Paramyotonia congenita

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Paramyotonia congenita
Other namesParamyotonia congenita of von Eulenburg or Eulenburg disease [1]
Autosomal dominant - en.svg
This condition is inherited in an autosomal dominant manner.
Specialty Neurology   OOjs UI icon edit-ltr-progressive.svg

Paramyotonia congenita (PC) is a rare congenital autosomal dominant neuromuscular disorder characterized by "paradoxical" myotonia. [2] This type of myotonia has been termed paradoxical because it becomes worse with exercise whereas classical myotonia, as seen in myotonia congenita, is alleviated by exercise. PC is also distinguished as it can be induced by cold temperatures. Although more typical of the periodic paralytic disorders, patients with PC may also have potassium-provoked paralysis. PC typically presents within the first decade of life and has 100% penetrance. Patients with this disorder commonly present with myotonia in the face or upper extremities. The lower extremities are generally less affected. While some other related disorders result in muscle atrophy, this is not normally the case with PC. This disease can also present as hyperkalemic periodic paralysis and there is debate as to whether the two disorders are actually distinct. [3]

Contents

Symptoms and signs

Patients typically complain of muscle stiffness that can continue to focal weakness. This muscle stiffness cannot be walked off, in contrast to myotonia congenita. These symptoms are increased (and sometimes induced) in cold environments. For example, some patients have reported that eating ice cream leads to a stiffening of the throat. For other patients, exercise consistently induces symptoms of myotonia or weakness. Typical presentations of this are during squatting or repetitive fist clenching. Some patients also indicate that specific foods are able to induce symptoms of paramyotonia congenita. Isolated cases have reported that carrots and watermelon are able to induce these symptoms. The canonical definition of this disorder precludes permanent weakness in the definition of this disorder. In practice, however, this has not been strictly adhered to in the literature.[ citation needed ]

Pathophysiology

Paramyotonia congenita (as well as hyperkalemic periodic paralysis and the potassium-aggravated myotonias) is caused by mutations in a sodium channel, SCN4A. The phenotype of patients with these mutations is indicated in Table 1. These mutations affect fast inactivation of the encoded sodium channel. There are also indications that some mutations lead to altered activation and deactivation. The result of these alterations in channel kinetics is that there is prolonged inward (depolarizing) current following muscle excitation. There is also the introduction of a "window current" due to changes in the voltage sensitivity of the channel’s kinetics. These lead to a general increase in cellular excitability,[ citation needed ] as shown in figure 1.

Figure 1. Theoretical simulation of a muscle membrane potential in response to 150 ms depolarizing pulse of -45 pA. (A) Normal muscle produces only a single action potential due to such stimulus. This is due to inactivation of sodium channels, preventing their further activation even during depolarization. (B) Myotonic muscle, however, is hyperexcitable and able to produce action potentials for the duration of the stimulus pulse. This model adapted from Cannon, 1993. Myotonia figure.png
Figure 1. Theoretical simulation of a muscle membrane potential in response to 150 ms depolarizing pulse of −45 pA. (A) Normal muscle produces only a single action potential due to such stimulus. This is due to inactivation of sodium channels, preventing their further activation even during depolarization. (B) Myotonic muscle, however, is hyperexcitable and able to produce action potentials for the duration of the stimulus pulse. This model adapted from Cannon, 1993.

There has been one study of a large number of patients with paramyotonia congenita. Of 26 kindreds, it found that 17 (71%) had a mutation in SCN4A while 6 (29%) had no known mutation. There is no large difference between these two groups except that patients with no known mutation have attacks precipitated less by cold but more by hunger, are much more likely to have normal muscle biopsies, and show less decreased compound muscle action potentials when compared to patients with known mutations. [5]
 

