Myasthenia Gravis Flashcards
What is myasthenia gravis?
AI disease affecting the neuromuscular junction producing weakness in skeletal muscles
Describe. the aetiology of MG
Impairment of NMJ transmission
Most commonly due to autoantibodies against the nicotinic ACh receptor
Describe the epidemiology of MG
Prevalence: 9/100,000
F > M at younger ages
Equal gender distribution in middle age
Give 3 types of symptoms of MG
Muscle weakness: worsens with repetitive use/ at end of day
Ocular: Drooping eyelids + Diplopia
Bulbar: Facial weakness, Disturbed hypernasal speech, Difficulty smiling, chewing or swallowing
List 3 signs seen in the eyes in MG
Bilateral ptosis (may be asymmetrical)
Complex ophthalmoplegia
Check for ocular fatigue by asking pt to sustain an upward gaze for 1 min + watch the progressive ptosis that develops
What test involving the eyes may be performed in MG?
Ice on Eyes Test
Placing ice packs on closed eyelids for 2 mins can improve neuromuscular transmission + reduce ptosis
+ve when ptosis improves by >2mm
What bloods should be investigated in MG?
CK: exclude myopathies
Serum ACh receptor antibody (+ve in 80%)
Anti-voltage gated Ca2+ channel antibody (in Lambert-Eaton syndrome)
What imaging should be performed in MG?
CT Thorax/ CXR: visualise thymoma in the mediastinum or lung malignancies
What other investigations should be performed in MG?
Nerve Conduction Study: Repetitive stimulation shows decrements of muscle action potential. Can differentiate between MG + Lambert Eaton
EMG: Jitter (variability in latency from stimulus to muscle potential) indicates fluctuation in neuromuscular conduction
How can signs of MG be classified?
May be generalised (affecting many muscle groups)
May be bulbar (affecting bulbar muscles)
May be ocular
What is myasthenia gravis associated with?
Other AI conditions (e.g. pernicious anaemia) Thymoma development (breakdown of immune tolerance occurs in thymus)
What bulbar signs may be elicited in MG?
Reading aloud may cause dysarthria or nasal speech
What signs should you look for in the limbs in MG?
Test the power of a muscle before + after repeated use of the muscle
What occurs in the Tensilon Test in MG?
Short-acting anti cholinesterase (edrophonium) increases ACh levels + causes a rapid + transient improvement in clinical features
Risk of bradycardia: so generally avoided
What is Lambert-Eaton myasthenic syndrome?
AI disorder of neuromuscular junction.
May occur as a paraneoplastic disorder a/w cancer (small cell carcinoma of the lung), or without cancer, as part of a general AI state
Antibodies against VGCC; impairs neuromuscular transmission by inhibiting inward Ca2+ current + subsequent release of ACh into the synaptic cleft.
