Myofascial pain syndrome in dogs
Myofascial pain syndrome is a difficult-to-diagnose and seldom-treated condition in dogs. This is despite the fact that it's been a recognized pain issue for more than 400 years and entered mainstream human medicine almost 80 years ago. It's rarely taught in the university setting, and there are no books about it.
The trick of trigger pointsMyofascial pain syndrome is always associated with a taut band of muscle fibers within the affected muscle. Multiple trigger points—extremely tender spots—are within these fibers where the myosin and actin have become fixed in the contracted state. This constant contraction can result in both loss of motor function and chronic pain.
The motor component may occur secondary to injury, but, in my experience, it more often is the result of a constant low-grade contraction of the affected muscle. When a muscle is used in slight contraction, fibers always are the first to be put to work and the last to get rest.
We've all experienced this when we try to hold our arms overhead to perform a task. Very quickly it feels as though we're holding a heavy weight; this is because we're using only a few muscle fibers for a simple task, and as those fibers become exhausted, there are no others helping out. The constant contraction eventually causes the working fibers to use all the ATP (energy) required to "un-contract" the muscle. It also puts pressure on local blood vessels, causing a localized ischemia that blocks sources of energy (oxygen-saturated blood) from reaching the affected area when it's needed most. This becomes an insidious pattern resulting in trigger points that can last for many months.
The sensory component of a trigger point can be appreciated by direct palpation of that point. Gentle palpation of the taut band or trigger point often leads to the dog reacting with a jump or with a look toward the side of palpation. More vigorous palpation commonly results in vocalization with a menace and escape response.
Why do animals develop trigger points? Consider an orthopedic procedure or osteoarthritis of the hindlimb. To keep weight off the painful bone or joint, a slight contraction of muscles, such as the iliopsoas, sartorius or rectus femoris, occurs whenever the dog is standing. This protects the painful bone or affected joint but eventually causes trigger points to form. Trigger points usually are multiple and clustered within a muscle. In fact, it's not unusual to final a dozen points in a single muscle.