Neuromuscular disorders include a large range of diseases affecting the skeletal/voluntary muscle (muscle that we can control) function either directly related to muscle pathologies or indirectly related to problems with the nerves or the neuromuscular junctions. Voluntary movements of skeletal muscles are brought out by electrical signals from the brain. The signals first travel down from the brain via upper motor neurons to the spinal cord where they are further transported to the lower motor neurons. The lower motor neurons will send their axonal processes in the peripheral nerves in our limbs in which the electrical signals are further transmitted down to end of the nerves causing release of special chemicals into the neuromuscular junctions. These chemicals are then taken up by receptors in the muscles and the muscles will contract to produce the wanted movements. The peripheral nerves also contain sensory neurons which carry the information received from the environment via senses like touch, pain, temperature etc. in the form of internal electric signals and bring them back to the brain via the spinal cord.
The causes of neuromuscular disorders are heterogeneous which could be genetic/inherited related or acquired or due to unknown causes. The genetic /inherited causes again encompass a broad group of diseases involving malfunctions of different parts of the neuromuscular system stated above. It can be problems with structural components of muscles like the membrane proteins, filaments, myofibril, endomysium etc. (e.g. muscular dystrophies and congenital myopathies.). They also include those involving malfunctions of: ion channels (e.g. myotonia congenita, paramyotonia congenita, hyperkalemic/hypokalemic periodic paralysis ), energy production of muscle cells ( e.g. mitochondrial diseases and other inborn error of metabolism ), the lower motor neurons (e.g. spinal muscular atrophies, amyotrophic lateral sclerosis ), the peripheral nerves ( e.g. Charcot-Marie-Tooth diseases, Giant axonal neuropathy) and neuromuscular junctions ( Congenital myasthenic syndromes). In the acquired causes, the different components of the neuromuscular system can be affected due to disease processes from autoimmune ( e.g. myasthenia gravis, polymyositis), inflammatory (e.g. Guillain Barre syndrome, dermatomyositis, inclusion body myositis), paraneoplastic (e.g. Lambert-Eaton myasthenic syndrome), endocrine ( e.g. hyper-/hypo-thyroid myopathy), infective (e.g. viral myositis, botulism, tetanus), metabolic (e.g. electrolyte disturbances, acquired mitochondrial dysfunction), nutritional (e.g. vitamin B12 deficiency), iatrogenic (e.g. statin myopathy), traumatic and environmental aetiologies (e.g. insecticide poisoning, heavy metal poisoning).
Neuromuscular disorders can present at any age depending on the causes. The symptoms include the followings:
Muscle weakness
Easy muscle fatigue
Muscle loss/atrophy
Muscle pain
Muscle twitching (fasciculations)
Cramps
Numbness
Paresthesia
Seeing doubles
Drooping of eyelids
Breathing problem
Chewing, swallowing problem
Joint contractures
The diagnoses are made based initially on the history, symptoms and physical examinations. Then followed by blood testing including checking the electrolytes, muscle enzymes, hormones, inflammatory markers etc. and in appropriate clinical settings, the auto-antibodies. Imagings of the muscles and nerves by ultrasound or magnetic resonance imaging (MRI) are further investigations to be done. Nerve conduction studies and electromyographies (studies measuring electrical activities in muscles) also aid the diagnostic processes. When further information about the microstructure of muscles and nerves are needed, muscle +/- nerve biopsies will be the next step in the investigation process. In certain disease groups, checking the lung functions, heart functions and swallowing functions are also necessary. When a genetic /inherited cause is suspected, genetic testing will be performed.
Management and treatment are much dependent on the underlying causes. In the acquired neuromuscular disorders, treatments are directed to the underlying pathologies, for examples immuno-suppressants in autoimmune related ones, hormonal replacements and nutritional supplements in endocrine and nutritional deficiency related ones respectively. In most of the genetic/ inherited related neuromuscular diseases, there are no cure but in some of them, drug treatments are available to treat the symptoms, to slow down the disease progresses and delay the deteriorations in functions like steroid treatment in Duchenne muscular dystrophies. Physiotherapy and occupational therapy are general measures to improve functions and in certain situations, surgery can help that too e.g. joint contracture release and scoliosis surgeries.
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