EMERGENCY:Patients with Myotonia Congenita undergoing surgery — suxamethonium (succinylcholine) is CONTRAINDICATED. Alert the anaesthetic team before any procedure —call040-46324365immediately — 24 hours, 7 days— 24/7
Myotonia Congenita in Hyderabad: Muscle Stiffness That Loosens Up With Use — Diagnosing the Warm-Up Phenomenon and Separating It From Myotonic Dystrophy
Expert clinical assessment, EMG, CLCN1 genetic testing, and mexiletine treatment for non-dystrophic myotonia at KIMS Neuroscience, Secunderabad, Hyderabad.
Myotonia Congenita is a genetic channelopathy of the skeletal muscle chloride channel (ClC-1, encoded by the CLCN1 gene on chromosome 7q35). It causes myotonia — delayed muscle relaxation after contraction — without progressive weakness, cardiac involvement, or other systemic features. The warm-up phenomenon is the clinical hallmark: muscle stiffness is maximal after a period of rest and improves within seconds to minutes of repeated movement, as the muscle's electrical hyperexcitability gradually normalises during sustained activity. Two genetic forms: (1) Thomsen disease (autosomal dominant, usually milder, from early childhood, no significant weakness or hypertrophy); (2) Becker myotonia congenita (autosomal recessive, more severe, transient weakness on first contractions that resolves with warm-up, visible muscle hypertrophy especially calves). MC is distinguished from Myotonic Dystrophy (DM1) by the ABSENCE of progressive weakness, cardiac arrhythmias, cataracts, diabetes, cognitive slowing, and multi-system involvement. If any of these features are present in a patient with myotonia, DM1 must be excluded with CTG repeat sizing before MC is diagnosed. Treatment: membrane-stabilising medications (mexiletine first-line, carbamazepine second-line). Mexiletine requires a baseline ECG before starting (QT prolongation risk). Succinylcholine CONTRAINDICATED (severe sustained myotonic contraction, compromising airway management).
- Warm-up phenomenon — the defining feature (molecular mechanism)
- Muscle stiffness is maximal after rest (sitting, sleeping, inactivity) and improves within seconds to minutes of repeated movement. Molecular mechanism: in Myotonia Congenita, the ClC-1 chloride channel is dysfunctional. ClC-1 accounts for approximately 80% of the resting membrane's electrical conductance and rapidly repolarises the sarcolemma after each contraction. Without functional ClC-1, the membrane fires repeated spontaneous action potentials after a single voluntary contraction (myotonia). During exercise, potassium accumulates between muscle fibres and adaptive sodium channel changes gradually restore membrane excitability to near-normal levels. The warm-up phenomenon is this gradual biochemical normalisation during sustained activity.
- Thomsen vs Becker — the two genetic forms
- THOMSEN DISEASE (autosomal dominant): typically milder myotonia present from early childhood; no significant weakness; no visible muscle hypertrophy; the affected parent has the same condition. BECKER MYOTONIA CONGENITA (autosomal recessive): more severe; characteristic transient weakness on first contractions that resolves completely with warm-up (the brief weakness that disappears, unlike the fixed weakness of muscular dystrophy); visible muscle hypertrophy, especially the calves, quadriceps, and forearms (the muscle fibres hypertrophy in response to the sustained electrical activity of myotonia). Both are caused by CLCN1 mutations; the inheritance pattern differentiates them.
- MC vs DM1 — the critical separation at KIMS
- Both MC and DM1 cause myotonia. MC causes ONLY myotonia. DM1 causes myotonia PLUS: progressive muscle wasting and weakness, cardiac conduction defects (which can cause sudden death), respiratory failure, posterior subcapsular cataracts, insulin resistance/diabetes, excessive daytime sleepiness, cognitive slowing, and GI dysmotility. At KIMS: any patient presenting with myotonia is asked about, and examined for, the DM1 multi-system features. ANY cardiac symptom, cataract, cognitive slowing, or first-degree relative who died suddenly from a cardiac event = CTG repeat sizing in DMPK is performed before the diagnosis of pure Myotonia Congenita is accepted.
