Author(s): Sara Liegel, MD, Deanna Jewell, DO
Originally published: November 10, 2011 Last updated: June 6, 2024
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Disease/Disorder

Definition

  • Myotonic dystrophy (MD) is a form of muscular dystrophy that is classified into two types: Myotonic Dystrophy type 1 (DM1), and Myotonic Dystrophy type 2 (DM2).  DM1 and DM2 have differences in genes affected, age of onset, severity, pattern of muscle weakness, muscle fiber involvement, and other clinical features. (Table 1)
  • DM1, also known as Steinert disease, is the most common inherited neuromuscular disorder identified in adults and is a multisystem disease with hallmark distal weakness and myotonia (the inability to relax a muscle after activation).
  • DM1 is an autosomal dominant disorder caused by cytosine-thymine-guanine (CTG) trinucleotide expansion in the myotonic dystrophy protein kinase (DMPK) gene on chromosome 19. 
  • DM1 is a multisystemic disorder with variable age of onset.  Congenital myotonic dystrophy is the most severe form of DM1 with symptoms present at birth.

TABLE 1: Genetics and Clinical Manifestations of DM1 versus DM21

 DM1DM2
InheritanceAutosomal dominantAutosomal dominant
GeneticsDMPK gene on chromosome 19CNBP gene on chromosome 3
Genetic AnticipationYesNo
Age of OnsetBirth to AdultAdult 
Congenital FormYesNo
Muscle Fibers AffectedType 1 fibersType 2 fibers
Predominant Muscles AffectedDistal weaknessProximal weakness
Life ExpectancyReducedNormal
Facial and Bulbar WeaknessProminentRare
Gastrointestinal SymptomsProminentRare
Respiratory FailureProminentRare
Cognitive dysfunctionProminentNot Apparent
Behavioral DisorderProminentNot Apparent

Etiology

  • Age of onset and severity of DM1 correlate with the number of CTG repeats. CTG length with greater than 34 repeats is abnormal.2
  • Slightly increased number of repeats (35-49) is known as a premutation, where the individual is asymptomatic, but is at risk of transmission of longer trinucleotide repeats to successive generations.
  • More than 50 CTG repeats is associated with symptoms of DM1.  Greater than 800 repeats can lead to childhood DM1.
  • 1000 or more repeats are seen only with congenital DM1. 
  • Children of affected individuals tend to have a more severe presentation because CTG repeats tend to increase in successive generations, this is termed anticipation.

Epidemiology including risk factors and primary prevention

  • Congenital DM1 occurs in about 2.1 to 28.6 /100,000 live births3 and children with DM1 (congenital and childhood onset) represent an estimated 10-30% of overall DM1 patients.
  • Males and females are equally affected by DM1.
  • 90% of cases of congenital DM1 are maternally inherited.  
  • Parents of children with congenital DM1 are commonly asymptomatic or not yet diagnosed with DM1 at the time of birth of the affected child.  If a parent is detected to have positive molecular testing for DM1, a first-degree family member has a 50% risk of being affected by DM1; however, less than 10% of affected mothers give birth to congenitally affected children.4 Advisement for prenatal or pre-implantation testing is possible.  Fetal cell DNA can be assessed at 15-18 weeks of gestation and chorionic villus sampling can be done at 10-12 weeks of gestation to test for DM1.

Patho-anatomy/physiology

The CTG repeats, located in the 3′ untranslated region of the DMPK gene, are believed to produce a toxic RNA which is retained in cell nuclei.  This leads to dysfunctional transcription of a variety of genes, not only reducing the DMPK transcript itself on the affected allele, but also affecting multiple organ systems.  Organ systems affected include skeletal muscles via CLCN (chloride channel 1) and TNNT3 (troponin 3), heart, eye, smooth muscle, brain, and hormonal pathways particularly those involved in circadian rhythm and insulin sensitivity.5 The role of having a reduced amount of the DMPK protein itself, a serine-threonine protein kinase, is unclear.

