A 72 year old man complains of drooping of lids for last 2 months. On awakening he is able to open his eyes, but few hours later his ptosis progressively worsens with increasing diplopia most likely diagnosis?

Myasthenia gravis is a disease characterized by generalized muscular weakness and rapidly developing fatigue of the muscles due to an auto-immune disorder in which damage of the acetylcholine receptors takes place at the post-synaptic membrane. Anti-acetylcholine receptor antibodies are found in approximately 80% of patients with myasthenia, the levels correlating with the severity of the disease. The ptosis is nearly always bilateral and is increased by prolonged fixation or attempts to look upwards; effective compensation by overaction of the frontales is impossible. Ophthalmoplegia externa, paial or complete, occurs in 50% of the cases, but the intrinsic muscles are not affected. Nystagmoid jerks are not uncommon.