- Succinylcholine CONTRAINDICATED — the anaesthetic emergency
- Succinylcholine (suxamethonium), the depolarising muscle relaxant commonly used in India for rapid-sequence induction of anaesthesia, triggers severe and sustained myotonic contraction in Myotonia Congenita patients. This sustained contraction: (1) makes intubation impossible (the jaw and throat muscles contract continuously); (2) generates massive heat and can trigger rhabdomyolysis; (3) can compromise the airway and prevent mask ventilation. The consequence can be a "can't intubate, can't oxygenate" airway emergency. At KIMS, every MC patient receives an anaesthetic alert card. Any planned surgical procedure must be communicated to the anaesthetic team BEFORE the procedure to allow alternative agents and trigger-free protocols.
- Mexiletine — first-line anti-myotonic with the ECG precondition
- Mexiletine (sodium channel blocker) is the most evidence-based anti-myotonic agent for both Myotonia Congenita and Myotonic Dystrophy. It reduces the abnormal sarcolemmal electrical excitability underlying myotonia. Important precondition at KIMS: baseline ECG is required before starting mexiletine because it can prolong the QT interval and produce proarrhythmic effects. This is particularly important in DM1 patients (where cardiac conduction disease coexists) but also relevant in MC patients with any baseline cardiac history. Carbamazepine is the alternative when mexiletine is not available or tolerated.
- Cold worsens myotonia — the Hyderabad AC problem
- Myotonia Congenita worsens significantly in cold temperatures. Reduced temperature slows membrane repolarisation and exacerbates muscle hyperexcitability. Patients note that their stiffness is dramatically worse in cold water (swimming), in air-conditioned offices and vehicles, and in air-conditioned operating theatres. In Hyderabad, where indoor air conditioning is widespread, this cold-sensitivity has practical consequences: MC patients learn to avoid prolonged exposure to heavily air-conditioned environments or to exercise during such exposure to maintain the warm-up effect. At KIMS, cold-sensitivity is specifically discussed in the management counselling as a practical lifestyle point.
- Percussion myotonia — the bedside diagnostic test
- Percussion of the thenar eminence (the muscle mass at the base of the thumb) with a reflex hammer produces a visible and sustained dimple or mound as the muscle contracts and fails to immediately relax. This is percussion myotonia — a simple, reliable bedside test that can be performed at the initial clinical assessment without any equipment beyond a reflex hammer. In Myotonia Congenita, percussion myotonia is present in virtually all affected individuals. In Myotonic Dystrophy, it is also present but may be milder.
About This Condition
The Condition Where the First Movement Is the Hardest
A 16-year-old boy in Hyderabad has been complaining for several years that his legs are stiff and almost unresponsive when he first stands up from his seat. He stumbles when he starts walking. If he sits still through a class at school and is then asked to stand quickly, he freezes momentarily — unable to move his legs. His classmates think he is clumsy or acting. After walking for a few seconds, the stiffness disappears and he moves normally.
He has noticed the same pattern when he closes his fist — the first clench is tight and releasing it is difficult, but after opening and closing his hand several times it moves freely. His general physician has tested his thyroid (normal), his CK (mildly elevated at 280 U/L), and arranged physiotherapy for what was labelled “muscle cramps”. The physiotherapy has not helped.
At KIMS Neuroscience, the symptom pattern is recognised immediately. Stiffness that is worst after rest and improves within seconds to minutes of repeated movement is the warm-up phenomenon — the clinical hallmark of myotonia.
Importantly, Myotonic Dystrophy also causes progressive muscle wasting, cardiac arrhythmias that can produce sudden death, respiratory failure, cataracts, diabetes, and cognitive slowing. At KIMS, the two diagnoses are always separated explicitly by clinical examination, EMG pattern, and genetic testing before any management is commenced.
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040-44885000Surgical / anaesthetic emergency: 040-46324365The Biology of Myotonia Congenita — A Chloride Channel Problem
The skeletal muscle membrane (sarcolemma) contains multiple types of ion channels that control the electrical excitability of the muscle. In normal muscle, after an action potential triggers contraction, the membrane rapidly repolarises and the muscle relaxes. The chloride channel ClC-1 — encoded by the CLCN1 gene — accounts for approximately 80% of the resting membrane's electrical conductance and plays a critical role in stabilising the membrane after each contraction.
