Myotonia Congenita
Myotonia congenita is a hereditary skeletal muscle channelopathy caused by mutations in the voltage-gated chloride channel (CLCN1) gene, resulting in impaired muscle relaxation after voluntary contraction.
Myotonia Congenita
Myotonia congenita (MC) is a non-dystrophic myotonic disorder — an inherited skeletal muscle channelopathy caused by loss-of-function mutations in the CLCN1 gene encoding the voltage-gated chloride channel (ClC-1) of skeletal muscle. The cardinal feature is myotonia: a delayed relaxation of skeletal muscle after voluntary contraction or mechanical/electrical stimulation, resulting in muscle stiffness.
Let's break down the name:
- "Myo-" = muscle (Greek mys)
- "-tonia" = tone/tension (Greek tonos)
- "Congenita" = present from birth (Latin congenitus)
So the name tells you: muscle stiffness that is present from birth (or early life).
Key Conceptual Distinction — Myotonia vs Dystrophy
Myotonia congenita is a non-dystrophic myotonia. This means the muscle fibres are NOT progressively degenerating (unlike myotonic dystrophy). The problem is purely one of ion channel dysfunction → abnormal electrical excitability → delayed relaxation. The muscle itself is structurally intact and may even be hypertrophied ("Herculean" appearance). This is fundamentally different from myotonic dystrophy (DM1/DM2), which is a trinucleotide repeat disorder with progressive muscle wasting AND multisystem involvement [1][2].
Myotonia: delayed muscle relaxation after contraction [2]
Causes: myotonic dystrophy, myotonia congenita, paramyotonia congenita, proximal myotonic myopathy [2]
- Prevalence: approximately 1 in 100,000 worldwide, though estimates vary (some Scandinavian populations report higher prevalence up to 1 in 10,000 due to founder effects).
- No strong sex predilection overall, but:
- Thomsen disease (autosomal dominant, AD): affects males and females equally.
- Becker disease (autosomal recessive, AR): more clinically apparent in males; females may be carriers or have milder symptoms due to variable expressivity.
- Age of onset:
- Thomsen: typically presents in infancy or early childhood (often noted when a toddler has difficulty releasing grip or falls due to stiff legs).
- Becker: onset usually in later childhood to early adolescence (5–12 years), and tends to be more severe despite its later onset.
Myotonia congenita: AD/AR inheritance, due to voltage-gated Cl⁻ channel mutation (7q35) [2]
S/S: infancy/childhood onset, generalized myotonia which ↑ in cold and ↓ by warmth/exercise, minimal weakness [2]
- In Hong Kong, myotonia congenita is rare but recognized. Genetic testing is increasingly available through clinical genetics services at QMH and the HKU Department of Paediatrics/Medicine. The AR (Becker) form is more common globally and is the more clinically significant variant.
3. Anatomy and Function: The Skeletal Muscle Chloride Channel
To understand why a chloride channel defect causes myotonia, you need to understand normal muscle electrophysiology from first principles:
- Resting membrane potential of skeletal muscle is approximately –80 to –90 mV.
- This resting potential is maintained by:
- K⁺ leak channels (major contributor — K⁺ flows out, making inside negative)
- Chloride channels (ClC-1) — these contribute approximately 70–80% of the resting membrane conductance of skeletal muscle.
- When a motor neuron fires, acetylcholine is released at the neuromuscular junction → nicotinic receptors activate → Na⁺ influx → depolarization → action potential propagates along the sarcolemma and into T-tubules → voltage-gated Ca²⁺ channels (dihydropyridine receptors) activate → ryanodine receptors on SR release Ca²⁺ → actin-myosin cross-bridge cycling → contraction.
- Relaxation requires:
- Ca²⁺ reuptake into SR (via SERCA pumps)
- Repolarization of the membrane back to resting potential — this is where ClC-1 is critical.
- Gene: CLCN1, located on chromosome 7q34-35
- Protein: ClC-1, a voltage-gated chloride channel
- Structure: functions as a homodimer, with each subunit having its own pore (a "double-barrelled" channel)
- Location: densely expressed on the sarcolemma of skeletal muscle fibres
- Function: Cl⁻ conductance stabilizes the resting membrane potential. After an action potential, chloride conductance rapidly brings the membrane back towards the resting potential, preventing re-firing.
Why is ClC-1 so important? Because in skeletal muscle (unlike nerve or cardiac muscle), the T-tubule system is extensive and K⁺ can accumulate in the narrow T-tubular lumen during repetitive firing. Without the massive Cl⁻ conductance to "clamp" the membrane near rest, accumulated K⁺ would depolarize the membrane enough to trigger more action potentials. ClC-1 essentially acts as an electrical brake.
4. Etiology and Pathophysiology
Myotonia congenita is caused by loss-of-function mutations in the CLCN1 gene (chromosome 7q34-35). Over 300 different mutations have been identified (missense, nonsense, splice-site, frameshift, deletions).
Two classical forms are recognized based on inheritance pattern:
| Feature | Thomsen Disease | Becker Disease |
|---|---|---|
| Inheritance | Autosomal dominant (AD) | Autosomal recessive (AR) |
| First described | Julius Thomsen, 1876 (described in his own family) | Peter Emil Becker, 1977 |
| Prevalence | Rarer | More common (3–10× more prevalent) |
| Age of onset | Infancy/early childhood | Later childhood/adolescence |
| Severity | Milder | More severe |
| Transient weakness | Absent or rare | Present (characteristic) |
| Muscle hypertrophy | Common | Very common ("Herculean" build) |
Don't Confuse Becker Myotonia Congenita with Becker Muscular Dystrophy
These are completely different diseases by different Dr. Beckers. Becker myotonia congenita = AR chloride channelopathy with myotonia and no dystrophy. Becker muscular dystrophy (BMD) = X-linked dystrophinopathy with progressive muscle weakness and wasting. The only thing they share is a surname.
4.2 Pathophysiology — From Ion Channel to Symptom
This is the core concept and is best understood step by step:
- Mutations in CLCN1 → reduced or absent functional ClC-1 channels on the sarcolemma
- Resting chloride conductance drops from ~80% to < 20% of total membrane conductance
- Without ClC-1 to stabilize resting potential, the membrane becomes hyperexcitable
- After a normal action potential, instead of cleanly repolarizing, the membrane hovers near threshold
- Small depolarizing stimuli (e.g., K⁺ accumulation in T-tubules during repeated firing) now trigger repetitive, self-sustaining action potentials that the muscle fibre cannot shut off
- These repetitive action potentials cause sustained muscle contraction = myotonia
- The muscle contracts normally but relaxes abnormally slowly because the electrical activity persists long after voluntary drive has ceased
- This is why patients describe "stiffness" — the muscle stays contracted for seconds after they try to relax
Mutations in ion channels → leaky channels → failure to maintain stable resting action potential [2]
Minor leakage (10-15mV) → myotonia [2]
Major leakage (20-30mV) → periodic paralysis [2]
High Yield: Pathophysiology Framework for Channelopathies
The GC lecture and senior notes [1][2] present a unifying framework:
- Chloride channel (ClC-1) mutations → myotonia congenita (minor membrane instability → repetitive firing → stiffness)
- Sodium channel (SCN4A) mutations → paramyotonia congenita and hyperkalemic periodic paralysis (depending on severity of leak)
- Calcium channel (CACNA1S) mutations → hypokalemic periodic paralysis, malignant hyperthermia
The key principle: small leaks cause myotonia; large leaks cause paralysis (because massive depolarization inactivates Na⁺ channels → the fibre becomes completely inexcitable → flaccid weakness).
- A hallmark of myotonia congenita (especially Thomsen type) is that myotonia improves with repeated muscle use — this is the "warm-up" phenomenon.
- Why? With repeated contractions, there is gradual partial inactivation of Na⁺ channels and some adaptive membrane stabilization. Additionally, increased muscle blood flow washes out accumulated K⁺ from T-tubules. The net effect is that after a few contractions, subsequent ones are smoother and less stiff.
- This is opposite to paramyotonia congenita, where myotonia worsens with exercise ("paradoxical myotonia" — hence the name "para-myotonia").
Features [of myotonia]: [2]
- Painless
- Often temperature related (↑ when cold)
- Warming-up/down phenomenon (↓ by repeated muscle contraction)
- In the AR (Becker) form, the loss of chloride conductance is more severe
- At the start of activity, the intense burst of repetitive action potentials can partially depolarize the membrane to a level that inactivates some Na⁺ channels
- This transient Na⁺ channel inactivation produces brief weakness at the onset of movement (lasting seconds to minutes) before the warm-up phenomenon kicks in
- This transient weakness is NOT seen (or is very mild) in the AD (Thomsen) form
- Chronic repetitive involuntary muscle fibre activation acts as a form of isometric exercise
- Over time, muscle fibres hypertrophy → patients may have an impressively muscular ("Herculean") build, especially in the legs, buttocks, and shoulders
- This is paradoxical: the patient looks strong but complains of stiffness
- Thomsen (AD): The mutant ClC-1 subunit exerts a dominant-negative effect. Since ClC-1 functions as a homodimer, one mutant subunit can impair the function of the dimer even when paired with a normal subunit. However, enough functional channels remain → milder phenotype.
- Becker (AR): Both alleles carry loss-of-function mutations → near-complete loss of ClC-1 function → more severe myotonia + transient weakness.
Myotonia congenita is classified within the broader category of hereditary myotonias (non-dystrophic myotonias):
Important examples [of ion channelopathies]: [2]
- Cl⁻ channel: myotonia congenita
- Na⁺ channel: paramyotonia congenita, hyperkalemic (K⁺ sensitive) periodic paralysis
- Ca²⁺ channel: hypokalemic periodic paralysis, malignant hyperthermia
Muscular dystrophies [GC 056 classification]: [1]
- Duchenne (DMD), Becker (BMD)
- Facioscapulohumeral (FSHD)
- Limb-girdle (LGMD)
- Myotonic dystrophy (MyoD)
Classification Context from GC 056
The GC lecture slide on muscular dystrophies [1] lists myotonic dystrophy (MyoD) under the dystrophic category. Myotonia congenita is distinct — it is a non-dystrophic myotonia. Both share the symptom of myotonia but differ fundamentally: MC has no progressive muscle degeneration and no multisystem involvement.
