Question 14: should sensory and/or motor stimulation be performed before radiofrequency ablation?
Rationale for sensory stimulation and evidence
The success of RFA of lumbar medial branch nerves is dependent on correctly identifying patients whose pain is mediated via the medial branch nerves and by providing a thermal lesion that adequately coagulates the nerves, thereby preventing conduction of nociceptive information along the nerves. Integral to this second point is that the RF cannula must be in close enough proximity to the intended target nerves to result in ablation. Additionally, to avoid or minimize complications, the procedure should avoid coagulation of the ventral ramus or other unintended structures. Although some physicians believe that these goals can be accomplished through appropriate needle placement to the intended anatomical target based on fluoroscopic landmarks, others advocate for the use of sensory and/or motor testing in addition to anatomic landmarks to achieve optimal placement. The justification for this is based on anatomical variations in the location of the medial branches and the multiple articular branches emanating from each nerve.
261 265–267
Sensory stimulation is typically carried out at 50 Hz. Patients are asked to inform the treating physician when they identify a sensory change (eg, tingling, buzzing, vibration, pain). Traditionally, an acceptable threshold is <0.5 V.
7 18 81 84 If sensory threshold is in fact being used to determine optimal placement, however, the cannula should be advanced in all three dimensions (anterior-posterior, cephalad-caudal and medial-lateral) to determine exactly where the stimulation threshold is lowest. In clinical practice, most physicians do not modify placement once an acceptable threshold is reached. Additionally, sensation may be evoked by local muscle stimulation even when the nerve is not close enough to be incorporated into a thermal RF lesion. This is particularly true since the shape of the lesion is known to extend circumferentially along the active tip. In this regard, suboptimal needle placement technique (perpendicular rather than parallel trajectory) can result in adequate sensory stimulation while the lesion may be insufficient for coagulation of the nerve and relief of pain. A prospective study in 61 patients who underwent lumbar facet RFA after a positive block found no correlation between average sensory threshold and treatment results.
268 However, the authors concluded that because sensory testing was optimized for each patient by adjusting the electrode in multiple planes before lesioning, the results should not be misinterpreted as meaning sensory testing should not be done. Rather, sensory testing is just one of many factors that include age, gender, genetics, sedation and baseline analgesics and comorbidities (eg diabetes) that could affect medial branch sensory perception. In a small observational study by Dreyfuss
et al,
147 the authors found no correlation between the degree of multifidus muscle atrophy and treatment outcome or levels treated 17–26 months after denervation. In two small placebo-controlled studies performed in the cervical and lumbar spine that yielded positive results, the investigators did not use sensory testing, instead creating four to six empirical lesions per level based on anatomic landmarks.
82
