I read your question carefully and I understand your concern. Actually I don t see it as indicative towards any of those two diagnoses. It certainly doesn t indicate MS in any way, it causes neurological deficits such as weakness of the limb, not only of a thumb and it doesn t manifest with shaking or twitching either. Neither does ALS seem likely. While it can show muscle involuntary contractions called fasciculations usually they come at a later stage accompanying preceding muscle atrophy and weakness which would have been noted by his doctor. While I hope to have comforted you regarding those two diagnoses, it is hard to express my opinion as to what those episodes actually are as physical examination is vital in this case, also it is not clear whether these shaking/twitching episodes are on one or both sides, involve only the thumbs or spread to other fingers, manifest at rest or during action. A diagnosis of tremor is well possible, but there are different kinds of tremor with different causes and treatments so if he s not improving it should be re-evaluated by the doctor, if he s unsure preferably a neurologist consult is advisable. I hope to have been of help.