Author(s): Vikki A. Stefans, MD
Originally published: September 22, 2015 Last updated: July 10, 2020
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Disease/Disorder

Definition

Myotonia is essentially failure of muscle relaxation following contraction, leading to clinical spasm and specific patterns of prolonged electrophysiological activity.  If of muscular origin, it is characterized on EMG by a distinctive pattern of rapid spontaneous firing of muscle fibers waxing and waning in frequency and amplitude, while in neuromyotonia this may occur in irregular bursts. 1   Clinically, it can manifest as painful muscular tension in affected muscles, inability to relax after voluntary contraction, classically an inability to promptly and easily release after a strong grip, or percussion myotonia reflecting muscle irritability. The underlying pathology is dysfunction of ion channels on the muscle membranes or in peripheral nerve.

Etiology

The majority of conditions with myotonia are hereditary (genetic), and may be congenital or appearing later in life. Hereditary myotonia is commonly divided into dystrophic and nondystrophic types. Nonhereditary or acquired myotonia can be drug induced, or occur in the setting of other conditions such as hypothyroidism. As noted, myotonia is not a single disease entity, and a clinical appearance of myotonia may not have underlying electrophysiologic myotonia.

Though presentation may be similar, there are electrophysiologially distinct conditions associated with abnormal sustained muscle contraction, such as myokymia, continuous motor activity, complex repetitive spontaneous discharges including bizarre high-frequency discharges, fibrillations, fasciculations, or electrically silent contracture.  These other conditions with delayed relaxation, which lack true electrophysiological myotonia, may referred to as neuromuscular hyperexcitability disorders or“pseudomyotonia”.2,3,  Specific treatment for each of these disorders differs substantially.  Examples include:  

  • McArdle’s disease (glycogenosis type V) with parapsinal and other fibrillations and sometimes contracture, treated with carbohydrate and excercise mangement.
  • Hoffman’s disease with CPK elevation, with myoedema or muscle hypertrophy and complex spontaneous discharges sometimes including true myotonia, due to hypothyroidism.
  • Brody myopathy is an autosomal recessive disorder of relaxation in fast twitch muscle associated with deficiency of ATP2A1, the sarcoplasmic reticulum calcium ATPase responsible for reuptake of calcium post contraction.  It typically presents with exercise–induced initially painless contracture or cramp.  Dantrolene and calcium channel blockers are usually helpful.  It may be called a pseudomyotonia as myotonic discharges are typically absent.
  • Tetanus11 shows continuous discharge of motor subunits and shortening of refractory periods between action potentials. Strychnine poisoning is also a close mimic clinically. Both occur due to loss of function of spinal inhibitory neurons.  Tetany related to hypocalcemia may include high frequency doublet and triplet discharges.  
  • Myokymic discharges are single or a few motor potentials that fire spontaneously in a burst pattern, and can be associated with a variety of conditions, including motor neuron disease.
  • Stiff-person syndrome presents with progressive rigidity and waxing and waning spasm with stimulation, primarily of proximal muscles, sometimes with myoclonus. EMG shows continuous involuntary motor unit activity that is abolished during sleep. It is most commonly due to anti-GAD antibodies which affect GABA-ergic neurons, and may be associated with thyroiditis or type I diabetes. It responds to benzodiazepine and baclofen and sometimes anticonvulsants and may also be treated with plasmapheresis or immune suppression with corticosteroid or IVIG, though recovery is often incomplete. 
  • Isaacs syndrome, aka Isaacs-Merton syndrome, or Morvan syndrome if associated with encephalopathy, is characterized by continuous muscle fiber activity on EMG that persists during sleep, and also shows fasciculations, myokymia, and complex repetitive discharges. Etiology is paraneoplastic or other autoimmune, having associations with VGKC, LGI1 and CASPR antibodies, thymoma, SCLC and other conditions. It has been called acquired neuromyotonia but this may be inaccurate. Cramp fasciculation syndrome may be a component of this condition. It may respond to sodium channel blocking anticonvulsants (phenytoin, carbamaezpeine) but is also treated with plasmapheresis and/or immune suppression as well. Some sources consider KCNA1 mutations causing episodic ataxia plus neuromyotonia and myokymia as a hereditary cause of this condition, and it may respond to magnesium supplementation.4
  • Hyperekplexia syndromes, which are primarily hereditary and related to glycine signaling, most commonly GLRA1, are sometimes referred to as stiff-baby syndrome because of early presentation with excess generalized stiffness, but later are more characterized by exaggerated startle and are the basis of the “jumping Frenchman syndrome.” Rarely this can be acquired due to brainstem encephalitis. EMG does not show abnormal potentials but abnormal spread and occurrence of muscular activities.  It often responds to clonazepam and other anticonvulsants.
  • Rippling muscle syndrome or disease due to caveolinopathy, has usually electrically silent percussion-induced muscle mounding, or may have bursts of short‐duration, low‐amplitude spikes.  The underlying disorder may require attention to preventing rhabdomyolysis and managing proximal weakness if present (LGMD1c).
  • CNS (upper motor neuron) disorders causing spasticity, rigidity or other dystonia, typically have poor relaxation and overflow but normal muscle potentials with or without minimal muscle membrane irritability.

