Intraoperative Monitoring of the Diaphragm Function During Cervical Dorsal Root Entry Zone-Lesioning Illustration With a Case Report

Abstract

BACKGROUND AND IMPORTANCE:

Microsurgical DREZotomy (MDT) involves lesioning the dorsal root entry zone at the cervical spinal level, carrying a potential risk of iatrogenic spinal cord injury. Misidentifying the C4 root can lead to impaired diaphragmatic contractions and respiratory compromise. This report is the first MDT procedure in our Central Asian region and introduces the simultaneous analysis of 3 ventilator waveforms—capnography, pressure time, and flow time—as a novel, rapid-response intraoperative neurophysiological monitoring for definitive C4 root identification.

CLINICAL PRESENTATION:

Traumatic brachial plexus avulsion is an upper limb injury resulting in severe complications such as paralysis and neuropathic pain. Patients with refractory, pharmaco-resistant pain may be candidates for cervical MDT. Intraoperative neurophysiological monitoring during MDT is beneficial because it allows for anatomical mapping from C4 to Th1 nerve roots. The C4 root should not be targeted during MDT because it is cardinal for diaphragmatic muscular activity. Three ventilation curves simultaneously displayed synchronous changes during C4 ventral root stimulation, indicative of phrenic excitation and the diaphragm's reaction against the anesthesia machine. The expected C4 ventral root responses completely aligned with the stimulation patterns observed across all 3 monitored parameters.

CONCLUSION:

The simplicity of this method and its high practical applicability make it valuable for identifying phrenic nerve fibers during various surgical procedures where nerve roots responsible for diaphragmatic muscle contraction may be at risk.

ABBREVIATIONS:

DREZ

dorsal root entry zone

EMG

electromyography

MDT

microsurgical DREZotomy.

Chronic neuropathic pain significantly reduces the quality of life for neurologically handicapped patients. Since 1972, microsurgical DREZotomy (MDT) has been used to treat specific types of localized pain and has undergone continuous refinement over time.¹ This technique involves microsurgical coagulation of the dorsal horn in the dorsal root entry zone (DREZ) using bipolar micro-forceps. The lesioning extends to a depth of 3 mm with a 35-degree angle in the dorsal horn. To mitigate complications, these operations can be accompanied by intraoperative neurophysiological monitoring.² A crucial aspect of monitoring during the main surgical stage is the identification and preservation of the C4 root, which is responsible for diaphragmatic innervation.^3-5 We present a case of a 53-year-old female patient with chronic post-traumatic pain syndrome secondary to brachial plexus avulsion, who underwent cervical MDT with intraoperative mapping of neural structures.

CLINICAL PRESENTATION

The patient is a 53-year-old woman suffering from monoparesis of her right upper limb and severe chronic post-traumatic pain. By February 2020, when she was referred, she had developed resistance to analgesics due to a brachial plexus injury sustained in a traffic accident 3 years prior. Given the nature of this retrospective case report, describing clinical management, specific Institutional Review Board approval was not required as per our institutional guidelines. Written informed consent was obtained from the patient for the publication of this case report and accompanying images.

Her pain intensity during the day varied, with a visual analogue scale score plateauing at 8/10 and paroxysmal crises reaching 10/10. Upon examination, she exhibited trophic changes and dysesthesia in the C5-C8 dermatomal distribution.

• The patient's clinical history and intervention unfolded over several years:

• 2017: Patient sustained the brachial plexus injury in a traffic accident.

• 2017-2020: 3 years of refractory, pharmacoresistant neuropathic pain.

• Early 2020: Patient referred to the neurosurgery department.

• 2020: Cervical MDT procedure was performed.

• Immediate Postoperative: Pain was reported as immediately resolved.

• Long-Term Follow-up: Pain relief maintained through the immediate postoperative period.

