IONM in Cranial Surgery

Cortical mapping

The resection of tumors located within or near the eloquent cortex can result in postoperative sensory, motor and language deficits. Cortical brain mapping techniques have been used for many years to help the neurosurgeon identify and avoid these important brain structures. For tumors that impact the primary motor cortex, the main concern includes loss of motor functions in the contralateral face and upper and lower extremities. Two techniques have been developed for intraoperative cortical and subcortical mapping of the corticospinal tracts - the Penfield and Taniguchi techniques. Sufficient stimulation of the motor cortex along the homunculus elicits compound muscle action potentials in lower motor neurons that innervate contralateral muscle tissue. Electromyography is used to record these electrical responses. The Penfield technique uses low frequency stimulation. The current is delivered between the two points of the probe with trains of monophasic cathodal pulses. In contrast, the Taniguchi technique uses high frequency stimulation. The current is delivered between the probe and a reference electrode placed on the skull close to the motor strip with trains of monophasic anodal pulses. In either case, it is essential to determine the motor response threshold - the lowest intensity required to elicit a motor response in muscle tissue. Determination of the motor response threshold provides information on (1) the level of excitation of the motor cortex and (2) the distance between the stimulation site and the motor cortex.

The following are stimulation parameters and information for the Penfield and Taniguchi techniques:

                                    Penfield technique                 

(1) Stimulator type: Bipolar (2) Pulse type: Bi or Monophasic cathodal (3) Frequency: 50 Hz (4) Pulse width: 300-1000 microsec (5) Pulse number: 5 (6) Intensity: 2-20 mA (7) Duration: 2-5 sec with a 10-20 sec interval

The Penfield technique is particularly effective at eliciting motor responses in tumors that do not infiltrate the motor cortex extensively and have sharp margins (reference).

The Penfield technique may induce intraoperative seizures or result in false negative mapping (reference).

                                    Taniguchi technique

(1) Stimulator type: Monopolar (2) Pulse type: Monophasic anodal (3) Frequency: 250-500 Hz (4) Pulse width: 500 microsec (5) Pulse number 5 (6) Intensity: 2-20 mA (7) Duration: 20 microsec

Some difficult cases may require adjustment of these parameters, such as increasing the number of pulses to 7-9 or increasing the pulse width up to 800 microsec).

Alternatively, some have suggested using a bipolar probe with the Taniguchi technique to increase the spatial resolution of stimulation. This method may be useful for cases involving the primary motor cortex or supplementary motor cortex (Rossi et al., 2021).

Subcortical mapping

Subcortical mapping is used for tumors that are located within or near the descending subcortical motor pathways. A monopolar stimulating electrode is used to determine the motor threshold, indicating the relative distance from the electrode tip to the motor pathways. Every 1 mA change is approximately 1 mm distance change from the motor pathways. It is recommended that the surgeon not proceed further if a motor threshold of 7 mA is reached. Resection past this point may result in postoperative motor deficits.

The following are stimulation parameters and information for subcortical mapping: Intensity: 2-20 mA, Stimulation type: monopolar cathodal stimulation, pulse duration: 0.5-1.0 ms, Frequency: 250-500 Hz, Pulse number: 4-5, Interstimulus interval: 3-4 ms.

Monopolar cathodal stimulation has been shown to be more effective than bipolar stimulation for eliciting subcortical MEPs (Szelenyi et al., 2011).

Neurovascular information

The brain is very sensitive to changes in blood flow. Cerebral blood flow is maintained and regulated by a homeostatic process called autoregulation. Cerebral blood flow is maintained at approximately 40-70 ml per minute for every 100 g of brain tissue, which occurs over a wide range of arterial blood pressures (~60 - 160 mm Hg) in a healthy brain. To maintain constant cerebral blood flow, several homeostatic mechanisms converge to maintain a balance between vasoconstriction and vasodilation, which includes myogenic, neurogenic, metabolic, and endothelial components (Armstead, Anesthesiol Clin. 2016; 34(3): 465–477).

Cranial nerve monitoring

The cranial nerves are monitored with for a variety of surgical procedures. The modalities that are monitored typically include spontaneous and triggered EMG recordings and motor evoked potentials.

1. The recurrent laryngeal nerve, a branch of the vagus nerve (CN X), is monitored for thyroidectomies. The recurrent laryngeal nerve, one of approximately 11 branches of CN X, innervates the intrinsic muscles of the larynx. This nerve is monitored using an endotracheal tube that is lined with recording electrodes.

2. The facial nerve (CN VII) is monitored for parotidectomies, tympanoplasties, mastoidectomies, microvascular decompressions, etc. Major branches of CN VII include the temporal, zygomatic, buccal, marginal mandibular, and cervical. The temporal branch innervates the frontalis, orbicularis oculi and corrugator supercilii muscles. The zygomatic branch innervates the orbicularis oculi muscle. The buccal branch innervates the orbicularis oris, buccinator and zygomaticus muscles. The marginal mandibular branch innervates the mentalis muscle. The cervical branch innervates the platysma, a sheet of muscle fibers extending from the collarbone to the jaw. Pairs of recording electrodes are typically placed at the muscles of each branch.

3. The glossopharyngeal (CN IX) can be monitored for microvascular decompressions and acoustic neuroma resections. For acoustic neuroma resections, multiple cranial nerves could be at risk of injury depending on the size of the tumor, including the vestibulocochlear (CN VIII), facial (CN VII) nerve, vagus nerve (CN X), hypoglossal nerve (CN XII), and accessory nerve (CN XI).