Introduction .................Neurological Aspects .. .......... Intervertebral Disc Disease........Bone & Joint Panosteitis

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Chronic lameness is a fairly common occurence in dogs and cats. Yet in many instances, the cause(s) are never fully elucidated. Most of the time we only treat the symptoms with analgesia, dietary modifications, "neutraceuticals" and/or, when appropriate, excercise restriction in order to make our patients comfortable. However, in fairness to fellow veterinarians and to clients, sometimes the clinical manifestations of lameness, in conjunction with age, breed (which narrow the list of probable causes), other health issues and client financial limitations makes a full diagnostic workup, which can be expensive, an academic excercise that does not alter the prognosis nor the treatment options. Nevertheless, I think I speak for most veterinarians and pet owners when I advocate a full diagnostic workup so that we are certain that the ameliorative treatment we provide does not mask an insidious and potentially serious underlying problem.

Acute onset lameness is most often precipitated by a traumatic event, though in the absence of a supportive history, metabolic, cardiogenic, vascular (thrombus?), infectious, neurological, neoplastic and immune-mediated causes must be seriously considered. Animals are frequently in dire pain and while addressing this it is imperative that the definitive diagnosis be established early to identify a potentially important underlying problem and to prevent the development of a progressive and potentially life-long affliction that jeopardizes the animal's quality of life.

The purpose of this page will be to provide a broad overview of small animal lamenesses, with illustrations if possible, that demonstrate anatomical and physiological relationships of those components which directly impact locomotion, namely the central and peripheral nervous systems, muscles, bones and joints and miscellaneous soft tissues. In addition, when appropriate, the physiological basis of metabolic, infectious, congenital and other more esoteric causes of lameness will be explained, hopefully in a manner that is readily understandable to any pet owner, as it should be. This project will take time and so, as has been the case often on this web site, "chapters" will be added one at a time over a period of days and perhaps weeks. I will certainly try to expedite the process as I am able.


Neurological Aspects of Locomotion

The first portion of this topic will try and cover the neuromuscular aspects of locomotion, so that you can appreciate how an injury or a disease process that may occur quite distant from the lame limb can be responsible for that lameness, nevertheless.

Motor Pathways

When muscle contracts in a coordinated manner, limbs move in a coordinated manner. Such motion (deemed "motor activity")--voluntary and involuntary--usually begins as electrical impulses originating in higher and lower motor centers in the brain. These impulses pass down the spinal cord in specific and highly organized tracts towards the target (second nerve then) muscle and ultimately initiate movement, as will be described. The spinal cord is housed and protected within a multiply segmented boney column--the vertebral column, sometimes referred to as the "backbone(s)". The spinal cord sits within this protective boney structure. Figure 1, though not of high resolution, illustrates this relationship: Vertebral column is divided into cervical (neck), thoracic (chest), lumbar (lower) and sacral (hip) segments.


Doggy Bones and Spine
Figure 1 The cord really sits within the vertebral column and can not be seen , but is drawn yellow here for illustration.

Notice that the nerve fibers actually leave the cord and pass from within the vertebral column to the "outside" through natural spaces between boney vertebral segments.


Figure 2 is a schematic illustration of what the cord would look like if half of each veterbral bone were removed (for visualization)



Spinal Cord & Verterbrae
Figure 2 Notice that nerve roots eminate from cord and nerve fibers pass out of
vertebral column


Figure 3 represents the terminal portion of Figure 2..the end of the cord. Notice that the cord ends before the vertebral column ends but the nerves emininating from the terminal aspects of the cord continue within the boney vertebral canal and exit the vertebral column further away.


Figure 3: Magnification of Figure 2. Note the end of the cord and the
continuation of nerves within the vertebral column.


As the figures try to show, nerve roots eminate from the spinal cord all along its length. Specific roots form specific nerves. Branches of these connect (more appropriately "synapse") to the ends of separate nerve fibers that, in turn, stimulate an impuse in either another nerve or to various and very specific soft tissue structures. In the case of a limb muscle, the stimulation provided by the input electrical impulse that originated higher up (e.g. in the brain) causes changes in the movement of specific charged molecules in the cell membrane. This, in turn, stimulates exquisitely intricate chemical activity within the muscle cell that, eventually results in the shortening (contraction) of muscle fibers. If the muscle's tendon is attached to a bone, contraction of the muscle exerts force causing the bone to move in the general direction of contraction. The exact magnitude and direction of movement will depend on what other bone and soft tissue associations are concurrently involved.


