I can’t help you with the Avonex, as I myself am not on DMDs.
However, I don’t think it’s necessarily true that none of the “weird” symptoms could be MS.
Achilles tendonitis and/or repeated achilles injuries (despite being something of a couch potato) were one of my earliest symptoms.
If your calf muscles are very tight - typical of multiple sclerosis - a knock on effect is that it can put strain on your achilles tendons. This is actually a rather classic problem, and not “weird for multiple sclerosis”.
Long before I was diagnosed, I couldn’t understand how I was getting so many stupid injuries, despite doing no high intensity activities. My then doctor was dismissive, and said it was just bad luck, and I didn’t need to be doing anything sporty, as it could have happened “stepping off a kerb”.
I eventually saw a sports physio privately - despite being sure it hadn’t happened through sport - and he remarked I had the shortest calf muscles he’s ever seen, and asked if I’d been ill. At the time, I had no reason to suspect so, so denied it completely, and didn’t really understand what he was talking about.
Looking back, I’m sure he either knew or suspected I had MS, but that must have put him in a difficult professional position. It was quite clear I didn’t suspect, and he wasn’t a neurologist, or even part of the NHS, so was it for him - an unqualified person - to tip someone off they might have something serious? Looking back, I feel he could have gently suggested I ought to return to my doctor - but for whatever reason, he didn’t. Perhaps he thought it would be enough for me to read between the lines, but it wasn’t. Instead, I just went away thinking: “Ill? Pah! What does he know, and what a cheek!”
I am now sure the achilles tendon stuff was linked to my then-undiagnosed MS, and he was the first person to spot what was really wrong. It wasn’t “bad luck stepping off a kerb”, or “wearing silly shoes”, or “having one leg longer than the other” - all of which had been cited as the reason at various times.
Also backache is common with MS. Our core muscles tend to be weak, so don’t support our backs properly - especially in a standing position.
I also get painful thumbs - general problem with grip strength - thumbs tire easily from simple manual tasks.
MS definitely (or so I’ve been told) doesn’t cause dry-eye - although I’ve had an inexplicably sore eye from time-to-time, too. Definitely not optic neuritis - it’s the external surface of the eye (always the same eye). I get attacks of what appears to be conjunctivitis, but no infection has ever been found.
But anyway, I’ve undergone full rheumatology screening - twice.
Once was years ago, when I began having the feet/leg problems, and once was routine procedure on the way to an MS diagnosis. Neither time were any rheumatology markers found, so I’m confident everything - with the possible exception of the eye - stems from the only thing that’s ever been diagnosed, which is MS.
I don’t think you should quit a DMD solely because of these symptoms, as I have most of them too, and I’ve never been on a DMD - and am as certain as I can be I don’t have RA, after being tested twice, years apart (funny - RA was the only thing I ever suspected - MS hadn’t crossed my mind!)
I’ve never heard of any upper limit on how long anyone was supposed to be on Avonex, but others who’ve been on it will know better than I.
After at least 17 years with MS, I would expect there to be some progression, even on Avonex - it isn’t claimed to stop it in its tracks. So I wonder if most of what you’re experiencing isn’t fairly par for the course, after years with MS.
I’d certainly be more inclined to think it was the MS than the Avonex.