Penetrance

Penetrance is age-dependent and approaches 100%.

Source: GeneReviews — "Myotonic Dystrophy Type 2"

Diagnostic Criteria

Consensus-based recommendations on the molecular diagnosis of myotonic dystrophy type 2 (DM2) have been published (full text).

Suggestive Findings

DM2 should be suspected in individuals with the following clinical, electrophysiologic, imaging, and laboratory findings and family history.

Clinical findings

• Muscle weakness initially in the neck and finger flexors, and later in the hip-girdle muscles (iliopsoas muscle)

• Myotonia (sustained muscle contraction) that can manifest as grip myotonia, percussion myotonia, and/or leg myotonia

• Posterior subcapsular cataracts. Nonspecific vacuoles and opacities on direct ophthalmoscopy or as pathognomonic posterior subcapsular red-and-green iridescent opacities on slit lamp examination

Electrophysiologic and echocardiogram findings

Source: GeneReviews — "Myotonic Dystrophy Type 2"

Differential Diagnosis

Differentiating myotonic dystrophy type 2 (DM2) from other neuromuscular disorders relies on a thorough clinical examination and ancillary and genetic testing. Myotonic dystrophy type 1 (DM1)

Source: GeneReviews — "Myotonic Dystrophy Type 2"

Management Guidelines

Detailed international guidelines and recommendations for evaluation of individuals with myotonic dystrophy type 2 (DM2) have been published .

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with DM2, the evaluations summarized (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Table 4.

Myotonic Dystrophy Type 2: Recommended Evaluations Following Initial Diagnosis

System/Concern | Evaluation | Comment

Neurologic

| • Clinical eval of muscle strength functional status

• Assess for myotonia myalgia.

| The quick motor function test can be helpful as baseline to allow long-term monitoring.

Ophthalmologic

| Exam by ophthalmologist familiar w/posterior subcapsular cataracts epiretinal membranes | To establish baseline

Sleep dysfunction/ Fatigue

| Assess for daytime sleepiness, insomnia, REM sleep behavior disorders, restless legs syndrome. |

Endocrine

| Fasting lipid profile, glucose, glycosylated Hgb concentrations | To assess for evidence of insulin insensitivity diabetes mellitus

Thyroid studies | Hypothyroidism from any cause has been assoc w/ muscle weakness myotonia.

Serum testosterone FSH concentrations | In postpubertal males to assess gonadal function

Hypogammaglobulinemia

| Serum protein electrophoresis immunoprotein electrophoresis | To establish baseline prevent misinterpretation of future studies demonstrating hypogammaglobulinemia

Audiologic

| Hearing assessment |

Cardiologic

| Cardiac eval inc...

Source: GeneReviews — "Myotonic Dystrophy Type 2"

Clinical Guidance

Increased weakness has been associated with the use of certain cholesterol-lowering medications; some strength can return if statin-type cholesterol-lowering medications are eliminated . Note: Not all individuals with DM2 have an adverse response to statin medications; thus, the diagnosis of DM2 is not an absolute contraindication to the use of these drugs. Anesthetic risk is lower in individuals with DM2 than DM1 . In a large series, general anesthesia was well tolerated in DM2; severe perioperative complications were rare (1%), likely reflecting the relatively spared respiratory function. Nonetheless, caution is advised: avoid medications that can exacerbate myotonia (depolarizing muscle relaxants) and monitor ventilation before, during, and after anesthesia.

Source: GeneReviews — "Myotonic Dystrophy Type 2"

Surveillance Guidelines

Detailed guidelines for surveillance are provided in international care recommendations for DM2 .

Table 6.

Myotonic Dystrophy Type 2: Recommended Surveillance

System | Look for | Frequency

Neurologic

| • Clinical eval w/neurologist, OT, PT to assess muscle strength functional status incl use of quick motor function test

• Assess for myotonia myalgia.

| Annually, starting w/onset of first clinical manifestation(s); earlier in those w/family history of DM2

Eye | Ophthalmology eval incl slit lamp exam for cataract, vision assessment, eval for retinal changes | Annually, starting w/onset of first clinical manifestation(s)

Sleep issues

| • Assess for excessive daytime sleepiness, apnea, restless legs syndrome, periodic limb movement syndrome, REM sleep behavior disorders incl use of Epworth, Stanford, /or excessive daytime sleepiness scales.

• Polysomnography to assess for obstructive central sleep apnea hypoventilation

• Measure serum vitamin D.

Endocrine

| • Assess BMI.

• Assess for clinical manifestations of diabetes mellitus thyroid disorders.

• Fasting serum glucose, Hgb A1c

• Glucose tolerance test

• Free T3, free T4, TSH

• Cholesterol, LDL, HDL, triglycerides

• Parathyroid hormone

• Assess for clinical manifestations of hypogonadism.

• Serum testosterone, inhibin B, LH, FSH, DHEA-S

| Annually in males, starting w/onset of first clinical manifestation(s)

Hearing

| Audiometry eval | Annually, starting w/onset of first clinical manifestation(s)

Cardiovascular

| • Assess for palpitation, syncope, dyspnea, orthopnea, e...

Source: GeneReviews — "Myotonic Dystrophy Type 2"