MRI identified avulsion of the right C5-C8 roots at the DREZ. Electromyography (EMG) showed signs of complete denervation of the deltoid, biceps brachialis, and triceps muscles, corresponding to the upper and middle trunks of the brachial plexus and segments C5-C7. Simultaneously, signs of reinnervation of the supraspinatus muscle (upper trunk of the plexus, segments C5, C6) were recorded. Innervation of the hand (lower plexus trunk, segment C8) was preserved. Based on the results of all examinations, the neurosurgical team proposed MDT, a procedure performed for the first time in Kazakhstan and the Central Asian region.

The surgery was performed in February 2020. The patient was positioned in a prone “concord” position. A skin incision was made from the C3 to Th1 levels. Hemilaminotomy of C3-C7 was performed. After opening the dura mater, it was observed that the dorsal roots of C5, C7, and partially C8 were avulsed at the DREZ (Figure 1). On the right side, green (C5), red (C7), and yellow (partially avulsed C8) arrows indicate the ruptured dorsal roots. After identifying the C4 ventral root, the operating table was rotated 35° to the microscope's viewing field. An incision, 3 mm in depth, was made in the dorsolateral sulcus from the C4 root down to the Th1 root entry zone, encompassing the C5-C8 medullary segments. The red arrow indicates the intervertebral foramen of the right C7 nerve, while the green arrow shows the dentate ligament between the C6 and C7 intervertebral foramina (Figure 2). To obtain muscle responses during intraoperative mapping, total intravenous anesthesia was administered, free from neuromuscular blocking agents except for short-acting forms used during induction and intubation.⁶ Stimulation intensity parameters for ventral roots ranged between 0.2 and 1 mA, with common frequency of 2 Hz.

FIGURE 1.

Green lines illustrate the dorsolateral sulcus beneath the ruptured dorsal roots. The green arrow points to the existing C5 root, the red arrow to C7, and the yellow arrow to the partially avulsed C8 root.

FIGURE 2.

Red arrow indicates the intervertebral foramen of the right C7 nerve, while the green arrow shows the dentate ligament between the C6 and C7 intervertebral foramina.

Responses from the diaphragm were monitored on the anesthesia machine. The study focused on 3 ventilation curves: capnography, pressure/time, and flow/time (Figure 3). In addition, to exclude potential damage during the main stage of the operation, motor evoked potentials, somatosensory evoked potential, raw EMG, and triggered EMG were performed.^2,4 As can be concluded from the graph, pressure and flow-time curves respond earlier than capnography. At C4 ventral root stimulation, capnography exhibited abrupt falls in the baseline (red arrow, Figure 4). These changes signify expiratory flow obstruction or involuntary contractions of the respiratory muscles. Pressure-time (yellow arrow, Figure 4) and flow-time waveforms (white arrow, Figure 4) demonstrated real-time changes in their modified baseline. The advantage of pressure-time and flow-time curves is their real-time recording, whereas the capnography curve is recorded with a slight delay (approximately half a respiratory cycle). This delay was confirmed during intraoperative neurophysiological monitoring. As a beneficial result, we can easily and accurately determine the level of the C4 root/phrenic nerve.

FIGURE 3.

Changes in 3 ventilation waveforms (first capnography; second pressure-time curves; and third flowtime) during C4 root stimulation.

FIGURE 4.

Intraoperative stimulation of the C4 root demonstrating expiratory flow obstruction/involuntary contractions of the respiratory muscles: red arrow (capnography), yellow arrow (pressure-time), white arrow (flowtime).

The patient was monitored postoperatively for any signs of respiratory or motor deficits, with a particular focus on the C4 function. Immediately after the procedure, the patient reported that the severe neuropathic pain had completely resolved. Tolerated well; successful outcome assessed by the immediate and complete disappearance of target pain without new neurological deficits. There was no deterioration in the patient's existing motor or sensory neurology. The successful outcome, characterized by the immediate and complete disappearance of the target pain without new deficits, indicated the MDT procedure was successful and the intraoperative monitoring technique effectively preserved the C4 phrenic nerve pathway.