When ClC-1 is dysfunctional, the sarcolemma becomes hyperexcitable — it fires repeated spontaneous action potentials after a single voluntary contraction instead of returning to its resting state. This electrical storm in the muscle membrane is myotonia: the muscle cannot relax because the membrane is firing continuously.
Why repeated movement helps
During exercise, potassium ions accumulate in the small space between muscle fibres. In normal muscle, ClC-1 channels counteract this accumulation. In Myotonia Congenita, the dysfunctional ClC-1 channels cannot perform this initially — but as exercise continues, other adaptive mechanisms (including changes in sodium channel activity) gradually reduce membrane hyperexcitability. The muscle biochemistry normalises during sustained activity. This is the warm-up phenomenon at the molecular level.
Two genetic forms — Thomsen and Becker
Thomsen disease (autosomal dominant) usually produces milder stiffness from early childhood with no significant weakness and no muscle hypertrophy. Becker myotonia congenita (autosomal recessive) is generally more severe — with transient weakness on first contractions that resolves with warm-up, and often visible muscle hypertrophy, especially of the calf muscles.
The distinction that changes everything
Myotonic Dystrophy is caused by a CTG repeat expansion in DMPK and produces multi-system involvement — progressive weakness, cardiac conduction disease (which can cause sudden death), respiratory failure, cataracts, insulin resistance, cognitive slowing. Myotonia Congenita produces ONLY myotonia. At KIMS, any cardiac symptom, cataract, endocrine abnormality, or family history of sudden cardiac death in a patient with myotonia prompts CTG repeat sizing — never assumed to be Myotonia Congenita without this exclusion.
Clinical Features — What Myotonia Congenita Feels Like
Myotonia presents in patterns that are highly specific once you know what to look for. These are the features that lead to a clinical diagnosis at KIMS — before genetic testing confirms it.
The warm-up phenomenon — the defining symptom
Muscle stiffness that is maximal after rest (sitting, sleeping, inactivity) and that diminishes progressively within seconds to minutes of repeated movement. The first few steps after sitting are rigid; after a few seconds of walking, movement normalises. The first clench of the fist is stuck; after several openings and closings, the hand moves freely. This pattern is highly specific to myotonia and should be specifically asked about in any patient with muscle stiffness.
Grip myotonia and percussion myotonia
Grip myotonia: the patient shakes hands with the examiner and cannot release the handshake promptly — the fingers remain flexed for 1–3 seconds before relaxing. Percussion myotonia: tapping the thenar eminence (thumb muscle pad) with a reflex hammer produces a sustained dimple — the muscle contracts and remains contracted for several seconds before relaxing. This is the most specific bedside sign of a chloride or sodium channelopathy.
Muscle hypertrophy — the "Hercules" appearance
In Becker myotonia congenita (autosomal recessive form), persistent muscle hyperactivity from the repeated involuntary contractions leads to muscle hypertrophy — particularly prominent in the calf muscles, thighs, and forearms. The patient may appear unusually muscular. This is genuine hypertrophy (increased muscle fibre bulk) — not pseudohypertrophy from fat infiltration as in Duchenne MD. The hypertrophied muscles are strong between myotonic episodes.
Transient weakness in Becker MC
In Becker myotonia congenita there is a characteristic transient weakness at the initiation of movement following prolonged rest — in addition to the stiffness. The patient cannot stand at all for the first second or two after prolonged sitting. This resolves completely with warm-up and does not represent true progressive myopathy. It is caused by accumulation of intracellular sodium during the myotonic discharge, temporarily reducing the muscle's excitability below threshold.
Eyelid myotonia and facial stiffness
Closing the eyes forcefully (as in crying or squeezing the eyes shut) can produce eyelid myotonia — difficulty opening the eyes promptly after the closure. Facial stiffness may be noted when smiling, chewing, or talking after a period of facial rest. This is more prominent in paramyotonia congenita (an SCN4A-related condition aggravated by cold) than in typical Myotonia Congenita, but can occur in both.
Cold sensitivity — variable but relevant in India
Cold exposure worsens myotonia in many Myotonia Congenita patients. This can be relevant in air-conditioned environments (offices, hospitals, vehicles), cold-water exposure during ablutions in winter mornings, and certain occupational settings. Cold sensitivity is much more prominent in Paramyotonia Congenita (SCN4A mutations) than in Myotonia Congenita (CLCN1 mutations) — its presence helps distinguish the two conditions at KIMS.
