Dr. Jan Steiss at Auburn University: Coccygeal Muscle Injury in English Pointers (Limber Tail). Reference is The Journal of Veterinary Internal Medicine 1999, 13:540-548.
For those new to the list, limber tail (also called cold or dead tail) is where an adult dog suddenly develops a limp or flaccid tail. The tail either hangs down from the tail base or is held out horizontally for several inches from the tail base and then hangs down.
The study was small: 3 control (unaffected) and 4 affected dogs. The dogs were all English Pointers, but the article specifically stated that the findings applied to Labrador Retrievers as well. Even though the study was small, it was very thorough. They took biopsies of the tail muscle, did CT scans and MRIs to look for structural abnormalities, electromyography and thermography.
Without going into detail on each test, I'll summarize the new findings. Old knowledge is that the occurrences seem to be in dogs after prolonged cage transport, a hard workout the previous day or change in climate (especially cold and wet). Less than half the dogs have a recurrence. It is felt that the dogs were either underconditioned or overworked when the episode occurred.
With that said, some of us have had dogs that did not fit the above conditions. The article did not say that these were the only precipitating factors, just ones they had seen repeatedly in past studies as well as this study. An interesting note is that 2 of the control dogs went through the exact workout regimen that 2 of the affected dogs did. So, it is still unknown why some dogs develop this and some do not. The study addressed the physical changes found in the affected dogs.
It was felt that the Pointer was prone to the syndrome because it has such an "active tail". This would apply to Labradors also. The muscle groups affected most were the ones that did the side to side movement.
Apparently, after all the testing was done it was determined that there is actual muscle damage in the tail muscles. The damage varied and seemed to be directly related to the amount of pain and length of recovery period. Most dogs recover spontaneously in a few days, but some may take several weeks.
They did not see anatomical differences in the control and affected dogs.
There was no nerve damage found.
None of the affected dogs had impacted anal glands or prostate problems (this had been reported anecdotally).
Now, here's the thing that was very interesting. It had been thought that the syndrome was similar to muscle fatigue similar to human's overworking a muscle--delayed onset muscle soreness. This study found the syndrome was more similar to acute compartment syndrome. This is significant because compartment syndrome is extremely painful in humans. Pain is made worse by stretching the involved compartment---in this case moving the tail increases the pain.
Anecdotal reports suggest that anti-inflammatory drugs administered within 24 hours after onset hasten recovery.
In cases were people are not familiar with the condition, it can be misdiagnosed as fracture, spinal cord disease, impacted anal glands or prostatic disease. Other forms of tail trauma should be ruled out. Diagnosis is made on history and clinical signs. Tests to confirm would be thermography (2-3 degree decrease in temp.) and elevated serum CK (creatine kinase). Biopsy should not be done.
Personally, I wouldn't do the above tests if the history and signs fit the picture of cold tail and other tail trauma can be ruled out.
So, while they still aren't sure which dogs will get it and why, they do know more about what is actually going on physically in the tail muscles.
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