Table 1. Summary of mutations found in patients diagnosed with paramyotonia congenita and their resulting phenotypes
Mutation Region Myotonia Weakness References
Cold Exercise/
Activity
Potassium Cold Exercise/
Activity
Potassium
R672C D2S4  ?  ?  ?  ?  ?  ? [5]
I693T D2S4-S5 N  ?  ? Y Y Y [6]
T704M* D2S5 Y  ?  ? Y Y Y [7] , [8] , [9] , [10]
S804F** D2S6 Y Y Y  ? Y N [11]
A1152D D3S4-S5 Y  ?  ?  ?  ?  ? [12]
A1156T* D3S4-S5 Y  ?  ?  ? Y  ? [3] , [11]
V1293I D3S4 Y Y N  ?  ? N
G1306V** D3-4 Y Y Y  ?  ? Y [13] , [14]
T1313A D3-4 Y Y N Y Y N [15]
T1313M D3-4 Y Y N Y Y**** N [13] , [16]
M1360V* D4S1  ?  ?  ? Y Y  ? [17]
M1370V* D4S1 Y Y N N N Y [18]
L1433R D4S3 Y Y Y  ? Y***** N [16]
R1448C D4S4 Y Y N N Y N [6] , [10] , [19] , [20]
R1448H D4S4 Y Y Y Y Y  ? [10] , [16] , [19] , [20]
R1448P D4S4 Y Y  ? Y  ? N [21]
R1448S D4S4 Y Y N  ? Y N [22]
R1456E D4S4 Y Y N N N N [23]
V1458F*** D4S4  ?  ?  ?  ?  ?  ? [24]
F1473S*** D4S4-S5  ?  ?  ?  ?  ?  ? [24]
M1592V* D4S6 Y Y Y Y Y Y [10] , [16] , [25] , [26] , [27] , [28] , [29]
E1702K C-term  ?  ? N  ?  ? N [5]
F1795I C-term Y  ?  ?  ?  ?  ? [30]
 
*
**
***
****
*****
 
Symptoms of both PC and hyperKPP (Periodica paralytica paramyotonica)
Also diagnosed as a Potassium-aggravated myotonia
Original case reports unpublished.
When exercised in a cold environment
After muscles were cooled
This table was adapted from Vicart et al., 2005. [31] "Cold" refers to symptoms either occurring or significantly worsening with cold temperatures. Likewise, "Exercise/Activity" refers to symptom onset or severity worsening with exercise and/or more general movement like hand clenching. "Potassium" refers to ingestion of food high in potassium or other disorders which are known to increase serum potassium levels. Mutation region nomenclature is: domain number (e.g., D1) followed by segment number (e.g., S4). Thus, D2S3 indicates that the mutation is in the 3rd membrane spanning loop of the 2nd domain. Some mutations occur between segments and are denoted similarly (e.g., D4S4-S5 occurs between the 4th and 5th segments of the 4th domain). Other mutations are located between domains and are denoted DX-Y where X and Y are domain numbers. C-term refers to the carboxy-terminus.

Diagnosis

Diagnosis of paramyotonia congenita is made upon evaluation of patient symptoms and case history. Myotonia must increase with exercise or movement and usually must worsen in cold temperatures. Patients that present with permanent weakness are normally not characterized as having PC. Electromyography may be used to distinguish between paramyotonia congenita and myotonia congenita. [32] , [33] Clinicians may also attempt to provoke episodes or myotonia and weakness/paralysis in patients in order to determine whether the patient has PC, hyperkalemic periodic paralysis, or one of the potassium-aggravated myotonias. Genomic sequencing of the SCN4A gene is the definitive diagnostic determinant.[ citation needed ]

Treatment

Some patients do not require treatment to manage the symptoms of paramyotonia congenita. Others require treatment for their muscle stiffness and often find mexiletine to be helpful. Others have found acetazolamide to be helpful as well. [34] Avoidance of myotonia triggering events is also an effective method of myotonia prevention.[ citation needed ]

Epidemiology

Paramyotonia congenita is considered an extremely rare disorder, though little epidemiological work has been done. Prevalence is generally higher in European-derived populations and lower among Asians. Epidemiological estimates have been provided for the German population. There, it was estimated that the prevalence of PC is between 1:350,000 (0.00028%) and 1:180,000 (0.00056%). [20] However, the German population of patients with PC is not uniformly distributed across the country. Many individuals with PC herald from the Ravensberg area in North-West Germany, where a founder effect seems to be responsible for most cases. [20] [35] The prevalence here is estimated at 1:6000 or 0.017%.[ citation needed ]

History

Originally thought to be separate from hyperkalemic periodic paralysis and the sodium channel myotonias, there is now considerable disagreement as to whether these disorders represent separate entities or overlapping phenotypes of a complex disorder spectrum. It was once thought that paramyotonia congenita was more common in males. Observation of the most recent generation has shown this to be untrue. On average, half of children in a family inherit the disorder regardless of gender. [36]

Related Research Articles

Myotonia is a symptom of a small handful of certain neuromuscular disorders characterized by delayed relaxation of the skeletal muscles after voluntary contraction or electrical stimulation.