Disease progression including natural history, disease phases or stages, disease trajectory (clinical features and presentation over time)

  • Earliest symptoms of congenital DM1 are reduced fetal movement, talipes equinovarus, and polyhydramnios. 
  • Premature delivery is common and the condition can be mistaken for or even coexist with cerebral palsy.
  • Initial neonatal symptoms include hypotonia, generalized atrophy, and myopathic facies such as tented upper lip, high-arched palate, and triangular-shaped mouth. 
  • Oromotor weakness is common leading to failure to thrive and necessitating tube feeding.
  • Myotonia is usually clinically absent in infancy and may or may not be present electromyographically.
  • 50% develop neonatal respiratory distress and require mechanical ventilation. 
  • Very severely involved newborns may not survive the neonatal period due to respiratory failure. 
  • There is a gradual improvement in motor function during childhood, and children with congenital DM1 may walk. However, strength will decrease due to progressive myopathy during the second decade of life.
  • Developmental delays are seen with motor milestones, speech, and feeding
  • Intellectual disability is seen in 50-65% of individuals with congenital DM1.6 Autism spectrum disorder, ADHD, mood/anxiety disorders, and abnormal visual-spatial abilities have been reported.
  • Weakness is typically seen first in distal muscles and later in more proximal muscles.  This is in contrast to many dystrophies, including DM2, which present with proximal weakness first. 
  • Children typically require bracing for ambulation but can become wheelchair dependent due to progression of weakness.
  • Decline is due to progressive loss of functional muscle fibers, decreased endurance, pulmonary function, and cardiac impairment. 
  • Average age of death in DM1 is 45 years old. 25% of patients with congenital DM1 die before 18 months of age, and 50% die before their mid-30s.7
  •  Younger age of onset, more severe muscle weakness, and cardiac arrhythmias are associated with increased risk of death. 

Specific secondary or associated conditions and complications

  • Musculoskeletal: Myotonia is absent in infants and presents later in life. Distal weakness tends to progress along with temporal wasting, symptomatic and sometimes painful cramping and difficulty releasing grip (known as grip myotonia).  Infants are commonly born with talipes equinovarus.
  • Cardiac: Conduction defects occur in up to 90%, while dilated cardiomyopathy happens more rarely.
  • GI: Dysphagia, constipation, irritable bowel syndrome mimic, delayed gastric emptying, gallstones, mildly elevated LFTs.
  • CNS: Fatigue, hypersomnia, early age-related cognitive decline in some adults, lower full-scale IQ and frontal-parietal deficits on formal testing; higher risk of ADD, anxiety, depression, and other pathologic personality traits. Limited facial expression should not be mistaken for dullness or depression.
  • Ophthalmologic: Palpebral ptosis, and “Christmas tree” cataracts, usually not symptomatic until thirties; hyperopia, strabismus and astigmatism being more common in children.
  • Endocrine: Thyroid dysfunction, insulin resistance with eventual type 2 diabetes, male infertility due to testicular atrophy.
  • Respiratory: Progressive impairment of respiratory function and central and obstructive sleep apnea are common.  Hypersensitivity to respiratory depressants and severe reaction to succinylcholine with hyperthermia and hyperkalemia, somewhat distinct from malignant hyperthermia, can complicate surgery and anesthesia. 
  • Dermatology: Seborrheic Dermatitis
  • HEENT: Cavities, gingivitis
  • OBGYN: High risk-pregnancies
  • Oncology: Shown to have increased risk of malignancy

Essentials of Assessment

History

Important historical information includes understanding:

  • Family history of similar conditions
  • Perinatal history (as indicated by APGAR scores, birth weight and history of asphyxia)
  • Comprehensive developmental history including ages at which developmental milestones were met or if any were lost, with loss of achieved milestones suggesting a neurodegenerative condition.
  • Nutritional status and feeding history
  • Duration and type of respiratory support
  • Frequency of respiratory infection
  • Atypical behavior for age
  • Excessive fatigue or hypersomnia

Physical examination

Assess for:

  • Profoundly hypotonic infant.
  • Eyelid ptosis and facial weakness, with “carp” or “tented” mouth configuration.
  • Temporal muscle atrophy, flat facies, or limited facial expression.
  • Oral motor weakness and signs or symptoms of dysphagia
  • Signs and symptoms of respiratory insufficiency. 
  • Grip myotonia or percussion myotonia (percussion of the thenar eminence with a reflex hammer resulting in thumb adduction and flexion) 
  • Distal muscle atrophy in older patients.
  • Clubfoot with or without hip dislocation.
  • Sensation is often normal.
  • Scalp or other pilomatrixomas or epitheliomas may be noted.
  • Abnormal rhythm on cardiac exam. 
  • Cataracts, and/or testicular atrophy. 