Sensory Pathways


The next figure (Figure 4) represents the cross-sectional view of the spinal cord and includes labels and arrows whose purpose is to illustrate the approximate location and direction and consequence of some electrical impulses generated from sensory (e.g. pain, touch, itch, stretch) stimuli ( yellow ). Notice that there is coexistence, divergence and even mixing of sensory pathways (i.e. a single touch to a point on a limb (a leg) elicits concurrent electrical pathways from limb-to-spine-to-brain and from limb-to-spine-to limb (...a reflex sensory-to-motor pathway). An example of the latter is the patella reflex: the knee ligament is gently tapped, stretching ligaments and tendons and sensory nerves within them; the impulse travels in the sensory (yellow) branches of the nerves to the spinal cord and stimulates the motor (blue) branch in the same segment of the spinal cord to send an impulse back to the muscles of the leg to "jerk" (kick). Note that the original stimulus also sends an impulse up the spinal cord towards the brain...thus the brain will know that the knee has been tapped. Upon close inspection of Figure 4, you see that this reflex can occur without any input from the brain (it is a simple arc); however, the motor fibers eminating from the brain intermingle with the motor fibers of the arc; it is, therefore possible (in fact quite probable) that the brain, which was also made aware of the stimulus, has some influence on some characteristic(s) of the reflex arc. This particular concept can be useful in physically localizing the presence of a neurological cause of lameness to a specific region of the spinal cord as well as assessing the severity of the problem.


Crude Spinal Cord Cross Section

Figure 4: Yellow=Sensory; Blue=Motor

(Back to sensory text) is important to realize that many kinds of lesions that perturb the spinal cord, the nerve roots and/or the sensory or motor nerves that connect to muscles, joints, bone and other tissues can result in abnormalities in gait or attitude that we label as "lameness". The next section will touch upon a specific example of this: intervertebral disk disease. Then the topic will digress to non-neurological causes of lamesness, such as inherited or acquired orthopedic syndromes--joint, bone, ligament---as well as other inherited, metabolic and infectious diseases that cause muscle weakness, lameness and/or pain (manifest as lameness). This is a big topic...I intend to slowly hone in on the information you need to help your pet...I simply want to be sure you have the foundation to fully appreciate it

Intervertebral Disk Disease and Lameness 

Figure 5:Cross-section of Verterbral Canal: Illustration of Intervertebral Disk.
Note bulging of ruptured or degenerated disk material (place cursor over image)
into spinal canal, with potential for pressure/damage to spinal cord

Figure 5 is a schematic representation of the relationships of spinal cord, boney vertebral canal, ligamentous support structure (the dorsal longitudinal ligament is shown here) and the corresponding intervertebral disc. The latter is composed of complex cartilage and, in theory, each serves as a buffer/cushon between the adjacent boney vertebrae.

In some animals, trauma or simple degenerative changes to cartilage result in the bulging or extrusion of disc material into the small canal containing the spinal cord. This exerts pressure on the cord, it's blood and nutrient supplies; inflammation and swelling ensues, and exacerbates the damage to the integrity of the cord and it's related structures, as well as the structures innervated by it. The net effect is loss of functionality, and too often, there is considerable pain. Afflicted animals may demonstrate weakness and abnormal gait (staggering, falling down) and even paralysis if damage is severe.

Treatment depends upon the severity of clinical signs and radiological findings. Medical management involves aggressive use of anti-inflammatory medications and alleviation of pain. Quite often, medical resolution is not possible and to prevent irreversible loss of neurological integrity, surgical intervention is required.

The next discussions will explore bone and joint abnormalities that can result in lamenesses; after that diseases of muscle!

A table is available to review which will be indexed for easy reference

Selected: Bone-Joint-Ligament Lamenesses

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