Mechanosensitive Desenitization and Nociceptive Sensitization: The most Common Clinical Presentation Seen in Practice

by Christopher J. Colloca, D.C.    

A 1995 cum laude graduate of Life College, and a 1990 graduate of Ithaca College, Dr. Colloca directs a full time private practice and clinical research facility in Phoenix, AZ. His original research has been presented at several scientific conferences and published in numerous peer-reviewed biomedical journal articles. He is a co-author of Activator Methods Chiropractic Technique (Mosby, 1997) and has lectured extensively throughout the U.S. and around the World, providing over 100 post-graduate education seminars for over 5,000 chiropractors. In 2000, he formed Neuromechanical Innovations, LLC to continue his vision with new continuing education programs, products, and services for chiropractors.

 

       The majority of patients who present to my office have no idea what “event” caused them to land in my exam room. I’ll bet your patients don’t either. Patients want answers and doctors are at a loss of what to tell them. In the consultation, the patient’s confusion and questions only worsen as the eager to explain doctor makes up cockamamie scenarios that could have caused the patients predicament. “Well you see, Mrs. Smith, you may have sprained your back while you were cleaning the house.” The problem is that Mrs. Smith has cleaned the house without incident for many years. Despite her confirmatory understanding as she nods her head, the puzzled look in her eyes tells you that she doesn’t quite believe you or understand. Unfortunately, you don’t understand it yourself, but don’t know what else to say to pacify the patient, and still come across as an expert. The patient’s question remains unanswered and we proceed. Is it any wonder why our patients don’t “get” chiropractic, don’t stay as long or refer as much as we’d like?

      Somewhere along the line, we resigned ourselves to the fact that “injuries” cause patients to seek our care. While macrotrauma occurs in our patients, the majority of patients seeking our care suffer from prolonged overloading and overuse of their spinal tissues. For example, certain occupational activities that involve cyclic or prolonged anterior flexion loading as seen in factory and warehouse workers, masons, mechanics and others are known to result in a 10-fold increase of exposure to low back injury.1, 2 In such individuals, however, the injury often occurs after the work is completed while they are performing simple, unloaded movements.3 Understanding the contemporary concepts of specific biomechanical and neurophysiological events that precipitate spinal dysfunction and ultimately pain will enable you to better communicate and manage your patients. First, we will begin with an understanding of spinal stability.

 

Spinal Stability

      Prolonged anterior flexion, or repetitive loading has been shown to induce creep deformation in the various passive tissues of the spine (disc, ligaments, facet capsule) increasing the laxity of the functional spinal unit (FSU), allowing increased relative motion, destabilizing their natural alignment.3 Surprisingly however, a trend in the literature appears to support an inability of spinal ligaments to significantly contribute to maintaining spinal stability.4-6 The major player, in contrast, is the musculature who’s co-contraction has been shown repeatedly to be the major structure generating forces capable of maintaining spinal stability.3, 7-9 Just how the muscles are regulated and how they contribute to injury causation is vital to patient communications.

 

The Role of Receptors and Stability

      Data now solidifies the extensive innervation of discoligamentous and other spinal soft-tissues.10-12 Mechanosensitive receptors transmit proprioceptive and kinesthetic information while nociceptive afferents communicate the presence of noxious stimuli (Figure 1). Through these neural networks stress/strain relationships, articulation position/angle velocity, and injury potential (via inflammatory presence and subsequent nociceptive stimulation) are communicated through the nervous system. Through the work of Solomonow et al.,13, 14 Indahl et al.,15-17 Pickar,18 and others, ligamento-muscular reflexes have been demonstrated to develop forces that stabilize the spine (Figure 2). When alteration to this reflexogenic stabilization occurs, the potential for tissue injury rises from destabilization, creating chiropractic vertebral subluxation with its clinical accompaniments including pain and dysfunction. It commonly happens through mechanosensitive desensitization, which brings to consciousness a contemporary understanding of vertebral subluxation.

