Monday, June 29, 2009
Discogenic Spine Pain
When I started in this industry, almost 20 years ago, my very first case involved a lumbar disc injury with a subsequent discectomy and fusion surgery. Since that first case, I've probably worked on hundreds if not thousands of similar cases. Any medical illustrator who works for attorneys will probably agree that intervertebral disc cases are the most common projects we receive. If you've been handling personal injury or medical malpractice cases for a while you've probably seen more than a few disc cases yourself.
Thankfully, new developments in the sciences of disc injury and surgical techniques have kept this topic from getting stale. Where once we saw fixation accomplished with things like Harrington rods or Cotrel-Debousset instrumentation that could easily be confused with medieval torture devices, we now see pedicle screw fusion hardware, and even percutaneous discectomy procedures that do not require large open incisions. While once there were only different shapes of harvested bone grafts, there is now a variety of titanium and synthetic cages or spacers that can be employed to replace an offending disc.
Today, I'd like to discuss spinal discogenic pain. Discogenic pain is pain that comes from the disc itself. Although some medical experts have been discussing the issue for years, it is a fairly groundbreaking concept that is relatively new in our industry. This is not just the standard discussion of disc bulges and disc herniations that can compress the adjacent nerves, but rather a discussion of the pain that comes from the disc itself when it is damaged.
Traditionally, many experts discounted intervertebral disc cases that showed clear evidence of disc injury if there was not also evidence of spinal cord or nerve root compression. Their rationale was that nerves generate and transmit pain, therefore if there is no compression of a nerve then there can not be real pain. However, it is now becoming clear that there are at least two ways that pain can originate from an injured disc that does not involve direct compression of the spinal cord or nerve roots.
The first mechanism of discogenic pain involves the recurrent meningeal nerves. These nerves branches off of the ventral rami of the nerve roots outside of the spinal canal and then course back in through the neural foramen to innervate the posterior annulus of the discs as well as the posterior longitudinal ligament. These nerves sense when the annulus or posterior longitudinal ligament is stretched, strained or torn. Nerve impulses then flow through the recurrent meningeal nerve, through the nerve root, up the spinal cord and into the brain to stimulate the pain centers. Injuries to the discs are no different than injuries to the fingers, toes or any other part of the body. Pain originates wherever there are nerves to transmit sensory impulses from damaged tissue.
The second mechanism discussed in recent literature concerns the caustic effects of the fluid normally contained within an intervertebral disc. When the annulus of the disc is torn, fluid from within the disc may leak out into the spinal canal (passageway of spinal cord) or neural foramen (passageway of nerve roots). This fluid contains inflammatory cytokines, chemicals that have an inflammatory effect. Inflammatory cytokines act as a strong irritant causing severe inflammation and pain. Recent studies on rats show that chemicals extracted from the nucleus of a damaged intervertebral disc which results in the rat walking with a limp, implying that the rat is experiencing pain.
Also, when these chemicals come into contact with a nerve root, it can cause typical radiculopathy, paresthesia, weakness, and sensory and motor loss symptoms even though the MRI may show no nerve root impingent at all. It is believed that this inflammation is helpful in the reabsorption and healing of a herniated disc, but these same chemicals can have a harmful effect on the adjacent nerve roots. This is one explanation why a patient who's herniation may appear to have healed on a subsequent MRI may still be experiencing significant symptoms.
I mention all this as a means of illustrating that new science is constantly bringing forth new questions and new answers in all areas, even those we think we have seen a thousand times before. It is vital that we continue to keep abreast of new theories and new discoveries to be sure that we are utilizing the best tools at hand to understand the subtleties of the issues in each case. I will continue to try to keep you informed of some of these issues through this forum in the weeks and months ahead.