Removal of Perirolandic Cavernoma with Direct Cortical Stimulation and Neuronavigation with DTI
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.314528
Source of Support: None, Conflict of Interest: None
Keywords: Awake craniotomy, cavernoma, eloquent cortex lesions, intraoperative neuromonitoringKey Message: Intraoperative monitoring has become an indivisible component of a neurosurgeon's armamentarium. Even if a standard evoked potential monitoring system is unavailable, simple NIM system with monopolar probe stimulator can be used for surface mapping during resection of eloquent cortex lesions.
Intraoperative neuromonitoring (IONM) has become one of the integral parts of the operating room setup in neurosurgery. Not just oncological procedures, but all eloquent region surgeries are unjustified without neuromonitoring in the present-day world. Neuronavigation is a surgical adjunct commonly used for planning and guiding procedures the world over. We are demonstrating the use of both IONM and navigation systems for surgical excision of perirolandic cortex cavernoma.
We used a simple NIM© nerve monitoring system ([email protected] NIM response 4.0) for direct cortical stimulation with a monopolar nerve stimulator probe for mapping the motor cortex. This is a useful alternative and equally effective method to NIM eclipse system with motor evoked potentials (MEPs) monitoring for mapping the motor cortex.
A 17-year-old-boy, presented with focal motor seizures in the right upper limb for 3 years and an episode of sudden onset of headache 1 month back. He had weakness of right handgrip and pronator drift in right upper limb on neurologic examination. Magnetic resonance imaging (MRI) brain showed a large left parietal cavernoma in the sensory cortex with bleed and surrounding edema. Classical “popcorn” appearance was seen with bleed of different ages within the lesion with a hypointense rim of hemosiderin. He also had other multiple lesions in the left atrium, and the left and right cerebellum. On processed functional MRI (fMRI) images, the right-side hand motor cortex was seen anterior to the lesion with lightening up of left supplementary motor area (SMA). In the coronal reconstructed diffusion tensor imaging (DTI), the left-side corticospinal tracts were seen pushed medially to the lesion and in the depth of the lesion, remarkably close to the medial surface.
He was started on levetiracetam by a physician at a local place for his seizures. As the lesion was surfacing at pia and he presented with seizures, surgery was considered a primary form of treatment.
He underwent awake left parietal craniotomy under neuronavigation guidance with tractography images. Awake surgery was considered due to the proximity of the motor cortex and corticospinal tracts to the lesion in depth. We followed standard asleep–awake–asleep protocol for awake craniotomy. Neuronavigation was used for planning craniotomy and intraoperatively for localizing corticospinal tracts in depth.
Circumferential scalp block was administered before the application of the Mayfield head clamp. He was positioned supine with head flexion as per the patient's comfort, supported with pillows for support. On the Brainlab workstation, corticospinal tracts were constructed from DTI images using the fiber tracking feature preoperatively. Brainlab navigation system (Brainlab AG, Munich, Germany) was registered using surface marking. The left parietal square flap was marked and based laterally, and the local area was shaved, prepped, and draped in a standard manner.
Direct cortical stimulation was used for localizing the motor cortex with the NIM© nerve monitoring system ([email protected] NIM response 4.0). Response electrode was inserted in the right abductor pollicis brevis (APB) and ground electrodes in the sternum (as shown in the still image in the video). We used the monopolar nerve stimulator probe of the NIM© system for direct cortical stimulation. The stimulation settings on the NIM system were adjusted to match as closely as possible to the Taniguchi method. We chose Stimulator 1 (red arrow). The compound motor action potential (CMAP) response of the abductor pollicis brevis was elicited using the NIM stimulating probe (setting: pulse width = 250 μs, train frequency = 7 pulses/s, current = 6–20 mA), and twitch of the corresponding thumb and index finger was noted.
Video link: https://youtu.be/LwXIRhPt16o
Video timeline with audio transcript (Minutes):
We will be showing a novel technique of using direct cortical stimulation for mapping motor cortex in removal of perirolandic cavernoma.
(0.00.07) A 17-year-old-boy presented with focal motor seizures in the right upper limb for 3 years and an episode of sudden onset headache 1 month back. He had weakness of right handgrip and pronator drift in right upper limb on neurologic examination. MRI brain imaging showed a large left parietal cavernoma in the sensory cortex with bleed. He also had other multiple lesions in the left atrium, left and right cerebellum. On processed fMRI images, right-side hand motor cortex is seen anterior to the lesion with lightening up of left SMA. In the coronal reconstructed DTI image, left-side corticospinal tracts are seen pushed medially to the lesion and in the depth of the lesion, remarkably close to the medial surface.
(0.00.51) Preoperatively corticospinal tract was reconstructed on Brainlab navigation system by placing a region of interest (ROI) on the left cerebral peduncle to generate fibers traversing it.
(0.00.53) Although the lesion was in an eloquent cortex but was surfacing to the pial edge, hence we considered for surgery under neuromonitoring. As it was located just posterior to the motor cortex, we considered for awake surgery.