Functional assessment

  • Typical fine and gross motor function developmental testing, and clinical ADL scales.
  • Gait observation and manual muscle testing once the child can cooperate.
  • Formal neuropsychological and psychoeducational testing should be done later on to guide programming. 
  • The Muscular Impairment Rating Scale (MIRS) is specific to DM1 and may be beneficial for assessment.8

Laboratory studies

  • CPK is usually mildly elevated (hundreds).
  • In infants, EMG does not typically have myotonic discharges, but may show fast runs of single fiber discharges
  • Muscle biopsy may show type 1 fiber atrophy and type 2 fiber hypertrophy, ring fibers, increased internalized nuclei, and sarcoplasmic masses.9
  • The gold standard test for diagnosis is DMPK molecular genetics for number of CTG repeats.
    • Whole exome sequencing cannot reliably detect repeat expansions, thus targeted DM1 genetic testing is recommended.
    • Avoid more expensive panels including DM2 and myotonia congenita genes as those conditions would not clinically be in the differential for affected infants.

Imaging

  • Neuroimaging can be normal or may show mild cortical atrophy and occasionally white matter abnormalities. In some cases, there will be periventricular leukomalacia as this occurs as a complication of prematurity. 
  • Pelvic x-ray for developmental hip dysplasia should be considered if clinical signs.
  • Spine x-ray if scoliosis is clinically noted.

Supplemental assessment tools

  • Pulmonary function testing, (especially FVCs).
  • ECG (at diagnosis and annually), echocardiogram (at diagnosis and every 2-4 years), and ambulatory electrocardiographic monitoring (at diagnosis and interval follow up).10
  • Evaluate for possible sleep apnea with overnight oximetry or polysomnogram.
  • Formal dysphagia evaluation and swallowing video fluoroscopy should be done for any possible nutritional or respiratory symptoms.
  • Baseline audiometry exam should be performed at diagnosis.
  • Hemoglobin A1c, TSH and Free T4 should be obtained at baseline and every 3 years.
  • Ophthalmologic evaluation at diagnosis and at least yearly.
  • Lipid profile every 3 years.   Use statins only with close monitoring for muscle-related impacts
  • Vitamin D can be tested annually. 
  • Neuropsychological testing beginning at preschool age and repeated at grade school and college age.
  • Typical oncologic screening should be done.

Early predictions of outcomes

  • Larger CTG repeat size may be associated with clinical severity but does not provide information about prognosis.
  • The presence of cardiac involvement is associated with decreased muscle strength and higher likelihood of need for non-invasive ventilation.11 Cardiac conduction abnormalities, respiratory depression, and dysphagia with aspiration can lead to mortality if not identified and addressed in a timely manner.

Environmental

  • Level surfaces for ambulation and limited requirement for stair climbing or long-distance ambulation would be an appropriate practical consideration. 
  • Generator availability and adequate power outlets for respiratory support or CPAP should be assured.
  • Maintaining a warm environment in surgical areas minimizes risks of muscle dysfunction that can occur with shivering.

Social role and social support system

Cognitive effects of parental disease can lead to difficulties with low income, organizing or providing care, and keeping follow-up appointments.  A disproportionate number of affected children are in grandparental care.  Waiver, respite, case management, and similar supports should be sought out and assistance be provided in making applications.  MDA offers many services that could fill these needs.

Professional issues

It is very important to realize many parents and most grandparents have gone unrecognized as having DM1 themselves, and the individual must be approached cautiously and gently.  This may encourage them to seek care for themselves and avoid provoking unmerited guilt for transmitting a genetic condition or being unaware of their condition.