Neuromuscular junction Junction between the axon of a motor neuron and a muscle fiber

A neuromuscular junction is a chemical synapse between a motor neuron and a muscle fiber.

Hyperkalemic periodic paralysis is an inherited autosomal dominant disorder that affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium, heat or cold, can lead to uncontrolled shaking followed by paralysis. Onset usually occurs in early childhood, but it still occurs with adults.

Andersen–Tawil syndrome Rare autosomal dominant genetic disorder

Andersen–Tawil syndrome, also called Andersen syndrome and long QT syndrome 7, is a rare genetic disorder affecting several parts of the body. The three predominant features of Andersen–Tawil syndrome include disturbances of the electrical function of the heart characterised by an abnormality seen on an electrocardiogram and a tendency to abnormal heart rhythms, physical characteristics including low-set ears and a small lower jaw, and intermittent periods of muscle weakness known as hypokalaemic periodic paralysis.

Fainting goat American breed of meat goat

The myotonic goat or Tennessee fainting goat is an American breed of goat. It is characterised by myotonia congenita, a hereditary condition that may cause it to stiffen or fall over when excited or startled. It may also be known as the fainting goat, falling goat, stiff-legged goat or nervous goat, or as the Tennessee wooden-leg goat. Four goats of this type were brought to Tennessee in the 1880s.

Channelopathy Diseases caused by disturbed function of ion channel subunits or the proteins that regulate them

Channelopathies are a group of diseases caused by the dysfunction of ion channel subunits or their interacting proteins. These diseases can be inherited or acquired by other disorders, drugs, or toxins. Mutations in genes encoding ion channels, which impair channel function, are the most common cause of channelopathies. There are more than 400 genes that encode ion channels, found in all human cell types and are involved in almost all physiological processes. Each type of channel is a multimeric complex of subunits encoded by a number of genes. Depending where the mutation occurs it may affect the gating, conductance, ion selectivity, or signal transduction of the channel.

Myotonia congenita is a congenital neuromuscular channelopathy that affects skeletal muscles. It is a genetic disorder. The hallmark of the disease is the failure of initiated contraction to terminate, often referred to as delayed relaxation of the muscles (myotonia) and rigidity. Symptoms include delayed relaxation of the muscles after voluntary contraction (myotonia), and may also include stiffness, hypertrophy (enlargement), transient weakness in some forms of the disorder, severe masseter spasm, and cramping. The condition is sometimes referred to as fainting goat syndrome, as it is responsible for the eponymous 'fainting' seen in fainting goats when presented with a sudden stimulus. Of note, myotonia congenita has no association with malignant hyperthermia (MH).

Voltage-gated Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's plasma membrane.

Periodic paralysis is a group of rare genetic diseases that lead to weakness or paralysis from common triggers such as cold, heat, high carbohydrate meals, not eating, stress or excitement and physical activity of any kind. The underlying mechanism of these diseases are malfunctions in the ion channels in skeletal muscle cell membranes that allow electrically charged ions to leak in or out of the muscle cell, causing the cell to depolarize and become unable to move.

Hypokalemic periodic paralysis Medical condition

Hypokalemic periodic paralysis (hypoKPP), also known as familial hypokalemic periodic paralysis (FHPP), is a rare, autosomal dominant channelopathy characterized by muscle weakness or paralysis when there is a fall in potassium levels in the blood. In individuals with this mutation, attacks sometimes begin in adolescence and most commonly occur with individual triggers such as rest after strenuous exercise, high carbohydrate meals, meals with high sodium content, sudden changes in temperature, and even excitement, noise, flashing lights, cold temperatures and stress. Weakness may be mild and limited to certain muscle groups, or more severe full-body paralysis. During an attack, reflexes may be decreased or absent. Attacks may last for a few hours or persist for several days. Recovery is usually sudden when it occurs, due to release of potassium from swollen muscles as they recover. Some patients may fall into an abortive attack or develop chronic muscle weakness later in life.

