A basic research on orofacial pain originating from cervical structure: Referred pain

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Patients occasionally present with a complaint of orofacial pain originating from cervical structures. The mechanisms of orofacial pain with cervical origin are not well known. We studied the etiology of this type of pain on a cervical nerve injury model. 6 SD rats underwent two ligation s on C2-C5 cervical nerves followed by section between them (CN group). 6 SD rats received sham-operation (SO group). Another 6 rats received cervical sympathectomy before nerve injury (SE group). Escape behavior was observed before and after operation (POD14) in all rats. Expression of fos-like immunoreactive protein in cells (Fos-LI cells) was observed with tactile stimulation on ipsilateral preauricular skin on POD14. The results were as follows. 1. Cervical nerve injury resulted in allodynia in trigeminal territory. 2. Fos-LI cell expression was observed not only Vi/Vc and Vc/C levels but also middle Vc and C3. 3. Cervical sympathectomy led to no significant behavioral changes. 4. Sympathectomy resulted in significant expression of Fos-LI cells Vi/Vc and Vc/C levels. These results revealed that cervical nerve injury may elicit hypersensitization of secondary neurons in wide area from Vi/Vc to upper cervical spine. This central sensitization may be due to convergence of the trigeminal nerve and cervical nerves. Cervical sympathectomy may modulate signal transduction in nociceptive neurons in uppercervical spine.