Some medications cause or aggravate EMG myotonia with or without clinical myotonia, including the herbicide 2,4-dichlorophenoxyacetate, chloroquine, colchicine, asthma medications, such as terbutaline, fenoterol, beta-adrenergic agonist inhalers, some immunosupresants, including  penicillamine and cyclosporine, lipid lowering agents such as HMG-CoA reductase inhibitors, clofibrate, and 20, 25-diazacholesterol (currently not used therapeutically in humans.)5 Cases induced by 20, 25-diazacholesterol can present with myotonia and cramps several months after start of medications.7 Colchicine, monocarboxylic acids, anthracene-9-carboxylic acid, 2,4-dichlororophenoxyacetic acid and chloroquine can also cause myotonia.5 Heavy metal exposure is reported as a possible cause in a few sources. Treatment is to stop exposure whenever possible.

Electrophysiologic myotonia may also be seen in association with other disorders:

  1. Metabolic: acid maltase deficiency (Pompe’s disease) which is treatable with enzyme replacement therapy, and debrancher enzyme deficiency type III aka Forbes-Cori disease.
  2. Inflammatory: Dermatomyositis/polymyosits is treated with immunosuppresion, primarily corticosteroids.
  3. Inclusion body myopathies, either idiopathic or hereditary forms such as the autosomal recessive GNE myopathy or Paget disease of bone with dementia due to heterozygous mutation of the VCP gene for valosin-containing protein; none of those have specific pharmacological treatment at this time.
  4. Congenital myopathies, especially centronculear or myofibrillar.
  5. Muscular dystrophies such as Emery-Dreifuss or DNAJB6 myopathy
  6. Schwartz-Jampel syndrome, a complicated dwarfism syndrome aka chondrodystrophic myotonia, is due to biallelic perlecan (HSPG2) mutation, with some controversy whether it is considered a true form of myotonia.6,7 The spasms may be treatable with anticonvulsants such as phenytoin or carbamazepine or antiarrhythmics such as mexiletine, procainamide, quinidine or quinine, but the blepharospasm also can benefit from botulinum toxin injection or surgery.
  7. Neuromyotonia and axonal neuropathy (NMAN) due to biallelic HINT1 mutation, also treatable with phenytoin or carbamazepine.  
  8. Systemic disorders: malignant hyperpyrexia, hypothyroidism.
  9. Cases of severe denervation of any etiology.8

The rest of this article will focus on the true primary myotonic disorders.

Detailed Pathoanatomy/Physiology and Etiologies

Hyperexcitability of skeletal muscle fibers can be due to alterations in voltage gated chloride, sodium, calcium, or potassium channels. In the nondystrophic disorders, mutations in their genes alter the channel structure, conductivity and function.3 In myotonic dystrophy, the toxic RNA produced by the excess repeats in a gene leads to defective mRNA splicing of the Cl channels,, along with other more widespread gene expression in other organs causing a complex multisystem disease.  

