Myotonia is a symptom of muscle disease, the essence of which is an obstructed diastasis of the muscle and its stiffness. This symptom occurs in the course of various disease entities. Myotonic activity originates in the muscle cell and clinically manifests as active myotonia or percussive myotonia. Myotonic syndromes are divided into non-dystrophic myotonia and myotonic dystrophies. The diagnosis of myotonic syndromes is based on electromyographic (EMG) examination.
Table of contents:
- How does myotonia arise?
- What are the symptoms of myotonia?
- Diagnostic tests
- Myotonic syndromes
- Treatment of myotonia
How does myotonia develop?
Impaired muscle relaxation is caused by electrical instability and hyperactivity of the muscle fibre membrane, resulting in repeated electrical discharges of the cell, which clinically manifests as active and percussive myotonia. Myotonic activity originates in the muscle cell as it persists despite blocked neuromuscular conduction.
What are the symptoms of myotonia?
Clinically, a distinction is made between active myotonia and percussive myotonia. Active myotonia can affect many muscles, most commonly the lower limbs, hands, eyelids or masseter. Active myotonia can be most easily visualised with a simple clinical test by instructing the patient to clench and relax their fist. Myotonic symptoms are most clearly seen in the muscles of the hand, impairing the performance of precise movements such as shaking hands or opening doors by the handle. The activity reveals very slow and impaired muscle diastole after free contraction. Percussion myotonia manifests as a prolonged contraction after hitting the muscle belly. It is induced by striking the muscle belly. Most commonly, this test is carried out on the belly of the finger extensor muscles on the forearm, the glenoid muscles of the thumb or the tongue muscle.
The appearance of myotonia symptoms is influenced by exertion and cold. In the majority of patients, its symptoms decrease with repeated movements - a phenomenon known as warm-up. In some patients, however, the opposite phenomenon is observed - myotonia increases with repeated movements (paradoxical myotonia). The warm-up phenomenon occurs mainly in chloride channelopathies and paradoxical myotonia in sodium channelopathies. An additional symptom in patients with some myotonic syndromes is transient muscle weakness, improving with repetition of free movements. Muscle weakness or paralysis may also occur under the influence of cold, as in congenital paramyotonia.
Diagnostic tests
In addition to the characteristic features on neurological examination, electromyographic examination is the basis for the diagnosis of myotonic syndromes. In EMG, strings of myotonic discharges are present. These are high-amplitude, variable-frequency discharges that occur during diastole and are the cause of its prolongation and obstruction. Discharge trains consist of fibrillations, sometimes positive sharp waves or small three-phase potentials of the motor unit with frequencies up to 150 Hz. The amplitude and frequency of the strings decrease to about 20-30 Hz, causing a characteristic appearance and producing the sound of a diving bomber.
Myotonic bands
In myotonic syndromes, a distinction is made between non-dystrophic my otonia and myotonic dystrophies. Non-dystrophic myotoniae are caused by mutations in genes encoding chloride and sodium ion channels. Chloride channel myotonia includes Thomsen's and Becker 's myotonia, whereas sodium channel myotonia includes congenital paramiotonia, fluctuant myotonia, permanent myotonia and potassium-dependent myotonia.
Thomsen's myotonia is inherited autosomal dominantly. It occurs much less frequently than the recessive form, Becker's myotonia. The symptoms of the disease occur very early, sometimes from birth. Stiffness usually affects the muscles of the lower limbs, which can lead to falls in the child, as well as the eyelids, hands and masseter muscles. The stiffness decreases with repeated movements - a warm-up phenomenon. Myotonia is non-painful, although some patients may suffer from painful muscle cramps. Muscle strength is normal or even greater than average. The athletic physique of patients is characteristic. They may participate in sports requiring strength but not speed. Some patients may experience an exacerbation of their symptoms during pregnancy.
Becker's myotonia is a generalised myotonia with recessive inheritance. It is characterised by a later onset (4.-12 years of age) and more severe and generalised symptoms at times than in Thomsen's myotonia. Patients' appearance is notable for an athletic physique with hypertrophy of mainly the muscles of the lower limbs and buttocks and a deepened lumbar lordosis. In addition to stiffness, most patients experience muscle weakness, to the greatest extent after rest, which decreases with repetitive movements.
Myotonia, photo: shutterstock
Congenital paramyotonia is inherited in an autosomal dominant manner. Symptoms can present from birth. Exposure to cold intensifies the symptoms. Muscle stiffness is accompanied by muscle weakness and even muscle paralysis. The weakness occurs immediately after exertion and can last for 1.5 hours. Facial muscle stiffness causes difficulty opening the eyes and speaking. With repeated movements, symptoms worsen - paradoxical myotonia.
In myotonia with fluctuating symptoms, the disease pattern is analogous to congenital myotonia, but with variable severity over days. Symptoms are provoked by exertion, are very sensitive to potassium and do not change under cold. Muscle weakness is never present.
Patients with fixed-symptom myotonia have severe persistent stiffness and marked muscle hypertrophy, especially of the neck and shoulders. Respiratory distress may also be present.
Steinert's myotonic dystrophy type 1 is a multisystem disease with onset of symptoms in infancy. It manifests with atrophy of the temporalis muscles, masseter muscles, and the limbs - mainly in the periphery. Atrophy of the facial muscles results in a characteristic appearance - the face is narrow, myopic in appearance, sometimes with slightly drooping eyelids. Muscle stiffness affects the hands and forearms, is less severe compared to myotomies and decreases with repeated movement. Percussive myotonia is present on the tongue and on the knuckle of the thumb.
Extramuscular symptoms include the following signs:
- ocular
- cataracts,
- ocular palsy,
- retinal pigmentary degeneration,
- endocrine
- testicular atrophy,
- diabetes mellitus,
- pituitary disorders,
- cutaneous
- frontal alopecia,
- cardiovascular
- hypotension,
- syncope,
- bicuspid valve prolapse,
- sudden death,
- respiratory
- sleep apnoea,
- excessive sleepiness,
- mental retardation.
Myotonic dystrophy type 2, differs from myotonic dystrophy type 1 in the later onset of symptoms - it does not find them in childhood. Muscle weakness predominates in the iliac rim. Myotonic symptoms increase with heat and decrease with cold. Extramuscular symptoms are the same as in myotonic dystrophy type 1.
Neuromyotonia caused by pathology of potassium channels in nerve endings can present in congenital and acquired forms. It is characterised by muscle stiffness as a result of spontaneous sustained muscle fibre activity. Clinical manifestations of the syndrome include stiffness, myokymias, cramps and excessive sweating. Nerve blockade with xylocaine results in resolution of the myotonic activity. Carbamazepine is used in the treatment, and plasmapheresis has a beneficial effect in immune-mediated cases.
Schwartz-Jampel syndrome combines features of myotonia and bone dysplasia. The course of the disease is stationary, with symptoms present from early childhood.
Myotonia, photo: shutterstock
Treatment of myotonia
The aim of myotonia treatment is to stabilise the muscle fibre membrane. A number of drugs have this effect, including phenytoin, disopyramide, tocainide and mexiletine. These drugs do not affect the muscle weakness element, only the stiffness.