6. Clinical Features
| Symptom | Pathophysiological Basis |
|---|---|
| Muscle stiffness (myotonia) — the cardinal symptom. Patients say "my muscles lock up" or "I can't let go." Worse in hands, legs, and eyelids. | Loss of Cl⁻ conductance → membrane hyperexcitability → repetitive action potentials → sustained involuntary contraction → delayed relaxation |
| "Warm-up" phenomenon — stiffness improves with repeated movement. "The first few steps are stiff, but then I loosen up." | Repeated contractions → gradual Na⁺ channel inactivation + K⁺ washout from T-tubules → membrane stabilizes → myotonia diminishes |
| Worsening with cold — patients report stiffness is much worse in cold weather or after cold exposure | Cold slows Cl⁻ channel kinetics further (already reduced) and enhances Na⁺ channel recovery from inactivation → net effect is more repetitive firing → worse myotonia |
| Worsening after prolonged rest — "First movement of the morning is the worst" | After rest, no warm-up effect; K⁺ may accumulate in T-tubules; first contraction meets a fully hyperexcitable membrane |
| Transient weakness at onset of movement (mainly Becker form) — brief weakness (seconds) before warm-up kicks in | Severe Cl⁻ loss → massive initial depolarization → partial Na⁺ channel inactivation → transient inexcitability → weakness; then warm-up gradually restores function |
| Difficulty with sudden movements — e.g., cannot quickly stand up, catch a ball, or start running; risk of falling | Myotonia locks muscles at initiation of movement; voluntary force is generated but relaxation lag means antagonist muscles resist |
| Difficulty releasing grip — classic complaint (e.g., shaking hands, opening jar) | Hand intrinsic muscles stay contracted after gripping; repeated opening/closing improves (warm-up) |
| Eyelid stiffness / inability to open eyes quickly after forced closure | Orbicularis oculi myotonia → delayed lid opening |
High Yield Clinical Pearl: Stiffness, Not Weakness
In myotonia congenita, the chief complaint is stiffness, NOT weakness. Strength is generally normal or even above average (due to muscle hypertrophy). The transient weakness in Becker MC is brief and at the onset of movement only. If a patient has myotonia WITH progressive weakness AND multisystem features, think myotonic dystrophy instead.
| Sign | How to Elicit | Pathophysiological Basis |
|---|---|---|
| Grip myotonia | Ask patient to grip your fingers tightly for 5 seconds, then release. Observe delayed opening of the hand. | Sustained involuntary contraction of finger flexors due to repetitive muscle action potentials |
| Percussion myotonia | Tap the thenar eminence briskly with a tendon hammer. A sustained dimple/contraction appears and slowly relaxes over several seconds. | Direct mechanical stimulation triggers action potentials in hyperexcitable muscle → sustained contraction |
| Eyelid myotonia ("lid-lag" variant) | Ask patient to look up, then quickly look down. The upper lid lags or the patient cannot open eyes quickly after forceful closure. | Myotonia of levator palpebrae superioris or orbicularis oculi |
| Muscle hypertrophy ("Herculean" or "athletic" build) | Inspect. Particularly prominent in calves, thighs, buttocks, shoulders, forearms. | Chronic involuntary isometric exercise of hyper-excitable fibres → work hypertrophy |
| Normal or near-normal muscle strength on formal testing | Standard MRC grading of muscle power | Unlike dystrophies, the contractile apparatus is intact; the problem is purely electrical |
| Normal deep tendon reflexes | Standard reflex testing | Lower motor neuron arc is intact; muscle itself is intact |
| No muscle wasting | Inspect | No degeneration/necrosis of fibres; in fact, fibres are hypertrophied |
| No multisystem signs (no cataracts, no cardiac conduction defects, no endocrinopathy, no cognitive impairment) | General examination | This distinguishes MC from myotonic dystrophy, which is a multisystem disease |
Clinical tests [for myotonia]: [2] — percussion myotonia, grip myotonia, eyelid myotonia
Warm-Up Phenomenon — The Clinical Test
To demonstrate the warm-up phenomenon at the bedside:
- Ask the patient to make a fist and release — the first release is slow (myotonia).
- Ask them to repeat this 5–10 times rapidly.
- By the 5th–10th repetition, the release is noticeably faster/smoother. This is pathognomonic for chloride channel myotonia (MC) and helps distinguish it from paramyotonia congenita (where repeated contractions make myotonia worse).
| Feature | Myotonia Congenita | Paramyotonia Congenita | Myotonic Dystrophy (DM1) |
|---|---|---|---|
| Channel | Cl⁻ (ClC-1) | Na⁺ (Nav1.4) | RNA toxicity (CTG repeat) |
| Gene | CLCN1 (7q35) | SCN4A (17q23) | DMPK (19q13.3) |
| Inheritance | AD (Thomsen) or AR (Becker) | AD | AD (with anticipation) |
| Onset | Infancy–childhood | Infancy | 15–40 years |
| Warm-up | Yes (myotonia improves with repeated use) | No — paradoxical (worsens with use) | Variable (may have some warm-up) |
| Cold sensitivity | Yes (worsens) | Yes (markedly worsens, triggers episodes) | Mild |
| Weakness | Minimal (transient in Becker) | Yes (can be prolonged after cold exposure) | Progressive, distal > proximal |
| Muscle wasting | No (hypertrophy instead) | No | Yes (temporal, sternomastoid, distal limb) |
| Multisystem | No | No | Yes (cataracts, cardiac, DM, gonadal, cognitive) |
| EMG | Myotonic discharges | Myotonic discharges + decremental CMAP with cooling | Myotonic discharges |
Myotonic dystrophy (MyD) [2]:
- Cause: CTG expansion at 19q13.3, AD inheritance
- Weakness + wasting in distal muscles, temporalis, sternomastoid, facial and jaw muscles
- Myotonia
- Multisystem involvement incl. Eyes: cataract; Cardiac: cardiomyopathy, conduction disorders; Endocrinopathy: DM, testicular atrophy; Smooth muscle disorder; Cognitive dysfunction
- Characteristic myotonic facies
High Yield: Why Distinguish MC from Myotonic Dystrophy?
This is a common exam question. The key differentiators:
- MC: stiffness WITHOUT weakness, NO wasting, NO multisystem features, warm-up phenomenon present, muscle hypertrophy, benign prognosis.
- Myotonic dystrophy: stiffness WITH progressive weakness + wasting + cataracts + cardiac conduction defects + endocrinopathy + cognitive dysfunction + characteristic facies (hollow temples, ptosis, long face). Progressive and potentially life-threatening (cardiac arrhythmias, respiratory failure).
High Yield Summary
Myotonia Congenita — Key Points for Exams:
- Definition: Non-dystrophic hereditary myotonia due to loss-of-function mutations in CLCN1 (voltage-gated Cl⁻ channel, chromosome 7q35).
- Two forms: Thomsen (AD, milder, earlier onset) and Becker (AR, more severe, later onset, transient weakness).
- Pathophysiology: Reduced Cl⁻ conductance → membrane hyperexcitability → repetitive action potentials → delayed relaxation (myotonia). Minor leakage → myotonia; major leakage → paralysis.
- Cardinal symptom: Muscle stiffness (NOT weakness). Strength is preserved or even enhanced (muscle hypertrophy).
- Warm-up phenomenon: Myotonia improves with repeated contraction — this distinguishes MC from paramyotonia congenita (where myotonia paradoxically worsens).
- Cold worsening: Myotonia increases in cold temperatures.
- No multisystem features: Unlike myotonic dystrophy, there are NO cataracts, cardiac conduction defects, endocrinopathy, or cognitive decline.
- Signs: Grip myotonia, percussion myotonia, eyelid myotonia, muscle hypertrophy ("Herculean build"), normal reflexes, no wasting.
- Prognosis: Generally benign; life expectancy is normal. The stiffness is the main source of disability.
- Channelopathy framework: Cl⁻ channel → MC; Na⁺ channel → paramyotonia congenita / hyperK PP; Ca²⁺ channel → hypoK PP / malignant hyperthermia.
Active Recall - Myotonia Congenita
[1] GC 056. Generalized muscle weakness.pdf (Muscular dystrophies classification slide) [2] Ryan Ho Neurology.pdf (pp. 193–194: Myotonic Dystrophy, Ion Channelopathies, Non-dystrophic myotonic syndromes, Periodic paralysis) [3] Block A - Inherited Cardiac conditions.pdf (Cardiomyopathy and neuromuscular associations) [4] MBBS Final MB (Medicine) (Felix PY Lai).pdf (Periodic paralysis and channelopathies overview)
Differential Diagnosis of Myotonia Congenita
The clinical approach to a patient presenting with myotonia (delayed muscle relaxation after contraction) requires systematic differentiation from conditions that mimic or share this symptom. The DDx also encompasses the broader scenario where a patient presents with muscle stiffness, difficulty releasing grip, or "locking up" of muscles, since these are the presenting complaints that bring a myotonia congenita patient to the clinic.
We will organize this in two tiers:
- Differential diagnosis of myotonia (the symptom) — i.e., what else causes delayed muscle relaxation?
- Differential diagnosis of muscle stiffness (the broader complaint) — i.e., non-myotonic causes of stiffness that can mimic MC.
Tier 1: Differential Diagnosis of Myotonia (the Symptom)
Causes [of myotonia]: myotonic dystrophy, myotonia congenita, paramyotonia congenita, proximal myotonic myopathy [1][2][3]
These are the conditions where true electrical myotonia exists — i.e., EMG shows myotonic discharges (repetitive, waxing-and-waning discharges with a characteristic "dive-bomber" sound). The key is distinguishing them from each other.
These share the feature of myotonia WITHOUT progressive muscle wasting and WITHOUT multisystem involvement. They differ by the ion channel involved and specific clinical nuances.
| Condition | Channel/Gene | Inheritance | Key Distinguishing Features |
|---|---|---|---|
| Myotonia Congenita (Thomsen) | Cl⁻ / CLCN1 (7q35) | AD | Onset infancy; generalized myotonia; warm-up phenomenon; mild; no weakness; muscle hypertrophy |
| Myotonia Congenita (Becker) | Cl⁻ / CLCN1 (7q35) | AR | Onset later childhood; more severe; transient weakness at onset of movement; marked muscle hypertrophy |
| Paramyotonia Congenita | Na⁺ / SCN4A (17q23) | AD | Onset infancy; paradoxical myotonia (worsens with repeated use — opposite of warm-up); markedly worsened by cold; associated with flaccid weakness after cold exposure; may overlap with hyperK PP |
| Sodium Channel Myotonia (myotonia fluctuans, myotonia permanens, acetazolamide-responsive myotonia) | Na⁺ / SCN4A | AD | Variable severity; fluctuating myotonia; may respond to acetazolamide; no periodic paralysis |
Myotonia congenita: AD/AR inheritance, due to voltage-gated Cl⁻ channel mutation (7q35) [2] S/S: infancy/childhood onset, generalized myotonia which ↑ in cold and ↓ by warmth/exercise, minimal weakness [2]
Paramyotonia congenita: AD inheritance, due to voltage-gated Na⁺ channel mutation (17q23) [2] S/S: infantile onset, attacks of myotonia which ↑ in cold and hyperK, ↓ by hypoK, a/w flaccid weakness [2]
High Yield: Warm-Up vs Paradoxical Myotonia
The single most important clinical test to distinguish myotonia congenita from paramyotonia congenita at the bedside:
- MC: Repeated grip → myotonia improves (warm-up phenomenon). This is because repeated use gradually inactivates Na⁺ channels and clears K⁺ from T-tubules.
- PMC: Repeated grip → myotonia worsens (paradoxical). This is because the mutant Na⁺ channel in PMC has defective inactivation — more use means more channels stuck open → more depolarization → more myotonia.
The name "para-myotonia" literally means "beyond/against myotonia" — the myotonia behaves opposite to what you'd expect [2].