Myotonia Congenita vs Myotonic Dystrophy vs Paramyotonia Congenita
All three conditions cause myotonia — but only one is benign. Distinguishing them changes everything about monitoring, treatment, and prognosis.
| Feature | Myotonia Congenita | Myotonic Dystrophy (DM1) | Paramyotonia Congenita |
|---|---|---|---|
| Gene | CLCN1 (chloride channel) | DMPK (CTG repeat) | SCN4A (sodium channel) |
| Inheritance | AR (Becker) or AD (Thomsen) | Autosomal dominant | Autosomal dominant |
| Myotonia | Yes — dominant symptom | Yes — but weakness may dominate | Yes — worse with cold |
| Warm-up | Classic — improves with use | Present but less prominent | WORSENS with repeated use in cold |
| Muscle weakness | None (Thomsen) or transient only (Becker) | Progressive — distal and proximal | Periodic — can develop permanent weakness |
| Cardiac risk | None | High — arrhythmias and SCD risk | Low |
| Cataracts | None | Yes — posterior subcapsular | None |
| Cold sensitivity | Mild to moderate | Mild | Severe — key diagnostic feature |
| Muscle hypertrophy | Yes (Becker form) | No — wasting predominates | No |
| EMG | Myotonic discharges; waning with warm-up | Myotonic discharges | Myotonic discharges; worsening with cold |
| Life expectancy | Normal | Reduced (cardiac/respiratory) | Near-normal |
Diagnosis at KIMS
Diagnosis at KIMS — From Bedside Warm-Up Test to CLCN1 Sequencing
The KIMS diagnostic pathway moves from clinical examination, to EMG, to ECG, to a single channelopathy gene panel that simultaneously confirms Myotonia Congenita and excludes the two conditions it can be confused with — Myotonic Dystrophy and Paramyotonia Congenita.
The clinical, electrophysiological, and genetic findings are always considered together — never in isolation. This is what allows confident family counselling, safe long-term medication planning, and an explicit anaesthetic alert for every confirmed patient.
Clinical warm-up test — the first diagnostic step
The KIMS assessment begins with a structured clinical warm-up test: ask the patient to make a tight fist for 5–10 seconds and time how long it takes to open; repeat 5 times in succession and observe whether the time to open improves; ask the patient to squeeze their eyes closed tightly and then open them quickly; test grip myotonia by shaking the patient's hand and observing delayed release. A positive warm-up test — where the first attempt shows difficulty that improves within 5 repetitions — is strongly diagnostic of myotonia.
Serum CK
CK in Myotonia Congenita is normal or mildly elevated — typically 1–4 times the upper limit of normal. A CK significantly above 4 times normal in a patient with myotonia should prompt consideration of Myotonic Dystrophy, LGMD, or inflammatory myopathy. The mild CK elevation in MC reflects the persistent low-level muscle activity from myotonic discharges, not progressive muscle destruction.
EMG — the dive-bomber discharge
EMG produces the characteristic myotonic discharge on needle insertion and voluntary contraction: a high-frequency burst of electrical activity from the muscle membrane that waxes and wanes in amplitude and frequency, producing the distinctive dive-bomber sound on the audio output. EMG in MC also shows that myotonic discharges DECREASE with repeated stimulation (the warm-up at the electrical level) — whereas in Paramyotonia Congenita, discharges increase with cooling.
ECG — to exclude cardiac involvement
A resting 12-lead ECG is arranged at KIMS for every patient presenting with myotonia, before the diagnosis of Myotonia Congenita is confirmed. The purpose: to exclude the cardiac conduction abnormalities that characterise Myotonic Dystrophy. A normal ECG in the context of isolated myotonia without systemic features supports Myotonia Congenita, but genetic testing is still required for confirmation.
CLCN1 genetic testing — definitive confirmation
Next-generation sequencing of the CLCN1 gene confirms the diagnosis and defines the inheritance pattern (heterozygous = Thomsen dominant; homozygous or compound heterozygous = Becker recessive). At KIMS, the channelopathy gene panel includes CLCN1, SCN4A, and DM1/DM2 repeat sizing in a single test from one blood sample — establishing the diagnosis and excluding the differentials in one step.
