Penetrance

Penetrance is high (nearly 100% by age 50 years) when all manifestations of the disease, even those that are subtle, are sought. However, mild cases (e.g., persons with only cataracts) may be missed .

Source: GeneReviews — "Myotonic Dystrophy Type 1"

Diagnostic Criteria

Suggestive Findings

Myotonic dystrophy type 1 (DM1) should be suspected in adults with the following:

• Muscle weakness, especially of the distal leg, hand, neck, and face

• Myotonia (sustained muscle contraction), which often manifests as the inability to quickly release a hand grip (grip myotonia) and which can be demonstrated by tapping a muscle (e.g., the thenar muscles) with a reflex hammer (percussion myotonia)

• Posterior subcapsular cataracts detectable as red and green iridescent opacities on slit lamp examination

DM1 should be suspected in neonates with some combination of the following:

• Hypotonia

• Facial muscle weakness

• Generalized weakness

• Positional malformations including clubfoot

• Respiratory insufficiency

Establishing the Diagnosis

The diagnosis of DM1 is established in a proband with id...

Source: GeneReviews — "Myotonic Dystrophy Type 1"

Differential Diagnosis

Hypotonia in infancy is seen in many disorders, including Prader-Willi syndrome, multiminicore disease (OMIM 606210, 180901), nemaline myopathy (OMIM PS161800), X-linked centronuclear myopathy, other centronuclear myopathies (OMIM PS160150), and maternal uniparental disomy for chromosome 14. Myotonic dystrophy type 2 (DM2) is characterized by myotonia (90% of affected individuals) and muscle dysfunction (weakness, pain, and stiffness) (82%), and less commonly by cardiac conduction defects, iridescent posterior subcapsular cataracts, insulin-insensitive type 2 diabetes mellitus, and testicular failure.

Source: GeneReviews — "Myotonic Dystrophy Type 1"

Management Guidelines

Evaluations Following Initial Diagnosis To establish the extent of disease and needs in adults with classic myotonic dystrophy type 1 (DM1), the evaluations summarized (if not performed as part of the evaluation that led to the diagnosis) are recommended. Consensus-based care recommendations have been published both for adults and for persons with congenital- and childhood-onset DM1 and are available at www.myotonic.org. Table 3. Recommended Evaluations Following Initial Diagnosis in Adults with Myotonic Dystrophy Type 1

Clinical Guidance

Statins used to lower cholesterol may sometimes cause muscle pain and weakness. noted that "[n]umerous cases of perioperative complications in patients with DM have been reported. Hazards have been associated with the use of thiopentone, suxamethonium, neostigmine, and halothane. Most complications were pulmonary. The likelihood of perioperative pulmonary complications (PPC) was not related to any specific anesthetic drug. Because of the increased risk of PPC, careful monitoring during the early postoperative period, protection of the upper airways, chest physiotherapy, and incentive spirometry are mandatory in all symptomatic patients with DM, particularly those with a severe muscular disability or those who have undergone an upper abdominal surgery." and have reviewed appropriate anesthetic care for individuals with DM1 and DM2. Malignant hyperthermia during anesthesia including the use of vecuronium has been reported in DM1 but is very uncommon . (See Malignant Hyperthermia Susceptibility.) Aggressive doxorubicin-based chemotherapy for lymphoma in a person with DM1 produced sudden atrial fibrillations . presented evidence that obesity, tobacco smoking, physical inactivity, and alcohol / illicit drug consumption are lifestyle risk factors associated with more severe DM1 phenotypes.

Source: GeneReviews — "Myotonic Dystrophy Type 1"

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Source: GeneReviews — "Myotonic Dystrophy Type 1"

Surveillance Guidelines

provide guidelines for surveillance. The following are appropriate:

• Annual EKG to detect asymptomatic cardiac conduction defects. Some centers perform annual 24-hour Holter monitoring of individuals with DM1 who do not have cardiac symptoms . Tissue Doppler monitoring has also been proposed .

• Annual measurement of fasting serum glucose concentration and glycosylated hemoglobin concentration, with treatment for diabetes mellitus if indicated

• Ophthalmologic examination every two years to evaluate for cataract formation

• Attention to nutritional status including mastication and trouble eating

• Polysomnographic follow up of sleep complaints

Source: GeneReviews — "Myotonic Dystrophy Type 1"