A 29-year-old woman presents complaining of weakness, particularly of her arms....

A 29-year-old woman with a past history of hypothyroidism presents to the surgery complaining of weakness, particularly of her arms, for the past four months. She has also developed double vision towards the end of the day, despite having a recent normal examination at the opticians. What is the most likely diagnosis?

A. LambertEaton myasthenic syndrome
B. Polymyositis
C. Polymyalgia rheumatica
D. Multiple sclerosis
E. Myasthenia gravis

Answer: E. Myasthenia gravis

Discussion: 

Myasthenia gravis is an autoimmune disorder resulting in insufficient functioning acetylcholine receptors. 

Antibodies to acetylcholine receptors are seen in 85-90% of cases.(antibodies are less commonly seen in disease limited to the ocular muscles)

 Myasthenia is more common in women (2:1) 

Clinical Features: The key feature is muscle fatigability - muscles become progressively weaker during periods of activity and slowly improve after periods of rest: 

 extraocular muscle weakness: diplopia 

 proximal muscle weakness: face, neck, limb girdle 

 ptosis 

 dysphagia 

Associations 

 thymomas in 15%

 autoimmune disorders: pernicious anaemia, autoimmune thyroid disorders, rheumatoid, SLE 

 thymic hyperplasia in 50-70%

Investigations 

 single fibre electromyography: high sensitivity (92-100%) 

 CT thorax to exclude thymoma 

 CK normal 

 autoantibodies: around 85-90% of patients have antibodies to acetylcholine receptors. In the remaining patients, about about 40% are positive for anti-muscle-specific tyrosine kinase antibodies  Tensilon test: IV edrophonium reduces muscle weakness temporarily - not commonly used anymore due to the risk of cardiac arrhythmia 

Management 

 long-acting anticholinesterase e.g. pyridostigmine 

 immunosuppression: prednisolone initially 

 thymectomy 

Management of myasthenic crisis 

 plasmapheresis 

 intravenous immunoglobulins