Rehabilitation Management and Treatments

Available or current treatment guidelines

Consensus-based care recommendations for children with congenital DM1 were developed in 2019 in a project organized by the Myotonic Dystrophy Foundation.12 Similar consensus-based care recommendations also exist for adults with DM1.2

Overall treatment guidelines are based on the different medical complications that can occur due to the congenital myotonic dystrophy.  These include:

  • Assessment of neurological involvement via motor testing.
  • Performing functional tests such as six-minute walk tests and hand grip.
  • Following up with questions about limitations of ambulation and transfers. 
  • Children with congenital DM1 experience progressive improvement in proximal strength until at least adolescence and should be encouraged to participate in moderate aerobic and strengthening activities.  For many patients, aerobic exercise is difficult given their muscle weakness and likely poor endurance. Aquatic therapies can be considered for those with low overall strength.
  • As patients get weaker, different bracing options should be considered.  It is advised to continue contracture prevention; however, wheelchair dependence likely occurs in the setting of weakness, not contractures. Contractures are typically due to poor positioning and some are inevitable. 
  • For myotonic symptoms, mexiletine has been shown to be effective (dose 1.5 to 3.0 mg/kg every 8-12 hours)).  However, mexiletine may have an arrhythmogenic effect and ECG should be obtained prior to start of therapy and at serial intervals thereafter.  Other medications to consider to treat myotonia include phenytoin, carbamazepine, clomipramine, imipramine, amitriptyline, nifedipine, flecainide, acetazolamide, and taurine. 
  • Pain is a common complaint and should be addressed with analgesics.
  • Many patients present with fatigue and modafinil 50 -200 mg 1-2 times per day may be helpful. The Epworth sleepiness scale should be conducted.
  • Assessment and management of arrythmias as per cardiology.  Patients may require ICDs or pacemakers as appropriate.
  • Pulmonary conditions should be addressed regularly by pulmonology.   Patients may require CPAP or other forms of non-invasive ventilation. Influenza and pneumococcal vaccinations are recommended.
  • A Gastrotomy tube may need to be placed for dysphagia.  Gallstones are treated conventionally
  • Constipation, diarrhea, fecal incontinence, and abdominal pain are common, and symptoms can mimic irritable bowel syndrome.  Fiber supplements should be used as first line treatment, with gentle laxatives added for constipation.
  • Pregnant patients should follow with a high-risk pregnancy team.
  • Cognitive impairments should be assessed via neuropsychological testing, and patients referred to a mental health professional for of autism spectrum disorders, attention deficit disorders, alexithymia, and/or other behavioral problems. 
  • Patients will need to be followed by ophthalmology as cataracts may need to be removed.  Eyelid crutches can be inserted into glasses to manage ptosis prior to considering surgical intervention.

At different disease stages

New onset/acute

  • Confirm diagnosis.
  • Discuss implications for parents including genetic counseling
  • Assess respiratory insufficiency, dysphagia
  • Assess developmental delay
  • Educate about long-term medical care needs, surgical/anesthetic risks
  • Orthopedic and physiatric management including bracing, scoliosis surveillance, contracture management
  • Refer to ophthalmology, cardiology, and pulmonology

Subacute

  • Refer for early intervention and therapy with focus on feeding/dysphagia, gross and fine motor delay and weakness, dysarthria and potential for augmentative and alternative communication (AAC) needs, and language acquisition delays.
  • Consider orthotic management for positioning and weight bearing or ambulatory support.
  • Treat symptomatic myotonia with mexiletine or anticonvulsant (off-label).
  • Treat attention deficit and hypersomnolence with stimulants such as modafinil or methylphenidate.  Use caution in individuals with cardiac arrhythmia.
  • Provide neuropsychology assessments starting at preschool age.to help to develop appropriate Section 504 plans or IEPs, and guide advocacy for services and accommodations.
  • Obtain baseline audiometry at school age.

Chronic/stable

  • Follow cardiopulmonary status, to prevent sudden death, cardiac failure, aspiration or pneumonia.
  • Follow endocrine function and detect progression to diabetes.
  • Prevent undernutrition or obesity.
  • Encourage parental self-care, support for affected and/or aging caregivers.
  • Encourage participation in community, MDA, special camp settings, self-advocacy.
  • Prepare adolescents for transition to adult care, aid in obtaining guardianship when appropriate.

Pre-terminal or end of life care

  • Provide for home health care and equipment needs as appropriate; be aware of need to modify opiate and anxiolytic protocols.
  • Palliative care should be introduced at the time of diagnosis.

Coordination of care

Specialists may include physiatrist, orthopedist, cardiologist, pulmonologist, endocrinologist, ophthalmologist, palliative care, GI, sleep medicine, genetics/genetic counselor, PT, OT, SLP, nutritionist, social worker, and psychologist. Communication with primary care or medical home physician; MDA or other clinic setting with advanced coordination of screening tests and ancillary services as possible is ideal.

