What can make a dog’s back legs weak, or cause a dog’s back legs to give out?
That’s a question I had to answer recently for the anxious parents of a middle-aged Labrador Retreiver.
Murphy is your typical, eager 6-year-old Lab. He’s a happy soul whose tail is always wagging. However, this lively ball-chasing dog had woken up that morning unable walk.
Dog’s Back Legs Suddenly Not Working
Murphy’s concerned humans phoned in saying he was struggling to walk but weren’t sure which leg he was limping on.
Thirty minutes later, in the waiting room, Murphy was completely unable to stand and had to be carried into the exam room.
As always, a history is vital.
Murphy’s story was remarkable in how simple it was:
- He had been fine the previous day but woken that morning with a weak back end.
- As he bravely tried to stand, he kept collapsing, although his tail was still wagging. Typical Lab!
Murphy didn’t seem to be in pain, but the dog’s back legs couldn’t support his weight. With his person supporting him in a standing position, I examined the poor dog.
Murphy’s Physical Exam
With no history of trauma, broken bones seemed unlikely in Murphy’s case.
Even so, it’s important to check for fractures and dislocations, but there were none, and neither did Murphy have painful joints.
A basic test flipping Murphy’s back paw upside down showed it stayed that way. This is called the “placing reflex” and tests the nerves going from his paw up into the spinal cord and brain stem. When the dog isn’t aware the paw is upside down, the vet concludes:
- The dog doesn’t know the paw is upside down (the placement nerves aren’t working) OR
- He knows it’s upside down but the leg doesn’t get the nerve message to flip it over (the do-something-about-it nerves aren’t working) OR
- His muscles are too weak to respond to the nerve messages.
Given that Murphy had good muscle tone, it looked like his problems were neurological.
Next on the list was to check more nerve reflexes to confirm this as a neurological problem and pinpoint the location. To do this, I had Murphy lie on his side and tested one back leg at a time in this way:
- Toe pinch: This sounds unfair, but it’s an important test of “deep pain sensation.” If the dog is unaware of the skin between his toes being pinched, this is a sign of serious nerve damage. Happily, Murphy pulled back and had sensation on both hind legs (also a good thumbs-up for muscle tone).
- Patellar reflex: You may have had this done to you. It’s where the doctor taps the ligament immediately below the kneecap. In Murphy’s case, his lower leg kept vibrating for way longer than it should have. This indicates that the brain isn’t getting a message to dampen down the reflex and usually indicates a spinal lesion.
- Skin sensation: A hypodermic needle is used to touch the skin in various places. When full sensation is present, you expect the skin to ripple. For Murphy, this was hit-or-miss. There was an absence of sensation along his lower back, thighs and the tops of his paws, but he had sensation on part of the lower back legs.
Which brings us back to the $6 million question: Why were this dog’s back legs weak?
Why Are My Dog’s Back Legs Suddenly Not Working?
When faced with a problem, the vet draws up mental list of the likely problems (in vet-speak, this is a list of “differential diagnoses”). For Murphy, mine went something like this:
Disc disease is a frequent cause of a dog’s back legs suddenly not working. The puzzling thing was Murphy didn’t seem in pain, since normally this is a bloodcurdling, painful condition.
Fibrocartilaginous Embolism (FCE)
This is where a small piece of cartilage enters the bloodstream and blocks the blood supply to the spine, causing swelling. Again, slightly puzzling was that both Murphy’s back legs were affected — usually, this is a one-sided problem, with one good leg and one bad leg.
This was certainly a possibility, but the signs should have come on more slowly.
This is an inflammation of the spinal cord. The dog is usually unwell and possibly feverish, none of which applied to Murphy, who was happily begging for treats.
Spinal Cord Hemorrhage
A blood clot can damage the spinal cord. This is usually linked to trauma (which didn’t apply to Murphy) or problems with blood clotting. But Murphy had nice pink gums and no signs of internal bleeding.
Hmmm, Murphy didn’t fit neatly into any of these options.
The Next Step: Tests for When a Dog’s Back Legs Are Weak
What test would most likely to give us the answer when presented with a dog whose back legs are weak?
In short, an MRI or CT scan:
- Both will give a great picture of not only the bone but also the nerves.
- This is superior to X-rays, which tell us only about bone problems, leaving the nerves a gray area.
This meant referring Murphy to a specialist center for a state-of-the-art scan. The dog’s caretakers didn’t hesitate and straight away gave consent to refer him.
A couple of phone calls later, and I had a veterinary neurological specialist on the phone.
He was optimistic that Murphy’s problem was a fibrocartilaginous embolism (FCE). Indeed, this condition is a “Labrador thing,” with the breed being especially at risk.
The symptoms come on suddenly, and the dogs aren’t in pain. Happily, FCE carries a good prognosis. With great nursing care, many dogs go on to make a full recovery.
However, there was still a faint possibility of disc disease that couldn’t be ruled out without an MRI scan.
I discussed this with Murphy’s people, and quite rightly, they preferred to get Murphy seen by the specialist, just in case something else was going on.
Watch this dog strengthen her back legs:
A Twist in the Tale
About 2 days later, I got a preliminary report back from the specialist.
Guess what? Murphy had not 1 but 2 slipped discs.
He’d had surgery to remove the discs and was recovering well. Prompt action meant the damage to his nerves was limited, and — fingers crossed — he should make a good recovery.
Things don’t always go the way the signs suggest, and confirming (or ruling out) a hunch is the best policy where feasible. Keep that tail wagging, Murphy, and next time, how about something straightforward, like a snagged claw?
If your dog’s back legs are weak, please see your veterinarian to learn the cause.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed May 18, 2018.
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