Myasthenia Gravis

Question 1

Define:

Answer 1

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

Question 2

Aetiology/risk factors:

Answer 2

Antibody against the nicotinic acetylcholine receptor which interferes with the transmission via depletion of post synaptic receptors.

Question 3

What is lambert-eaton syndrome:

Answer 3

Paraneoplastic subtype of myasthenia gravis

Tested by anti-voltage gates calcium channel antibodies

Question 4

Epidemiology:

Answer 4

8-9/100,000

More common in females under 50

More common in males over 50

Question 5

Symptoms:

Answer 5

Muscles weakness gets worse as the day progresses –> Fatiguability.
Order of muscle weakness: extraocular- bulbar -face - neck - limb girdle- trunk
o NOTE: in Lambert-Eaton syndrome, muscle weakness improves after repeated use

• Ocular symptoms
o Drooping eyelids
o Diplopia

• Bulbar symptoms
o Facial weakness (myasthenic snarl)
o Disturbed hypernasal speech
o Difficulty smiling, chewing or swallowing

Question 6

Signs:

Answer 6

• May be generalised (affecting many muscle groups)
• May be bulbar (affecting the bulbar muscles i.e. those associated with cranial nerves 9, 10, 11 and 12)
• May be ocular

Eye Signs
o Ptosis
o Complex ophthalmoplegia
o Check for ocular fatigue by asking the patient to sustain an upward gaze for 1 min and watch the progressive ptosis that develops
• Ice on Eyes Test
o Placing ice packs on closed eyelids for 2 mins can improve neuromuscular transmission and reduce ptosis

Bulbar Signs
o Reading aloud may cause dysarthria or nasal speech – on counting to 50, the voice fades

Limbs
o Test the power of a muscle before and after repeated use of the muscle
o Tendon reflexes are normal

Question 7

Investigations:

Answer 7

Tensolin test is DIAGNOSTIC

o CK - exclude myopathies
o Serum acetylcholine receptor antibody (positive in 90%)
If seronegative, look for MUSK antibodies (multiple specific tyrosine kinase)
o TFTs (it is associated with hyperthyroidism)
o Anti-voltage gated calcium channel antibody (in Lambert-Eaton syndrome)

• Nerve Conduction Study
o Repetitive stimulation shows decrements of muscle action potential

EMG

CT Thorax/CXR - visualise thymoma in the mediastinum or lung malignancies – found in 20% of MG patients