Myotonic dystrophy (MyD): [2][3]
- Incidence: 5/100k births
- Cause: CTG expansion at 19q13.3, AD inheritance
- S/S: usually onset at 15-40y, slowly progressive
- Weakness + wasting in distal muscles, temporalis, sternomastoid, facial and jaw muscles
- Myotonia
- Multisystem involvement, incl.:
- Eyes: cataract
- Cardiac: cardiomyopathy, conduction disorders
- Endocrinopathy: DM, testicular atrophy
- Smooth muscle disorder: e.g. gut motility disorder, constipation, poor bladder emptying
- Cognitive dysfunction
- Characteristic myotonic facies
| Condition | Gene/Mechanism | Inheritance | Key Distinguishing Features from MC |
|---|---|---|---|
| Myotonic Dystrophy Type 1 (DM1) (Steinert disease) | CTG trinucleotide repeat in DMPK (19q13.3) | AD (with anticipation) | Onset 15-40y; progressive distal weakness + wasting (temporalis, sternomastoid, distal limbs); myotonic facies (long, expressionless, ptosis, temporal hollowing); multisystem: cataracts, cardiac conduction defects, DM, testicular atrophy, cognitive decline; frontal balding |
| Myotonic Dystrophy Type 2 (DM2) (PROMM — Proximal Myotonic Myopathy) | CCTG tetranucleotide repeat in CNBP (3q21) | AD | Onset adulthood; proximal weakness (unlike DM1 which is distal); milder myotonia; similar but generally milder multisystem features; no congenital form |
Myotonic dystrophy (AD) [from Maksim notes]: [5] Type 1: CTG trinucleotide repeat in DMPK gene Type 2: CCTG tetranucleotide repeat in CNBP gene Distal muscle weakness; Myotonia: hand grip, close eyes, percussion myotonia; Myopathic facies (tented open mouth, elongated face, expressionless); Frontal baldness, temporalis wasting, bilateral ptosis, cataract, cardiomyopathy
The Most Important DDx: MC vs DM1
This is the number one differential to get right. Both have myotonia — but:
| Feature | Myotonia Congenita | Myotonic Dystrophy (DM1) |
|---|---|---|
| Muscle bulk | Hypertrophied ("Herculean") | Wasted (temporal hollowing, thin sternomastoids) |
| Weakness | Absent or transient | Progressive, distal |
| Multisystem | None | Cataracts, cardiac, DM, gonadal, cognitive |
| Face | Normal | Myopathic facies (long, expressionless, ptosis) |
| Prognosis | Normal lifespan | Reduced lifespan (cardiac/respiratory) |
| Genetics | CLCN1 (Cl⁻ channel) | DMPK (CTG repeat, RNA toxicity) |
If a patient with myotonia has cataracts or a cardiac conduction defect, it is NOT MC — it is DM1 until proven otherwise.
Some periodic paralyses have inter-ictal myotonia (myotonia between paralytic attacks). These are important to consider because a patient with hyperkalemic periodic paralysis may present with myotonia as the predominant complaint.
Important examples [of ion channelopathies]: [2]
- Cl⁻ channel: myotonia congenita
- Na⁺ channel: paramyotonia congenita, hyperkalemic (K⁺ sensitive) periodic paralysis
- Ca²⁺ channel: hypokalemic periodic paralysis, malignant hyperthermia
| Condition | Channel/Gene | Key Distinguishing Features |
|---|---|---|
| Hyperkalemic Periodic Paralysis | Na⁺ / SCN4A | Episodes of flaccid weakness (minutes to hours) triggered by fasting, K⁺ intake, cold, rest after exercise; inter-ictal myotonia on EMG; onset in first decade; serum K⁺ elevated during attacks |
| Hypokalemic Periodic Paralysis | Ca²⁺ / CACNA1S (or Na⁺ / SCN4A) | Episodes of flaccid weakness triggered by CHO load, rest after exercise; NO myotonia; serum K⁺ low during attacks |
The key distinction: In MC, weakness is absent or very brief and the dominant problem is stiffness. In periodic paralyses, the dominant problem is episodic flaccid weakness, and myotonia (if present) is a secondary feature.
Certain drugs can cause electrical myotonia on EMG or clinical myotonia:
| Drug/Toxin | Mechanism |
|---|---|
| Statins (e.g., simvastatin, atorvastatin) | Membrane destabilization via cholesterol depletion in sarcolemma → altered Cl⁻/Na⁺ channel function |
| Chloroquine / Hydroxychloroquine | Lysosomal myopathy with secondary myotonic discharges |
| Colchicine | Myopathy with myotonic discharges |
| Fibrates (e.g., clofibrate) | Direct sarcolemmal effect |
| Propranolol (rare) | Na⁺ channel blockade effect |
| 2,4-D herbicide | Chloride channel inhibition |
In clinical practice, always take a drug history in any patient presenting with new myotonia. Drug-induced myotonia is an acquired, reversible cause that must be excluded before labelling a condition as inherited.
Hypothyroidism can cause pseudomyotonia — clinically similar to myotonia (slow muscle relaxation) but the EMG shows electrically silent contraction (no myotonic discharges). The mechanism is impaired Ca²⁺ reuptake by SERCA into the SR due to low thyroid hormone → slow cross-bridge uncoupling → slow mechanical relaxation. This is distinct from true myotonia because the problem is mechanical (slow Ca²⁺ handling), not electrical (repetitive action potentials).
- The classic sign is delayed relaxation of deep tendon reflexes ("hung-up" reflexes), especially the ankle jerk.
- Check TFT in any patient with apparent myotonia to exclude hypothyroidism.
When a patient says "my muscles are stiff," they may not have true myotonia. Other causes of muscle stiffness include:
| Condition | Mechanism | Key Distinguishing Features |
|---|---|---|
| Stiff-person syndrome (SPS) | Anti-GAD65 antibodies → impaired GABAergic inhibition in spinal cord → continuous motor neuron firing → sustained contraction | Progressive axial stiffness and rigidity; episodic painful spasms triggered by noise/touch; antibodies to GAD65 or amphiphysin; associated with T1DM; EMG shows continuous motor unit activity (NOT myotonic discharges) |
| Neuromyotonia (Isaacs syndrome) | Anti-CASPR2/LGI1 antibodies (or KCNA1 mutations) → voltage-gated K⁺ channel dysfunction at peripheral nerve → hyperexcitability → continuous muscle fibre activity | Muscle stiffness + visible rippling/myokymia + delayed relaxation; EMG shows neuromyotonic discharges (very high frequency, > 150 Hz, decrementing) rather than myotonic discharges; peripheral nerve origin |
| Cramp-fasciculation syndrome | Peripheral nerve hyperexcitability | Muscle cramps + fasciculations; stiffness is intermittent and painful (unlike painless myotonia in MC) |
| Spasticity (UMN lesion) | Loss of descending inhibition → increased tone, velocity-dependent | Clasp-knife pattern; hyperreflexia; Babinski positive; distribution follows UMN pattern; no percussion myotonia |
| Rigidity (extrapyramidal) | Basal ganglia dysfunction → co-contraction of agonist/antagonist | Lead-pipe or cogwheel; NOT velocity-dependent; associated with bradykinesia/tremor; no myotonic discharges |
| Contracture (metabolic myopathy, e.g., McArdle disease) | Glycogen storage disease type V → cannot break down glycogen → ATP depletion during exercise → failure of cross-bridge detachment | Muscle stiffness/hardening during exercise; electrically silent on EMG (no action potentials at all — a true contracture, not myotonia); associated with exercise intolerance, myoglobinuria |
True Myotonia vs Pseudomyotonia vs Contracture vs Neuromyotonia
| Feature | True Myotonia | Pseudomyotonia | Contracture | Neuromyotonia |
|---|---|---|---|---|
| EMG | Myotonic discharges (waxing/waning, "dive-bomber") | Electrically silent slow relaxation | Electrically silent | Very high-frequency decrementing bursts |
| Example | MC, DM1, PMC | Hypothyroidism | McArdle disease | Isaacs syndrome |
| Origin | Muscle membrane (sarcolemma) | Impaired SR Ca²⁺ reuptake | Metabolic (ATP depletion) | Peripheral nerve terminal |
| Warm-up | Yes (in MC) | No | No (worsens with exercise) | No |
This distinction is critical because the approach to investigation and management differs entirely.
| Condition | Channel/Cause | Myotonia? | Warm-up? | Weakness? | Multisystem? | Key Distinguisher |
|---|---|---|---|---|---|---|
| Myotonia congenita | Cl⁻ (CLCN1) | Yes | Yes | Minimal/transient | No | Muscle hypertrophy, benign |
| Paramyotonia congenita | Na⁺ (SCN4A) | Yes | Paradoxical | Yes (cold-induced) | No | Worsens with cold AND exercise |
| HyperK periodic paralysis | Na⁺ (SCN4A) | Yes (inter-ictal) | Variable | Episodic flaccid | No | K⁺ elevated during attacks |
| DM1 | CTG repeat (DMPK) | Yes | Variable | Progressive distal | Yes | Facies, cataracts, cardiac, cognitive |
| DM2 (PROMM) | CCTG repeat (CNBP) | Yes (milder) | Variable | Proximal | Yes (milder) | Proximal weakness pattern |
| Hypothyroidism | Impaired SERCA | Pseudomyotonia | No | Proximal | Yes (systemic) | Slow reflexes, electrically silent, ↑TSH |
| McArdle disease | Myophosphorylase deficiency | No (contracture) | No | Exercise-related | No | Electrically silent; second-wind phenomenon |
| Neuromyotonia | VGKC antibodies | No (neuromyotonic) | No | Variable | May have (Morvan) | Myokymia, rippling; peripheral nerve origin |
| Drug-induced | Various | Yes/pseudo | No | Variable | No | Drug history! Reversible on cessation |
When you see a patient with apparent myotonia, the clinical reasoning proceeds as follows:
- Confirm true myotonia — percussion myotonia, grip myotonia, EMG confirmation (myotonic discharges with waxing/waning amplitude and frequency).
- Exclude acquired/drug causes — drug history (statins, chloroquine), TFTs (hypothyroidism).
- Assess for dystrophic features — any weakness? Wasting? Cataracts (slit-lamp)? Cardiac conduction abnormalities (ECG)? Endocrinopathy? Cognitive change? If yes → myotonic dystrophy.
- Characterize the myotonia — does it warm up (MC) or paradoxically worsen (PMC)? Is it predominantly cold-triggered with subsequent weakness (PMC)?
- Assess for episodic paralysis — if present, think periodic paralysis with myotonia (hyperK PP).
- Confirm with genetic testing — CLCN1 for MC, SCN4A for PMC/hyperK PP, DMPK/CNBP for DM1/DM2.
Muscular dystrophies [from GC 056 classification]: [4] Duchenne (DMD), Becker (BMD); Facioscapulohumeral (FSHD); Limb-girdle (LGMD); Myotonic dystrophy (MyoD)
Note: the muscular dystrophies listed in the GC 056 slide [4] are differential diagnoses in the sense that myotonic dystrophy (MyoD) is the dystrophic condition that must be excluded when evaluating a patient with myotonia. The other dystrophies (DMD, BMD, FSHD, LGMD) do NOT have myotonia and present primarily with progressive weakness — they enter the DDx only if the chief complaint is "weakness" rather than "stiffness."
Differential Diagnosis of Myopathy [from medicine neurology lecture]: [6] Infective: Viral (HIV, CMV, EBV...), pyomyositis Neoplastic: Paraneoplastic Inflammatory: Rheumatoid arthritis, Sjögren's syndrome Congenital: Muscular dystrophy Autoimmune: Dermatomyositis, necrotising autoimmune myositis Trauma/toxin: Crush injuries/seizures causing rhabdomyolysis, glucocorticoids, colchicine, statins Endocrine: Hypothyroidism, Cushing's syndrome, hypokalemia
These are the broad DDx for myopathy (weakness). Most of these do NOT cause myotonia and are therefore distinguished from MC by the absence of grip/percussion myotonia and the presence of progressive weakness. They are relevant mainly when the clinician is casting a wide net for "what is wrong with this patient's muscles?"
