Bilateral Globe Penetration From Electromyography Electrode Placement for Intraoperative Neurophysiologic Monitoring

Abstract

Purpose:

This interventional case report discusses inadvertent bilateral temporal globe penetration during placement of intramuscular wire electrodes to the lateral rectus muscles for intraoperative neurophysiological monitoring (IONM) via electromyography.

Methods:

An 11-year-old girl underwent surgical resection of massive medulloblastoma within the fourth ventricle, requiring IONM. Placement of an electrode in each lateral rectus muscle resulted in bilateral globe penetration, with choroidal rupture, retinal tears, and hemorrhage.

Results:

Sterile needle perforation of the globe did not result in endophthalmitis. Encircling laser retinopexy was performed, and no retinal detachments occurred.

Conclusions:

Insertion of needle electrodes without guidance imaging can potentially lead to globe penetration and incorrect electrode placement. Direct visualization with ultrasound, electromyography, or other advanced image-guided systems may offer a safe solution for electrode placement to avoid injury. Verbal patients should be made aware of postoperative warning signs of globe penetration. For nonverbal patients, a postoperative dilated exam is warranted.

Introduction

Intraoperative neurophysiological monitoring (IONM) with electromyography (EMG) provides continuous real-time monitoring feedback to evaluate potential cranial nerve involvement or injury during intracranial tumor resection. ¹ To enable stable EMG analyses, hypodermic needles are used to load and insert an intramuscular wire electrode within the extraocular muscle belly (Figure 1). ² This procedure is most commonly performed by pediatric ophthalmologists and neurosurgeons. Given the close proximity of the lateral rectus muscle belly to the ocular surface, electrode placement requires careful consideration to prevent globe injury. We present a case of bilateral temporal globe penetration with hypodermic needles used during placement of intramuscular wire electrodes for IONM.

Figure 1.

Sterile hypodermic needle with paired hookwire electrodes with subdermal ground electrode and subdermal stim return. (Medtronic Xomed Inc, reference No. 8226326.)

Methods

An 11-year-old girl presented to the emergency department with nausea and vomiting that awoke her in the mornings and a 6-month history of progressively worsening diplopia. Magnetic resonance imaging (MRI) showed a large mass that extended from the floor of the cerebral aqueduct to the foramen magnum. She was taken urgently to the operating room for decompression and then resection of the brain lesion. In preparation for the 15-hour facedown procedure, neurophysiological electrodes were placed for cranial nerve monitoring, with somatosensory evoked potential through bilateral cranial nerves VI-XII. A paired hookwire electrode with subdermal ground electrode (Medtronic Xomed Inc) was loaded into a sterile 25-gauge needle (Figure 1). A freehand lateral transconjunctival approach was taken for placement of the needle with the loaded electrode into the lateral rectus muscle belly. The Medtronic Stealth-guided posterior occipital craniotomy with tumor resection and subsequent external ventricular drain placement was successful, and the patient was admitted to the pediatric intensive care unit for monitoring. Pathology showed medulloblastoma. On postoperative day 2, ophthalmology was consulted by the primary team because of ocular concern. The patient was intubated but able to communicate with head movements. Her vision was count fingers in both eyes. Pupil response was sluggish but without a relative afferent pupillary defect. Intraocular pressures were normal. Ocular motility was limited to a vertical V-pattern distribution with –4 abduction in both eyes and limited adduction. Ocular surface examination showed mild bitemporal subconjunctival hemorrhage. Dilated examination showed grade 4 papilledema, macular edema, and mild vitreous hemorrhage in both eyes. In the right eye, a 4 mm × 1 mm linear yellow and white scleral, choroidal, and retinal penetration injury was seen pointing to the posterior globe with surrounding retinal hemorrhage. A larger 6 mm × 1 mm linear yellow and white scleral, choroidal, and retinal penetration injury was seen in the left eye, similarly directed toward the posterior globe (Figure 2). Indirect laser retinopexy was performed to encircle the diseased areas. During her 3-month hospitalization, the patient did not develop a retinal detachment, endophthalmitis, or proliferative vitreoretinopathy. The patient continued her hospital course with external-beam radiation and vincristine as an adjuvant radiosensitizer.

Figure 2.

Bilateral temporal globe penetration of hypodermic needles during placement of intramuscular wire electrodes for intraoperative neurophysiological monitoring via electromyography. (A) Right eye and (B) left eye with complete globe penetration and retinal tears temporally treated with barrier laser, without traction or retinal detachment 6 months from incidence.

Results

At her 6-month outpatient follow-up, the patient was able to talk, and visual acuity had improved to 20/25 in the right eye and 20/60 in the left eye. Her extraocular motility had significantly improved; papilledema had resolved, leaving a mild degree of optic pallor (which was responsible for the remaining vision deficit), and her maculas were no longer edematous, as determined by spectral-domain optical coherence tomography. The retinal tears were stable after treatment with barrier laser, without proliferative vitreoretinopathy (Figure 2).

Conclusions

Skull-base, brainstem, anterior fossa, or cavernous sinus and orbit surgical cases can benefit from intraoperative neurophysiological monitoring to prevent cranial nerve injury. ² Surface and subcutaneous electrodes placed around the eyes are less desirable because of missed motor unit potentials and inaccuracies in identifying the correct muscle generating the EMG. Intramuscular electrodes are more reliable because of an improved signal-to-noise ratio. ³ The 2 most common intramuscular electrodes are electroencephalogram (EEG) needles and wire electrodes. EEG needles are not ideal for extraocular muscle placement given that they are usually not insulated, are short, and can be displaced from their target muscle with eye movements, resulting in poor measurement and injury to surrounding tissues. Intramuscular wire electrodes are more reliable because of better stability and fixation in the eye muscle. These probes are hooked to the beveled side of a 25-gauge hypodermic needle, followed by transconjunctival or percutaneous injection past the orbital septum and then into the belly of the extraocular muscle. Once in the muscle, the wire probe is advanced and hooked to muscle tissue. The needle is then removed, leaving the electrode stable within the muscle for neurophysiological monitoring. ²

This report emphasizes that the placement of electrodes in extraocular muscles for IONM carries the risk of globe penetration. Globe penetration with needles has been reported with botulinum toxin injections for strabismus, ⁴ retrobulbar injections, ⁵ and acupuncture ⁶ ; however, there has been little direct attention paid to potential complications with placement of needle electrodes.

Placement of electrodes can be performed in a free-hand transconjunctival approach using orbital structures to guide placement. ³ However, alternative approaches including ultrasound, EMG, and advanced image-guided systems may help avoid globe penetration through improved feedback for more accurate positioning. Examples include orbital ultrasound with regular needle probes or advanced insulated telescoped steel cannulas, which have been reported to improve needle entry visualization. ^3,7 Additionally, EMG guidance to confirm correct placement within extraocular muscles for botulinum injections has been reported. ⁸ Furthermore, a direct instrument-tracking method using infrared light flashes integrated into a camera system that detects instrument location with reflective markers on superimposed previously acquired computed tomography or MRI images has reportedly guided placement of electrodes. ^9,10

Use of ultrasound, EMG, or other advanced image-guided systems may offer a safe solution for electrode placement to avoid injury. In addition, in verbal patients, providers and patients should be made aware of postoperative warning signs of photopsia, new floaters, or peripheral vision changes that warrant ophthalmologic evaluation. In patients who are not verbal (owing to age, developmental delay, intubation, or sedation), a postoperative dilated examination is warranted during the early recovery period to confirm intact ocular structures.