Myopathies are a heterogeneous group of disorders of the structure or function of the skeletal muscles, that is, the muscles that attach to the bones and consciously move our joints.
Their main common clinical feature is the appearance of muscle weakness and they must be distinguished from neuropathies, motor neuron diseases, or disorders of the neuromuscular synapse.
In an attempt to categorize myopathies, they can first be distinguished into hereditary and acquired, which in turn are briefly distinguished, as below.
HEREDITARY MYOPATHIES
Relatives: usually present in neonatal, infancy, or childhood, with generalized symptoms, typically without progressive deterioration with a characteristic delay in the development and acquisition of the individual's motor skills (eg walking).
Muscular Dystrophies: caused by mutations in the gene encoding dystrophin, a protein important for maintaining the cell membrane of muscle fibers. This includes Duchenne and Becker muscular dystrophies, myotonic dystrophies type I and II, facial-shoulder-brachial, and oropharyngeal muscular dystrophy.
Mitochondrial: due to mutations of mitochondrial DNA. Disorders of the central nervous system, heart, and digestive tract often coexist. Typical examples are MELAS, MERRF, and Kearns-Sayre syndromes.
Metabolic: Glucogenoses (eg Pompe disease) and myopathies due to disorders of lipid metabolism.
Channelopathies: Familial periodic paralysis (hypokalemic or hyperkalemic), rare conditions characterized by episodic muscle weakness, often after exercise.
Congenital myotonias (Steinert's disease and Thomsen's disease) with myotonic phenomena (difficulty relaxing the muscles after their contraction) as the main feature.
ACQUIRED
Autoimmune or Idiopathic inflammatory (eg polymyositis, dermatomyositis, inclusion bodies).
Toxic or medicinal (eg statins, corticosteroids, alcohol, cyclosporine, amiodarone, etc.).
Infectious etiology: bacterial (Lyme disease), viral (HIV, Coxsackie, influenza), parasites (e.g. toxoplasmosis), fungal.
Endocrine: Diabetes mellitus and disorders of the thyroid (hyper- or hypothyroidism), parathyroid (hyperparathyroidism), and adrenal function (Cushing's syndrome and Addison's disease).
From electrolyte disorders (potassium disorders, hypercalcemia, hypermagnesemia, and hypophosphatemia).
Associated with other systemic diseases: eg amyloidosis, sarcoidosis, or paraneoplastic etiology due to underlying malignancy.
SYMPTOMS
As mentioned above, a characteristic clinical symptom of myopathies is the decline in muscle strength, i.e. the appearance of muscle weakness, usually of a centro-medial distribution, with symmetrical involvement of the muscles of the shoulder girdle and pelvic girdle. Many patients describe difficulty rising from a sitting position, difficulty or inability to climb stairs, difficulty combing hair, and difficulty raising upper limbs above shoulder level.
Another common symptom is deep muscle pains (myalgias) of the large muscle groups of the limbs, which should not be confused with the much more common myoperitoneal syndrome, a regional syndrome of chronic pain, which is due to strain or injury to one or more muscles, poor posture and exposure to environmental factors (humidity, cold) and is characterized by the existence of one or more trigger points. The association of myalgias with the production of muscle work is characteristic.
Many patients describe fasciculations (irregular, arrhythmic, involuntary "twitching" muscle fiber contractions that can be felt under the skin), cramps (involuntary painful muscle contractions), and a feeling of general tiredness and fatigue. There is a possibility of diplopia and/or ptosis, dysphagia and dysarthria, abnormal trunk posture (pterygoid scapula, hyperlordosis, scoliosis), dyspnea, and hyperpigmentation of urine. Sensory disturbances are characteristically absent.
DIAGNOSIS
The diagnosis of a myopathy based on a corresponding clinical suspicion includes the following:
Receive an individual and family diagnosis. Among other things, the patient should be asked about any co-morbidities, taking pharmaceutical preparations such as statins, difficulties in everyday life, weight loss, hyperpigmentation of urine, cramps, and myalgias, temporal evolution of symptoms, as well as similar clinical manifestations in relatives.
Thorough clinical examination to look for true muscle weakness as well as its distribution, disorders of muscle trophicity, typically atrophies, but less often hypertrophy (myotonias) or pseudohypertrophy (muscular dystrophies), characteristic rashes (dermatomyositis), signs of arthritis (polymyositis), exophthalmos, etc. a
Extensive laboratory testing. Crucial is the determination of creatine kinase (CK). Increases above ten times normal should be seriously considered. An increase of this size in young people strongly suggests myopathy. Of course, it should not be forgotten that a persistent increase in CK, even in the absence of related symptoms, may hide a latent myopathy and should always be investigated.
Cardiac check-up due to possible coexisting heart disease (electrocardiogram, Holter rhythm, ultrasound, and/or cardiac MRI).
Electromyogram (EMG) to look for a characteristic myopathic pattern. It should be performed in all patients with clinically suspected myopathy.
Study of rates of treatment to rule out neuropathy and myasthenic syndromes.
Magnetic resonance imaging (MRI) of muscles, showing fatty degeneration of the affected muscles.
Muscle biopsy. It is the cornerstone for the diagnosis of mitochondrial, metabolic, and inflammatory myopathies. MRI and EMG can be used to select the appropriate muscle for biopsy.
Genetic testing in the case of hereditary myopathies (eg muscular and myotonic dystrophies).
FORECAST
The prognosis depends on the type of myopathy. In hereditary myopathies, systemic complications such as arterial hypertension, sensorineural deafness, cataracts, seizures, infections, rhabdomyolysis, cardiac arrhythmias, cardiorespiratory and renal failure may appear progressively. Typical is the case of Duchenne muscular dystrophy (DMD), which mainly affects males, with onset of symptoms typically at the age of 4, progressive deterioration with inability to walk around the age of 12, average life expectancy of 28 - 30 years and main cause of death the cardiorespiratory complications of the disease.
The symptoms of some of the hereditary myopathies can be slowed down or even significantly improved (e.g. administration of corticosteroids in DMD). Usually, however, the treatment is simply supportive and symptomatic (appropriate diet, physical therapy, occupational therapy, speech therapy). Genetic counseling is crucial in these cases. On the contrary, acquired myopathies can be successfully treated in a variety of ways, such as by treating the triggering factor (e.g. correcting an electrolyte disorder, restoring thyroid function, stopping a responsible drug) or administering immunomodulatory or immunosuppressive treatment, as in the case of autoimmune or idiopathic inflammatory myopathies.
In conclusion, people with symptoms compatible with a myopathy should be addressed as soon as possible to a specialist neurologist with similar experience and the ability to perform a neurophysiological test, since the diagnosis is often difficult, the required clinical laboratory and imaging tests are extensive, the differential diagnosis is wide and the timely therapeutic approach is vital.
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