High Yield Summary — DDx of Myotonia Congenita
- Always confirm true myotonia (EMG myotonic discharges) — exclude pseudomyotonia (hypothyroidism), contracture (McArdle), and neuromyotonia (Isaacs).
- The #1 DDx is myotonic dystrophy (DM1 > DM2) — look for progressive weakness, wasting, cataracts, cardiac, endocrine, and cognitive features. If ANY of these are present, it is NOT MC.
- Among non-dystrophic myotonias, the main DDx is paramyotonia congenita — distinguished by paradoxical myotonia (worsens with use) and Na⁺ channel gene (SCN4A).
- HyperK periodic paralysis can have inter-ictal myotonia but is distinguished by episodic flaccid weakness with elevated K⁺.
- Drug-induced myotonia (statins, chloroquine) is acquired and reversible — always take a drug history.
- Warm-up phenomenon = MC; paradoxical worsening = PMC — this is the bedside differentiator.
Active Recall - DDx of Myotonia Congenita
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 145: Myotonic Dystrophy, causes of myotonia) [2] Senior notes: Ryan Ho Neurology.pdf (pp. 193–194: Ion channelopathies, non-dystrophic myotonic syndromes, periodic paralysis) [3] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p. 706: Differential diagnosis of myopathies) [4] Lecture slides: GC 056. Generalized muscle weakness.pdf (Muscular dystrophies classification slide) [5] Senior notes: Maksim Medicine Notes.pdf (p. 276: Myopathy approach, myotonic dystrophy features) [6] Lecture slides: Neurology - Two cases of lower limb weakness.pdf (p. 38: Differential diagnosis of myopathy)
Diagnosis of Myotonia Congenita: Criteria, Algorithm, and Investigations
There are no universally codified "diagnostic criteria" for myotonia congenita in the way that, say, the Jones criteria exist for rheumatic fever. Instead, the diagnosis is reached through a clinical-electrophysiological-genetic triad. In practice, a confident diagnosis of MC requires:
| Component | Requirement |
|---|---|
| 1. Clinical | History of painless muscle stiffness (myotonia) from infancy or childhood, with warm-up phenomenon, no progressive weakness, no multisystem features, ± family history consistent with AD or AR inheritance |
| 2. Electrophysiological | EMG demonstrating myotonic discharges — the hallmark waxing-and-waning, high-frequency repetitive discharges with the characteristic "dive-bomber" sound; normal NCS |
| 3. Genetic | Pathogenic mutation identified in CLCN1 (chromosome 7q34-35) on genetic testing — this is the gold standard confirmatory test |
All three components reinforce each other, but genetic testing is the definitive confirmation. A patient can be diagnosed clinically + electrophysiologically with high confidence even without genetics, but genetic confirmation is increasingly standard (and essential for genetic counselling and family screening).
High Yield: Why Clinical Alone Is Insufficient
Clinical myotonia alone does NOT make the diagnosis. You must:
- Exclude dystrophic myotonia (DM1/DM2) — because both have myotonia, but DM1 has progressive weakness + multisystem involvement and a completely different prognosis.
- Exclude paramyotonia congenita — because both are non-dystrophic channelopathies, but PMC involves SCN4A (Na⁺ channel) and may require different management.
- Exclude acquired causes — drug-induced myotonia, hypothyroidism.
The EMG and genetic testing provide the specificity needed.
Dx [of myotonic dystrophy]: usually clinical ± genetic testing [1][3]
This principle applies similarly to MC — clinical suspicion followed by confirmatory genetic testing [2].
The following algorithm represents the systematic approach to a patient presenting with the chief complaint of muscle stiffness or delayed relaxation, leading to a diagnosis of myotonia congenita.
3. Investigation Modalities — Detailed Interpretation
These are performed before any laboratory or electrophysiological investigations. They are the first-line screening tools.
| Test | Technique | Positive Finding in MC | Pathophysiological Basis |
|---|---|---|---|
| Grip myotonia | Ask patient to make a tight fist for 5 seconds then open quickly | Delayed opening — fingers slowly unfurl over 2–10 seconds; improves on repeated trials (warm-up) | Repetitive action potentials in finger flexors due to reduced Cl⁻ conductance → sustained contraction |
| Percussion myotonia | Tap thenar eminence sharply with tendon hammer | Sustained dimple or thumb adduction/opposition lasting several seconds before slowly relaxing | Direct mechanical stimulation of hyperexcitable sarcolemma → burst of repetitive APs |
| Eyelid myotonia | Ask patient to close eyes tightly for 5 seconds, then open | Delayed lid opening; may see a brief scleral flash as the lid slowly retracts | Orbicularis oculi myotonia |
| Warm-up test | Repeat grip-release 10 times rapidly | Progressively faster release with each repetition | Gradual Na⁺ channel inactivation + K⁺ clearance from T-tubules with repeated use |
| Cold provocation (optional) | Immerse hand in cold water for 5 minutes, then test grip myotonia | Worsening of myotonia after cold exposure | Cold slows residual Cl⁻ channel kinetics and enhances Na⁺ channel recovery from inactivation → more repetitive firing |
Clinical tests [for myotonia]: [grip myotonia, percussion myotonia, eyelid myotonia] [1][2]
Myotonia: hand grip, close eyes, percussion myotonia [5]
Exam Tip: Describing Clinical Tests
For clinical exams (OSCE/long case), when asked to demonstrate myotonia, describe all three tests: grip myotonia, percussion myotonia, and eyelid myotonia. Then specifically demonstrate the warm-up phenomenon by asking the patient to repeat grip-release. State: "Myotonia that improves with repeated contraction is consistent with chloride channel myotonia (myotonia congenita) and argues against paramyotonia congenita."
| Investigation | Expected Finding in MC | Why This Test? | Interpretation |
|---|---|---|---|
| Serum CK | Normal or mildly elevated (typically < 2–3× ULN) | CK is released when muscle membranes are damaged. In MC, muscle fibres are NOT degenerating — the architecture is intact. Hence CK is NOT significantly elevated. | CK > 1000 → think inflammatory myopathy, DMD/BMD, rhabdomyolysis [3][5]. CK 200–1000 → non-specific, can be seen in vigorous exercise, after EMG, or mild myopathy [3]. In MC, CK is typically normal to very mildly raised because there is no structural muscle damage — only electrical dysfunction. |
| Serum electrolytes (K⁺, Na⁺, Ca²⁺, Mg²⁺, PO₄³⁻) | Normal | To exclude electrolyte-mediated muscle dysfunction (hypoK, hyperK) and periodic paralysis | If K⁺ is abnormal, consider periodic paralysis or secondary causes. In MC, electrolytes are normal between and during episodes of stiffness. |
| Thyroid function tests (TFT) | Normal | To exclude hypothyroid pseudomyotonia and thyrotoxic periodic paralysis | Hypothyroidism causes slow relaxation (pseudomyotonia) that is electrically silent. Hyperthyroidism can cause periodic paralysis (especially in Asian males). Both must be excluded. |
| Liver function / Renal function | Normal | Baseline; exclude systemic causes of myopathy | Drug-induced or metabolic myopathies may have deranged LFT/RFT. |
Investigations [for myopathy]: [3][7]
- CK, ALT, LDH
- CK: 200-1000 for most myopathies; > 1000 for inflammatory myopathies, rhabdomyolysis, DMD/BMD
- NCS: to rule out neuropathy
- EMG: polyphasic, low-amplitude motor unit potential [in myopathy]
- Muscle biopsy
- Ix for underlying causes
3.3 Electrophysiology — NCS and EMG
This is the most important investigation after clinical assessment. It confirms true electrical myotonia and helps narrow the differential.
| Parameter | Expected Finding in MC | Interpretation |
|---|---|---|
| Motor NCS | Normal — normal CMAP amplitude, distal latency, conduction velocity | Confirms the pathology is NOT in the peripheral nerve (excludes neuropathy). The motor neuron and axon are intact; the problem is in the muscle membrane itself. |
| Sensory NCS | Normal | Confirms no sensory nerve involvement (myotonia is a purely motor phenomenon). |
| F-waves | Normal | Normal proximal conduction. |
NCS: to rule out neuropathy (can also cause reduced motor response) [5]
This is where the diagnosis is clinched electrophysiologically. EMG in MC shows myotonic discharges — these are the single most important electrophysiological finding.
| Feature | Description | Why It Occurs |
|---|---|---|
| Myotonic discharges | Repetitive, high-frequency (20–150 Hz) discharges of single muscle fibres that wax and wane in both amplitude and frequency. Produce the characteristic "dive-bomber" sound on the audio speaker. | Each discharge represents a single muscle fibre firing repeatedly due to membrane hyperexcitability from reduced Cl⁻ conductance. The waxing-and-waning pattern reflects the fluctuating membrane potential hovering near threshold — sometimes recruiting more fibres (waxing), sometimes fewer (waning). |
| Triggered by | Needle insertion (insertional activity), percussion, voluntary contraction, or movement of the needle | Mechanical perturbation of a hyperexcitable membrane triggers a burst of repetitive APs |
| Distribution | Widespread — found in multiple muscles, both proximal and distal | ClC-1 is expressed throughout skeletal muscle; the defect is generalized |
| Voluntary MUAPs | Normal morphology — normal amplitude, normal duration, normal recruitment | Unlike myopathy (which has small, short, polyphasic MUAPs) or neuropathy (which has large, long MUAPs with reduced recruitment), MC has structurally normal muscle — the motor units fire normally. The abnormality is the EXTRA involuntary discharges between and after voluntary activation. |
High Yield: EMG Pattern Recognition
| Condition | EMG Finding | Sound |
|---|---|---|
| Myotonia congenita | Myotonic discharges (waxing/waning) | "Dive-bomber" |
| Myotonic dystrophy | Myotonic discharges + small polyphasic MUAPs (myopathic) | "Dive-bomber" + myopathic units |
| Paramyotonia congenita | Myotonic discharges ± decremental CMAP with cooling | "Dive-bomber" |
| Normal myopathy (e.g., polymyositis) | Small, short, polyphasic MUAPs; early recruitment; NO myotonic discharges | No dive-bomber |
| Neuropathy | Large, long MUAPs; reduced recruitment; fibrillation potentials | No dive-bomber |
The presence of myotonic discharges with normal voluntary MUAPs (no myopathic changes) strongly favours a non-dystrophic myotonia (MC or PMC) over myotonic dystrophy [2][5].
