My sister and I rescue, rehab, and rehome horse’s frequently. One of our newest additions is an 11 yr old paint gelding. He Hasan issue with his front left shoulder. It has lost a lot of muscle tone (I looked at pics of Sweeney shoulder and they look about dead on) and he seems very stiff and sore in the shoulder joint. The lady we got him from had him for about a year and he had this problem when she got him. We think he may be a horse we saw in a video from a local rodeo (same markings and paints can be distinctive). In the video, they were running in a pony express event and the 2 horses collided fairly violently. The injuries/ damage match up to issues with this horse, Draco. At this point, all my research has said acupuncture, electro stim, all of that, has to be done quickly, but we are at least 1 1/2 years out from whatever injury caused this. Does anyone have any suggestions for possibly physical therapy, stretches, massage… or is this poor boy hopeless like our farrier suggests? 
In my limited experience, if you’re a 1 1/2 years out from the injury and you can visibly tell he has Sweeney, you’re wayyyy down a bad path.
One of my friend’s horses recovered from Sweeney, but she caught it the minute he started losing muscle tone, took him to Rood and Riddle monthly for almost a year, and still was holding her breath to see if he’d be rideable again (he is now, over a year later).
Once the trauma has settled in, it’s past hope of recovery back to “normal.” It is what it is.
Consult with a veterinarian rather than a farrier. Sweeney used to be, and perhaps still is, primarily a draft horse problem.
Sweeney might be more common in drafts as the collars can cause the damage, but any horse who collides with something - another horse, fence post, stall wall/doorway, gets kicked, etc - can suffer the damage. A friend’s Quarab was kicked, developed sweeney, and was eventually put down as a result. It’s not a good prognosis, unfortunately, especially this far out. Definitely talk to a vet, or get a referral to a big hospital who might have seen more of it. Probably the best you can hope for is a comfortable pasture pet until it gets to be too much 
“Sweeney” is still relatively common secondary to trauma (horse falls down, gets kicked, runs into fence etc.) and is due to damage of the suprascapular nerve. We tell people that if you rest the horse for 6 months in a stall/small paddock and it shows improvement, there may be hope. 18 months out with no improvement has pretty much a 99.9% chance of never improving, unfortunately.
One of the horses at the barn has this… he was kicked during a trail ride years ago. He is retired. He is doing ok, but you can tell his leg is stiff and he does not know where “his foot is”.
I sometimes see the owner’s niece (18) sitting on him which I think is not very smart (but they have done other things which confirm this). Anyway, he is enjoying being a pasture ornament and does like to play with other horses.
A horse in my barn had this, he was older, was lame for eons, had surgery and ultimately was put down…
