Corona Radiata – an overview
Hemispheric gliomas located within or adjacent to functional areas, such as Rolandic cortex, supplementary motor area, corona radiata, internal capsule, and uncinate fasciculus, constitute the major indications for intraoperative motor mapping. Because of the tendency of infiltrative gliomas to invade underlying white matter tracts, it is important to identify both cortical motor sites and their descending pathways. Regardless of the gross appearance and consistency of the tumor, functional tissue may also reside within the mass itself and must be identified with stimulation mapping before definitive resection.
For intraoperative mapping, the patient is placed in the position appropriate for the area to be exposed. Special care is given to pad and protect all extremities. The core temperature is kept above 96.5 °F (35.83 °C) by using a heating blanket. Cortical stimulation mapping will be difficult if the patient’s temperature drifts below that point, especially in patients under general anesthesia. An intravenous propofol or alfentanil drip maintains the sedative-hypnotic anesthesia. Oxygen is administered through a nasal cannula in case of a decrease in arterial oxygen saturation. Regardless of the need for osmotic diuretics, a Foley catheter is inserted. Prophylactic antibiotics are used routinely and are given during the induction phase of anesthesia.
The head is shaved as needed and washed, and the incision is marked. As always, a preoperative pause by the surgical team is necessary to confirm the operative site before incision. The area of the scalp around the incision is infiltrated with a local anesthetic consisting of lidocaine (0.5%) and Marcaine (0.25%) with sodium bicarbonate as a buffer. The craniotomy should be sufficient to expose the tumor and, in some cases, the surrounding brain, including areas where language is likely to be located, providing adequate cortex for language mapping. Because the dura is pain-sensitive, the area around the middle meningeal artery should be infiltrated with the lidocaine–Marcaine mixture to alleviate the patient’s discomfort while awake.
For patients with an intractable seizure disorder, electrocorticography is often performed before tumor resection. Strip electrodes are used to record from mesiobasal structures. In appropriate patients recording along the hippocampus is obtained after removal of the lateral temporal cortex and entry into the temporal horn of the lateral ventricle. Strip electrodes can also be used for the orbitofrontal cortex or under the bone flap if the cortical exposure is inadequate. The preresection recording requires 5–15 min. An intravenous infusion of methohexital (0.5–1 mg/kg) may be used to induce ictal discharges if epileptiform activity is sparse.
After the dura is opened, stimulation mapping should begin by first identifying the motor cortex. A bipolar electrode (5-mm spacing) on the surface for 2–3 s with a current amplitude between 2 and 16 mA is typically used. A constant-current generator is used to produce biphasic square-wave pulses at 60 Hz and a 1.25-ms single-peak (pulse) duration. The current necessary to evoke motor movement depends on the anesthetic condition of the patient, with lower currents used when the patient is awake. The motor strip is stimulated with the patient asleep at a starting current of 4 mA, which is then reduced to 2 mA when stimulating with the patient awake. The amplitude of the current is adjusted in 1- to 2-mA increments until motor movements are identified. The use of multichannel electromyographic recordings, in addition to visual observations of motor movements, improves sensitivity, permitting the use of lower stimulation levels to evoke motor activity. It has never been necessary to use a current above 16 mA to evoke a sensory or motor response. At this point in the operation, ice-cold Ringer’s lactate should be immediately available for irrigation of the stimulated cortex in case a focal motor seizure develops. Rapid cortical irrigation at the stimulation site with ice-cold Ringer’s solution is the best management of intraoperative stimulation-induced focal motor seizures. Its application abruptly stops seizure activity originating from the irrigated cortex without using short-acting barbiturates.
After the motor cortex has been identified, the descending tracts may be found using similar stimulation parameters. Descending motor and sensory pathways may be followed into the internal capsule and inferiorly to the brainstem and spinal cord. This process is especially important during resection of infiltrative glial tumors because functioning motor, sensory, or language tissue can be located within macroscopically obvious tumor or surrounding infiltrated brain. Determination of the subcortical pathways is important when removing a deep tumor within or adjacent to the corona radiata, internal capsule, insula, supplementary motor area, and thalamus. Because current spread from the electrode contacts is minimal during bipolar stimulation, resection is stopped when movement occurs or paresthesias are evoked.
Cortical language localization, through object naming and reading, varies across individuals and does not follow any reproducible pattern across the population. The traditional concept regarding the cortical representation of language function involves an anterior language site, Broca’s area (posterior part of the inferior frontal gyrus) and a posterior site, Wernicke’s area (perisylvian in the temporoparietal cortex). This concept was challenged by some early studies in which electrocortical stimulation was used. In addition, dominant temporal lobe resections guided by standard neurosurgical landmarks – that is, restricting the temporal lobe resection to within 4 cm of the temporal tip and limiting removal of the superior temporal gyrus – have been associated with permanent postoperative language deficits.
Thus, after bone removal under propofol anesthesia, these patients are kept awake during language mapping. The electrocorticography equipment is placed on the field and attached to the skull after the motor pathways have been identified. The recording electrode–cortex contact point is stimulated using the bipolar electrode with the electrocorticogram in progress. This stimulation may cause after-discharge potentials to appear on the monitor. The presence of such after-discharge potentials indicates that the stimulation current is too high and must be decreased 1 or 2 mA until no after-discharge potentials follow stimulation. At this point, the patient is asked to count from 1 to 50 while the bipolar stimulation probe is placed near the inferior aspect of the motor strip to identify Broca’s area. Interruption of counting (i.e., complete speech arrest without oropharyngeal movement) localizes Broca’s area. Speech arrest (e.g., complete interruption of counting) is usually localized to the area directly anterior to the portion of the motor cortex devoted to the face.
Using this ideal stimulation current, object-naming slides are presented and changed every 4 s. The patient is expected to name the object correctly during stimulation mapping. The answers are carefully recorded. To ensure that there are no stimulation-induced errors in the form of anomia and dysnomia, each cortical site is checked three times. All cortical sites essential for naming are marked on the surface of the brain with sterile numbered tickets. Consistent application of this protocol vastly improves the reliability of a negative stimulation map. However, with the exception of those brain tumor surgeons whose specialized practices routinely employ intraoperative mapping, a generous exposure is necessary to enable stimulation of a positive control, typically along the motor cortex.
A final postresection stimulation of cortical sites should be performed to confirm that the pathways are intact. It also ensures that the underlying functional tracts have been preserved if subcortical responses have not been obtained. Even if the patient’s neurological status is worse postoperatively, the presence of intact cortical and subcortical motor pathways implies that the deficit will be transient and will resolve in days to weeks. The distance of the resection margin from the nearest language site is the most important factor in determining improvement in preoperative language deficits, the duration of postoperative language deficits, and whether the latter are permanent. Significantly fewer permanent language deficits occur if the distance from the resection margin to the nearest language site is more than 1 cm.