Mapping language-eloquent cortex

Speech mapping is a technique to locate the areas in the brain critically involved in language processing and speech production. rTMS temporarily disturbs normal brain function in the cortex underlying the coil. rTMS can be thought of as creating a temporary lesion—turning a specific cortical area "off" for a fraction of a second.

When an area of the brain vital for speech or language is stimulated, rTMS can cause hesitation, loss of fluency or even the inability to speak. Often, the patient is not aware of any interference or change, and it may be difficult for the operator to discern. However, since the entire mapping session is recorded, post-mapping video analysis can reveal even minor perturbations in normal speech production.

In patients with high- or low-grade gliomas, macroscopically complete resection improves survival. However, many gliomas, are located in eloquent areas of the brain, including areas important for language or speech. Because patients with glioma benefit significantly from surgical treatment, these lesions should be resected with functional monitoring in order to avoid damage to the vital areas. Most centers performing resections in language-eloquent tumors use direct cortical stimulation during awake surgery.

After an initial set-up process which aligns the patient head to a 3D of the patient’s MRI head scan, NBS is used to find the primary motor cortex and determine the patient’s resting motor threshold (RMT), typically from the hand muscle representation area in the right hemisphere. Stimulation intensity is nominally set to 100% of RMT, but can be lowered for tolerability. Accurate measurement of the patient’s cortical excitability is important for optimizing the stimulator output for language mapping for tolerance.

Speech function is tested by the patient’s ability to name everyday objects from a series of pictures, under normal conditions without stimulation. A subset of familiar objects then forms an initial baseline set, unique for each patient. During mapping, the object pictures familiar to the patient are re-shown with time-locked delivery of rTMS from the NBS System. Initially, a baseline “language threshold” is established for the individual patient by defining the optimal number of pulses and the frequency of the rTMS pulse train needed to elicit language errors. Using patient-specific thresholds, presumed language areas can then be mapped in a similar manner as in awake surgery. As in intraoperative mapping, language-positive locations are typically defined when speech disturbance occurs in at least two out of three consecutive stimulations.

A speech mapping may last several hours in total, but the patient may take breaks, as needed, and the system can be quickly re-aligned by the operator prior to resumption of mapping.

The mapping session is recorded on video. Matching the stimulation locations to speech disturbances allows the neurosurgeon to determine which areas of the brain need to be preserved so that the ability to speak is not harmed. Speech mapping results are a significant help for both patients and doctors when they come to discuss treatment options and any possible trade-offs between risks and expected benefits of surgery.