Specific Myotonic Disorders – Dystrophic

These are both autosomal dominant disorders due to repeat expansion, type 1 having much greater anticipation or worsening in terms of earlier onset and greater severity with each generation. They affect about 1 in 8,000 people worldwide with DM1 being the most prevalent. The repeat expansion is associated with a toxic RNA which interferes with regulation of splicing and transcription most notably of CLCN1, via binding and inactivating muscleblind-like proteins particularly MBNL1.10

  1. Myotonic Dystrophy Type I (DM1) aka Steinert disease
  2. Myotonic Dystrophy type II (DM2) aka PROMM (proximal myotonic myopathy)

DM1 is the most common adult onset muscular dystrophy, but can present at any age; its severity and earlier age of onset are roughly proportional to the number of CTG repeats in the DMPK (dystrophica myotonica protein kinase) gene on chromosome 19q13, which tends to expand with each generation. Normally there are fewer than 35-37 repeats; the premutation range with risk of disease transmission though asymptomatic is 38-50, adult onset cases may have 50 to several hundred, and younger onset cases several hundred to over 1000.  The congenital form is essentially always maternally transmitted, and some mothers are unaware of their condition. This is because male fertility is affected in more severe cases, while in affected females, fertility is preserved even though mothers have myotonia and myopathic facies and may even be cognitively affected enough that child care is delegated to a grandparent who is either unaffected or very mildly symptomatic.

These infants may be hypotonic with marked myopathic facies, bilateral clubfoot, respiratory weakness, inability to feed with oral and pharyngeal dysphagia, and are often premature due to maternal effects of the condition. This can be mistaken for cerebral palsy or may coexist with cerebral palsy due to the prematurity. Cases with pediatric onset are typically associated with significant intellectual disability and learning differences, attention deficit and psychological disorders, but higher level executive functions including judgement and organizational skills are often affected in adults as well.11  

For DM2 the repeat expansion is a CCTG in CNBP (cellular nucleic-acid-binding protein, formerly ZNF9) on chromosome 3q21. There is a founder effect for increased prevalence in some geographical areas, most notably in Finland. Though repeat numbers can be much higher, anywhere form over 75 to over 10,000, severity of the condition is much less clearly related and the expansion size tends to be more stable over generations. Age of onset is always in adulthood and there is no congenital form.

Clinical Course/Prognosis

The clinical manifestations of these disorders are protean and myotonia may or may not be the clinically most important feature. 

Cardiac arrhythmia is common and many adults eventually require pacemaker placement.  Accelerated atherosclerosis is also noted in many cases.

Cataracts are nearly universal in adults and the history of cataract before age 50 in prior generations may be a clue to diagnosis.

Insulin resistance is high and type 2 diabetes with onset in early middle age is common. Other endocrinopathies may occur, especially hypothyroidism and hypogonadism.

Disturbance of circadian rhythm with poor sleep and daytime somnolence is very common. Obstructive and/or central sleep apnea contributes to this when present, but low neurologic arousal is a primary issue. Cognitive dysfunction and psychiatric issues are rare in DM2 but common for DM1.

Restrictive pulmonary disease due to respiratory muscle weakness is also rare in DM2 but common for DM1.  

Sensorineural hearing loss commonly occurs in adulthood as well in 20-30% of cases.

Anesthetic considerations are present with risk of a malignant hyperthermia-like condition and predisposition to severe respiratory depression.

With DM2 most patients are able to function, work, and perform activity of daily living through adulthood, though tremors are prominent and muscle weakness becomes more severe at age 60-85. Life expectancy is much closer to normal. Cardiopulmonary disease can be life-limiting for adults with DM1.

