Hello. If you've read the stickies, you know twitching does not ALS make. As for atrophy, it doesn't affect skin. It affects muscles. I'd suggest talking to your GP to see if perhaps you have a pinched nerve somewhere affecting your hand. Ulnar nerve trapped is the most common cause of issues in the thumb/forefinger.
If there was atrophy, there would also be some level of function loss.
ALS kind of works like this:
1. Clinical weakness
The fascis seen in ALS are caused by muscles trying to repair themselves. Thus, almost always seen with some level of dysfunction.
Let me close by saying you should see your GP, as if it's a simple nerve trapped, the sooner it's fixed, the better the outcome.