ELECTROMYOGRAM AND DISORDER OF SKELETAL MUSCLE


INTRODUCTION

 

– The study of electrical activity of the muscle is called Electromyography.

– The graphical representation of electrical activity of muscle is called Electromyogram.

 

• Electromyography Technique :

-Electromyogram is recorded by the use of cathode ray oscilloscope or a polygraph.

– Electrodes used in this technique are ;

(1) Surface electrode or skin electrode -for studying activity of muscle.

(2) Needle electrodes – for studying activity of single motor unit.

 

• Electromyogram :

– Motor unit is the structural basis for Electromyogram.

– Motor unit potential is the electrical potential developed by the activation of the motor unit .

– Duration : 5 to 8 milliseconds.

– Amplitude : 0.5 mV.

– It is monophasic.

 

• Uses of Electromyogram :

– Neuromuscular diseases such as motor neuron lesions , peripheral nerve injury and myopathies can be diagnosed with the help of Electromyogram.

 

• Disorders of skeletal muscles – Myopathy

– Muscular weakness caused due to dysfunction of muscle fiber is called as Myopathy.

– It can be acquired or genetically derived.

– It may or may not involve nervous system.

 

✓ Common diseases of Skeletal muscle are ;

(1) Muscular dystrophy 

(2) Diseases involving muscle tone

(3) Fibrillation and denervation hypersensitivity

(4) Myasthenia gravis

(5) Lambert – Eaton syndrome

(6) McArdle’s disease

(7) Mitochondrial myopathy

(8) Nemaline myopathy.

 

1) Muscular Dystrophy :

– It is characterized by the progressive degeneration of muscle fibers , without the involvement of nervous system.

– Hereditary in origin.

– Muscle fails to regenerate.

– Progressive weakness confine to wheelchair.

– Eventually death can occur.

– Types of muscular dystrophy ;

(1) Duchenne muscular dystrophy.

(2) Becker muscular dystrophy.

 

*Duchenne muscular dystrophy :

✓ It is  Sex- linked disorder characterized by degeneration and necrosis of muscle fibers.

✓ Caused due to absence of the dystrophin in X chromosome.

✓ Dystrophin is necessary for sarcolemma stability.

✓ Muscular weakness is the common symptom.

✓ In severe cases the respiratory muscles becomes weak which results in difficulty in breathing and death.

 

* Becker muscular dystrophy :

✓ It is a sex linked disorder occurs due to reduction in quantity of dystrophin .

✓ Features ;

– Slow progressive weakness of legs and pelvis .

– Pseudohypertrophy of calf muscles

– Difficulty in walking

– Fatigue

– Mental retardation.

 

2) Diseases involving muscle tone :

– It includes ;

√ Hypertonia

√ Hypotonia

√ Myotonia

 

* Hypertonia :

 

– Also called hypertono city , characterized by increased muscle tone and inability of muscle to stretch.

 

– Causes : 

✓ During upper motor neuron lesion , inhibition of lower motor neuron is lost and causes exaggeration of lower motor neuron activity resulting in hypertonia.

✓ In children it is associated with cerebral palsy and the motor pathway is affected.

Such children usually have speech and language delays , with lack of communication skills.

 

– Hypertonia and spasticity :

✓ Hypertonia can occur with or without spasticity.

✓ Spasticity is a motor disorder and due to continuous contraction  stiffness of certain muscle can occur .

✓ Paralysis occurs due to hypertono city is called spastic paralysis.

✓ Hypertonia resists passive movements.

 

* Hypotonia :

– Decrease in muscle tone is called as Hypotonia .

– muscle provide little resistance to stretch.

– Flaccidity can be seen .

 

Causes ;

✓ LMN lesion is the main cause.

✓ Flaccid paralysis results in wasting of muscles.

✓ It can occur due to central nervous system dysfunction , genetic disorders or Muscular disorders.

Clinical condition associated with Hypotonia ;

(1) Down syndrome

(2) Myasthenia gravis

(3) Kernicterus

(4) Congenital cerebellar ataxia

(5) Muscular dystrophy

(6) Congenital hypothyroidism

(7) Hypervitaminosis D

(8) Rickets 

(9) Infant botulism

 

*Myotonia :

– It is a Congenital disease.

– Characterized by continuous contraction of muscle and slow relaxation even after cessation of voluntary act.

– Feature : muscle stiffness sometimes referred as cramps.

✓ More common type of myotonia

✓ Autosomal recessive disorder produced by defective genes contributed by both the parents.

 

3) Fibrillation and denervation hypersensitivity:

– Skeletal muscle degeneration causes Fibrillation with flaccid paralysis and denervation hypersensitivity.

√ Fibrillation : Fine irregular contraction of individual muscle fibers.

√ Denervation hypersensitivity : Due to degeneration , muscle becomes highly sensitive to acetylcholine released from neuromuscular junction.

 

4) Myasthenia Gravis :

– Autosomal disease of neuromuscular junction caused by antibodies to cholinergic receptors .

– Grave weakness of muscle due to inability of neuromuscular junction to transmit impulse from nerve to muscle.

– Causes : Due to development of autoantibodies ( IgG autoantibodies ) against the receptors of acetylcholine.

 

– Symptoms : 

✓ Slow and weak muscular contraction

✓ Inability to maintain prolonged skeletal muscle contraction.

✓ Quick fatigability.

✓ Weakness and fatigability of arms and legs.

✓ Double vision and droopy eyelids

✓ Difficulty in swallowing

✓ Difficulty in speech .

✓ In severe cases , there is a paralysis of muscles .

✓ Patient can die due to paralysis of respiratory muscles.

 

– Treatment : Administration of cholinesterase inhibitors such as neostigmine and pyridostigmine.

 

5) Lambert – Eaton syndrome :

– It is a Neuromuscular disorder caused by development of antibodies against calcium channel in nerve terminal .

– This disease associated with carcinoma is called carcinomatous myopathy.

– Patient have blurred vision and dry mouth.

 

6) McArdle’s disease :

– Glycogen storage disease due to mutation of genes.

– It involves muscle glycogen phosphorylase, necessary for breakdown of glycogen in muscles.

– Common features : muscular pain and stiffness .

 

7) Mitochondrial myopathy :

– It is a inherited disorder occurs due to defect in mitochondria.

 

8) Nemaline myopathy :

– It is a Congenital myopathy characterized by microscopic changes and small rod like structures can be seen in muscle fibers.

– Also called Nemaline rod myopathy.

– Features : Delayed development of motor activities and weakness of muscles.

 

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