Essentials of Assessment

History

Patient should be asked about symptoms of pain, cramping, weakness, and limitation of mobility. Distal muscles are usually more affected in DM1 and difficulty releasing grip may be noted, or patients may have successful, even subconscious, strategies for avoiding this. Proximal muscle pain and weakness are is more common in DM2. A thorough review of systems is needed to address all organs with special attention to cardiac, endocrine, special sensory, gastrointestinal, respiratory and sleep symptoms. Family history should include any neuromuscular disease, family history of cataracts and of heart disease or arrhythmias with age of onset. Birth, developmental and academic history is important as well as occupational history and disability status.

Physical examination

Clinically, myotonia is best tested by having a patient make a tight fist and then try to open hands quickly. In the case of myotonia, there will be a few second delay in hand opening. Similarly, after forced eye closure there will be delay in eyelid opening.12 Percussion of thenar eminence, wrist extensors, quadriceps, gastrocnemius or tongue, are classically used to test for percussion myotonia. Symptoms of myotonia are generally aggravated by cold. 

On ophthalmologic exam, the presence of iridescent posterior subcapsular opacities are almost pathognomonic. Ptosis and strabismus may also be noted.

DM1: Characteristic facial appearance with frontal balding, atrophy in the temporal muscles, facial weakness and myopathic lack of expression, with ptosis and tented upper lip when more severe. Speech intelligibility may be affected and may be worsened in patients with hearing loss. Assessing cognitive versus audiologic reasons for difficulty with following commands may be difficult.  Distal muscles are more affected and reflexes may be brisk with hypertonia, but Babinski reflexes will usually be downgoing. There is often a “warm up” phenomenon where myotonia improves with repeat activation. Gait may reveal poor balance, subtalar deviation, and sometimes foot drop. Heel cord or hamstring tightness which require stretching may be identified. Sensory and motor neuropathy may occur but is usually mild unless there is long-standing diabetes. Vibratory and position sense testing will usually detect this.

DM2: There may be a jerky or ratchet quality to grip.13 Pain, unrelated to myotonia, occurring spontaneously or tenderness elicited by palpation is common in proximal muscle groups. Proximal muscles are more affected than distal muscles or facial muscles. Cognitive deficits, when present, are generally mild.

Clinical Functional Assessment

Difficulty with ambulation, climbing stairs,11 manual dexterity, opening or closing eyelids11 and releasing a handshake, which can be socially embarrassing. Issues with employability, self-care and household management should be assessed.

Laboratory Studies

Laboratory testing for diagnosis should include creatinine kinase, which will be modestly elevated or possibly within upper limits of normal, plus specific genetic testing for DM1 alone first, followed by DM2 if that is negative.  Regular annual or semi-annual follow-up of fasting blood sugar or hemoglobin A1c is absolutely necessary, lipid panels should be monitored, and TSH, follicular stimulating hormone, should also be considered with any symptoms.

Annual EKG, Holter if symptomatic, periodic echocardiogram, monitoring of pulmonary function and sleep study are also recommended.

Muscle biopsy may be abnormal particular for dystrophic versus non-dystrophic myotonias, but is typically non-specific. Electrodiagnosis is not necessary if the clinical phenotype is present and genetic tests confirm the diagnosis. Biopsy findings for DM1 may include small type I fibers, sarcoplasmic masses, ring fibers and internalized nuclei, and for DM2 there may be atrophic type 2 fibers and nuclear clumps. Other dystrophic features such as degeneration-regeneration, necrosis and inflammation are not generally observed. 

Electrodiagnosis is more essential if there is concern for pseudomyotonia or other disorders. Note that myotonia may not be present in the infant or young child with DM1 initially. Electrodiagnostically, the myotonia is classic and has been likened to hearing a revving engine or dive bomber airplane. Morphologically, the spontaneous muscle fiber action potentials have characteristics of positive waves or fibrillations5 and can be stimulated by voluntary contraction, motor nerve stimulation, or mechanical stimulation including percussion or just the needle.2 Characteristic results with short and long exercise tests help to differentiate between the various causes of myotonia. Myotonia in DM1 is more easily evoked in most muscles compared to in DM2 where it is more easily evoked in proximal muscles, such as vastus lateralis and tensor fascia lata.  Amplitudes of CMAP and myotonic potentials increase with exercise and decrease with rest.8

Imaging14,15

MRI of muscles shows fatty hypertrophy correlating with clinical weakness. Brain MRI may show widespread white matter changes. In DM1 the soleus and medial gastrocnemius are most affected and affected first, and in DM2 vastus lateralis is first affected with involvement of erector spinae and gluteus maximus also common.17 Milder brain MRI changes may be found in some advanced or severe cases.

