OVERVIEW
Metabolic myopathies are a group of disorders resulting from abnormal energy supply within muscle cells. They are characterized by abnormalities in glycogen and lipid metabolism in skeletal muscle, and are primarily associated with abnormalities in glycogen, lipid, or mitochondrial metabolism, as well as abnormalities in nuclear genes. The final diagnosis of the disease requires pathologic examination, and specific typing requires biochemical and genetic testing, with different types of metabolic myopathies having different characteristics. The three main types of disease include mitochondrial disease, lipid metabolic myopathy and glycogen metabolic myopathy.
Etiology
The causes of different types of myopathies are related to different factors: (1) mitochondrial disease is a group of diseases caused by genetic abnormalities resulting in defects in mitochondrial metabolic enzymes; (2) lipid metabolic myopathies are caused by defects in the oxidation process of long-chain fatty acids in muscle; and (3) glycogen metabolic myopathies are a group of diseases caused by inherited disorders of glycogen metabolism.
Symptoms
The main clinical manifestations of mitochondrial disease are multi-system disease with brain and muscle involvement, mostly seen in children and adolescents. Muscle damage mainly manifests as extreme intolerance of fatigue in skeletal muscles; neurological manifestations mainly include extraocular muscle paralysis, stroke, recurrent epileptic seizures, myoclonus, migraine, ataxia, mental retardation, and optic neuropathy, etc. Other systemic manifestations may include heart block, cardiomyopathy, diabetes mellitus, renal insufficiency, pseudo-intestinal obstruction, and short stature.
Lipid metabolism myopathy is common in children, but it can also occur in adults. Most of the cases start slowly and mainly involve skeletal muscles. Symmetric muscle weakness is present in the limbs, and a few may have mild muscle atrophy. In addition, cervical muscles, chewing muscles, swallowing muscles and tongue muscles can be involved. The weakness is aggravated by prolonged muscle movement and is accompanied by muscle swelling and pain. The general course of the disease is from several months to several years. If the disease is caused by carnitine deficiency, and is systemic, in addition to progressive weakness of the proximal skeletal muscles of the limbs, accompanied by cardiomyopathy, and often accompanied by low ketogenic hypoglycemia symptoms.
Glycogen metabolic myopathy is characterized by exercise intolerance, spasmodic pain, and myosinophilia. Other organs, including the liver, kidneys, heart, and brain, may also be involved.
Examination
1. Routine examination
Elevated creatine phosphokinase level, abnormal forearm ischemia test and minimal exercise test.
2. Electromyography
Most of them show myogenic damage, a few are normal, and some are neurogenic damage.
3. Other auxiliary tests (according to clinical manifestations)
Peripheral nerve conduction velocity, evoked potentials, electrocardiogram and imaging studies.
4. Muscle biopsy
Morphologic changes showing abnormalities in mitochondrial, glycogen, and lipid metabolism, and lack of other pathologic changes.
5. Molecular examination
Abnormal decrease in metabolic enzyme activities or mutations in related genes.
Diagnosis
The diagnosis of metabolic myopathy should follow the principle from clinical to auxiliary examination, and both of them should be analyzed comprehensively to reach a diagnostic conclusion.
Presence of skeletal and damage manifestations or accompanied by brain, heart, liver and other organs damage; elevated creatine phosphokinase level, abnormal forearm ischemia test and minimal exercise test; electromyography shows the presence of myogenic damage or neurogenic damage; abnormal changes in peripheral nerve conduction velocity, evoked potentials, electrocardiography and imaging; abnormal mitochondrial, tonicity and fat metabolism in muscle biopsy; molecular examination Abnormal decrease in metabolic enzyme activity or presence of mutations in related genes.
Treatment
1.Mitochondrial disease
CoQ10 is effective in the treatment of mitochondrial disease. In addition, Vitamin B2, L-carnitine, Vitamin K3, Vitamin K1 are also commonly used in the treatment of mitochondrial disease.
2. Lipid metabolism myopathy
Vitamin B2 combined with carnitine, L-carnitine and medium chain triglycerides can be used.
3. Glycogen metabolic myopathy
There is no effective treatment at present. Only a few cases have been reported in which the symptoms improved to some extent with the use of a high protein diet.
Prognosis
The prognosis of this disease is poor.
Dietary management
Glycogen metabolic myopathy is treated with a high protein diet such as 50% carbohydrates, 20% fat and 25% to 30% protein. For lipid metabolizing myopathies, a lifelong low-protein, low-fat, high-carbohydrate diet is recommended.