Myotonic Dystrophy and Myotonias

Prepared and reviewed by the PrepElephant Academic Review Team

First published 14 Oct 2026 · Spot an error?

Reference basis: Adams and Victor's Principles of Neurology; Harrison's Principles of Internal Medicine

On this page

Direct answer
What you must remember
Separating myotonic dystrophy from channelopathy myotonias
Where the exam tries to catch you
Frequently asked questions
Related topics

Direct answer

Myotonic dystrophy type 1 is an autosomal dominant multisystem disorder caused by an expanded CTG repeat in the DMPK gene, producing myotonia, distal weakness, cataract, frontal balding and cardiac conduction defects. Anticipation makes severity and earlier onset worsen across generations, most markedly when the congenital form comes from the mother. Myotonias caused by ion-channel mutations are separate conditions, and mexiletine is the usual treatment for symptomatic myotonia.

What you must remember

Genetics: DM1 is a CTG expansion in DMPK on chromosome 19, while DM2 is a milder CCTG expansion in CNBP with predominantly proximal weakness.
Anticipation: repeat length increases in successive generations, and the congenital form usually follows maternal transmission.
Myotonia: delayed relaxation after grip or percussion, most noticeable in the hand and tongue, and often out of proportion to the weakness.
Classic features: frontal balding, ptosis, facial weakness, dysphagia, posterior subcapsular cataract, testicular atrophy and insulin resistance.
Cardiac risk: conduction defects and arrhythmias cause sudden death, so ECG surveillance and pacing or defibrillator decisions are essential.
Anaesthesia caution: weak respiratory and bulbar muscles make sedatives, opioids and general anaesthesia risky.
Channelopathy myotonias: chloride channel mutations cause Thomsen and Becker disease, and sodium channel mutations cause paramyotonia, with mexiletine the usual symptomatic drug.

Separating myotonic dystrophy from channelopathy myotonias

A 40-year-old man has difficulty releasing his handshake, frontal balding and a cataract on slit-lamp examination. His father had early balding and a known heart rhythm problem. The multisystem picture raises concern for a dominant disorder rather than an isolated channelopathy.

Feature	Myotonic dystrophy type 1	Thomsen myotonia	Paramyotonia congenita
Mechanism	DMPK CTG expansion	CLCN1 chloride channel	SCN4A sodium channel
Inheritance	Autosomal dominant	Autosomal dominant (Becker form is recessive)	Autosomal dominant
Key clue	Cataract, balding, conduction defect	Muscle hypertrophy, warm-up effect	Cold-induced stiffness, worsening with repetition
Multisystem disease	Yes	No	No

The multisystem features and the repeat expansion make myotonic dystrophy the diagnosis that cannot be missed. Channelopathies cause myotonia alone, and the response to cold and repeated movement narrows the diagnosis. EMG showing myotonic discharges supports any type, while genetic testing confirms the specific mutation.

At the bedside, check grip release, the eyes, the heart rhythm and the family history in turn. Each step looks for a different organ system involved in the disorder, and a single normal finding does not exclude it.

Where the exam tries to catch you

Most errors come from treating myotonic dystrophy as a muscle problem alone or underestimating the heart and breathing.

Trap: managing myotonic dystrophy as an isolated muscle disease Correct: it is multisystem, so cardiac conduction, cataract, endocrine and cognitive involvement must be checked.
Trap: treating myotonia as the main problem Correct: weakness, arrhythmia and respiratory failure often determine prognosis more than stiffness does.
Trap: sedating patients without respiratory planning Correct: weak respiratory muscles make sedation and anaesthesia dangerous, so careful planning is needed.

Frequently asked questions

What causes myotonic dystrophy type 1?

Myotonic dystrophy type 1 is caused by an expanded CTG trinucleotide repeat in the untranslated region of the DMPK gene on chromosome 19. Repeat length correlates with severity and age of onset. Testing for the expansion confirms the diagnosis.

Why does congenital myotonic dystrophy usually come from the mother?

Congenital myotonic dystrophy usually follows maternal transmission because the CTG repeat expands most markedly when passed through the mother. Affected infants show hypotonia, respiratory difficulty and feeding problems from birth. This maternal pattern is a classic exam association with anticipation.

Which eye finding is characteristic of myotonic dystrophy?

Posterior subcapsular cataract is the characteristic eye finding in myotonic dystrophy and often appears early in the disease. Slit-lamp examination should therefore be part of the work-up. Ptosis and facial weakness are also commonly seen.

What drug is used to treat symptomatic myotonia?

Mexiletine is the usual drug for symptomatic myotonia because it blocks sodium channels in muscle fibres. Carbamazepine and phenytoin are alternatives used in some patients. Treatment must consider cardiac safety, since mexiletine can affect cardiac conduction.

How does a channelopathy myotonia differ from myotonic dystrophy?

Channelopathy myotonias such as Thomsen disease are caused by ion-channel mutations and produce myotonia without multisystem disease. Myotonic dystrophy adds cataract, cardiac conduction defects, endocrine problems and cognitive effects. The presence of these multisystem features is the key distinguishing point.