There a few diseases that mimic ALS pretty good:
1) Multifocal motoric neuropathy (MMN) - weakness, fasciculations (not too much) but NO atrophy and very slow progression
2)Postpoliomyelitic syndrome - Late consequences of poliomyelitis. Weakness, fasciculations, tireness.
3) Myasthenia gravis -no need to comment
4) Polymyositis - weakness etc, no fascics
5) Spondylogennc cervical myelopathy - hand atrophies, sensory disorders on limbs.
I dont know the spelling of these diseases in english, I do my best..