Proximal Myopathy

Question 1

Definitions:

1. Myopathy : disease of voluntary muscles
2. Muscular dystrophy : inherited disorders of muscles cells
3. Myasthenia : fatiguable (worse on exercise) weakness
4. Myotonia : sustained contraction / slow relaxation

Answer 1

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Question 2

What inflammatory conditions can cause proximal myopathy?

Answer 2

1. Polymyositis

2. Dermatomyositis

Question 3

What metabolic and endocrine conditions may lead to proximal myopathy?

Answer 3

1. Prolonged high dose steroid use and Cushing’s syndrome may lead to proximal weakness
2. Thyrotoxicosis : severe proximal myopathy
   => assoc. of thyrotoxicosis and myasthenia gravis or hypokalaemic periodic paralysis
3. Hypocalcaemia, rickets, osteoporosis
4. Alcohol & drugs => severe proximal myopathy with muscle pain and necrosis in severe acute alcohol excess

Question 4

What is myasthenia gravis?

Answer 4

Myasthenia gravis is an autoimmune disorder of neuromuscular junction transmission.

Achr Antibodies destroy Ach receptor at post synaptic membrane causing proximal limb weakness and fatiguability as well as bulbar and ocular muscle weakness.

Women > men [2:1] ; age of onset ~30yrs

Myasthenia gravis is assoc with other autoimmin disorders i.e. thyroid disease, rheumatoid disease, pernicious anaemia and SLE

Question 5

What are the clinical features of myasthenia gravis?

Answer 5

Proximal limb weakness and fatiguability

Bulbar and ocular muscle weakness

Speech, facial expression muscles and muscles of mastication affected

Symptoms worse towards the end of the day

Respiratory difficulty

Reflexes initially preserved but disappear after repetitive activity

Thyoma / thymic hyperplasia common

Question 6

What investigations are carried out for myasthenia gravis?

How do you treat it?

Answer 6

Serum anti-AChR antibodies

Repetitive nerve stimulation

Mediastinal MRI or CT for thyomas

Treatment:

=> Oral anticholinesterases i.e. Pyridostigmine => prolongs ACH action by inhibiting its breakdown

=> Immunosuppressant drugs

=> Thymectomy

Question 7

Lambert-Eaton myasthenia-myopathic syndrome is a paraneoplastic manifestation of small-cell bronchial carcinoma due to defective ACH release at the neuromuscular junction.

=> proximal limb weakness
=> weakness improves after a few minutes of contraction
=> Diagnosis confirmed by repetitive nerve stimulation

Answer 7

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Question 8

What is Duchenne muscular dystrophy?

What are the clinical signs?

What is the management?

Answer 8

X-linked recessive disorder

Absence of dystrophin protein essential for cell membrane stability

Leads to proximal limb weakness with calf psuedohypertrophy ; myocardium affected

Creatinine kinase hugely increased

No cure but steroids may delay progression ; physiotherapy helps

Carrier detection in mothers

Question 9

What investigations are carried out in proximal myopathy presentation?

Answer 9

Serum muscle enzymes i.e Creatine Kinase (CK)
=> marker of muscle fibre damage
=> greatly elevated in dystrophies

Electromyography (EMG)
=> Myopathy : short duration spiky polyphasic muscle action potential ; spontaneous fibrillation

Muscle biopsy
=> easy to biopsy
=> provides definitive diagnosis

MRI