Rehabilitation Management and Treatments

Available or current treatment guidelines

There is currently an AAN publication by 66 experts in collaboration with the MDF (Myotonic Dystrophy Foundation) entitled “Consensus-based care recommendations for adults with myotonic dystrophy type 1 accessible via https://cp.neurology.org/content/8/6/507. Briefly, multidisciplinary treatment with education, genetic counseling, physical, occupational and speech therapy as well as multispecialty medical monitoring and treatment for associated conditions is the standard of care. There is no medication specifically approved for this condition but there is evidence for safety and efficacy for treatment of myotonia when it is functionally bothersome or painful. In the past, phenytoin, clomipramine and imipramine, and taurine were considered potentially helpful but the preponderance of more recent evidence exists favors mexiletine in the absence of cardiac contraindications.16      

It is recommended to evaluate at least annually for swallowing and speech difficulties, mobility, balance, and falls, activities of daily living and activities in home, school, work, and community.

Pulmonary followed by cardiac issues are the most common causes of death for this population. Cardiac management is defined in detail by a recent AHA publication, also open access at https://www.ahajournals.org/doi/10.1161/JAHA.119.014006 and consultation with cardiology is recommended for use of both stimulant and anti-myotonic medications.  Symptoms of hypersomnolence are thought to be primarily caused by a central dysfunction of sleep-wake regulation rather than poor sleep, but sleep-related breathing disorders, sleep fragmentation, and restless legs symptoms may also occur. Pulmonology and/or sleep clinic consultation is recommended. PFT showing significant restrictive disease or peak cough flow under 270 l/min should trigger pulmonology referral. There is some evidence for treatment with either methylphenidate or modafinil; 17melatonin is not contraindicated and can be tried for insomnia. Precautions are needed during and post anesthesia as patients have a risk for an MH-like reaction and high sensitivity to respiratory depressants such as benzodiazepines and opioids. Weak bulbar muscles, prolonged gastric emptying time, and blunted response to C02 may require awake extubation, postoperative noninvasive support, and careful use of supplemental oxygen. Muscle relaxants and hyperkalemia should be avoided and regional anesthesia is preferred when possible. Cognitive status is also relevant to surgical tolerance and recovery.18 

Endocrine systems dysfunctions require annual plus symptomatic monitoring of glycemic status and hepatic function (GGT, biliruibin and alkaline phosphatase to avoid alarm due to mildly elevated SGPT and SGOT which are enzymes also found in muscle), and thyroid function (T4 and TSH) testing and lipid profiles every three years or if symptomatic.  Recommendations for moderate active exercise, both resistance and cardiovascular, would appear to be safe and may improve insulin sensitivity and reduce risks of frailty and debility, though strength benefits may not be dramatic and care should be taken not to increase fatigue in individuals who are already active.  Gastrointestinal complaints are also frequent and may be severe, often requiring GI specialist, speech therapist, and nutritionist input for dysphagia, constipation or diarrhea. Dermatologic concerns include susceptibility to pilomatrixoma which may require surgical excision. Ophthalmologic evaluation is required at least annually including slit lamp examination. Ptosis crutches or surgery are options. Pregnant women require referral to high risk obstetrics. Psychological, psychiatric and social screening is also needed. 

Genetic counseling and options for in vitro fertilization and pre-implantation diagnosis may reduce the 50% transmission risk, which may not be well understood even in large affected families.

 For pediatric patients, school situation and appropriateness of individualized educational plans and/or section 504 accommodations is an additional concern, as well as transition to adulthood and possible needs for full or limited guardianship depending on individual neuropsychological assessment. Maximizing independence and continence require assessment and encouragement for caregivers.