Myotonic Dystrophy exclusion — mandatory
Before any management plan is finalised, Myotonic Dystrophy must be formally excluded. At KIMS this means CTG repeat sizing for DM1 (DMPK gene) and CCTG repeat sizing for DM2 (CNBP gene). If either is expanded, the diagnosis is Myotonic Dystrophy — requiring cardiac monitoring, respiratory assessment, and full multi-system management. If both are normal and CLCN1 is mutated, Myotonia Congenita is confirmed.
Treatment
Management — Mexiletine, Alternatives, and Drugs to Avoid
The goal of management is functional: less stiffness, easier first movements, and safe surgical care over a lifetime. Not every patient needs medication — many are well-managed by structured exercise warm-up and cold avoidance alone. When stiffness is functionally limiting, mexiletine is the most effective drug.
Not all patients need medication
Many patients — particularly with the Thomsen (dominant) form — have mild stiffness that is adequately managed by the warm-up phenomenon alone. Structured exercise warm-up before any demanding activity (a 5-minute light walk before sport, a hand warm-up before writing), avoidance of prolonged rest before anticipated activity, and cold avoidance are sufficient for many patients. The decision to start medication is based on functional disability — not on the genetic diagnosis alone.
Mexiletine — the most effective drug
Mexiletine, a sodium channel blocker (the same mechanism as lidocaine), is the most effective pharmacological treatment for myotonia in Myotonia Congenita and is the current evidence-based first choice. It reduces the repetitive firing of the hyperexcitable sarcolemma by blocking the sodium channels that drive the myotonic discharge.
How mexiletine is used at KIMS
Starting dose: 150 mg once daily, titrated upward over 2–4 weeks based on response and tolerance
Maintenance: most patients respond at 150–300 mg two to three times daily (maximum 300 mg three times daily)
Pre-treatment ECG mandatory — performed at KIMS before starting and at dose escalations; QRS or QT abnormalities require cardiac review
Side effects: nausea and gastrointestinal discomfort (take with food), transient lightheadedness, tremor at higher doses — all monitored at follow-up
Efficacy: most patients describe meaningful improvement in stiffness severity within 1–2 weeks of reaching therapeutic dose
When mexiletine is not tolerated
Carbamazepine (200–400 mg twice daily) — commonly available in India for epilepsy. Anti-myotonic effect less potent than mexiletine but adequate for mild to moderate myotonia. Requires monitoring of blood levels and full blood count with long-term use
Lamotrigine (50–200 mg daily) — evidence from a double-blind randomised trial (Andersen et al., Brain 2017) showing specific antimyotonic benefit in non-dystrophic myotonia. Useful as an alternative or add-on
Phenytoin (200–300 mg daily) — historically used, still effective. Less preferred due to cosmetic side effects (gum hypertrophy, hirsutism) with long-term use, but an option in resource-limited settings
Drugs to avoid in Myotonia Congenita
Suxamethonium (succinylcholine) — the depolarising muscle relaxant used in many Indian anaesthetic protocols for rapid sequence intubation — is CONTRAINDICATED. It causes severe, sustained myotonic contraction of all muscles including jaw and larynx, producing masseter spasm and potential airway obstruction
Non-depolarising relaxants (vecuronium, rocuronium) are safe alternatives and should be specified in the surgical plan
Adrenaline (epinephrine) and beta-adrenergic agonists can worsen myotonia — clinically important because adrenaline is used in dental local anaesthetics and in resuscitation. Request adrenaline-free local anaesthetic for dental work
Propranolol and other beta-blockers can also worsen myotonia and should be avoided where alternatives exist
Every confirmed Myotonia Congenita patient at KIMS receives a written anaesthetic alert card and a KIMS letter for their surgical and dental teams
Genetic Counselling at KIMS
Once the CLCN1 mutation pattern is known, the inheritance implications for the wider family can be explained clearly — including the chance that siblings or children carry the same mutation, and the practical options for predictive or carrier testing.
Because Myotonia Congenita is a non-progressive condition with a normal life expectancy, the conversation is reassuring — and the practical guidance for school, sport, occupation, and surgery can be given to the whole family at once.
