Disorders of skeletal muscles

 

DISORDERS OF SKELETAL MUSCLES – MYOPATHY

Myopathy is a muscular disorder in which the dysfunction of muscle fiber leads to muscular weakness. Myopathies may be acquired or genetically derived. These diseases may or may not involve the nervous system.

skeletal muscles

Common diseases of skeletal muscles are:

1. Muscular dystrophy

2. Diseases involving muscle tone

3. Fibrillation and denervation hypersensitivity

4. Myasthenia gravis

5. LambertEaton syndrome

6. McArdle disease

7. Mitochondrial myopathy

8. Nemaline myopathy.

MUSCULAR DYSTROPHY

Muscular dystrophy is a disease characterized by progressive degeneration of muscle fibers, without the involvement of nervous system. Mostly it has a hereditary origin. The muscles fail to regenerate, resulting in progressive weakness and confinement to a wheelchair. Eventually, death occurs. Common types of muscular dystrophy are Duchenne muscular dystrophy and Becker muscular dystrophy.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy is a sexlinked recessive disorder. It is due to the absence of a gene product called dystrophin in the X chromosome. Dystrophin is necessary for the stability of sarcolemma. This disease is characterized by degeneration and necrosis of muscle fibers. The degenerated muscle fibers are replaced by fat and fibrous tissue. Common symptom is the muscular weakness. Sometimes, there is enlargement of muscles (pseu do hypertrophy). In severe conditions, the respiratory muscles become weak, resulting in difficulty in breathing and death.

Becker Muscular Dystrophy

Becker muscular dystrophy is also a sexlinked disorder. It occurs due to the reduction in quantity or alteration of dystrophin. Common features of this disorder are slow progressive weakness of legs and pelvis, pseudohypertrophy of calf muscles, difficulty in walking, fatigue and mental retardation.

DISEASES INVOLVING MUSCLE TONE

Hypertonia

Hypertonia or Hypertonia occurs in upper motor neuron lesion. During the lesion of upper motor neuron, inhibition of lower motor neurons (gammamotorneurons in the spinal cord) is lost. It causes exaggeration of lower motor neuron activity, resulting in hypertonia. In children, hypertonia is associated with cerebral palsy (permanent disorder caused by brain damage, which occurs at or before birth and is characterized bynicity is a muscular disease characterized by increased muscle tone and inability of the muscle to stretch.


Hypertonia and spasticity

Hypertonia may be related to spasticity, but it is present with or without spasticity. Spasticity is a motor disorder characterized by stiffness of the certain muscles due to continuous contraction. Hypertonicity is one of the major symptoms of spasticity. Paralysis (complete loss of function) of the muscle due to hypertonicity is called spastic paralysis.

In hypertonia, there is a resistance to passive movement and it does not depend on velocity (the speed at which the movement occurs), where as in spasticity there is an increase in resistance to sudden passive movement. It is velocity dependent, i.e. faster the passive movement stronger the resistance.

Hypotonia

Hypotonia is the muscular disease characterized by decreased muscle tone. The tone of the muscle is decreased or lost. Muscle offers very little resistance to stretch. Muscle becomes flaccid (lack of firmness) and the condition is called flaccidity.

Causes

The paralysis of muscle with hypotonicity is called flaccid paralysis and it results in wastage of muscles. Hypotonia may also occur because of central nervous system dysfunction, genetic disorders or muscular disorders.

Clinical conditions associated with hypotonia are:

i. Down syndrome (chromosomal disorder, charact erized by physical and learning disabilities)

ii. Myasthenia gravis

iii. Kernicterus (brain damage caused by jaundice in infants

iv. Congenital cerebellar ataxia (incoordination)

v. Muscular dystrophy

vi. Congenital hypothyroidism

vii. Hypervitaminosis D

viii. Rickets

ix. Infant botulism (paralysis due to botulinum toxin).

 

Myotonia

Myotonia is a congenital disease characterized by continuous contraction of muscle and slow relaxation even after the cessation of voluntary act. The main feature of this disease is the muscle stiffness, which is sometimes referred as cramps. Muscle relaxation is delayed. This type of muscular stiffness with delayed relaxation causes discomfort during simple actions like walking, grasping and chewing. The muscles are enlarged (hypertrophy) because of the continuous contraction. Myotonia sets in during early to late childhood and it is not progressive.

Cause

Myotonia is caused by mutation in the genes of channel proteins in sarcolemma. Such disorders are called channelopathies.

Types

Myotonia is of two types:

i. Becker-type myotonia or generalized myotonia, which is more common than Thomsentype myotonia. It is an autosomal recessive disorder produced by defective genes contributed by both the parents

ii. Thomsen-type myotonia is relatively rare and it is an autosomal recessive disorder produced by defective gene contributed by one parent.

MYASTHENIA GRAVIS

Myasthenia gravis is an autoimmune disease of neuromuscular junction caused by antibodies to cholinergic receptors. It is characterized by grave weakness of the muscle due to the inability of neuromuscular junction to transmit impulses from nerve to the muscle. It is a serious and sometimes a fatal disease.

Causes

Myasthenia gravis is caused due to the development of autoanti bodies (IgG autoantibodies) against the receptors of acetylcholine. That is, the body develops antibodies against its own acetylcholine receptors. These antibodies prevent binging of acetylcholine with it receptors or destroy the receptors. So, though the acetylcholine release is normal, it cannot execute its action.

Symptoms

Muscles which are more susceptible for myasthenia gravis are muscles of neck, limbs, eyeballs and the muscle responsible for eyelid movements, chewing, swallowing, speech and respiration.

Common symptoms are:

i. Slow and weak muscular contraction because of the defective neuromuscular activity

ii. Inability to maintain the prolonged contraction of skeletal muscle

iii. Quick fatigability when the patient attempts repeated muscular contractions

iv. Weakness and fatigability of arms and legs

v. Double vision and droopy eyelids due to the weakness of ocular muscles

vi. Difficulty in swallowing due to weakness of throat muscles

vii. Difficulty in speech due to weakness of muscles of speech.

In severe conditions, there is paralysis of muscles. Patient dies mostly due to the paralysis of respiratory muscles.

Treatment

Myasthenia gravis is treated by administration of cholinesterase inhibitors such as neostigmine and pyridostigmine. These drugs inhibit cholinesterase, which degrades acetylcholine. So acetylcholine remaining in the synaptic cleft for long period can bind with its receptors.

LAMBERT-EATON SYNDROME

Lamberteaton syndrome is a disorder of neuromuscular junction caused by development of antibodies against calcium channel in the nerve terminal, resulting in reduction in the release of quanta of acetylcholine. This disease is commonly associated with carcinoma. So, it is also called carcino ma tous myopathy. This disease is characterized by several features of myasthenia gravis. In addition, the patients have blurred vision and dry mouth.

McARDLE DISEASE

McArdle disease is a glycogen storage disease (accumulation of glycogen in muscles) due to the mutation of genes involving the muscle glycogen phosphorylase, necessary for the breakdown of glycogen in muscles. Muscular pain and stiffness are the common features of this disease.

MITOCHONDRIAL MYOPATHY

Mitochondrial myopathy is an inherited disease due to the defects in the mitochondria (which provide critical source of energy) of muscle fibers.

NEMALINE MYOPATHY

Nemaline myopathy is a congenital myopathy characterized by microscopic changes and formation of small rod-like structures in the muscle fibers. It is also called nemaline-rod myopathy. The features are delayed development of motor activities and weakness of muscles.

Post a Comment

0 Comments