can contribute to muscle stiffness, pain, prolonged hand grip, speech and swallowing difficulties, and GI issues. Myotonia in DM1 is most prominent in the forearm and finger muscles, where it causes locking of the grip (“grip lock”). It sometimes affects tongue and jaw muscles, leading to difficulty with speech and chewing.

Weakness and atrophy of the jaw and facial muscles has the greatest functional impact and results in a number of manifestations

• Reduced facial expression.

• Weakness of the facial, tongue and palatal muscles, leading to dysarthria, dysphagia, and jaw tightness.

• Weakness of the eyelid muscles, leading to drooping of the eyelids (eyelid ptosis).

• Orthopedic complications may occur in children with CDM. The most common complication is talipes equinovarus, though children may develop knee or hip contractures and scoliosi.

Delayed relaxation after grip or percussion, difficulty related to activities of daily life, progressive speech impairment, or profound irritable bowel symptoms.

Discuss the following tests with your doctor:

• Grip myotonia can be observed by tightly gripping the clinician’s fingers, and then relaxing the hand after a sustained grip; the hand muscles will typically take 20 seconds or more to fully relax. Percussion myotonia can be demonstrated by a sustained contraction after the muscle is tapped with a reflex hammer. Electrical myotonia can also be demonstrated by abnormal, spontaneous muscle fiber discharges seen on a needle electromyogram (EMG).

• Physical, occupational, and speech therapy needs early and often with specific attention to:

  • Feeding concerns and dysphagia.

  • Gross motor delay.

  • Gross and fine motor weakness.

  • Dysarthria and potential augmentative and alternative communication (AAC) needs.

  • Language acquisition delays.

• Scoliosis; if necessary, consider bracing or referral to orthopedic surgeon.

• Crowded teeth that benefit from orthodontic treatment. The goal of such treatment is to facilitate oral hygiene. Caregivers should be counseled to proactively maintain oral hygiene.

Surgical correction of talipes equinovarus or other contractures early in evaluation if these changes prevent appropriate biomechanical alignment for mobility, cause pain, or limit functional mobility.

Speech therapy targeting speech, language, and communication from a very early age.

Augmentative and alternative communication (AAC) therapy individualized for their linguistic and cognitive abilities if appropriate or necessary.

Newborns with CDM often have difficulty feeding and alternative nutrition should be considered/ After about a year of actively working with a speech therapist or OT, most children can generally start on PO feeding.

Children with CDM experience progressive improvement in their proximal strength until adolescence, at a minimum. Children should therefore be encouraged to participate in physical activity.

Assessment of joint arthokinematics and range of motion should be observed to manage development of joint contracture.

Prevention of joint contractures is key to management and should be closely monitored with early initiation of stretching.

Treatment of talipes equinovarus and other joint contractures should include initial stretching regiment and appropriate ankle bracing (for talipes equinovarus). Serial casting may be considered.

Drugs affecting ion channels, such as mexiletine (Mexitil), can improve myotonia, although their potential for causing cardiac arrhythmias must be weighed against their possible benefits.