Myasthenia Gravis

Question 1

Define Myasthenia Gravis

Answer 1

Autoimmune disease affecting the neuromuscular junction, causing weakness of skeletal muscles

Question 2

Aetiology of Myasthenia Gravis

Answer 2

Most commonly auto-antibodies against the nicotinic acetylcholine receptor (nAChR)

Lamber-Eaton (paraneoplastic): auto-antibodies against pre-synaptic calcium ion channels to impair acetycholine release

Associated with paraneoplastic cancer (small cell lung cancer)

Question 3

Risk factors for Myasthenia Gravis

Answer 3

Autoimmune conditions e.g. pernicious anaemia
Thymoma development (breakdown in immune tolerance)

Question 4

Symptoms of Myasthenia Gravis

Answer 4

Muscle fatiguability 
Muscle weakness that worsens with repetitive use or towards the end of the day (Lamber eaton: improves with repeated use)
Diploplia
Dysphagia
Dysarthria
Proximal limb weakness
Facial paresis (myasthenic snarl)
Difficulty smiling, chewing, swallowing)
SOB

Question 5

Signs of Myasthenia Gravis on examination

Answer 5

May be normal initially

Eyes:
Bilateral ptosis, may be asymmetrical
Complex ophthalmoplegia
Ocular fatiguability (ask patient to sustain upward gaze for 1 min)
“Ice on eyes” - place ice-packs on eyelids for 2 mins which can improve neuromuscular transmission to reduce ptosis

Bulbar:
Reading aloud provokes dysarthria or nasal speech after 3 minutes

Limbs:
Test power of muscles before and after repeated use e.g. 20 repetitions

Question 6

Investigations for Myasthenia Gravis

Answer 6

Serum AChR antibody analysis: +ve (80-90%)
MuSK antibody analysis: +ve (70% or AChR seronegative)
Tensilon testing: return of power within 1 minute (short acting acetylcholinesterase given to test for improvement)

CK: normal (exclude myopathy)
TFTs: associated hyperthyroidism

Nerve conduction study: decrements of muscle action potentials
EMG: “jitter”

CT-thorax, CXR: potential THYMIC enlargement - hyperplasia (70%) or thymoma (10%), may see small cell lung cancer