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A 45-year-old lady bank worker presents with intermittent episodes of bilateral ptosis and seeing two images (diplopia) which comes on gradually at times. Over the past few weeks it has happened when busy at work and she needs to stop and take a break. Her boss sent her home after the latest episode. It gets worse when reading and watching TV. At times she even has had to hold her eyelids open to read her newspaper espeically in the evening time. At times her voice changes in quality and she has weakness in swallowing at the often near the end of a meal.
1. What single blood test would be most useful
This is a fairly unique history for Myasthenia Gravis and we would check the
Acetylcholine receptor (AChR) antibody analysis.
Anti-Muscle specific kinase.
We might also organise an EMG and CT chest and ring a neurologist. He asks that she is admitted under their care for treatment as treatment can make things worse rather than better initially.
Antibodies are detectable in 80% to 90% of patients with generalised myasthenia gravis (MG) and up to 50% of patients with ocular MG. Other less common antibodies include autoantibodies to muscle-specific kinase which is also involved with the postsynaptic acetylcholine receptors.
2. What is the cause
Myasthenia is a disease of the neuromuscular junction affecting neuromuscular transmission and is caused by antibodies to the postsynaptic acetylcholine receptors. The presence of antibodies supports the diagnosis.
3. What are the classical findings
There is clinical fatigability - a strong muscle becomes weaker with repetitive usage. This can affect the muscles that move the eyes as causing ptosis and those involved with swallowing and there may be a generalised fatigable weakness often around the shoulders. The weakness may affect respiratory muscles and the patient may need ventilatory support. A minority have a thymoma on thoracic CT scan in the anterior mediastinum and removal may help younger patients. Repetitive stimulation during nerve conduction studies may show a characteristic decremental response
4. What is a tensilon test
The test may be to see if there is a resolution of symptoms and signs after giving a test dose of edrophonium which will give a temporary boost to Ach levels (Tensilon test) but false positives are seen. A placebo dose is often given and response assessed then a true dose. Patients need monitored as this can cause a bradycardia.
5. What is the treatment
Treatment is to boost acetylcholine levels with drugs that prevent their breakdown. These drugs inhibit the enzyme acetylcholinesterase. The most commonly used anticholinesterase drug is pyridostigmine. In more severe cases treatments involve immunosuppression such as IV Immunoglobulin which mops up antibodies, plasmapheresis which also removes antibodies, steroids, azathioprine and Mycophenolate mofetil.
6. What is thymectomy
All patients with MG with thymoma should undergo investigations such as a CT chest to look for a thymoma and then have surgery to remove the tumour and may not produce improvement in MG.
7. What important advice would you also give
Ensure that all know that she has Myasthenia gravis. Relapses can occur
Avoid drugs that can make MG much worse e.g. Gentamicin. They are listed in the BNF
Infections on immunosuppression need early effective treatment
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