Myasthenia Gravis

Question 1

What is the definition of myasthenia gravis?

Answer 1

An autoimmune disease affecting the neuromuscular junction producing weakness in skeletal muscles

Question 2

Explain the aetiology and risk factors involved with myasthenia gravis?

Answer 2

• Impaired transmission at NMJ
• Most commonly due to auto antibodies against nictonic AcH receptors
• Myasthenia gravis is associated with other autoimmune conditions such as pernicious anaemia

Question 3

What is Lambert Eaton syndrome?

Answer 3

A paraneoplastic subtype of myasthenia gravis caused by autoantibodies against pre-synaptic Ca2+ channels -> impaired AcH release

Question 4

What is the epidemiology of myasthenia gravis?

Answer 4

Prevalence: 8-9/100,000

More common in FEMALES at younger ages

Equal gender distribution in middle age

Question 5

What are the presenting symptoms of myasthenia gravis?

Answer 5

• Muscle weakness that worsens with repetitive use or towards end of the day
  (In Lambert-Eaton syndrome - muscle weakness improves after repeated use)
• Ocular symptoms: Drooping eyelids and diplopia
• Bulbar symptoms (affecting bulbar muscles - CN 9,10,11 and 12):
  Facial weakness (myasthenic snarl)
  Disturbed hypernasal speech
  Difficulty smiling, chewing or swallowing

Question 6

What are the signs of myasthenia gravis on physical examination?

Answer 6

May be generalised affecting many musc groups or bulbar (relating to medulla oblungata and the CN 9-12 nuclei are in the medulla)

May be ocular

• Eye signs (ptosis, complex opthalmoplegia, check for ocular fatigue - ask pt to sustain an upward gaze for 1 min and watch progressive ptosis develops)
• Ice on eye tests (placing ice packs on closed eye lids for 2 mins should improve neuromuscular transmission hence reduce ptosis)

Bulbar signs (reading aloud may cause dysarthria or nasal speech)

Limbs (test the power of a muscle after repeated use of the muscle)

Question 7

What are the appropriate investigations for myasthenia gravis?

Answer 7

Bloods

• CK - exclude myopathies
• Serum AcH receptor antibody (+ve in 80%)
• TFTs (if associated with hyperthydroidism)
• Anti-voltage gated calcium channel antibody (Lambert-Eaton syndrome)

Tensilon Test:
Short-acting anti-cholinesterase (edrophonium bromide) increases acetylcholine levels and causes a rapid and transient improvement in clinical features
Risk of bradycardia - so is generally avoided

Nerve Conduction Study:
Repetitive stimulation shows decrements of muscle action potential

EMG

CT Thorax/ CXR - Visualise thymoma in the mediastinum or lung malignancies