Translation into Practice and Clinical Pearls

Despite the multiple pathologies of this condition, diagnosis is often delayed for adults and even for children. Average time from onset of symptoms to diagnosis is over 5 years for DM1, and over 14 years for DM2, likely due to subtler symptoms and less clinician awareness.  Diagnostic workup should be considered even if patients are taking medications with known myotonia-aggravating potential as they can unmask an underlying disease. Cataracts and diabetes at a young age should prompt consideration. Be aware that symptoms vary, even with identical repeat numbers in the same family generation.19   

Specific Myotonic Disorders – Non-Dystrophic (Ion Channel)

Definition and Etiology: These conditions in general and more benign and do not have the complex comorbidities as seen with the myotonic dystrophies, and have either autosomal recessive or dominant transmission without anticipation.

Chloride Channel Disorders:  CLCN1-associated Myotonia Congenita.20 Thomsen disease refers to autosomal dominant and Becker disease to autosomal recessive cases, with the latter being generally more severe but with considerable overlap.  These present with muscle stiffness which may improve with exercise (“warm-up phenomenon”) and aggravation with cold, but on particular response to dietary intake. There may be neonatal to young adult onset. Diffuse muscular hypertrophy may be notable, but without the expected supra-normal or high normal strength that might be expected from appearance. This is not as marked as in the “infant Hercules syndrome” associated with Kocher–Debré–Semelaigne syndrome, adrenogenital syndrome, or myostatin mutations. There is an intriguing case report of a young man with this who was successful as a body-builder without doing any exercise.21 Mild fixed proximal weakness may develop.

Sodium Channel Disorders: SCN4a-associated phentoypes include a range of symptomatology which are allelic but were defined clinically as different entities until their genetic basis was known. Paramyotonia Congenita refers to the “paradoxical” situation where the myotonia is aggravated by exercise, but still cold-aggravated.  Myotonia fluctuans and permanens refer to variable symptoms during rest post-exercise versus more persistent severe symptoms that may interfere with respiration. Very severe infantile onset cases with life threatening respiratory involvement are rare and typically due to de novo mutations. Potassium-aggravated myotonias include both of those clinical entities and acetazolamide-responsive myotonia as well as hyperkalemic or normokalemic periodic paralysis (type 2) with myotonia. Different mutations in different parts of the gene can also result on gain versus loss of function of the channel, so that hypokalemic periodic paralysis type 2, less likely associated with myotonia is also associated. These are autosomal dominant conditions and the HOKPP, HYKPP or normokalemic designations refer to the potassium level during an attack of paralysis. HOKPP is precipitated by carbohydrate ingestion and rest after exercise, and helped by potassium supplementation. Biallelic mutations in this gene are associated with a congenital myasthenic syndrome, usually without myotonia, for which pyridostigmine and/or acetazolamide may be helpful.  Hypertrophy is not usually a prominent feature of any of these conditions, but mild persistent proximal weakness may also develop. 

Calcium and Potassium Channel Disorders

Hypokalemic periodic paralysis, usually without myotonia, is associated with CACNA1S as an autosomal dominant disorder, which may also confer susceptibility to malignant hyperthermia and hyperthyroid-related periodic paralysis; the latter is also associated with mutation in KCNJ8. KCNA1 is associated with Episodic Ataxia and nyokymia which may include neuromyotonia.4

Cutting Edge/Emerging Issues and Unique Concepts

All of these conditions are covered under the MDA umbrella, both dystrophic and  non-dystrophic. Children and families can benefit from support groups and information available through this organization as well as the specialized clinical centers, and children up to age 16 can attend special camps where their condition or disability will be little or no obstacle to participation, often for no charge and with medical and nursing staff available. Registries for both DM1 and DM2 and online support including web sites and social media groups are available as well, and clinical studies for natural history, pulmonary interventions and new drugs continue primarily for DM1.  RNA silencing or interference was promising but there are no currently active clinical trials; delivery and off-target effects are of concern.