Patient & family education

  • Families should understand the 50% chance of having an affected child with each pregnancy involving an affected parent, and worsening severity with each succeeding generation.
  • Every generation requires good interdisciplinary medical care to optimize lifespan and function along the lines described above, with the avoidance of the few specific medications that may cause harm.  These include succinylcholine, rocuronium, remifentanil, sedatives, and opioids. 
  • There is a Myotonic Dystrophy Family Registry to enter that helps identify participants for research studies and clinical trials. This can be found on clinicaltrials.gov.

Measurement of Treatment Outcomes including those that are impairment-based, activity participation-based and environmentally based

  • Wee-FIM or any standardized tool for mobility, ADL, and health-related QOL could be used.
  • Muscular Impairment Rating Scale (MIRS) can be used to monitor DM1 progression of muscular impairment.
  • Participation in regular classroom education with integration is usually ideal. Even if special education resources are needed, observance of least restrictive environment and avoidance of homebound services that are not medically necessary should be considered. 
  • Participation in online disease-specific support groups and registries could also be a measure of successful education and engagement.

Translation into Practice:  practice “pearls”/performance improvement in practice (PIPs)/changes in clinical practice behaviors and skills

  • Consider circumstances of the entire family when caring for children with early onset disease, particularly health and participation of caregivers facing related health issues with their own progression.
  • Differentiate congenital DM1 from other causes of hypotonia on the basis of clinical presentation, targeting the optimal genetic diagnostic strategy.
  • Provide direct help and/or referrals for the entire spectrum of physiological and psychosocial deficits associated with this condition to optimize well-being and desired outcomes.

Cutting Edge/Emerging and Unique Concepts and Practice

  • Small molecule therapeutics, antisense oligonucleotide (ASO)-based therapy, and genome editing targeting DNA, RNA, or downstream signaling pathways are being trialed. 
  • Measurements of spliced RNA products, used as biomarkers, are being assessed as well to assist with tracking disease response.
  • Tideglusib, a selective and irreversible glycogen synthase kinase 3 inhibitor, is undergoing phase 2/3 clinical trial and is recruiting participants aged 6-45 years old.  More information can be found on clinicaltrials.gov.
  • Agents affecting production and degradation of RNA particles, reducing toxicity of the repeats have had promising results preclinically.

Gaps in the Evidence-Based Knowledge

As of now, treatments are for symptoms and clinical conditions that develop subsequently to DM1, but there are no current disease modifying treatments.  Some are in clinical trials as above.  A specific myotonic-dystrophy measure of function or quality of life remains to be developed although the Individualized Neuromuscular Quality of Life Questionnaire appears to be a good proxy.

References

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  7. Reardon W, Newcombe R, Fenton I, Sibert J, Harper PS. The natural history of congenital myotonic dystrophy: mortality and long term clinical aspects. Arch Dis Child. 1993 Feb, 8481038, 68(2):177-81.
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  10. Russo V, Capolongo A, Bottino R, Carbone A, Palladino A, Liccardo B, Nigro G, Marchel M, Golino P, D’Andrea A. Echocardiographic Features of Cardiac Involvement in Myotonic Dystrophy 1: Prevalence and Prognostic Value. J Clin Med. 2023 Mar 1 and 12(5):1947.
  11. Mazzoli M, Ariatti A, Garuti GC, Agnoletto V, Genovese M, Gozzi M, Kaleci S, Marchioni A, Malagoli M, Galassi G. Predictors of prognosis in type 1 myotonic dystrophy (DM1): longitudinal 18-years experience from a single center. Acta Myol. 2020 Sep 1 and 39(3).
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Original Version of the Topic

Desiree Roge, MD. Congenital Myotonic Dystrophy. 11/10/2011

Previous Revision(s) of the Topic

Vikki A. Stefans, MD. Congenital Myotonic Dystrophy. 5/5/2016

Jeremy Roberts, MD, Ray Stanford, MD, Amy Tenaglia, MD, Vera Tsetlina, MD, Hana Azizi, MD. Congenital Myotonic Dystrophy. 4/29/2021

Author Disclosures

Sara Liegel, MD
Nothing to Disclose

Deanna Jewell, DO
Nothing to Disclose