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The most common reason for an otherwise healthy dog to suddenly become paralyzed or be unable to stand would be intervertebral disc disease. This is when one (or multiple) of the discs between the vertebrae become inflamed, swollen, rupture, or slip out of place. This places a varying amount of pressure on the spinal cord, causing it to malfunction. In milder cases, pain is the only symptom. One step more severe causes scuffing of the toes and delayed reflexes. One beyond that is muscle weakness in the legs (back legs only if the disc problem is behind the front legs but in front of the back legs), and the worst possible scenario is full paralysis and loss of pain sensation. In the more severe cases, the sooner they receive veterinary care, the more likely they will be to return to walking with normal function. If the vet suspects IVDD, and they are able to do advanced imaging, an MRI or CT scan are the methods of choice to diagnose these lesions, because the discs and spinal cord do not show up on X-rays. X-rays can be helpful for general information - especially looking for signs of bone fractures or destruction. Treatment for a disc that's compressing the spinal cord would be surgical removal of the disc material from the canal to decompress the cord. Sometimes in hindsight, owners had seen the more subtle signs of IVDD like toe scuffing and weakness coming on before their pet lost the ability to stand. Obviously in a large breed dog of her age, considering surgery is a stretch. That's not to say that this treatment can't be pursued, but most owners would take pause before embarking on an expensive and potentially invasive treatment plan for an aged pet.
Another potential cause would be FCE, which is a disease in which one of the blood vessels that supplies a portion of the spinal cord becomes blocked, causing a portion of the spinal cord to die and lose function. This again is a diagnosis via advanced imaging like a CT/MRI scan - it can be very difficult to tell from IVDD on a physical examination. Treatment for FCE is mostly supportive and nursing care while we wait to see if and how much function they will recover - it can take weeks to months to know how well they will recover.
Another potential that is less common would be a spinal infection or tumor - while these are far less common, they can cause the same symptoms. They can be diagnosed by advanced imaging, and blood work can also sometimes give us a clue about an infection being present. A spinal tap could also give info on the type of infection if one is present. Even more rare would be immune mediated (autoimmune) inflammatory spinal disease. Also, larger breed dogs (especially if they are overweight) can sometimes be taken down by orthopedic or soft tissue injuries that a younger or lighter dog would take in stride like a groin pull or a tweaked knee or even a torn ACL. In those cases, they just need time and TLC to get back on the right track.
If you want aggressive treatment, the best thing to do would be to get her to a veterinary emergency hospital ASAP. IVDD is the most common disease and timely treatment is most important when considering whether she can recover. Most owners would be electing a more conservative approach, as you have been doing already, that would consist of medical management. Usually this includes anti-inflammatories and pain medications in addition to massage and acupuncture. It's possible that her appetite has declined just because she's "down", but there is the very real possibility that she's got a systemic "whole body" illness at play here causing her to be ill and too weak to stand - something like diabetes, kidney failure, or cancer. If a physical exam doesn't reveal anything obvious, then blood work would be warranted to see what's going on and also to know if anything else can be done to support her.
There is a range of time in a pet's life when discussing euthanasia is appropriate - you are there. That certainly doesn't mean that it's your only option, as I've discussed multiple other options first, but if you were considering humane euthanasia because you feel that her quality of life is poor, that is absolutely OK. There's no single magic "right time" for discussing end of life care - it's always on the table for an older pet with a debilitating disease like what you're seeing in her.
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