 

Mechanosensitive Desensitization

      In the 1999 Volvo award-winning study in Biomechanics,3 Solomonow’s group reported their groundbreaking experiments that demonstrated the deleterious response of the spine to cyclic loading. The authors examined electromyography of the lumbar multifidus muscles in the cat while cyclic passive loading was applied to L4-L5 for 50 minutes, followed by a 10 minute rest, and a second 50 minute cyclic loading period. A “drastic” reduction in the muscular stabilizing reflex was observed (85% in the first 5 minutes) exposing the spine to destabilizing injury. The reduction in the protective muscular reflex was shown to be the direct manifestation of mechanoreceptor desensitization caused by laxity in the viscoelastic tissues of the spine. Furthermore, a 10-minute rest period was not long enough to restore the reflexive stabilizing effect of the musculature to a functional level. The authors believe that repeated stimulation of mechanoreceptors though repetitive cyclic loading causes them to desensitize, breaking the communication link between the receptor and the muscle. When the decreased sensitivity of receptors in the disc, facet, and ligaments add up, it is only reasonable to expect that the spine is exposed to significant risk of destabilizing injury and pain even when performing light activities after cyclic or prolonged loading.3

      In summary, the cyclic loading of the spine exposes it to potential instability (subluxation) and injury because of three proposed mechanisms: 1) A drastic reduction or elimination of the stiffening reflexive forces applied by the musculature caused by mechanosensitive desensitization in the discoligamentous tissues while subjected to laxity; 2) Increased laxity in the intervertebral joints caused by creep; and 3) an additional reduction in muscular forces when they are subjected to fatigue induced by prolonged active cyclic contraction. “The spine in such circumstances is virtually unprotected and fully exposed to instability and possible injury .”3 Understandably, our patients whom continually to overload their spines during common household and work activities likely fall into such a destabilization demise, subluxate, and at the point of tissue fatigue/failure the pain begins and the phone call to our offices for an appointment. Let’s understand the related pain mechanisms.

 

Nociceptive Sensitization

      Simultaneously during the repetitive loading events and mechanosensitive desensitization, nociceptive stimulation occurs from the mechanical loading and inflammation that is generated. Free nerve endings, termed nociceptors (free nerve endings) abound in the soft-tissues of the spine19 signaling the potential for tissue damage (noxious stimuli) in this manner. However, a linear relationship between nociceptive stimulation and action potential generation does not exist. Nociceptors sensitize both peripherally and centrally causing them to lower their thresholds to repeated stimulation.20 Occurring at the tissue level (disc, facet, ligament, muscle) this is termed peripheral sensitization. However, the dorsal horn neurons of the spinal cord are also known to become sensitized with repeated stimulation (termed central sensitization).21 It should be known that normally, nociceptors have very high thresholds of activation, yet when sensitized (through repeated stimulation Ñ mechanically and chemically) nociceptive nerve signals can be generated from normal movements subsequently being perceived as painful. With this understanding, we arrive at the patient’s clinical presentation to your office not knowing what caused their problem. Can chiropractic help?

 

The Chiropractic Approach

      Anecdotally, of course we can help. We see incredible results in our patients where other professions have failed every day. From a scientific standpoint, however, through some of the research projects that I have been involved in, we are beginning to document the benefits of chiropractic. We have demonstrated that adjustments result in neurophysiologic responses in the spinal nerve roots22 (Figure 3) and causes reflex activation of the trunk muscles.23 We have further shown that adjustments relate to an immediate 21% increase in trunk muscle strength.24 We believe that spinal adjustments are responsible for stimulating and possibly “resetting” the nervous system possibly contributing to underlying neuromuscular reflex mechanisms to contributing to spinal stability. We have several projects underway and planned to assist in answering these vital questions to chiropractic. In the meantime, what do we tell our patients?

 

Patient Communications

      The new patients presenting to our offices generally suffer local or referred pain, restricted motion, and abnormal posture. These are somatic referred pain patients. Yes, we see those suffering radiculopathy and paresthesia from nerve root compression, but more often the former. Being a clinician, I understand the challenges of patient communications and their understanding of what chiropractors do. The next time that a patient presents to your office baffled as to why they are in their current state tell them this:

      1) Pain is your body’s way of telling you that something is wrong, we’re glad that you came in to get checked.

      2) As with most of our patients, we cannot pinpoint one event or “injury” that is responsible for your condition. More often, it is things that you did at home (housework, shoveling, or gardening) or in your job repetitively that have caused your problem.

      3) Vertebral subluxation occurs when the spine loses its normal position. When muscles and joints are overused or abused and the muscles are not able to support the spine properly and this puts you at risk of injury or a “flare-up.”

      4) Wear and tear on your body creates inflammation that causing nerves to fire, and pain recognition.

      5) Anti-inflammatory medications may only treat the symptom, and not the cause of the inflammation (this is a nice time to distribute literature on their dangerous side-effects as well).

      6) Just like your car or house, your body needs tune-ups and preventative maintenance. Chiropractic adjustments help restore the structure and function of our bodies.

      Continue to reinforce this etiological view on consecutive visits until the patient can repeat it to you. You’ll notice a drastic increase in your patient compliance and their ability to intelligently discuss chiropractic and thus refer others to you.   

   

 

References

1.    McGill SM. The biomechanics of low back injury: implications on current practice in industry and the clinic. J Biomech 1997; 30:465-75.

2.    Pope M. Occupational hazards for low back pain. In: Weinstein J, Gordon S, editors. Low Back Pain. Rosemont, IL: AAOS; 1996:

3.    Solomonow M, Zhou BH, Baratta RV, Lu Y, Harris M. Biomechanics of increased exposure to lumbar injury caused by cyclic loading: Part 1. Loss of reflexive muscular stabilization. Spine 1999; 24:2426-34.

4.    Panjabi M, Abumi K, Duranceau J, Oxland T. Spinal stability and intersegmental muscle forces. A biomechanical model. Spine 1989; 14:194-200.

5.    Abumi K, Panjabi MM, Kramer KM, Duranceau J, Oxland T, Crisco JJ. Biomechanical evaluation of lumbar spinal stability after graded facetectomies. Spine 1990; 15:1142-7.

6.    McGill SM, Norman RW. Partitioning of the L4-L5 dynamic moment into disc, ligamentous, and muscular components during lifting. Spine 1986; 11:666-78.

7.    Gardner-Morse MG, Stokes IA. The effects of abdominal muscle coactivation on lumbar spine stability. Spine 1998; 23:86-91.

8.    Kumar S, Narayan Y, Zedka M. An electromyographic study of unresisted trunk rotation with normal velocity among healthy subjects. Spine 1996; 21:1500-12.

9.    Granata KP, Marras WS. The influence of trunk muscle coactivity on dynamic spinal loads. Spine 1995; 20:913-9.

10.   McLain RF, Pickar JG. Mechanoreceptor endings in human thoracic and lumbar facet joints. Spine 1998; 23:168-73.

11.   McLain RF. Mechanoreceptor endings in human cervical facet joints. Spine 1994; 19:495-501.

12.   Mendel T, Wink CS, Zimny ML. Neural elements in human cervical intervertebral discs. Spine 1992; 17:132-5.

13.   Stubbs M, Harris M, Solomonow M, Zhou B, Lu Y, Baratta RV. Ligamento-muscular protective reflex in the lumbar spine of the feline. J Electromyogr Kinesiol 1998; 8:197-204.

14.   Solomonow M, Zhou BH, Harris M, Lu Y, Baratta RV. The ligamento-muscular stabilizing system of the spine. Spine 1998; 23:2552-62.

15.   Indahl A, Kaigle AM, Reikeras O, Holm SH. Interaction between the porcine lumbar intervertebral disc, zygapophysial joints, and paraspinal muscles. Spine 1997; 22:2834-40.

16.   Indahl A, Kaigle A, Reikeras O, Holm S. Electromyographic response of the porcine multifidus musculature after nerve stimulation. Spine 1995; 20:2652-8.

17.   Indahl A, Kaigle A, Reikeras O, Holm SH. Sacroiliac joint involvement in activation of the porcine spinal and gluteal musculature. J Spinal Disord 1999; 12:325-30.

18.   Pickar JG, McLain RF. Responses of mechanosensitive afferents to manipulation of the lumbar facet in the cat. Spine 1995; 20:2379-85.

19.   Bogduk N.; Twomey L.T. Clinical Anatomy of the Lumbar Spine. 2nd ed. Melbourne: Churchill Livingstone; 1991.

20.   Casey KL. Nociceptors and Their Sensitization. In: Willis WD, editors. Hyperalgesia and Alodynia. New York: Raven Press, Ltd.; 2000:p. 13-5.

21.   Bonica JJ. The Management of Pain. 2 ed. Philadelphia: Lea & Febiger; 1990.

22.   Colloca CJ, Keller TS, Gunzburg R, Van de Putte K, Fuhr AW. Neurophysiological response to intraoperative lumbosacral spinal manipulation. J Manipulative Physiol Ther 2000; 23:447-57.

23.   Colloca CJ, Keller TS. Electromyographic reflex response to mechanical force, manually-assisted spinal manipulative therapy. Spine 2001: in press.

24.   Keller TS, Colloca CJ. Mechanical force spinal manipulation increases trunk muscle strength assessed by electromyography: A comparative clinical trial. J Manipulative Physiol Ther 2000; in press.