Generalized epilepsy with febrile seizures plus (GEFS+) is a syndromic autosomal dominant disorder where afflicted individuals can exhibit numerous epilepsy phenotypes. GEFS+ can persist beyond early childhood. GEFS+ is also now believed to encompass three other epilepsy disorders: severe myoclonic epilepsy of infancy (SMEI), which is also known as Dravet's syndrome, borderline SMEI (SMEB), and intractable epilepsy of childhood (IEC). There are at least six types of GEFS+, delineated by their causative gene. Known causative gene mutations are in the sodium channel α subunit genes SCN1A, an associated β subunit SCN1B, and in a GABAA receptor γ subunit gene, in GABRG2 and there is another gene related with calcium channel the PCDH19 which is also known as Epilepsy Female with Mental Retardation. Penetrance for this disorder is estimated at approximately 60%.

Na<sub>v</sub>1.4

Sodium channel protein type 4 subunit alpha is a protein that in humans is encoded by the SCN4A gene.

Na<sub>v</sub>1.5

NaV1.5 is an integral membrane protein and tetrodotoxin-resistant voltage-gated sodium channel subunit. NaV1.5 is found primarily in cardiac muscle, where it mediates the fast influx of Na+-ions (INa) across the cell membrane, resulting in the fast depolarization phase of the cardiac action potential. As such, it plays a major role in impulse propagation through the heart. A vast number of cardiac diseases is associated with mutations in NaV1.5 (see paragraph genetics). SCN5A is the gene that encodes the cardiac sodium channel NaV1.5.

Potassium-aggravated myotonia Medical condition

Potassium-aggravated myotonia is a rare genetic disorder that affects skeletal muscle. Beginning in childhood or adolescence, people with this condition experience bouts of sustained muscle tensing (myotonia) that prevent muscles from relaxing normally. Myotonia causes muscle stiffness, often painful, that worsens after exercise and may be aggravated by eating potassium-rich foods such as bananas and potatoes. Stiffness occurs in skeletal muscles throughout the body. Potassium-aggravated myotonia ranges in severity from mild episodes of muscle stiffness to severe, disabling disease with frequent attacks. Potassium-aggravated myotonia may, in some cases, also cause paradoxical myotonia, in which myotonia becomes more severe at the time of movement instead of after movement has ceased. Unlike some other forms of myotonia, potassium-aggravated myotonia is not associated with episodes of muscle weakness.

CLCN1

The CLCN family of voltage-dependent chloride channel genes comprises nine members which demonstrate quite diverse functional characteristics while sharing significant sequence homology. The protein encoded by this gene regulates the electric excitability of the skeletal muscle membrane. Mutations in this gene cause two forms of inherited human muscle disorders: recessive generalized myotonia congenita (Becker) and dominant myotonia (Thomsen).

Na<sub>v</sub>1.1

Nav1.1, also known as the sodium channel, voltage-gated, type I, alpha subunit (SCN1A), is a protein which in humans is encoded by the SCN1A gene.

Na<sub>v</sub>1.2

Navα1.2, also known as the sodium channel, voltage-gated, type II, alpha subunit is a protein that in humans is encoded by the SCN2A gene. Functional sodium channels contain an ion conductive alpha subunit and one or more regulatory beta subunits. Sodium channels which contain the Navα1.2 subunit are called Nav1.2 channels.

Ca<sub>v</sub>1.1 Mammalian protein found in Homo sapiens

Cav1.1 also known as the calcium channel, voltage-dependent, L type, alpha 1S subunit, (CACNA1S), is a protein which in humans is encoded by the CACNA1S gene. It is also known as CACNL1A3 and the dihydropyridine receptor.

SCN1B Protein-coding gene in the species Homo sapiens

Sodium channel subunit beta-1 is a protein that in humans is encoded by the SCN1B gene.

Louis Ptáček is an American neurologist and professor who contributed greatly to the field of genetics and neuroscience. He was also an HHMI investigator from 1997 to 2018. His chief areas of research include the understanding of inherited Mendelian disorders and circadian rhythm genes. Currently, Ptáček is a neurology professor and a director of the Division of Neurogenetics in University of California, San Francisco, School of Medicine. His current investigations primarily focus on extensive clinical studies in families with hereditary disorders, which include identifying and characterizing the genes responsible for neurological variations.

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Notes

Further reading