In research centres or specialized neuromuscular clinics, provocative electrophysiological protocols may be used:
| Protocol | Technique | Expected Finding in MC |
|---|---|---|
| Short exercise test | Brief (10-second) maximal isometric contraction of a muscle (e.g., abductor digiti minimi) with CMAP recording before and after | In MC: transient drop in CMAP amplitude immediately post-exercise (due to myotonia interfering with relaxation), followed by recovery. In Becker MC, there may also be a delayed CMAP decrement (reflecting transient weakness). |
| Long exercise test | 5 minutes of intermittent exercise (5 seconds on, 5 seconds off) with CMAP every minute for 40-50 minutes post-exercise | In MC: typically shows a pattern 3 response (transient CMAP drop with exercise that recovers quickly). In periodic paralysis: prolonged CMAP decrement. |
| Cooling protocol | Record CMAP/myotonia before and after cooling the limb to 20°C | In MC: increased myotonic discharges with cold. In PMC: dramatic increase in myotonia + CMAP decrement with cold (paradoxical myotonia). This helps differentiate Cl⁻ channelopathies from Na⁺ channelopathies. |
These tests are not routine but are helpful when the clinical picture is ambiguous (e.g., overlap between MC and PMC) or when genetic testing is inconclusive.
| Aspect | Details |
|---|---|
| Gene | CLCN1 (Chloride Voltage-Gated Channel 1) |
| Chromosome | 7q34-35 |
| Method | Sanger sequencing of all 23 exons of CLCN1; increasingly replaced by next-generation sequencing (NGS) gene panels that simultaneously test CLCN1, SCN4A, CACNA1S, KCNJ2, and other skeletal muscle channelopathy genes |
| Mutations | > 300 known pathogenic variants — missense, nonsense, splice-site, small insertions/deletions, whole-exon deletions |
| Interpretation | Finding a known pathogenic or likely pathogenic variant in CLCN1 confirms the diagnosis. Heterozygous pathogenic variant + AD phenotype = Thomsen disease. Homozygous or compound heterozygous pathogenic variants + AR phenotype = Becker disease. |
| Variants of uncertain significance (VUS) | A CLCN1 VUS does NOT confirm the diagnosis — clinical-electrophysiological correlation is essential. Functional studies or segregation analysis in family members may be needed. |
| Negative result | If CLCN1 sequencing is negative but clinical suspicion remains high, consider: (1) large deletions/duplications not detected by sequencing (MLPA may help); (2) deep intronic variants; (3) reconsider the diagnosis — could it be SCN4A (PMC) or DM2 (CNBP)? |
| Availability in HK | Available through Clinical Genetics Service at QMH/HKCH, or can be sent to accredited overseas laboratories. Turnaround time: weeks to months. |
Why Gene Panels Over Single-Gene Testing?
Modern practice increasingly uses multigene NGS panels for skeletal muscle channelopathies rather than sequential single-gene Sanger sequencing. A typical "non-dystrophic myotonia / periodic paralysis" panel includes CLCN1, SCN4A, CACNA1S, KCNJ2, KCNJ5, and RYR1. This approach is:
- More efficient (tests all relevant genes simultaneously)
- More cost-effective (one test instead of sequential tests)
- More likely to catch unexpected diagnoses (e.g., a patient thought to have MC who actually has an SCN4A variant)
These are not diagnostic OF MC but are needed to rule out mimics:
| Investigation | Purpose | Expected in MC |
|---|---|---|
| Slit-lamp examination | Exclude cataracts (DM1) | Normal — no cataracts |
| ECG | Exclude cardiac conduction defects (DM1) | Normal — no PR prolongation, no bundle branch block |
| Echocardiography | Exclude cardiomyopathy (DM1, DCM associated with neuromuscular disorders) | Normal |
| HbA1c / Fasting glucose | Exclude DM (DM1 association) | Normal |
| Gonadal hormones (in males) | Exclude testicular atrophy (DM1) | Normal |
| Cognitive assessment | Exclude cognitive decline (DM1) | Normal |
Neuromuscular disorders [associated with familial DCM]: Duchenne muscular dystrophy, Becker muscular dystrophy, Myotonic dystrophy [8]
ECG / echo: dilated cardiomyopathy [5] — important to screen for in dystrophic myotonias, but expected to be NORMAL in MC.
If all of the above systemic screens are normal, this strongly supports a non-dystrophic myotonia and argues against DM1/DM2.
| Aspect | Details |
|---|---|
| Role | Muscle biopsy is rarely needed in MC because the diagnosis is made clinically + EMG + genetics. Biopsy is reserved for atypical cases or when the diagnosis remains unclear after genetic testing. |
| Findings if performed | Non-specific: absence of dystrophic changes (no fibre necrosis, no regeneration, no fibrosis). May show type 2 fibre hypertrophy and absence of type 2B fibres (particularly in Becker MC). Occasional internal nuclei. These findings are non-diagnostic on their own. |
| Contrast with DM1 | DM1 biopsy shows dystrophic changes: increased internal nuclei, ring fibres, sarcoplasmic masses, type 1 fibre atrophy, fibrosis. |
Muscle biopsy [as investigation for muscle diseases] [3][5][7]
| Step | Action | Rationale |
|---|---|---|
| 1 | History + Examination — confirm clinical myotonia (grip, percussion, eyelid); assess warm-up; assess for weakness and multisystem features | Clinical phenotyping |
| 2 | Exclude acquired causes — drug history, TFT, electrolytes | Rule out reversible causes |
| 3 | EMG + NCS — confirm myotonic discharges; normal NCS; assess MUAP morphology | Electrophysiological confirmation of true myotonia; exclude neuropathy; distinguish dystrophic (abnormal MUAPs) from non-dystrophic (normal MUAPs) |
| 4 | Screen for multisystem features — slit-lamp, ECG, echo, HbA1c, gonadal hormones, cognitive assessment | Exclude myotonic dystrophy |
| 5 | Genetic testing — CLCN1 (or NGS channelopathy panel) | Definitive confirmation; determine AD (Thomsen) vs AR (Becker); enable genetic counselling |
| 6 | Family screening — clinical ± genetic testing of at-risk relatives | Identify presymptomatic carriers; inform reproductive counselling |
High Yield Summary — Diagnosis of Myotonia Congenita
- No formal diagnostic criteria exist — diagnosis is a clinical-electrophysiological-genetic triad.
- Bedside tests: grip myotonia, percussion myotonia, eyelid myotonia, warm-up phenomenon.
- CK: Normal or mildly elevated (< 2–3× ULN) — muscle structure is intact, unlike dystrophies.
- EMG: Myotonic discharges (waxing/waning, "dive-bomber" sound) with normal voluntary MUAPs. This pattern = non-dystrophic myotonia.
- NCS: Normal — confirms the problem is in the muscle membrane, not the nerve.
- Genetic testing: CLCN1 mutation analysis is the gold standard. AD = Thomsen, AR = Becker. NGS gene panels are now preferred.
- Exclude DM1: Slit-lamp (cataracts), ECG (conduction defects), echo (cardiomyopathy), HbA1c, gonadal hormones, cognitive screen — all should be NORMAL in MC.
- Muscle biopsy: Rarely needed; non-specific findings (no dystrophy, type 2 fibre hypertrophy).
- TFT: Must be checked to exclude hypothyroid pseudomyotonia.
Active Recall - Diagnosis of Myotonia Congenita
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 145: Myotonic dystrophy, causes and diagnosis of myotonia) [2] Senior notes: Ryan Ho Neurology.pdf (pp. 191–194: Diseases of muscles, ion channelopathies, non-dystrophic myotonic syndromes) [3] Senior notes: Adrian Lui Pediatrics Notes.pdf (pp. 134, 143: Approach to generalized weakness investigations, myopathy investigations) [4] Lecture slides: GC 056. Generalized muscle weakness.pdf (Muscular dystrophies classification) [5] Senior notes: Maksim Medicine Notes.pdf (p. 276: Myopathy investigations, CK interpretation, myotonic dystrophy features) [7] Senior notes: Ryan Ho Fundamentals.pdf (p. 336: Generalized weakness investigation approach) [8] Senior notes: Block A - Inherited Cardiac conditions.pdf (p. 5: Neuromuscular disorders associated with familial DCM)
Management of Myotonia Congenita
Before diving into specific therapies, it is important to establish the overarching principles. Myotonia congenita is a benign, non-progressive channelopathy with normal life expectancy. The muscle is structurally intact — the problem is purely electrical. This fundamentally shapes the management philosophy:
| Principle | Rationale |
|---|---|
| 1. There is no cure | The underlying genetic defect (CLCN1 mutation) cannot be corrected. Management is symptomatic. |
| 2. Not all patients require pharmacotherapy | Many patients with mild MC (especially Thomsen type) adapt well to their stiffness using the warm-up phenomenon and lifestyle modifications. Drug treatment is reserved for those whose myotonia significantly impairs daily function. |
| 3. The goal is to reduce myotonia | We aim to reduce the frequency and severity of muscle stiffness so the patient can function normally — open jars, start walking without falling, drive safely, perform work tasks. |
| 4. Avoid triggers | Cold exposure, prolonged rest, sudden movements — all worsen myotonia. Non-pharmacological measures targeting these triggers are first-line. |
| 5. Genetic counselling is integral | AD (Thomsen) or AR (Becker) inheritance has implications for family planning and screening of relatives. |
| 6. Multidisciplinary approach | Physiotherapy, occupational therapy, genetic counselling, and in paediatric cases, school support. |
Mx [of non-dystrophic myotonic syndromes]: procainamide, phenytoin, disopyramide, nifedipine, quinine sulphate [2]
No specific treatment [for myopathy in general]. Supportive care with multidisciplinary approach: PT, OT, genetic counselling [5]
3. Non-Pharmacological Management (First-Line for All Patients)
These measures exploit the known pathophysiology of MC and should be tried before any drug is introduced.
| Topic | Content | Rationale |
|---|---|---|
| Nature of the disease | Explain that MC is a lifelong but benign condition with normal life expectancy. The muscles are healthy — they are just "too excitable." | Reduces anxiety. Many patients (or parents of affected children) fear progressive disability akin to muscular dystrophy. Reassurance is powerful. |
| Warm-up technique | Teach the patient to perform gentle, repetitive contractions before any demanding activity — e.g., open and close the fist 10 times before gripping a tool; walk slowly for 30 seconds before jogging | Exploits the warm-up phenomenon: repeated use causes gradual Na⁺ channel inactivation and K⁺ clearance → membrane stabilizes → myotonia diminishes. This is the patient's built-in therapy. |
| Cold avoidance | Dress warmly; use gloves in cold weather; avoid cold water immersion; warm up indoors before going outside in winter | Cold worsens ClC-1 dysfunction and enhances Na⁺ channel recovery from inactivation → more repetitive firing → worse myotonia |
| Sudden movement avoidance | Avoid explosive starts from rest (e.g., sprinting from standstill). Always "warm up" first. | After prolonged rest, muscles are maximally stiff (no warm-up effect in play). Sudden explosive movement against stiff muscles can cause falls/injury. |
| Intervention | Details | Rationale |
|---|---|---|
| Regular aerobic exercise | Swimming (in warm water), cycling, walking — at moderate intensity | Regular activity keeps the warm-up effect partially maintained; improves cardiovascular fitness; the inherent muscle hypertrophy in MC means patients can be quite athletic once past the initial stiffness |
| Stretching programme | Gentle stretching of commonly affected muscle groups (calves, quadriceps, forearms) | Reduces subjective stiffness; maintains flexibility; does NOT directly treat myotonia but improves functional range |
| Physiotherapy referral | Especially for children or patients with Becker MC who have transient weakness | Tailored exercise programmes; gait training; fall prevention strategies |
| Intervention | Rationale |
|---|---|
| Adaptive tools (e.g., ergonomic grips, button hooks, elastic shoelaces) | Reduces functional impact of grip myotonia on daily activities |
| Workplace modifications | Avoiding cold environments; allowing warm-up time before tasks requiring fine motor control |
| School support (in children) | Educating teachers about the condition; allowing extra time for handwriting; avoiding embarrassment from visible stiffness |
| Topic | Details |
|---|---|
| Thomsen (AD) | Each child of an affected parent has a 50% chance of inheriting the mutation. Penetrance is high but expressivity variable. |
| Becker (AR) | Both parents are carriers (usually asymptomatic). Each child has a 25% chance of being affected, 50% chance of being a carrier, 25% chance of being unaffected and not a carrier. |
| Family screening | Offer clinical evaluation ± genetic testing to at-risk relatives |
| Prenatal/preimplantation diagnosis | Technically possible but rarely pursued given the benign nature of MC |
4. Pharmacological Management
Drugs are indicated when myotonia significantly impairs daily function despite non-pharmacological measures. All anti-myotonic drugs work by the same fundamental principle: they stabilize the sarcolemmal membrane by blocking Na⁺ channels, thereby reducing the repetitive firing that constitutes myotonia.
