Thank you Gill,
I agree that arthritis is very unlikely at his young age (and it tends to be a progressive instead of overnight issue). Instead, when we have a young cat with sudden in onset severe hind limb lameness, then trauma will always top our list of concerns. Further to this, if he were just wobbly like he ws drunk instead of uncomfortable, then we'd have to also worry about toxicities (ie antifreeze)
Now I am glad that he isn't dragging his legs, has a dropped/non-responsive tail, or placing his feet incorrectly, as these would all raise concerns about spinal based damage. Still one of the key things we have to monitor is his ability to urinate. When cats have had traumatic damage to their hind end, we can see damage to the nerves that tell the bladder to empty. (We can see temporary signs if the nerves are inflamed but it can be permanent if the nerves are damaged). This means these cats cannot consciously pee and this as urination is a critical life function there overall prognosis is poor. So, this is a key sign to monitor for him. If there is any doubt he can urinate or you see him dribbling without realizing, then we'd need him to see his ER vet as soon as possible.
Otherwise, based on your description of his gait, I would be a bit concerned that he may have a hip fracture. It is possible that this is just severe bruising and soft tissue damage (which is quite possible since I cannot see him) but if he is moving very oddly and arthritically, then this fracture is a concern here. Since there is so much muscle around the hips, cats don't usually go off their legs completely (as they wound with a fracture leg or spine). Instead they will often try to walk, usually oddly (some look to crab walk on the back legs or drop their hocks when walking) and often struggle to get up from rest. As well, it is quite possible that we'd see him worse right after the accident (with the shock playing a role) but now be a bit more mentally normal though still bruised and sore. And unlike dogs who will drag themselves around and become distressed, cats often will refrain from moving about (to avoid the pain of doing so).
In this situation, the first point of call is determining if he can urinate. If there is any doubt, then he needs to be seen as soon as possible. If he can, then his situation is urgent but not the same level of emergency. Ideally, we'd want his vet to have a feel of his hips (+/- xray if a fracture is suspected) and evaluate him. If he has just had a soft tissue trauma, then pain relief and rest would be indicated. But if has fractured a hip, we do usually manage these medically (cat safe pain relief like Metacam and strict cage rest for 4-6 weeks) since those muscles will hold the bones in place. That said, if he has a severe fracture of the hip (where the fractured bits are not in align) then surgery could be required.
So, do check for urine when you get home but do let Bonnington keep resting in a restricted area (where he doesn't have anything to climb or jump on). It sounds like he has the right idea at the moment but we do just want to make sure that he hasn't had any damage to his nerves that could worsen his situation and potential for recovery.
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