Gaps in Evidence-Based Knowledge

Genotype-phenotype correlations and variable penetrance remain somewhat mysterious. Better studies of benefits of exercise including specific types are needed, as well as additional studies ideally leading to FDA indication and wider formulary coverage for commonly used drugs targeting symptoms.  

References

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  2. Jarcho Leonard W.m Tylerm Frank H. Myxedema, Pseudomyotonia, and Myotonia Congenita. AMA Arch Intern Med. 1958;102(3):357-366. doi:10.1001/archinte.1958.00030010357003
  3. Grünberg, Walter, et. al. Pseudomyotonia, a Muscle Function Disorder Associated With an Inherited ATP2A1 (SERCA1) Defect in a Dutch Improved Red and White Cross-Breed Calf Neuromuscul Disord. 2010 Jul;20(7):467-70. doi: 10.1016/j.nmd.2010.04.010. Epub 2010 Jun 14.
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  6. Fowler WM Jr, Layzer RB, Taylor RG, Eberle ED, Sims GE, Munsat TL, et al. The Schwartz-Jampel syndrome. Its clinical, physiological and histological expressions. Journal of the Neurological Sciences 1974;22(1):127–46.
  7. Viljoen D, Beighton P. Schwartz-Jampel syndrome (chondrodystrophic myotonia). J Med Genet. 1992;29(1):58–62.
  8. Miller, Timothy. Differential Diagnosis of Myotonic Disorders. Muscle Nerve 37: 293–299, 2008.
  9. Heatwole CR, Statland JM, Logigian EL. The diagnosis and treatment of myotonic disorders. Muscle Nerve. 2013;47(5):632–48. doi:10.1002/mus.23683.
  10. Mankodi A, Takahashi MP, Jiang H, Beck CL, Bowers WJ, MoxleyRT, et al. Expanded CUG repeats trigger aberrant splicing of ClC-1 chloride channel pre-mRNA and hyperexcitability of skeletal muscle in myotonic dystrophy. Mol Cell 2002;10:35–44.
  11. Douniol M, et.al, Psychiatric and cognitive phenotype in children and adolescents with myotonic dystrophy, Eur Child Adolesc Psychiatry. 2009;18(12):705-15. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8749.2012.04379.x/epdf
  12. Alon M, Korczyn AD. Pupillary responses and blink reflex in myotonic dystrophy. Clin Auton Res 1992;2:17–19.
  13. Meols, Giovannit and Cardani, Rosanna. Myotonic Dystrophy Type 2: An Update on Clinical Aspects, Genetic and Pathomolecular Mechanism. J Neuromuscul Dis. 2015 Jul 22; 2(Suppl 2): S59–S71.
  14. Peric, S., Maksimovic, R., Banko, B. et al. Magnetic resonance imaging of leg muscles in patients with myotonic dystrophies. J Neurol 264, 1899–1908 (2017). https://doi.org/10.1007/s00415-017-8574-0
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  16. Logigian, E.L. et. al. Mexiletine is an effective antimyotonia treatment in myotonic dystrophy type 1. Neurology. 2010 May 4; 74(18): 1441–1448.
  17. Van de Meche’ et al, Treatment of hypersomnolence in myotonic dystrophy with a CNS stimulant, Article first published online: 13 OCT 2004 DOI: 10.1002/mus.880090410, Muscle & Nerve Volume 9, Issue 4, pages 341–344, May 1986.
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  19. Yum, Kevin, WAng, Eric T., and Kalsotra, Auinash. Myotonic Dystrophy: Disease Repeat Range, Penetrance, Age of Onset, and Relationship Between Repeat Size and Phenotypes. Curr Opin Genet Dev. 2017 Jun; 44: 30–37.
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Original Version of the Topic

Heakyung Kim, MD, Hannah Aura Shoval, MD, Jahannaz Dastgir, MD. Congenital and Acquired Myotonia. 9/22/2015.

Author Disclosure

Vikki A. Stefans, MD
Nothing to Disclose