Core Pharmacological Principle
Why Na⁺ channel blockers for a Cl⁻ channel disease? Because the myotonia in MC is caused by repetitive Na⁺-dependent action potentials (the problem is that reduced Cl⁻ conductance fails to prevent these). By partially blocking Na⁺ channels, you raise the threshold for firing and reduce the number of repetitive action potentials → less myotonia. You are treating the downstream electrical consequence, not the upstream Cl⁻ defect.
| Aspect | Details |
|---|---|
| Drug class | Class IB antiarrhythmic; oral Na⁺ channel blocker (use-dependent — blocks channels more when they are rapidly firing, which is exactly what happens in myotonia) |
| Name breakdown | Related to lidocaine ("mexi-" from its origin as a Mexican derivative of lidocaine; "-letine" = oral bioavailability modification) |
| Mechanism in MC | Binds preferentially to Na⁺ channels in the inactivated state → stabilizes inactivation → reduces repetitive firing → less myotonia. Use-dependent: the more the channel fires, the more drug binds → selectively targets hyperexcitable fibres without significantly affecting normal muscle function. |
| Evidence | The MYOMEX trial (2017) — a multicentre, randomized, placebo-controlled trial — demonstrated that mexiletine significantly reduces myotonia (measured by clinical grading and EMG) in patients with non-dystrophic myotonia including MC [current evidence base, 2017–present]. This is the highest-level evidence available. |
| Dose | Start low: 150 mg PO once or twice daily. Titrate up to a maximum of 200 mg TDS (600 mg/day) based on response and tolerability. |
| Onset | Clinical improvement typically within days to 1–2 weeks. |
| Side effects | GI (nausea, dyspepsia — most common, dose-limiting); dizziness; tremor; cardiac (QT prolongation, pro-arrhythmic potential). |
| Monitoring | ECG before starting (to check baseline QTc) and periodically during treatment. Avoid in patients with pre-existing cardiac conduction abnormalities (prolonged QT, heart block, structural heart disease). |
| Contraindications | Significant cardiac conduction disease; long QT syndrome; severe hepatic impairment (mexiletine is hepatically metabolized via CYP2D6 and CYP1A2). |
High Yield: Mexiletine = First-Line for Non-Dystrophic Myotonia
Mexiletine is the only drug with Level 1 evidence (RCT) supporting its use in non-dystrophic myotonia. It is the current first-line pharmacotherapy recommended by international neuromuscular guidelines. Always perform a baseline ECG before initiation and monitor QTc.
These are used when mexiletine is contraindicated, not tolerated, or insufficiently effective. Evidence for these is limited (case series, expert opinion, historical use).
Mx [of non-dystrophic myotonic syndromes]: procainamide, phenytoin, disopyramide, nifedipine, quinine sulphate [2]
| Drug | Class / Mechanism | Dose (typical) | Key Points |
|---|---|---|---|
| Lamotrigine | Anticonvulsant; Na⁺ channel blocker (voltage-dependent, blocks sustained repetitive firing) | 25 mg/day → titrate slowly to 100–300 mg/day | Favourable side-effect profile compared to Class I antiarrhythmics; avoids cardiac risk; effective in some patients; requires slow titration to avoid Stevens-Johnson syndrome (SJS). Increasingly used as preferred second-line. |
| Carbamazepine | Anticonvulsant; Na⁺ channel blocker (stabilizes inactivated state) | 200 mg BD → up to 600–800 mg/day | Effective for myotonia in some patients; side effects include drowsiness, ataxia, hyponatraemia (SIADH), bone marrow suppression; requires monitoring of FBC and Na⁺. HLA-B*15:02 screening recommended in Han Chinese before use (risk of SJS/TEN). |
| Phenytoin | Anticonvulsant; Na⁺ channel blocker | 100 mg BD–TDS | Historical use; narrow therapeutic index; side effects include gingival hypertrophy, hirsutism, cerebellar toxicity, osteomalacia; zero-order kinetics make dosing unpredictable. Less preferred in modern practice. |
| Acetazolamide | Carbonic anhydrase inhibitor → mild metabolic acidosis → membrane stabilization; also modifies Cl⁻/HCO₃⁻ exchange | 250 mg BD | Particularly useful in some Na⁺ channel myotonias ("acetazolamide-responsive myotonia"); may also help in MC; side effects: paraesthesia, renal stones, metabolic acidosis, hypokalaemia. |
| Quinine sulphate | Na⁺ channel blocker; also blocks K⁺ channels | 200–300 mg BD | Historical use; concerns about cardiotoxicity (QT prolongation), cinchonism (tinnitus, nausea, visual disturbance), thrombocytopenia. Largely fallen out of favour due to safety profile. |
| Procainamide | Class IA antiarrhythmic; Na⁺ channel blocker | Variable | Historical use; risk of drug-induced lupus (anti-histone antibodies), agranulocytosis; rarely used now. |
| Disopyramide | Class IA antiarrhythmic; Na⁺ channel blocker + anticholinergic | Variable | Anticholinergic side effects (dry mouth, urinary retention, constipation); may worsen glaucoma; cardiotoxic; rarely used. |
| Nifedipine | Calcium channel blocker (dihydropyridine) | 10–20 mg TDS | Mechanism in myotonia unclear — possibly indirect membrane stabilization via Ca²⁺ modulation; limited evidence; may cause hypotension, peripheral oedema, flushing. |
Drug Selection Hierarchy in Practice
Current practical approach (2024–2026 guidelines):
- First-line: Mexiletine (strongest evidence)
- Second-line if cardiac contraindication or intolerance: Lamotrigine (no cardiac risk, reasonable evidence)
- Second-line alternative: Carbamazepine (effective but more side effects; HLA-B*15:02 screening in Chinese)
- Third-line / historical: Phenytoin, acetazolamide, quinine
- Rarely used now: Procainamide, disopyramide (cardiac and systemic toxicity)
The lecture slide listing [2] reflects historical options — in modern practice, mexiletine and lamotrigine dominate.
This is a critical safety issue that every clinician managing a patient with MC must be aware of.
| Issue | Details | Rationale |
|---|---|---|
| Avoid succinylcholine (suxamethonium) | Absolutely contraindicated in MC | Succinylcholine is a depolarizing neuromuscular blocker. In MC, the hyperexcitable membrane responds to depolarization with massive, sustained myotonic contraction → rigid jaw (masseter spasm), generalized rigidity → impossible intubation, risk of rhabdomyolysis, hyperkalaemia, and cardiac arrest. This can be life-threatening. |
| Non-depolarizing agents are safe | Use rocuronium, vecuronium, atracurium, cisatracurium | These block the NMJ without causing depolarization → no myotonic trigger. They work normally in MC. Sugammadex for reversal is also safe. |
| Volatile anaesthetics | Generally safe (unlike malignant hyperthermia, which involves RYR1 mutations, NOT CLCN1). However, some reports suggest volatile agents may trigger or worsen myotonia in individual cases → use with caution. | MC is a Cl⁻ channelopathy, NOT an RYR1/DHP receptor disorder. MC is NOT associated with malignant hyperthermia. However, prudence dictates caution. |
| Cold | Maintain normothermia during surgery; warm IV fluids; warm operating theatre | Cold worsens myotonia → intraoperative stiffness can interfere with surgical positioning and ventilation |
| MedAlert bracelet / Anaesthetic alert card | All MC patients should carry one | Ensures any anaesthetist is aware of the condition in emergency situations |
Exam Trap: MC vs Malignant Hyperthermia
Students often confuse myotonia congenita with malignant hyperthermia (MH) because both involve "muscle stiffness" during anaesthesia. They are completely different:
| Feature | Myotonia Congenita | Malignant Hyperthermia |
|---|---|---|
| Gene | CLCN1 (Cl⁻ channel) | RYR1 or CACNA1S (Ca²⁺ release channel) |
| Trigger | Succinylcholine (depolarization) | Volatile anaesthetics (halothane, isoflurane) + succinylcholine |
| Mechanism | Repetitive APs → myotonic stiffness | Uncontrolled Ca²⁺ release from SR → sustained contraction → hypermetabolism |
| Features | Stiffness/rigidity, NO hyperthermia | Rigidity + hyperthermia + metabolic acidosis + rhabdomyolysis + hyperK |
| Treatment | Avoid trigger; supportive | Dantrolene (blocks RYR1 Ca²⁺ release) |
MC patients are NOT at increased risk of malignant hyperthermia. But succinylcholine must still be avoided [2].
| Aspect | Frequency | Details |
|---|---|---|
| Clinical review | Every 6–12 months (stable patients); more frequently during drug titration | Assess myotonia severity (subjective + clinical tests), functional impact, medication side effects |
| ECG | Before starting mexiletine; 1–2 weeks after initiation or dose change; annually thereafter | Monitor QTc interval. Discontinue or reduce dose if QTc > 500 ms or increases by > 60 ms from baseline. |
| Blood tests | For carbamazepine: FBC + Na⁺ every 3–6 months. For phenytoin: drug levels. | Monitor for bone marrow suppression (carbamazepine), hyponatraemia (carbamazepine), supratherapeutic levels (phenytoin) |
| Functional assessment | Annually | Assess impact on ADLs, work, school; refer to OT/PT if needed |
| Genetic counselling update | At transition to adulthood; before family planning | Review inheritance, reproductive options |
7. Special Populations
- MC often presents in childhood. Pharmacotherapy is considered when stiffness significantly impairs motor milestones, play, school performance, or causes falls.
- Mexiletine can be used in children (dose adjusted by weight); data are limited but clinical experience supports safety and efficacy.
- Lamotrigine is an alternative with a well-established paediatric safety profile (used widely in childhood epilepsy).
- Education of teachers and school staff is essential — the child may appear clumsy or slow to start movement but is NOT cognitively impaired.
- Myotonia may worsen during pregnancy (hormonal and physiological changes).
- Mexiletine: limited safety data in pregnancy (Category C). Use only if benefit outweighs risk; discuss with patient.
- Lamotrigine: better pregnancy safety data (widely used in epilepsy in pregnancy, but levels drop in pregnancy due to increased clearance → may need dose adjustment).
- Carbamazepine / Phenytoin: teratogenic (neural tube defects with carbamazepine; fetal hydantoin syndrome with phenytoin) — avoid if possible.
- Labour and delivery: Myotonia is NOT a contraindication to vaginal delivery. However, anaesthetists must be informed (avoid succinylcholine if general anaesthesia needed). Regional anaesthesia (epidural/spinal) is safe and preferred.
- Patients with the AR (Becker) form who have significant transient weakness at the onset of movement may benefit from pharmacotherapy even if the myotonia itself is tolerable, because the weakness can cause falls and injuries.
- The warm-up technique is particularly important in this group — "always start slow."
| Aspect | Details |
|---|---|
| Life expectancy | Normal — MC does not shorten life |
| Progression | Non-progressive — the degree of myotonia remains relatively stable over the lifespan (unlike DM1 which worsens) |
| Functional outcome | Most patients lead fully independent lives with appropriate management. Many are physically fit or even athletic (muscle hypertrophy is an advantage in some sports) |
| Complications | Rare — mainly related to falls (Becker MC with transient weakness) or anaesthetic incidents (if succinylcholine given inadvertently) |
High Yield Summary — Management of Myotonia Congenita
- No cure exists — management is symptomatic and supportive.
