The Children’s Hospital of Philadelphia is one of only a handful of pediatric hospitals in the country to offer transcranial magnetic stimulation (TMS) to localize the epileptogenic and sensorimotor cortical regions in order to improve family counseling regarding epilepsy surgery and help surgeons avoid those areas when operating.
For children with refractory epilepsy whose seizures cannot be controlled by the usual medication choices, surgically removing the part of the brain most responsible for causing seizures can be curative. But in many cases, the area causing the seizures is too close to language or motor cortex, making surgery risky. Being able to map cortical regions crucial for normal function prior to undergoing surgery is vital to ensuring the best outcomes.
TMS is a noninvasive technology that creates a precise map of the eloquent cortex superimposed on a patient-specific MRI. TMS paired with neuronavigation can stimulate precise targets in the cortex, localizing functional areas with the proven accuracy of direct cortical stimulation (DCS). The stereotactic camera provides visualization of the induced E-field, which is displayed in a 3-D rendering of the individual patient’s MRI. Placing surface electrodes on the desired muscles, the 6-channel EMG records motor-evoked potentials, amplitudes and latencies, induced by a TMS stimulus. Stimulation targets that elicit motor responses are marked as pegs color-coded as a heat scale, creating a map of the cortical somatotopy.
Language mapping is performed by inducing speech arrest or speech errors with brief trains of stimuli during a language task such as picture naming.
“Seizure freedom is very important, but so is maintaining a child’s optimum abilities,” says CHOP pediatric neurologist Sudha Kessler, MD. “If we see that the epileptogenic zone is directly on top of or intertwined with the motor areas, we can counsel the family that there’s a high likelihood their child will be left with a weak arm or won’t be able to use their hand, etc. So the family can weigh the potential benefits of surgical resection with the potential risk of a motor deficit.”
TMS offers a number of benefits over the standard preoperative workup, which includes inpatient electroencephalogram (EEG), magnetoencephalography (MEG), and 3 Tesla functional magnetic resonance imaging (fMRI). It affords a level of spatial precision and accuracy these modalities lack and is more tolerable for younger patients and disabled patients who have trouble remaining still for MEG and fMRI.
“Our approach is focused on getting the best outcomes for our refractory epilepsy patients, whether that’s surgery or not,” says Kessler. “Our goal is to tailor the surgery as narrowly as possible to avoid taking out important areas that have function while still trying to disrupt the epileptic network that’s responsible for triggering seizures. TMS is an important tool in helping us get at these subtleties so we can take the big picture into account.”