I am VERY concerned that your dog may be experiencing paralysis/paresis due to Intervertebral Disc Disease (IVDD, herniated disc).
This happens when the material in the discs between the boney vertebrae in the back ruptures out and presses on the spinal cord. There is a lot of internal swelling when this occurs, leading to pain and decreased nerve function. This can happen in conjunction with arthritis of the vertebrae, because that causes decreased flexibility between the vertebrae.
Basically, the vertebrae are bones that protect the spinal cord which runs through a hole in the vertebrae. Each vertebrae has a little "shock absorber" between it and the next vertebrae, called a disc. The disc it a lot like a jelly donut! It has a fibrous part (the bread of the donut) and then a squishy substance in the middle (the jelly). In SOME dogs, this jelly in the middle becomes chalky and hard as they age. Dachshunds and other small dogs are prone to this. So, when the back flexes and extends, instead of the jelly compressing and expanding, this chalky substance gets squeezed - and it does not compress, but instead it extrudes out and you thus get a herniated disc.
It comes on very suddenly, just as you are describing.
In order to confirm this suspicion, your dog should go see his veterinarian promptly. If he has ruptured a disc, the sooner treatment is begun, the better the prognosis. I recommend that you either take him immediately to an emergency veterinary hospital NOW or be on your vet's doorstep when they open this morning! Do not wait for an appointment time later today - this is an EMERGENCY!
Treatment for IVDD often involves anti-inflammatories, pain killers and/or steroids. The goal is to decrease the swelling which in turn decreases the pain and improves nerve function. Sometimes, however, they are not enough. In these situations, surgery can be done to go in and remove the disc material that is pressing on the spine. This is called "decompression" surgery.
In order to determine what is appropriate treatment for your dog, a veterinarian needs to perform a very thorough neurological examination. The vet looks for neurological deficits such as a delay in turning the back foot over if it is turned so the top of the foot is on the ground instead of the pads, while the dog is in a standing position. The vet also looks for "purposeful movement" which is a stepping motion of the hind legs when the vet supports the dog's weight so the legs can swing freely. There are a number of other neurological tests the vet does to test reflexes. Also, the vet manipulates each vertebrae in a way to find where there may be pain.
Often, if a painful area is located, the vet will recommend x-rays to look for a compression between the vertebrae. This confirms the diagnosis.
The prognosis for each patient depends on the symptoms, the results of the neurological examination, how long the problem has been present, and how the dog responds to treatment.
I am concerned about your dog because it sounds like he is paralyzed. I would strongly recommend a visit to your veterinarian immediately for examination and treatment. It would be helpful to take him on an empty stomach just in case they need to give a sedative/pain killer to take x-rays.
Here are some links with more information:
Until you can get him to the vet, please keep him as quiet as possible. Prevent him from moving by keeping him in a small area.
Until you see a vet, you could try a hot compress. Many people with back pain report that a warm compress is soothing, and your dog may appreciate that too. You can do this by making a wet towel compress. Place a small wet towel, folded into a zip-lok bag (unzipped!) and heat for about 2 minutes in the microwave. Remove and press all the air out. Make sure it is not too hot! You may want to put another towel around it, and then gently place over your dog's back.
Please let me know how your baby does!
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