Detection of acute femoral artery ischemia during neuroembolization by somatosensory and motor evoked potential monitoring

Introduction

Intraoperative neurophysiological monitoring (IONM) is the frequent-to-continuous assessment of neurological function with the aim of minimizing injury through the identification of real-time insults to neurological structures, such that intervention can be initiated to avoid or mitigate resulting injury. Neuromonitoring can be used to intraoperatively map out patient-specific neuroanatomy, define physiologic deficits secondary to specific pathology or intervention, or predict postoperative outcome. ¹

IONM employs the use of a combination of discrete and continuous tests to gain insight about nervous system function. Discrete electrophysiological tests include somatosensory evoked potentials (SSEPs), which involve stimulation of peripheral nerves while recording over the scalp, cervical spine, or peripheral nerves; transcranial motor evoked potentials (TcMEPs), where the corticospinal pathways are stimulated electrically or magnetically and neurogenic and myogenic potentials are recorded; and brainstem auditory evoked potentials (BAEPs), involving auditory stimulation and recording of brainstem potentials via scalp electrodes. Continuous neurological monitoring tests include electroencephalography/electrocorticography (EEG/ECoG) and electromyography (EMG), which serve as correlates of central and peripheral nervous system function, respectively.

The first recorded use of neuromonitoring in an anesthetized patient involved continuous EEG monitoring in a patient undergoing carotid endarterectomy. ² SSEPs were first recorded in patients undergoing awake thalamotomies in 1966. ³ More recent efforts have focused on the expansion of applications for neuromonitoring and enhancement of algorithms to record SSEPs in speedier, more reliable fashions.^4,5 Herein, we present a case demonstrating the utility of neuromonitoring in detecting neurophysiological changes secondary to peripheral insult caused by an occlusive vascular sheath.

Case report

A 61-year-old woman was admitted for repeat endovascular coiling of a recurrent basilar tip aneurysm following aneurysmal rupture and subarachnoid hemorrhage one year earlier. Interim cerebral angiography had revealed coil compaction with recanalization of the aneurysm base necessitating retreatment (Figure 1).

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Figure 1.

Angiographic findings. (a) AP angiographic image from a left vertebral artery injection demonstrating the previously coiled basilar artery aneurysm with recurrence at the base secondary to coil compaction. (b) Right common femoral artery (CFA) groin run at the time of first aneurysm treatment demonstrating normal filling of right superficial and deep femoral arteries around the 6-French sheath. (c, d) Right CFA groin run at the time of second aneurysm treatment. The 6-French sheath is occlusive in the vessel (c, arrowhead). Slow and delayed anterograde flow in the CFA is demonstrated (d, arrowhead) secondary to occlusive sheath.

During retreatment, general anaesthesia was induced, IONM electrodes were placed, and baseline BAEPs, SSEPs, and TcMEPs were reproducible bilaterally. The right common femoral artery was accessed using a 6-French, 45-centimeter vascular sheath, and the left vertebral artery was catheterized with a 6-French Envoy Guide catheter. Approximately 90 minutes following induction, the right lower extremity TcMEPs could no longer be elicited, and a 50% decrement in amplitude of cortical SSEPs following right leg stimulation was noted. As catheter location within the left vertebral artery did not correlate with the documented findings, the left internal carotid artery was catheterized but also demonstrated no angiographic abnormalities. Neuromonitoring electrodes were rechecked. TcMEP and SSEP tracings proved refractory to increases in systemic blood pressure, and embolization was aborted accordingly.

However, angiography obtained prior to groin closure demonstrated delayed flow in the common femoral artery resulting from a near occlusive vascular sheath at the access site. Following prompt removal, TcMEP and SSEP amplitudes corrected to baseline (Figure 2). Postoperatively, the patient was able to move all extremities spontaneously, without evidence of central or peripheral neurologic dysfunction.

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Figure 2.

Intraoperative neuromonitoring tracings. (a) Preoperative symmetric cortical and cervical spinal somatosensory evoked potentials (SSEPs) following bilateral upper and lower extremity stimulation with noted transcranial motor evoked potentials (TcMEPs) from the right hand, right leg (tibialis anterior), and right foot (abductor hallucis). (b) Loss of cortical SSEPs (red arrowhead, top) and right leg TcMEPs (red arrowheads, middle and bottom) following stimulation intraoperatively. (c) Return of right leg SSEPs and TcMEPs (blue arrowhead, top) with continued absence of right foot TcMEPs (red arrowhead, below) after removal of groin sheath.

Discussion

Neuromonitoring is a crucial aspect of care within the endovascular suite, as evident herein, where changes in evoked potentials alerted attention to an occlusive vascular sheath and ultimately prevented significant morbidity. As such, in addition to well-established reasons for changes in neuromonitoring potentials such as intracranial hemorrhage, ischemic stroke, or technical error, we advise consideration of peripheral ischemia as a potential cause for changes in SSEPs/MEPs.

We routinely utilize pulse oximetry on the distal limb ipsilateral to groin access to monitor for limb ischemia secondary to vascular access. However, our patient had nonpalpable pedal pulses at baseline identifiable only by Doppler, and pulse oximetry waveforms were deemed too suboptimal for appropriate intraoperative monitoring. The consideration of peripheral ischemia is especially salient in particular populations where blood vessels might be of diminished size, such as in infants and young children. ⁶ In retrospect, greater caution should also have been exercised in our case, where due to non-ambulatory status at baseline and associated disuse atrophy impacting limb vasculature, the patient was predisposed to marked reductions arterial caliber and complications thereof (Figure 1).

Neurophysiological changes from peripheral ischemia have been reported in the setting of crossclamping during aortic aneurysm surgery.^7,8 Gugino et al. described peripheral ischemia from a femoral artery embolus and decreasing femoral artery pressures resulted in lower extremity SSEP/MEP changes, reversed following embolectomy and increasing perfusion. ⁷ SSEP changes due to femoral artery ischemia have also been demonstrated in the setting of spine instrumentation.^9,10

In sum, we maintain that the benefits of neuromonitoring to potentially prevent adverse neurological outcomes in high-risk patients justify its additional cost. We recommend triaging cases where neuromonitoring will be most helpful, endovascular procedures included. At our institution, IONM is routinely employed during endovascular coiling, as monitoring can indicate ischemic changes in real-time and allow clinicians to modify interventions that result in adverse changes in evoked potentials. We have modified our protocol for recording lower extremity SSEPs and now routinely add additional electrodes to record peripheral nerve potentials from the popliteal fossa.

Conclusion

Neuromonitoring is an important adjunct of intraoperative care critical for detecting central and peripheral ischemic events. Use of the technology and recognition of potential changes in SSEPs/MEPs secondary to limb ischemia and other recognized causes is essential in the avoidance of permanent deficits secondary to central and peripheral iatrogenic neurologic insult.