Myasthenia Gravis Flashcards
Define Myasthenia Gravis
Chronic autoimmune disorder affecting the post-synaptic membrane at the NMJ in skeletal
muscle
Lambert-Eaton myasthenic syndrome (LEMS) – paraneoplastic syndrome caused by
autoantibodies to calcium ion channels -> impaired ACh release
What are the causes/risk factors of Myasthenia Gravis?
Autoantibodies to nAChR -> impaired NMJ transmission
• Autoimmune conditions e.g. pernicious anaemia
• Thymoma (breakdown in immune tolerance in thymus)
What are the symptoms of Myasthenia Gravis?
- Muscle weakness that worsens with activity (activity improves weakness in LEMS)
- Drooping eyelids
- Diplopia
- Facial weakness (myasthenic snarl)
- Disturbed hypernasal speech
- Difficulty smiling
- Dysphagia (chewing and swallowing)
- Nasal regurgitation of fluids
- SOB – myasthenic crisis
What are the signs of Myasthenia Gravis?
- Bilateral ptosis
- Complex ophthalmoplegia
- Ocular fatiguability (unable to sustain upward gaze for 1 min)
- Dysarthria/nasal speech after 3 mins of reading aloud
What investigations area carried out for Myasthenia Gravis?
• CK - normal, to exclude myopathies.
• Antibodies - Anti-nAChR Antibodies
- MuSK Antibodies if atypical features
- Lambert-Eaton Syndrome suspected: Anti-calcium channel Antibodies.
• Tensilon Test - short-acting anti-cholinesterase increases ACh by blocking its metabolism and causes rapid and transient improvement in clinical features.
- Generally avoided due to risk of bradycardia (atropine and cardiac resuscitation equipment must be kept at hand) and subjectivity of most clinical features.
• Nerve Conduction Studies - repetitive stimulation demonstrating decrements of the muscle action potential. May differentiate between MG and Lambert–Eaton syndrome.
• EMG - single-fibre EMG may demonstrate jitter (variability in latency from stimulus to muscle potential) indicating fluctuation in neuromuscular conduction.
• CXR/ CT Thorax - to visualise thymoma in the mediastinum or malignancies in the lung
