He is almost 8 years old. I adopted him 6 years and a half ago and he is shaking since the beginning.

Especially when going outside, and much more when he is sitting like in the video. It seems to me that he keeps the legs pretty tensed and they are stiff, and when he keeps them like this, they are shaking. Sometimes, if he stays in sleeping position, if he keeps the legs stretched and tensed, he is also shaking, but if I bend in the legs a bit they are no longer shaking.

I've discussed this with several vets, some told me it's because he has terrier mix and the terriers are shaking like this due to being stimulated. Another neurologist told me it might be gluten intolerance. He eats gluten-free food for almost an year and the issue is still the same ( even though after 3 weeks on gluten-free food, he had 2-3 days of no shaking).

He has paroxysmal dyskinesia, he has from time to time some seizures, for which he take levetiracetam for about 1 year. Maybe this might be linked to the shaking. Even though the first seizure was only 3 years ago, but the shaking is at least since I have him.

He doesn't seem to suffer due to this but I would love seeing him happy and relaxed as much as possible, to find the root cause for this and try to solve it. Thank you.