DISCUSSION

Surgical Precision and Physiological Monitoring

The described method of using intraoperative neuromonitoring to identify the C4 root can significantly enhance the precision of surgical procedures improve efficacy and reduce the risk of complications. The changes observed in the capnography curve correspond to the transient generation of inspiratory flow caused by partial contraction of the diaphragmatic muscle. The timing of the contraction, after C4 root stimulation, indicates the reliability of this technique. The anatomical location of the phrenic nerve primarily corresponds to the C4 root, although literature suggests its fibers can originate from C3 and C5.^5,7

Ventilator Waveform Analysis

Over the past 25 years, one of the simplest methods for determining the phrenic nerve has been manual palpation of the hypochondrium on the stimulation side.⁸ The anesthesia machine allows continuous recording of 3 ventilation waveforms: capnography, pressure-time curves, and flowtime (Figure 3). These simultaneously show phrenic irritation and changes during C4 root stimulation. In our case, these 3 waveforms displayed 100% sensitivity during root determination.⁹

In comparison with literature and clinical alignment, Bhagat H and colleagues proposed using capnography to confirm phrenic nerve stimulation.¹⁰ Their article detailed 3 clinical cases of patients undergoing surgery for post-traumatic brachial plexus recovery, where they combined capnography with observation of diaphragmatic muscle contractions. Our literature review revealed that very few cases describing the use of capnography for phrenic nerve determination have been reported.¹⁰ Our clinical case aligns with previously described cases where both parameters (capnography and flow-time ventilator waveforms) proved valuable for confirming diaphragmatic reactions during root stimulation. Responses in capnography significantly facilitate the initial identification of responses from the diaphragmatic muscles. Simultaneously, while the amplitude of responses in pressure-time and flow-time curves may be lower, they are more specific as the responses are recorded in real-time. This relationship between the curves enables a double-check to obtain true responses from the C4 root/phrenic nerve.^10,11

Limitations

As a single case study, this report constrains the generalizability of the findings and prevents the calculation of formal sensitivity and specificity metrics. A larger prospective case series is required to validate this rapid-response C4 identification technique.

Patient Perspective

The patient's primary complaint was severe, refractory neuropathic pain lasting 3 years. After surgery, she expressed profound relief that the pain (previously 8/10 to 10/10) was immediately gone. The successful pain cessation was considered a major life improvement, enhancing her quality of life despite existing motor deficits.

CONCLUSION

We emphasize the simplicity and value of this method for identifying diaphragm motor pathways during surgical procedures where the C4 root is at risk. Complex operations necessitate a collaborative team effort involving neurosurgeons, anesthesiologists, and neurophysiologists.

Funding

This study did not receive any funding or financial support.

Disclosures

The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.

COMMENTS

The authors should be commended in their performing the first DREZ procedure in Kazakhstan and Central Asian region and working to innovate to improve the safety profile for this procedure. This was clearly a multidisciplinary and collaborative team effort involving neurosurgeons, anesthesiologists, and neurophysiologists. Specifically, the authors demonstrate the use of electrical stimulation to identify the C4 nerve root by monitoring changes in ventilator waveform (capnography, pressure/time, and flow/time). The authors identify that pressure/time and flow/time parameters are first to respond to stimulation with capnography lagging in its response, with the pressure-time and flow-time responding near instantaneously. The rapid feedback offers expeditious surgical decision making and is a core innovation of this paper. This important technique may be applicable beyond the DREZ procedure (ie, complex avulsion injuries) as an adjunctive confirmatory test of the C4 level. However, there should be some caution in stating that this technique uniquely selects the C4 level as the phrenic nerve has innervation from C3-4-5. Without stimulating the C3,4,5 independently and showing that C4 is the only nerve that will reliably create this pulmonary response, it is challenging to say that this technique exclusively identifies the C4 root (despite its cardinal innervation). A larger prospective series in a wider range of pathologies would improve the validity and applicability of this study. Further, it would allow the authors to determine a more accurate sensitivity and specificity that could be more widely applicable for C4 nerve root identification. In summary, this paper is an important multidisciplinary contribution to the field of intraoperative monitoring techniques, and the authors should be lauded for highlighting the commitment to innovation within the Central Asian neurosurgical community.

Alexander Suarez-Jew

San Francisco, California, USA