What KIMS explains to the family
Thomsen disease (autosomal dominant): each child of an affected parent has a 50% chance of inheriting the mutation. The condition is typically mild and non-progressive. Predictive genetic testing offered to at-risk family members who wish to know
Becker disease (autosomal recessive): both parents are carriers. Each sibling of the affected patient has a 25% chance of being affected and a 50% chance of being a carrier. Carrier testing offered to siblings of both parents. Prenatal diagnosis available
Family counselling at KIMS includes a clear explanation of the inheritance pattern, the mild and non-life-shortening nature of the condition, and the practical implications for school, sport, occupation, and surgery
When Should You Seek a Myotonia Assessment?
Come to KIMS Neuroscience, Hyderabad if you or your child experience any of the patterns listed here. Patients labelled as having “cramps”, “clumsiness”, or non-specific stiffness for years are often found, on a structured 10-minute clinical assessment, to have a treatable channelopathy.
For surgical patients with confirmed or suspected myotonia, contact KIMS BEFORE the scheduled procedure — suxamethonium is contraindicated and the anaesthetic plan must be adjusted in advance.
Muscle stiffness that is worst after rest and gets better with repeated movement — the warm-up phenomenon — at any age
A child or young adult who freezes momentarily when standing up quickly from sitting or sleeping
Difficulty releasing a handshake or letting go after gripping an object
Visible muscle hypertrophy in a child or young adult, particularly of the calves, thighs, or forearms
A family history of muscle stiffness, particularly across multiple generations
Any patient with confirmed or suspected myotonia who is scheduled for surgery requiring general anaesthesia — call before the operation, not after
Any patient labelled as having 'cramps' or 'clumsiness' that has not responded to conventional treatment
Related Conditions
Myotonia Congenita belongs to the family of neuromuscular disorders — and shares features with other genetic and acquired muscle conditions assessed at KIMS:
FAQs
Frequently Asked Questions
| Fact | Details |
|---|---|
| Genetics | CLCN1 gene (chromosome 7q35) · Thomsen: autosomal dominant, milder · Becker: autosomal recessive, more severe with transient weakness + muscle hypertrophy · No cardiac, systemic, or progressive weakness |
| Mechanism | ClC-1 chloride channel: 80% of resting sarcolemmal conductance · Dysfunction: membrane hyperexcitable · Warm-up: potassium + adaptive Na-channel changes normalise excitability during exercise |
| vs DM1 | MC: myotonia only · NO progressive weakness, cardiac, cataracts, diabetes, cognitive slowing · DM1: all of the above + myotonia · ANY cardiac symptom or systemic feature = CTG sizing before MC diagnosis |
| Anaesthetic risk | Succinylcholine: CONTRAINDICATED (severe sustained myotonic contraction, airway compromise) · Anaesthetic alert card for all MC patients · TIVA or non-depolarising relaxants required · Alert surgical team before procedure |
| Treatment | Mexiletine (first-line, sodium channel blocker, requires ECG before starting for QT prolongation risk) · Carbamazepine (alternative) · Warm-up exercises before activity · Avoid cold exposure |
| Diagnosis | Clinical: warm-up phenomenon + percussion myotonia · EMG: myotonic discharges (dive-bomber sound) · CLCN1 genetic testing · Exclude DM1 with CTG repeat sizing if any systemic features |
| Contact | 040-44885000 · Pre-surgical anaesthetic alert: 040-46324365 |
Stiffness that loosens up with movement. A child who freezes for a moment when standing up quickly. A handshake that takes an extra second to let go. This is not laziness, not cramps, and not 'just the way they are' — it is a specific genetic condition with a specific test and a specific treatment.
At KIMS Neuroscience, Secunderabad, Hyderabad, Myotonia Congenita is assessed with the structured clinical warm-up test, EMG producing the pathognomonic dive-bomber myotonic discharge, and a channelopathy genetic panel that simultaneously tests CLCN1 (Myotonia Congenita), SCN4A (Paramyotonia Congenita), and excludes CTG repeat expansion (Myotonic Dystrophy) in one blood sample. Every confirmed patient receives a pre-treatment ECG, a personalised mexiletine plan, and a written anaesthetic alert letter for their surgical and dental teams.
International enquiries: info@kimsneurosciences.com