In the 1960s, the dorsal root entry zone (DREZ) of the spinal cord was discovered to be associated with nociceptive transmission and began to be explored as a target for surgical treatment of pain. 1979, Nashold et al. first reported the use of DREZ dissection for the treatment of pain after brachial plexus avulsion injury with good results [1]. Since then, several other patients with chronic neurogenic pain have undergone the procedure, including phantom limb pain and pain after spinal cord injury. With further research on spinal cord anatomy and the development of science and technology, some scholars have improved the procedure and developed radiofrequency, laser and ultrasonic disruption based on microsurgical dissection; moreover, with the development of electrophysiological monitoring of the spinal cord [2], the efficacy of the procedure has significantly improved and complications have decreased, which has led to the promotion of the application of DREZ disruption. Tao Wei, Department of Functional Neurosurgery, Xuanwu Hospital, Capital Medical University
I. Microsurgical anatomy
The DREZ includes the dorsal root branches, the posterior lateral tract, and lamina I-V of the posterior horn of the spinal cord. Each dorsal root is divided into 4-10 branches with diameters of 0.25-1.5 mm into the posterior horn of the spinal cord. Depending on the thickness and destination of the afferent nerve fibers of the dorsal root, they are reorganized in the DREZ, with the fine fibers transmitting nociception located around the thick fibers transmitting tactile sensation. The posterior lateral bundle, located posteriorly to the posterior horn, plays an important role in the modulation of pain stimulus afferent fibers, with its medial portion transmitting excitatory impulses from each dorsal root to neighboring segments and its lateral portion transmitting inhibitory influences from the central glia to neighboring segments. The posterior horn is where the first synaptic transmission of the sensory system occurs, with coarse afferent fibers projecting to layers III and IV and fine afferent fibers projecting to layers I, II, and IV. Injurious afferent signals are modulated in the posterior horn by interneuronal and downstream connections.
The extent of DREZ dissection should include (1) the small injurious fibers (fine fibers) in the portion surrounding the dorsal root branches, (2) the excitatory medial portion of the posterior lateral fasciculus, and (3) the outermost layer of the posterior horn (layers I to V of Rexed). The inhibitory structures in the DREZ should be preserved, i.e., the thalamic fibers (thick fibers) that reach the posterior horn and the contact fibers that travel the lateral portion of the posterior lateral bundle.
DREZ dissection is the permanent destruction of secondary neurons of the injurious afferent pathway, i.e., it disrupts the normal injurious conduction pathway, allowing pain relief due to injurious stimuli. Some scholars have also found that nociception, in addition to being a response to injurious stimulus afferents, is also associated with spontaneous neuronal firing within the central nervous system, with abnormal electrophysiological activity in the posterior horn of the spinal cord in some patients with afferent nerve block pain [3]. Disruption of the DREZ can simultaneously eliminate the abnormal electrophysiological activity in the posterior horn of the spinal cord, resulting in pain relief.
II. Indications and efficacy
The best indication for DREZ dissection is pain after a brachial plexus avulsion injury. Usually due to trauma, resulting in avulsion of the brachial plexus nerve from the spinal cord, 70% of these patients experience pain, of which, 20% experience chronic intractable pain that is less effective with conservative treatments such as medications and physiotherapy.DREZ dissection is more effective for post-avulsion pain of the brachial plexus, with pain relief rates ranging from 66% to 87% in long-term follow-up [4-6]. A report reviewed the data of 91 patients with pain after brachial plexus avulsion injury treated with DREZ dissection at Duke; early after surgery, 91% had complete pain relief; at long-term follow-up, 73% had satisfactory pain relief; patients taking oral opioids decreased from 85% to 38% preoperatively; and five cases had pain recurrence [7].
Pain after spinal or cauda equina injury is another good indication for DREZ dissection, and most of these patients have a history of spinal trauma or surgery. The procedure is more effective when the patient’s pain is segmental and the area of pain is consistent with the level and extent of the spinal cord injury. Satisfactory pain relief was observed in 60% to 84% of long-term follow-up [8-10].
DREZ dissection has also been applied to treat phantom limb pain, residual limb pain, and postherpetic pain. Its efficacy in residual limb pain and phantom limb pain has been reported less frequently, and some authors have concluded that the method is satisfactory for phantom limb pain and less satisfactory for patients with residual limb pain only [11-12].The efficacy of DREZ incision for postherpetic pain is unclear, and Friedman reported complete pain relief in 18% of patients and partial pain relief in less than 50% of patients in long-term follow-up, and he concluded that DREZ excision for postherpetic pain relief is often not durable [13]. Due to the small number of cases, the efficacy of DREZ incision for these intractable pains is still not very certain and further clinical studies are needed.
III. Surgical technique
The patient is under general anesthesia in prone position and the procedure is to be performed under microscope. Patients undergoing neck surgery are immobilized with a head frame to allow neck flexion. A hemilaminectomy or total laminectomy is performed at the corresponding painful segment, and the dura mater is incised longitudinally to reveal the posterior lateral aspect of the corresponding spinal cord segment on the affected side. Localization of the spinal cord segment is performed according to anatomy or with the help of electrophysiological monitoring (electromyography). Along the selected spinal cord surgical zone, the soft spinal membrane is incised ventral-lateral to the entrance of the small root branch into the posterior lateral sulcus longitudinally and bluntly separated along the DREZ zone with a microdebrider to the posterior horn, which can be identified by its color change to gray. The posterior lateral spinal artery travels in the posterior lateral sulcus and has a diameter of 0.1 to 0.5 mm, emanating from the posterior root artery and anastomosing caudally with the anterior descending branch of the Adamkiewicz artery through the Lazorthes spinal cone anastomosis ring, which must be freed and protected from the posterior lateral sulcus.
In patients with pain after brachial plexus avulsion injury, the dorsal roots of the corresponding segment are absent and the spinal cord is degenerative and atrophic, making identification of the posterior lateral sulcus difficult. Identification can be made by the adjacent normal dorsal roots above and below; the tiny root vessels entering the spinal cord can also help determine the location of the posterior lateral sulcus; if it is still difficult to determine the location of the posterior lateral sulcus, intraoperative stimulation of the tibial nerve for posterior column somatosensory evoked potential monitoring is also very helpful [2]. Intraoperatively, the DREZ dissection should not be limited to the injured segment only, but extended to the nerve roots of the adjacent segment, especially if the level of injury coincides with the painful area.
In addition to incision of the DREZ by microsurgical techniques, the DREZ can be disrupted by a specific radiofrequency electrode. This electrode has a diameter of 0.25 mm and an exposed tip of 2 mm. the disruption temperature and time determine the size of the disruption foci, usually using 75°C for 15 seconds and making disruption foci at 1 mm intervals [14].
IV. Complications
Complications of DREZ dissection are mainly spinal cord injury, most commonly ipsilateral mild proprioceptive deficits due to ipsilateral posterior column injury, or ipsilateral mild proprioceptive deficits due to corticospinal tract injury. , or mild ipsilateral limb weakness due to corticospinal tract injury, with an incidence of approximately 5% to 20%, commonly associated with surgery of the thoracic medulla [5,7,9]. Under the premise of strict indication, DREZ dissection has good efficacy for some chronic intractable neurogenic pain, especially pain after avulsion of the brachial plexus, pain after spinal cord or cauda equina injury, and phantom limb pain, and its efficacy for pain due to some other peripheral neuropathies still needs to be further explored.