Clinical neurophysiology and quantitative sensory testing in the investigation of orofacial pain and sensory function.

Chronic orofacial pain represents a diagnostic and treatment challenge for the clinician. Some conditions, such as atypical facial pain, still lack proper diagnostic criteria, and their etiology is not known. The recent development of neurophysiological methods and quantitative sensory testing for the examination of the trigeminal somatosensory system offers several tools for diagnostic and etiological investigation of orofacial pain. This review presents some of these techniques and the results of their application in studies on orofacial pain and sensory dysfunction. Clinical neurophysiological investigation has greater diagnostic accuracy and sensitivity than clinical examination in the detection of the neurogenic abnormalities of either peripheral or central origin that may underlie symptoms of orofacial pain and sensory dysfunction. Neurophysiological testing may also reveal trigeminal pathology when magnetic resonance imaging has failed to detect it, so these methods should be considered complementary to each other in the investigation of orofacial pain patients. The blink reflex, corneal reflex, jaw jerk, sensory neurography of the inferior alveolar nerve, and the recording of trigeminal somatosensory-evoked potentials with near-nerve stimulation have all proved to be sensitive and reliable in the detection of dysfunction of the myelinated sensory fibers of the trigeminal nerve or its central connections within the brainstem. With appropriately small thermodes, thermal quantitative sensory testing is useful for the detection of trigeminal small-fiber dysfunction (Adelta and C). In neuropathic conditions, it is most sensitive to lesions causing axonal injury. By combining different techniques for investigation of the trigeminal system, an accurate topographical diagnosis and profile of sensory fiber pathology can be determined. Neurophysiological and quantitative sensory tests have already highlighted some similarities among various orofacial pain conditions and have shown heterogeneity within clinical diagnostic categories. With the aid of neurophysiological recordings and quantitative sensory testing, it is possible to approach a mechanism-based classification of orofacial pain.