- Non-pharmacological measures are first-line: warm-up technique, cold avoidance, physiotherapy, occupational therapy, patient education.
- Mexiletine is the first-line drug (Class IB Na⁺ channel blocker, use-dependent). Start 150 mg OD/BD, max 200 mg TDS. Baseline ECG mandatory — monitor QTc.
- Lamotrigine is the preferred second-line (no cardiac risk, familiar paediatric safety profile).
- Historical options include procainamide, phenytoin, disopyramide, nifedipine, quinine sulphate [2] — mostly superseded by mexiletine and lamotrigine.
- Succinylcholine is absolutely contraindicated — causes life-threatening myotonic crisis. Non-depolarizing agents are safe.
- MC is NOT malignant hyperthermia — different gene (CLCN1 vs RYR1), different mechanism, different treatment.
- Genetic counselling is essential (AD for Thomsen, AR for Becker).
- Prognosis is excellent — normal life expectancy, non-progressive, most patients function independently.
Active Recall - Management of Myotonia Congenita
References
[2] Senior notes: Ryan Ho Neurology.pdf (pp. 193–194: Non-dystrophic myotonic syndromes management — procainamide, phenytoin, disopyramide, nifedipine, quinine sulphate; malignant hyperthermia) [5] Senior notes: Maksim Medicine Notes.pdf (p. 276: Myopathy management — no specific treatment, supportive care, PT, OT, genetic counselling)
Complications of Myotonia Congenita
Before listing complications, it is essential to frame the discussion. Myotonia congenita is fundamentally a benign, non-progressive channelopathy. The muscle fibre is structurally intact — there is no degeneration, no fibrosis, no necrosis. The heart, brain, endocrine organs, and eyes are all unaffected (unlike myotonic dystrophy). Therefore, the complication profile of MC is remarkably limited compared to other neuromuscular disorders.
Most "complications" of MC are functional consequences of myotonia itself or iatrogenic (related to drug treatment or anaesthesia). There are no life-threatening systemic complications intrinsic to the disease.
Myotonia congenita: AD/AR inheritance, due to voltage-gated Cl⁻ channel mutation (7q35) [2] S/S: infancy/childhood onset, generalized myotonia which ↑ in cold and ↓ by warmth/exercise, minimal weakness [2]
1. Musculoskeletal Complications
| Aspect | Details |
|---|---|
| Mechanism | Myotonia causes muscle stiffness at the initiation of movement. When a patient tries to stand up suddenly, start walking from rest, or respond to an unexpected event (e.g., stepping off a curb), the stiff muscles cannot respond quickly enough → the patient falls. In Becker MC, the transient weakness at the onset of movement compounds this risk — the legs are both stiff AND briefly weak. |
| Why it happens from first principles | At rest, the hyperexcitable membrane (due to reduced Cl⁻ conductance) has not undergone the warm-up process. The first contraction meets maximal membrane instability → intense myotonia → the patient's legs "lock up" → they topple forward. In Becker MC, the severe Cl⁻ loss causes massive initial depolarization that transiently inactivates Na⁺ channels → brief weakness precedes the warm-up. |
| Types of injury | Fractures (wrist, hip — from falls), dental injuries (facial falls), lacerations, head injuries. Children are particularly vulnerable (playground falls, sports). |
| Prevention | Teach the warm-up technique ("always start slow"); avoid sudden explosive movements; fall prevention strategies (OT assessment, home modifications if needed); consider pharmacotherapy if falls are recurrent. |
Clinical Pearl: Falls in Becker MC
Falls are the single most common "complication" leading to medical attention in Becker MC. The combination of stiffness + transient weakness at movement onset is a recipe for stumbles. This is a legitimate reason to start mexiletine even if the patient otherwise tolerates the myotonia — preventing fractures is a valid indication.
| Aspect | Details |
|---|---|
| Mechanism | Chronic involuntary repetitive muscle fibre activation acts as a form of isometric exercise → work hypertrophy of type 2 fibres. This produces the characteristic "Herculean" build. |
| Why this is usually NOT a complication | Most patients view their muscular build positively. Some MC patients are even competitive athletes. The hypertrophy reflects healthy, functioning muscle. |
| When it becomes a problem | Rarely, extreme hypertrophy of specific muscle groups (e.g., calves, masseter) can cause cosmetic concern or functional limitation. Masseter hypertrophy may contribute to temporomandibular joint (TMJ) strain. In children, prominent calf hypertrophy may initially be mistaken for Duchenne muscular dystrophy (pseudohypertrophy), causing diagnostic confusion and parental anxiety. |
| Aspect | Details |
|---|---|
| Mechanism | A small proportion of patients with Becker (AR) MC develop mild fixed proximal weakness later in life (typically after the 4th–5th decade). This is thought to result from decades of chronic hyperexcitability causing low-grade cumulative fibre damage, though the mechanism is incompletely understood. |
| Clinical significance | Generally mild (MRC grade 4/5 at worst); does NOT approach the severity of dystrophic myopathies; does not cause respiratory failure or loss of ambulation. CK may be mildly elevated during these episodes. |
| Contrast with DM1 | In DM1, progressive weakness is the hallmark and leads to significant disability. In MC, any late weakness is minimal and should prompt reconsideration of the diagnosis if severe. |
2. Anaesthetic Complications
This is arguably the most clinically important category of complications because it is preventable and potentially life-threatening.
| Aspect | Details |
|---|---|
| Mechanism | Succinylcholine (suxamethonium) is a depolarizing neuromuscular blocking agent used for rapid-sequence intubation. It works by depolarizing the motor endplate. In a patient with MC, this depolarization spreads across a hyperexcitable sarcolemma → massive, sustained myotonic contraction of all skeletal muscles. |
| Clinical features | Masseter spasm (rigid jaw — cannot open mouth for intubation); generalized rigidity; inability to ventilate (chest wall rigidity); risk of rhabdomyolysis → myoglobinuria → acute kidney injury; hyperkalaemia (K⁺ released from damaged/contracted muscle) → cardiac arrhythmia → cardiac arrest. |
| Why this differs from malignant hyperthermia | In MH (RYR1 mutation), volatile anaesthetics trigger uncontrolled Ca²⁺ release from SR → sustained contraction + hypermetabolism + hyperthermia. In MC (CLCN1 mutation), the problem is purely sarcolemmal hyperexcitability → myotonic stiffness. There is NO hypermetabolic state, NO true hyperthermia (unless secondary to prolonged sustained contraction), and dantrolene is NOT effective (dantrolene blocks RYR1, which is normal in MC). |
| Prevention | Absolute avoidance of succinylcholine in all MC patients. Use non-depolarizing agents (rocuronium, vecuronium, cisatracurium). All MC patients should carry a MedAlert bracelet or anaesthetic alert card. |
| Management if it occurs | Stop succinylcholine immediately; supportive ventilation (may require emergency tracheostomy if jaw rigid); IV fluids for rhabdomyolysis prevention; monitor and treat hyperkalaemia (IV calcium gluconate, insulin-dextrose, nebulized salbutamol); monitor renal function; consider IV mexiletine or IV phenytoin to reduce myotonic stiffness (limited evidence). |
Exam Must-Know: Succinylcholine is Absolutely Contraindicated
This is the single most testable complication of myotonia congenita. If asked "What is the most dangerous complication?", the answer is succinylcholine-induced myotonic crisis during general anaesthesia. It is preventable by using non-depolarizing agents. MC patients are NOT at risk of malignant hyperthermia (different gene, different channel).
| Aspect | Details |
|---|---|
| Mechanism | The operating theatre is typically cold (18–22°C). IV fluids may be unwarmed. Cold worsens ClC-1 dysfunction (slows channel kinetics of already reduced Cl⁻ channels) and enhances Na⁺ channel recovery from inactivation → more repetitive firing → increased myotonia. |
| Clinical consequence | Increased muscle stiffness during surgery → difficulty with positioning, surgical access, and mechanical ventilation (chest wall rigidity). |
| Prevention | Maintain normothermia: warm IV fluids, forced-air warming blankets, warm operating theatre. Alert the anaesthetic and surgical team pre-operatively. |
| Aspect | Details |
|---|---|
| Mechanism | After general anaesthesia, the patient may experience exaggerated myotonia upon emergence — due to cold exposure, stress hormones (catecholamine surge on waking), and lack of warm-up during the period of immobility. |
| Clinical consequence | Difficulty moving, shivering (which can itself trigger myotonia), patient distress. |
| Prevention | Maintain warmth; allow gradual emergence; inform recovery room staff; reassure patient. |
These are often under-recognized but significantly impact quality of life.
| Complication | Mechanism / Details |
|---|---|
| Impaired motor function in daily life | Grip myotonia → difficulty opening jars, turning keys, shaking hands, using tools, typing. Leg myotonia → difficulty starting to walk, climbing stairs, getting up from chairs. Eyelid myotonia → difficulty opening eyes quickly (can impair driving safety). These are the primary source of disability in MC. |
| Impaired sports performance | Despite their muscular build, MC patients may underperform in sports requiring sudden bursts of activity (sprinting, ball-catching, combat sports). After the warm-up period they may perform well, but the initial stiffness is a handicap. |
| Psychosocial impact in children | Children with MC may be teased for appearing "clumsy" or "slow." They may avoid physical activities or be excluded from sports. This can affect self-esteem, peer relationships, and school participation. Teacher and peer education is essential. |
| Anxiety and frustration | Adults may feel frustrated by the unpredictability of stiffness (especially in cold weather or after rest). Anxiety about falling in public, difficulty with handshakes in professional settings, or concerns about passing the condition to children. |
| Occupational limitations | Certain occupations requiring rapid reflexive movements (e.g., firefighting, military combat, some forms of manual labour) may be challenging. Career counselling may be needed. |
| Driving safety | If eyelid or leg myotonia is severe and causes impaired ability to react quickly (e.g., sudden braking), driving safety may be a concern. In most jurisdictions, MC per se is not a bar to driving, but patients should be counselled. |
These arise from the pharmacotherapy used to treat MC.
| Drug | Complication | Mechanism | Prevention |
|---|---|---|---|
| Mexiletine | QT prolongation → Torsades de Pointes → sudden cardiac death | Mexiletine blocks cardiac Na⁺ channels (Class IB antiarrhythmic) → can prolong QT in susceptible individuals, especially in combination with other QT-prolonging drugs | Baseline ECG; monitor QTc; avoid in pre-existing long QT or structural heart disease; avoid co-prescription with other QT-prolonging agents (e.g., macrolides, antipsychotics, fluoroquinolones) |
| Mexiletine | GI intolerance (nausea, dyspepsia) | Direct GI irritant effect; dose-dependent | Take with food; start at low dose; dose titrate gradually |
| Carbamazepine | Hyponatraemia (SIADH) | Carbamazepine stimulates ADH release and potentiates ADH action on collecting ducts → water retention → dilutional hyponatraemia | Monitor serum Na⁺ every 3–6 months; educate patient about symptoms (confusion, nausea, seizures) |
| Carbamazepine | SJS/TEN (Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis) | HLA-B*15:02 mediated hypersensitivity — particularly prevalent in Han Chinese and Southeast Asian populations | HLA-B*15:02 screening mandatory before use in Chinese patients — if positive, carbamazepine is contraindicated |
| Carbamazepine | Bone marrow suppression (agranulocytosis, aplastic anaemia) | Direct bone marrow toxicity (rare) | Monitor FBC periodically |
| Phenytoin | Gingival hypertrophy, hirsutism, cerebellar toxicity, osteomalacia | Fibroblast stimulation (gingiva), altered vitamin D metabolism (osteomalacia), cerebellar neurotoxicity at supratherapeutic levels; zero-order kinetics → small dose changes can cause large level changes | Monitor drug levels; dental hygiene counselling; vitamin D supplementation |
| Quinine sulphate | Cinchonism (tinnitus, nausea, visual disturbance), QT prolongation, thrombocytopenia | Direct ototoxicity and cardiac Na⁺/K⁺ channel blockade; immune-mediated platelet destruction | Largely fallen out of favour; avoid in modern practice |
Understanding what does NOT happen in MC is as important as knowing what does — especially to distinguish MC from myotonic dystrophy.
