Myotonic Dystrophies

The symptoms of myotonic dystrophies vary by type and severity:

Myotonic Dystrophy Type 1 (DM1) – 95% of cases:

• Congenital/Infantile Forms: severe neonatal hypotonia, respiratory difficulties, intellectual disability, “tented” upper lip, and feeding problems.

• Classic Adult Form: myotonia (difficulty relaxing the hand, eyelid, or tongue following contraction, perceived as stiffness, which improves with repetition—the “warm-up” phenomenon—and worsens with low temperatures), distal weakness (forearms, hands, facial muscles, neck flexors), atrophy of the temporalis and sternocleidomastoid muscles, and myotonic facies (ptosis, expressionless face).

DM1 Multisystemic Involvement:

• Cardiac: conduction defects, arrhythmias, and cardiomyopathy (the leading cause of mortality).

• Respiratory: diaphragmatic weakness, sleep apnea, and recurrent pneumonia.

• Ocular: early-onset cataracts, ptosis, and extraocular muscle weakness.

• Endocrine: diabetes, thyroid dysfunction, hypogonadism, and alopecia.

• Cognitive: apathy, dysexecutive syndrome, lack of insight, excessive daytime sleepiness, and possible depression or anxiety.

Myotonic Dystrophy Type 2 (DM2) – milder form:

• Muscular: often subclinical myotonia, proximal weakness (“LGMD-like” pattern), characteristic muscle pain (myalgia), and morning stiffness.

• Systemic: early-onset cataracts (often the first sign), diabetes; cardiac involvement is less frequent than in DM1.

Distinguishing Characteristics:

• DM1: exhibits genetic anticipation (worsening severity and earlier onset across generations).

• DM2: intergenerational stability (absence of the congenital form).

• Clinical Evaluation: Targeted family history (assessing for genetic anticipation in DM1); identification of spontaneous myotonia (lid myotonia, grip myotonia) or elicited myotonia (percussion of the thenar eminence, tongue, or forearm finger extensors); assessment of weakness patterns (distal in DM1, proximal in DM2) and characteristic facies.

• Laboratory Tests: Creatine Kinase (CK) levels are usually normal or slightly elevated in DM1, and moderately elevated in DM2 (2–10x the upper limit of normal).

• Genetic Testing: Specific molecular tests (PCR/Southern blot) to detect CTG expansions in DMPK (DM1) or CCTG expansions in CNBP (DM2); assessment of genotype-phenotype correlation (expansion size vs. clinical severity in DM1).

• Electromyography (EMG): Presence of characteristic myotonic discharges (the “dive-bomber” sound) and a myopathic pattern.

• Multisystemic Evaluations: ECG/Holter monitoring (to detect conduction defects and arrhythmias), ophthalmological examination (for subclinical cataracts), spirometry/polysomnography (for respiratory dysfunction), and endocrinological screening (diabetes, thyroid dysfunction).

• Diagnostic Strategy: Initial screening for DM1, followed by DM2 testing if the results are negative but the clinical picture remains suggestive.