Myasthenia Gravis Flashcards
What is Myasthenia Gravis?
An autoimmune disease affecting the neuromuscular junction producing weakness in skeletal muscles
Autoantibodies against postsynaptic nicotinic Ach receptors (AChR) at the neuromuscular junction
What are the different causes of Myasthenia Gravis?
- Most common form has antibodies against the nicotinic acetylcholine receptor which interferes with the neuromuscular transmission via depletion of post-synaptic receptor sites.
- Lambert-Eaton Syndrome - paraneoplastic subtype of myasthenia gravis caused by autoantibodies against pre-synaptic calcium channels, leading to impairment of acetylcholine release
- Can be paraneoplastic (from small cell lung cancer) or autoimmune - Myasthenia gravis is associated with other autoimmune conditions (e.g. pernicious anaemia)
Summarise the epidemiology of myasthenia gravis
Prevalence: 8-9/100,000
More common in FEMALES under 50
More common in MALES after 50
What are the presenting symptoms of myasthenia gravis
- Muscle weakness that worsens with repetitive use or towards the end of the day – FATIGUABILITY
- Order of muscle weakness: extraocular 🡪 bulbar 🡪 face 🡪 neck 🡪 limb girdle 🡪 trunk
- NOTE: in Lambert-Eaton syndrome, muscle weakness improves after repeated use - Ocular symptoms
- Ptosis (Drooping eyelids)
- Diplopia (double vision) - Bulbar symptoms
- Facial weakness (myasthenic snarl)
- Disturbed hypernasal speech
- Difficulty smiling, chewing or swallowing - THYMIC HYPERPLASIA
NORMAL NERVE- AFFECTS MUSCLES NOT NERVES
What signs of myasthenia gravis can be found on physical examination
- 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)
- NOTE: bulbar = relating to the medulla oblongata (cranial nerves 9, 10, 11 and 12 have their nuclei in the medulla) - May be ocular
- Eye Signs
- Ptosis
- Complex ophthalmoplegia
- 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
- Placing ice packs on closed eyelids for 2 mins can improve neuromuscular transmission and reduce ptosis - Bulbar Signs
- Reading aloud may cause dysarthria or nasal speech – on counting to 50, the voice fades - Limbs
- Test the power of a muscle before and after repeated use of the muscle
- Tendon reflexes are normal AFFECTS MUSCLES NOT NERVES - Lambert Eaton: gait difficulty before eye signs, autonomic involvement (dry eyes, constipation), hyporeflexia and weakness that improves after exercise – depressed tendon reflexes are because less transmitter is released but reflexes may increase after maximum voluntary contractions due to build up of transmitter in the synaptic cleft.
What investigations are used to diagnose/ monitor Myasthenia gravis?
- Serum AChR antibody analysis → positive. Diagnostic test. Most specific test.
- Electromyography (EMG) → shows decremental response to repetitive nerve stimulation. Most sensitive test.
- CT Thorax → exclude thymoma
- Muscle-Specific Tyrosine Kinase antibodies → positive
- Tensilon Test → IV edrophonium reduces muscle weakness temporarily
How is myasthenia gravis managed?
- Acetylcholinesterase Inhibitors (Long-Acting) → pyridostigmine
- If too much, can cause cholinergic crisis (too much ACh) - Corticosteroids → prednisolone (if symptoms not resolved with pyridostigmine)
- Thymectomy (removal of the thymus- responsible for the production and maturation of immune cells)
What complications may arise from myasthenia gravis?
myasthenic crisis [life-threatening manifestation of myasthenia gravis (MG) defined by respiratory insufficiency that requires the use of invasive or non-invasive ventilatio] (Tx = plasmapheresis and IV immunoglobulins), respiratory failure, impaired swallowing, acute aspiration, secondary pneumonia
