A 50-year-old male presented with complains of ptosis, difficulty in chewing and occasional difficulty in swallowing. There is no history of diplopia or visual loss. On examination, there is symmetric ptosis and mild restriction of extraocular muscle movement with finger abduction test 60deg. Nerve conduction study shows decremental response in orbicularis only. ERG revealed a myopathic pattern. Anti-AchR radioimmunoassay was negative. The most probable diagnosis would be: