We have observed previously unreported muscle atrophy in the rectus capitis posterior minor (RCPMI) muscles of a group of chronic pain patients. We hypothesize that chronic pain, in this select group of patients, is a consequence of tramua that occurs to the C1 dorsal ramus during whiplash. Damage to a peripheral nerve can result in motor and sensory dysfunction within peripheral and/or central structures, effecting a perception of pain that is consistant with the clinical symptoms of our chronic pain patients.

For all subjects, axial images consisting of 4 mm thick contiguous slices of data were collected inferior and superior to the posterior arch of the atlas using a 1.5-Tesla superconducting magnet (SignaTM; General Electric Medical Systems, Milwaukee, WI) with TR = 2000 msec, TE = 25 msec, a 12 cm field of view, a 128 x 256 matrix, NEX = 2, and a total scan time approximately equal to 8.5 minutes.

Figure 1 shows image data from one subject that was considered to be representative of the control group. The boundaries of muscles are well defined and the muscles are of uniform intensity. Figure 2 shows image data from one of the chronic pain subjects. There is a significant difference between the appearance of muscles between the two groups. In general, muscles from the chronic pain group have an appearance that is characteristic of skeletal muscle that has died and been replaced with fatty tissue. Comparison of muscles in PD-weighted images with muscles in T2-weighted images (TR = 2000 msec., TE = 80 msec.) confirmed that the infiltrating tissue was a fatty type.

We hypothesize that entrapment and traction of the C1 dorsal ramus during whiplash-type cervical distortions, results in denervation atrophy, characterized by fatty infiltration, of the RCPMI muscles. The C1 dorsal ramus, arising from the C1 spinal nerve as it crosses the superior aspect of the posterior arch of the atlas, branches to innervate the suboccipital muscles. The C1 dorsal ramus passes dorsolaterally through the suboccipital plexus of veins, describing an upward arch to enter the suboccipital triangle where it divides to form a branch for the RCPMI muscles (See Figure 3). Most commonly the suboccipital muscles are innervated on their dorsal surfaces, however the branch to the RCPMI muscles may innervate its muscle ventrally or by penetrating the RCPMA muscle. At this natural site of potential entrapment, the nerve root to the RCPMI muscles is vulnerable to traction during whiplash-type neck distortions.

The challenge of preventing chronic musculoskeletal pain begins with identifying factors that identify individuals who are at risk. The detection of skeletal muscle infiltrated with fatty tissue in chronic pain patients is a significant finding in that it supports our hypothesis that chronic pain within our patient group is a consequence of entrapment and traction of the C1 dorsal ramus during whiplash-type cervical distortions. Damage to peripheral nerves is known to produce motor and sensory dysfunction within peripheral and/or central structures, effecting a perception of pain that is consistant with the clinical symptoms of our chronic pain patients. We predict that damage to the dorsal ramus will constitute a risk factor, predicting development of idiopathic head and neck pain, that is detectable from EMG data early in the disease process.

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