Mystery Pain in Pony

I am asking for help from forum friends hoping to get some ideas on how to progress with my sweet pony and a mystery lameness. I have kind of a long story, but I would appreciate anyones time and input on my case! I’m desperate to find an answer.
Last August, my teenage paint pony was started showing signs of pain. My son had been riding him regularly and we had intentions to make it to a couple lower level dressage schooling shows. The pony had a pretty simple work load, taking care of a couple lightweight children and doing lower level dressage moves. The first day that we noticed discomfort, the pony seemed to carry his head and neck very long and low. He was resistant to go forward, and when he did trot, it was short strides and “lazy” for lack of a better word. He was also taking an occasional “hop” step and every so would lift his head and take a bouncey step. We gave him rest and bute for a few days, but it only got worse. He continued to hang his head very low and wanted to keep it turned to the right. He did not want turn, became more and more resistant to go forward, cantering seemed impossible for him under saddle. Other strange symptoms were that he even seemed to carry his head low in his stall, and there was a huge drop in his back. He is a pony with terrible confirmation to start - mutton withered, short swayed back, and plenty plump. But when this sudden onset of pain came on, his back seemed to drop very noticeably. (I’m confident saddle fit and pads were not an issue, I had a lot of help and did a lot of homework to get this guy fitted) I gave him a couple weeks off all together and he seemed to improve. So, one day I put him on the lunge to gauge his movement. He was worse than ever - he went to the left with his head looking right at the trot - although he seemed to track evenly. When asked to canter, and a very large circle, he made it through half a circle and then drop his nose to the ground and pulled as far to the outside as could be.
I took him to a very reputable vet for lameness eval. We jogged the pony in every which direction, on concrete, and he looked fine. They flexed him, tested him, and he gave no sign of pain. We tried to lunge on grass with no success, he only wanted to eat and I couldn’t make the signs that I see at home be clear at the vet. based on my disbelief and my complaints, we xrayed the hocks, the front feet, and even front shoulder/elbow (vet thought he had a lead after flexing. The vet was amazed at how good his joints were despite his age and even told me that if I had brought him for a prepurchase exam, he would probably pass! he suggested it could be a soft tissue injury or maybe a result of a crazy pasture injury. He advised I either rest him and do 60 days equioxx, or put him to work and see if I can get a red flag to be raised.
I opted for rest. To me, he was clearly uncomfortable when asked to work. However - when turned out, you would never guess there is anything wrong with him! So, 60 days go by and no major changes. He appears normal when allowed to pick his own path at w/t and even canter. But put him on a lunge line and he was still darting to the ground and pulling away. The neck position had improved, and the back had lifted to its usual state of sway. I contacted vet again and even sent him videos. he said that he was at a loss and that I should consider a nuclear scan.
I found another vet with a fantastic reputation and an indoor arena at the facility - so I chose to go there, knowing that I can make the vet see what I see. She reviewed x rays, flexed, jogged, and did get to see his nose to the ground movements on the lunge line. She tried blocking the suspensory, thinking that could be the cause for the pain in the corners, but the block brought no improvement. She then concluded that his X-rays weren’t 100% perfect and maybe it was just enough for him to complain. She wanted to inject hocks, upper and lower, inject RF coffin bone, and do the OSSPHOS injection. AT the same time, she told me that on a lameness scale of 1 to 5, he’s a 1 or 1.5 - and I don’t even think he’s rideable! I was overwhelmed and torn - how was her read of same X-rays so different from other vet? We talked and I decided to break it down and know if any one particular area was really the culprit and causing pain in other areas. She suggested that hocks would be best and that I should see improvement. So, I did the hocks - and absolutely no change. I was very discouraged and didn’t feel right about progressing with other injections.
I turned to my chiropractor - he visited a couple times throughout this process and after injections failed to bring results and another exam - he concluded that It is not an orthopedic issue and agreed that I should not pursue further injections. The pony shows no sign of pain anywhere during his exam and adjustment. He did watch him travel, and agreed that he is not even lame on a straight line, but the turn is painful He suggested bloodwork to check for metabolic syndrome or lyme.
So, I tried the bloodwork. Insulin tested fine, negative for lyme. But he did test baseline positive for Rocky Mountain Fever (which I had never even heard of!) A positive read is an 80 and he tested exactly that. The vet recommended Doxy treatment and we started right away. I questioned it, because while his symptoms are odd, they are so consistent. But, its worth a try! He has been on meds and rest for about 6 weeks now.
I was sure that he was looking better - he runs and plays like a mad man, carries his head high and looks pretty good. I let my youngest son sit on him the other day and he looked great. A couple days later, my other son wanted to give it a try and pony looked painful again. Now the last couple days, he seems to be as bad as ever! The trot was looking very solid, but now the “hop” step in back. And his feet just don’t hit the ground and drive off like they used to. He felt weak.
I’m feeling very defeated. I will finish the recommended round of meds for the tick borne suspicion and go back to full time rest, but after that, I feel like he may have to retire, which breaks my heart. He is such a good soul, and my boys just love him.
If you are still hanging with me here - is there any chance anyone has ever had such a strange lameness? What could it be that shows itself so significantly in the corners? My gut all along has told me there is something in the neck or at the wither causing this, because of how odd he carried himself when it first set on. But vets seem to dismiss that. Any thoughts on the Rocky Mountain Fever? Any leads to decide what to look at next? Thanks so much for any advice!

Honestly, my first thought is something neurological and/or something in his vertebrae - especially with the head turn and corners. I really think that looking into imaging and testing for neuro or neck/back issues would be a logical next step. Neurological issues can present in many different ways but your guy seems to have some key symptoms. I’m not sure why your vets are dismissing the idea of spinal/neurological involvement. I’d get another opinion and get some testing done. The sooner a spinal or neurological issue is diagnosed, the sooner proper treatment can be implemented thus increasing the chances of better recovery.

I think that because he is so insensitive to tough and flexing, they are ruling out neck and back pain. 3 vets that have seen him have all been confident that he doesn’t have back pain. And as time as gone by, he doesn’t carry his head to the right so obviously as he did when this first set on. But he does still consistently want to stretch his nose to the ground and pull as far wide as he can when asked to lunge, always at the canter to the left. On good days, he can trot both ways and canter to the right without complaint.

I still wouldn’t rule it out. I know with some neuro cases I’ve worked with as a vet tech, they can act completely normal during flexion and manual tests even though there is something wrong that’s an underlying issue. What did your chiro say about back and neck issues? Any new changes to feed, hay, or pasture? Could be a deficiency/toxicosis of a nutrient… It might be worth getting radiographs of his vertebrae just to rule out the obvious issues like a slipped or pinched disk. Some may not show pain from palpation or manipulating the area because of nerve involvement/stoic behavior. We had one case that had a badly pinched disc in his lumbar vertebrae who was negative to all testing but the issue was blaringly apparent with the x-rays.

My chiro felt confident that there was nothing to be concerned about with the back or neck. After his last exam of my pony, he was the one to recommend the bloodwork to test for EMS and Lyme. The vet that did the blood work suggesting testing for Rocky Mountain too, and I agreed. No new changes to feed, pasture, or living situation. But I did notice improvement in him in the fall - when there were concerns of EMS, I took him off grass for 30 days and did see improvement. But…that also added up to more time off and could have been coincidence.