Muscle disorders Flashcards by E Thomas

Describe and draw the structure of a skeletal muscle?

Muscle is made of fascicles

Fascicles made up of muscle fibres (the muscle cells)

Muscle fibres made of myofibrils

Myofibrils made of actin (thin) and myosin (thick) filaments

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What are the names of the membranes that make up skeletal muscle?

Epimysium surrounds muscle

Perimysium surrounds fascicle

Endomysium surrounds muscle fibres

Sarcolemma is found just under the endomysium, surrounding the muscle fibres (cells)

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What is the sarcolemma?

The cell membrane of the muscle fibre (cell)

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Which is thick and which is thin?

Actin or myosin?

Myosin is thick

Actin is thin

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What is a neuromuscular junction?

Where a nerve synapses with the muscle and an impulse from the brain activates the muscle to contract

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Name some muscle disorders?

Myasthenia Gravis

Muscular dystrophy:
- Duchenne’s / Becker’s

Myopathy
Myositis
Myotonia

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What is myopathy?

A general name for a disease of muscle tissue

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What is myositis?

Inflammation of muscle

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What is myotonia?

A disorder of muscle tone

Characterised by an inability to relax voluntary muscle after excercise

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Name the enzyme that is linked with muscle and is raised in muscular dystrophy and inflammatory conditions but not for M. gravis or others?

Creatinine Kinase

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What is myasthenia gravis?

An acquired autoimmune disease affecting the acetylcholine receptor

Causing weakness in muscles

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Which muscles are most commonly affected in MG?

Ocular
Bulbar

Proximal skeletal: shoulders, thighs

Axial skeletal: neck and trunk, muscles of respiration

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What are bulbar muscles?

Muscles of the mouth and throat that control swallowing and speaking

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What is the progression pattern of MG?

Fluctuating pattern of remission and crises

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Who gets MG?

Young women (20-35)
Older men (60-75)

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What causes MG?

We don’t really know but it is thought to have a genetic component

There are links with thymus too: thyoma is present in 75% of MG patients

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Pathophysiology of MG?

Autoantibodies are created which attach the postsynaptic acetylcholine receptors at the neuromuscular junction

Motor impulses are struggle to get from the nerves to the muscles to make them contract = weakness

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Clinical features of MG?

Variable between patients

Fatigability of muscle after exercise, that improves on rest

Often ocular muscles are affected: ptosis

The weakness moves down: difficulty chewing, talking, swallowing, respiratory difficulties

What is ptosis, and what features of ptosis would indicate to you the person may have MG?

Ptosis = drooping of eyelid

Unilateral
If it is improved by sleep or an ice pack

Investigations of MG?

Blood:

won’t have creatinine kinase
autoantibodies against Ach receptors

(TENSILON test - not used so much any more)

Electromyography: measure how fatigable muscle is

CT / MRI to look for thymus hyperplasia

What is the TENSILON test?

Test for muscle fatigability

Patients are given two drugs:
- edrophionium: prevents breakdown of ACh

atropine prevents damage to heart while doing this test

This rapidly improves the symptoms of MG, but only for a few minutes

Management of MG?

Oral acetylcholinesterase inhibitor

Immunosupression: steroids

DMARDS: methotrexate

Monitoring of thymus
Thymectomy

Why are oral actelycholinesterase inhibitors used in MG?

List one.

Prevent destruction of acetylcholine by acetylcholinesterase in the synaptic space

Pyridostigmine

How would you manage MG in a crisis phase?

IV immunoglobluins

Plasmapheresis

What is the prognosis of MG?

Near normal life expectancy, can occasionally be fatal due to respiratory failure, but this rare

What is thyoma? Which muscle disorder is it linked with?

Hyperplasia of thymus It is benign but can become cancerous It is seen in 75% of people with myasthenia gravis

What is muscular dystrophy? | What are the features?

An umbrella term for a number of inherited diseases Causing progressive weakness and wasting of muscles Replacement of muscle tissue with fibrous connective tissue Cardiac and resp involvement

Name the 2 main types of muscular dystrophy?

Duchenne's | Becker's

Describe Duchenne's muscular dystrophy?

Very severe form of muscular dystrophy Progressive wasting of muscles Most do not survive past adolescence

Who gets Duchenne's MD?

Boys only, it is an X linked condition

What is an X linked condition?

A condition caused by a mutation on the X chromosome inherited by a boy from his mother

Why are girls not affected by muscular dystrophy?

It is X-linked A girl inherits one X chromosome from her mother and one for her father, so if she inherits an X chromosome with the mutation on it, she'll have another X chromosome to back it up, so no disease is seen Boys only have 1 X chromosome so if it is mutated, there is not another one to back it up?

What is the mutation in Duchenne's?

The gene that codes for the molecule dystrophin is severely mutated so dystrophin is produced at all

What is dystrophin and what does it do?

Dystrophin is the molecule that is not produced in some boys, causing Duchenne's Without it, there is excessive calcium entry into the cell This causes excess stress within the cell, damaging the sarcolemma and resulting in cell death

Clinical features of Duchenne's?

Abnormal gait, tummy sticks forward Gower's sign Difficulty with standing up, walking, motor skills Pseudohypertrophy of calfs, due to replacement of muscle with fat Usually unable to walk at age 10 Death during teens or early 20s

What is Gower's sign?

A boy affected by muscular dystrophy, when he tries to stand up he will use his hands to 'walk up' his legs to stand upright

When is Duchenne's typically diagnosed?

Between 5 and 5 1/2

When do boys with Duchenne's usually: - become wheelchair bound - pass away?

By age 10 they are usually unable to walk They usually die during teenage years or early 20s

Investigations of Duchenne's?

Blood: - creatinine kinase raised Positive Gower's test Genetic tests, family history Muscle biopsy shows absence of dystrophin

Treatment of Duchenne's?

Corticosteroids: are aggressively used before the age of 10, this can dramatically reduce the aggressiveness of Duchenne's to more of a Becker's picture Physiotherapy: can prolong life of muscle Mechanical ventilation

What is the inheritance pattern of Duchenne's and Becker's?

Recessive X-linked

What is the difference between Becker's and Duchenne's?

In Becker's there is insufficient dystrophin produced In Duchenne's there is no dystrophin produced Duchenne's is more severe

What is the life expectancy of people with Becker's muscular dystrophy?

Around 40, onwards | Some live a nearly normal lifespan

Becker's is in general a much less severe condition that Duchenne's, what is more severe in Beckers?

Cardiac involvement tends to be worse in Becker's