(0.01.18) We used the monopolar nerve stimulator probe of the NIM© system for direct cortical stimulation. The machine was set at a pulse width of 250 μs, train frequency of 7 pulses/s, and the current strength of 6 to 20 mA, similar to the Taniguchi method for eliciting the CMAP response of the abductor pollicis brevis. Electrodes were inserted in the APB muscle to record the corresponding thumb and index finger's twitch during stimulation.
(0.01.48) He underwent awake left parietal craniotomy under neuronavigation guidance of tractography images with direct cortical stimulation using the NIM© monitor. Navigation helped in surface marking and planning of the craniotomy. After a craniotomy, navigation pointer was used to confirm the location of the lesion and adequacy of the extent of the craniotomy.
(0.02.09) Following durotomy, xanthochromic discoloration is seen surfacing posterior to the central vein in the sensory cortex. We performed direct cortical stimulation anterior to the lesion for the localizing hand motor cortex. In the PiP abductor pollicis brevis muscle MEP wave is seen on the monitor. We used the current strength of 10 mA at 100 μV, which is much higher than required for facial nerve stimulation. Similarly, direct cortical stimulation was performed on the surfacing part of the lesion to map the motor cortex for planning the entry point into the lesion.
(0.02.50) We entered the lesion from the posterior end away from the motor cortex. After cortisectomy, dissection continued in-depth along the gliotic plane. Blunt and sharp dissection continued to separate the lesion from the surrounding hemosiderin rim all around. Bipolar coagulation was performed minimally and at the lowest current especially anteriorly and in the depth to avoid injuring corticospinal tracts, along with continuous intraoperative monitoring of the motor power. As our patient presented with seizures, we made a preoperative decision to remove the hemosiderin rim.
(0.03.31) Complete excision was achieved with a clean resection cavity. Throughout the procedure, we kept assessing the motor power of the right upper limb, and mild worsening was noted after resection of the anteromedial part of the lesion. The weakness improved by evening. We followed a technique of asleep–awake–asleep for awake craniotomy. In the image, we can see surgical wound post skin closure.
(0.03.45) Post-op Day 1 MRI showing complete excision of sensory cortex cavernoma with surrounding post-op changes. Postoperatively, we performed MR spine screening as a baseline imaging for future reference, and it was normal.
(0.04.08) The technique of cortical mapping with DCS using the NIM© system was published earlier in 2017 by our neuroanesthesiologist for glioma. These are the other references cited in this manuscript.
(0.04.27) At 3 months follow-up, he was able to type on a computer and write legibly with mild weakness of flexors of the ring and little fingers. He could button and unbutton his shirt and tie his shoelaces. He achieved a neurological functional level the same as was in the pre-op period.
(0.04.59) This technique can be useful for localizing eloquent cortical regions in resource-constrained settings and guide simple superficial resections if used judiciously.
The handgrip weakness noticed intraoperatively resolved by evening. At 3 months follow-up, the patient achieved a neurological functional level the same as was in the pre-op period.
This type of direct motor cortex mapping using the NIM system gives surgeons more control than a dedicated neurophysiologist for IONM. The setup required is simple with broader availability, cheaper cost, and familiarity of neurosurgeons to it. Hence, it is an effective alternative method to transcranial MEP monitoring with the NIM eclipse system.
NIM system uses a direct stimulation technique with a monopolar stimulator electrode. It is possible only when an area of interest is exposed, that is, after durotomy. It is also an intermittent monitoring method, and surgical procedures must be temporarily interrupted for the surgeon to stimulate the area of interest.
This operative video demonstrated a simple and successful method for surface mapping of the motor cortex using the NIM nerve monitoring system. The system is easy to set up and is available at a lower cost without the need for a neurophysiologist's assistance. The system includes auditory alarms, check for technical errors such as high impedance of electrodes or displacement, cautery filter, and detection of artifacts.
We set up the system at lower train frequency stimulation, closely adjusted to the Taniguchi technique settings. With this method, we successfully mapped the motor cortex just anterior to the lesion during the procedure. Also, we could do negative mapping over the surface of the lesion before starting resection.
This system's limitations are intermittent stimulation with a lack of continuous monitoring using strip electrodes and prolongation of the surgical procedure due to intermittent stoppage for conducting the simulation. Also, this system precludes monitoring before exposure to the cortical surface. The number of muscle groups monitored is limited due to fewer channels available with the system. There is no option of waterfall display or record the amplitudes for serial comparisons. However, the event capture function allows the storage of baseline waveforms for subsequent comparisons. Most important, this system does not allow other modalities of IONM.
The NIM nerve monitoring system is a practical and easy alternative to the standard IONM system for mapping the cortical surfaces. It does not replace the standard multimodal IONM systems required for complex, eloquent region lesions. However, it is an additional tool in the armory of neurosurgeons for surface mapping in simple cases.
Declaration of patient consent
A full and detailed consent from the patient/guardian has been taken. The patient's identity has been adequately anonymized. If anything related to the patient's identity is shown, adequate consent has been taken from the patient/relative/guardian. The journal will not be responsible for any medico--legal issues arising out of issues related to patient's identity or any other issues arising from the public display of the video.
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Conflicts of interest
There are no conflicts of interest.