| Complication | Occurs in DM1? | Occurs in MC? | Why Not in MC? |
|---|---|---|---|
| Cataracts | Yes (posterior subcapsular) | No | Cataracts in DM1 are due to RNA toxicity affecting lens crystallins. In MC, the defect is limited to the skeletal muscle Cl⁻ channel → lens is unaffected. |
| Cardiac conduction defects (heart block, arrhythmias) | Yes (major cause of death) | No | ClC-1 is expressed almost exclusively in skeletal muscle, NOT in cardiac muscle. Cardiac muscle has different Cl⁻ channel subtypes. Therefore, the CLCN1 mutation does not affect cardiac electrophysiology. |
| Cardiomyopathy | Yes (dilated) | No | Same reason — cardiac muscle is not affected by CLCN1 mutations. |
| Respiratory failure | Yes (diaphragm/respiratory muscle weakness) | No | MC does not cause progressive muscle degeneration or respiratory muscle weakness. |
| Cognitive decline | Yes | No | CLCN1 is not expressed in the CNS. The RNA toxicity mechanism of DM1 (which affects neuronal function) is absent in MC. |
| Endocrinopathy (DM, testicular atrophy) | Yes | No | No multisystem RNA toxicity in MC. |
| Reduced life expectancy | Yes | No | Life expectancy in MC is normal. |
Multisystem involvement [in DM1], incl.: [1][2]
- Eyes: cataract
- Cardiac: cardiomyopathy, conduction disorders
- Endocrinopathy: DM, testicular atrophy
- Smooth muscle disorder: e.g. gut motility disorder, constipation, poor bladder emptying
- Cognitive dysfunction
High Yield: The Absence of Systemic Complications Is a Key Distinguishing Feature
If a patient with myotonia develops cataracts, cardiac conduction defects, or cognitive decline — reconsider the diagnosis. These features are inconsistent with MC and should prompt genetic testing for myotonic dystrophy (DM1 or DM2). The absence of systemic complications is what makes MC a benign condition with normal life expectancy.
| Category | Complication | Severity | Preventable? |
|---|---|---|---|
| Musculoskeletal | Falls and traumatic injury | Moderate (fractures possible) | Yes — warm-up technique, pharmacotherapy |
| Musculoskeletal | Fixed late myopathy (Becker, rare) | Mild | No — inherent to disease |
| Anaesthetic | Succinylcholine-induced myotonic crisis | Life-threatening | Yes — absolute avoidance of succinylcholine |
| Anaesthetic | Cold-induced intraoperative stiffness | Moderate | Yes — maintain normothermia |
| Functional | Impaired ADLs, sports, occupation | Variable | Partially — warm-up, pharmacotherapy, OT |
| Psychosocial | Low self-esteem, anxiety, social isolation (especially children) | Variable | Yes — education, counselling, peer support |
| Iatrogenic | Mexiletine QT prolongation | Potentially serious | Yes — ECG monitoring, avoid QT-prolonging co-medications |
| Iatrogenic | Carbamazepine SJS/TEN | Severe | Yes — HLA-B*15:02 screening in Chinese |
High Yield Summary — Complications of Myotonia Congenita
- MC is a benign condition — complications are few and mostly functional or iatrogenic.
- Falls are the most common complication (especially in Becker MC with transient weakness) → fractures, head injuries.
- Succinylcholine-induced myotonic crisis is the most dangerous complication — life-threatening but completely preventable by using non-depolarizing agents. All patients need a MedAlert bracelet.
- MC is NOT malignant hyperthermia — different gene, different channel, dantrolene does not help.
- No systemic complications: no cataracts, no cardiac conduction defects, no cardiomyopathy, no respiratory failure, no cognitive decline, no endocrinopathy. If these appear → rethink the diagnosis (likely DM1).
- Drug complications: mexiletine → QT prolongation; carbamazepine → hyponatraemia, SJS/TEN (HLA-B*15:02 screening in Chinese); phenytoin → gingival hypertrophy, cerebellar toxicity.
- Life expectancy is normal.
- Psychosocial impact (especially in children) is real and should not be neglected — education, counselling, and peer support are important.
Active Recall - Complications of Myotonia Congenita
References
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p. 145: Myotonic dystrophy multisystem features) [2] Senior notes: Ryan Ho Neurology.pdf (pp. 193–194: Non-dystrophic myotonic syndromes, ion channelopathies, malignant hyperthermia)
High Yield Summary
Myotonia Congenita — Key Points for Exams:
- Definition: Non-dystrophic hereditary myotonia due to loss-of-function mutations in CLCN1 (voltage-gated Cl⁻ channel, chromosome 7q35).
- Two forms: Thomsen (AD, milder, earlier onset) and Becker (AR, more severe, later onset, transient weakness).
- Pathophysiology: Reduced Cl⁻ conductance → membrane hyperexcitability → repetitive action potentials → delayed relaxation (myotonia). Minor leakage → myotonia; major leakage → paralysis.
- Cardinal symptom: Muscle stiffness (NOT weakness). Strength is preserved or even enhanced (muscle hypertrophy).
- Warm-up phenomenon: Myotonia improves with repeated contraction — this distinguishes MC from paramyotonia congenita (where myotonia paradoxically worsens).
- Cold worsening: Myotonia increases in cold temperatures.
- No multisystem features: Unlike myotonic dystrophy, there are NO cataracts, cardiac conduction defects, endocrinopathy, or cognitive decline.
- Signs: Grip myotonia, percussion myotonia, eyelid myotonia, muscle hypertrophy ("Herculean build"), normal reflexes, no wasting.
- Prognosis: Generally benign; life expectancy is normal. The stiffness is the main source of disability.
- Channelopathy framework: Cl⁻ channel → MC; Na⁺ channel → paramyotonia congenita / hyperK PP; Ca²⁺ channel → hypoK PP / malignant hyperthermia.
High Yield Summary — DDx of Myotonia Congenita
- Always confirm true myotonia (EMG myotonic discharges) — exclude pseudomyotonia (hypothyroidism), contracture (McArdle), and neuromyotonia (Isaacs).
- The #1 DDx is myotonic dystrophy (DM1 > DM2) — look for progressive weakness, wasting, cataracts, cardiac, endocrine, and cognitive features. If ANY of these are present, it is NOT MC.
- Among non-dystrophic myotonias, the main DDx is paramyotonia congenita — distinguished by paradoxical myotonia (worsens with use) and Na⁺ channel gene (SCN4A).
- HyperK periodic paralysis can have inter-ictal myotonia but is distinguished by episodic flaccid weakness with elevated K⁺.
- Drug-induced myotonia (statins, chloroquine) is acquired and reversible — always take a drug history.
- Warm-up phenomenon = MC; paradoxical worsening = PMC — this is the bedside differentiator.
High Yield Summary — Diagnosis of Myotonia Congenita
- No formal diagnostic criteria exist — diagnosis is a clinical-electrophysiological-genetic triad.
- Bedside tests: grip myotonia, percussion myotonia, eyelid myotonia, warm-up phenomenon.
- CK: Normal or mildly elevated (< 2–3× ULN) — muscle structure is intact, unlike dystrophies.
- EMG: Myotonic discharges (waxing/waning, "dive-bomber" sound) with normal voluntary MUAPs. This pattern = non-dystrophic myotonia.
- NCS: Normal — confirms the problem is in the muscle membrane, not the nerve.
- Genetic testing: CLCN1 mutation analysis is the gold standard. AD = Thomsen, AR = Becker. NGS gene panels are now preferred.
- Exclude DM1: Slit-lamp (cataracts), ECG (conduction defects), echo (cardiomyopathy), HbA1c, gonadal hormones, cognitive screen — all should be NORMAL in MC.
- Muscle biopsy: Rarely needed; non-specific findings (no dystrophy, type 2 fibre hypertrophy).
- TFT: Must be checked to exclude hypothyroid pseudomyotonia.
High Yield Summary — Management of Myotonia Congenita
- No cure exists — management is symptomatic and supportive.
- Non-pharmacological measures are first-line: warm-up technique, cold avoidance, physiotherapy, occupational therapy, patient education.
- Mexiletine is the first-line drug (Class IB Na⁺ channel blocker, use-dependent). Start 150 mg OD/BD, max 200 mg TDS. Baseline ECG mandatory — monitor QTc.
- Lamotrigine is the preferred second-line (no cardiac risk, familiar paediatric safety profile).
- Historical options include procainamide, phenytoin, disopyramide, nifedipine, quinine sulphate [2] — mostly superseded by mexiletine and lamotrigine.
- Succinylcholine is absolutely contraindicated — causes life-threatening myotonic crisis. Non-depolarizing agents are safe.
- MC is NOT malignant hyperthermia — different gene (CLCN1 vs RYR1), different mechanism, different treatment.
- Genetic counselling is essential (AD for Thomsen, AR for Becker).
- Prognosis is excellent — normal life expectancy, non-progressive, most patients function independently.
High Yield Summary — Complications of Myotonia Congenita
- MC is a benign condition — complications are few and mostly functional or iatrogenic.
- Falls are the most common complication (especially in Becker MC with transient weakness) → fractures, head injuries.
- Succinylcholine-induced myotonic crisis is the most dangerous complication — life-threatening but completely preventable by using non-depolarizing agents. All patients need a MedAlert bracelet.
- MC is NOT malignant hyperthermia — different gene, different channel, dantrolene does not help.
- No systemic complications: no cataracts, no cardiac conduction defects, no cardiomyopathy, no respiratory failure, no cognitive decline, no endocrinopathy. If these appear → rethink the diagnosis (likely DM1).
- Drug complications: mexiletine → QT prolongation; carbamazepine → hyponatraemia, SJS/TEN (HLA-B*15:02 screening in Chinese); phenytoin → gingival hypertrophy, cerebellar toxicity.
- Life expectancy is normal.
- Psychosocial impact (especially in children) is real and should not be neglected — education, counselling, and peer support are important.
Becker Muscular Dystrophy
Becker muscular dystrophy is an X-linked recessive muscular dystrophy caused by mutations in the dystrophin gene that produce a partially functional but reduced-quantity dystrophin protein, resulting in progressive proximal muscle weakness with later onset and slower progression than Duchenne muscular dystrophy.
Paramyotonia Congenita
Paramyotonia congenita is a rare autosomal dominant skeletal muscle sodium channelopathy characterized by cold-induced myotonia and paradoxical worsening of muscle stiffness with repeated activity, often followed by episodic weakness.