Transcranial Magnetic Stimulation in Epilepsy
Epilepsy is a brain disorder in which a person is prone to repeated seizures as a result of abnormal brain activity. Seizures may be due to a medical condition or head injury like stroke, trauma or infections. Sometimes, the cause is unknown and is termed primary or idiopathic epilepsy. Depending on which part of the brain is affected, patients may have obtrusive manifestations like violent shaking of limbs and loss of consciousness or may present with prolonged staring spells.
For patients with underlying causes for seizures, the primary goal is to treat the cause, whether it is surgery for a brain tumor or antibiotics for an infection. On the other hand, idiopathic epilepsy is not curable but the seizures are often managed with antiepileptic drugs. Treatment is usually chronic and medications may need frequent adjustments.
In an effort to understand the disease, transcranial magnetic stimulation (TMS) has been used as an investigative tool. Studies revealed that patients with generalized epilepsy have increased brain excitability with lowered thresholds for motor activity, making them more prone to seizures with less intense stimulation. Antiepileptic drugs like phenytoin and carbamazepine have been shown to increase the motor threshold.
For patients whose seizures are not controlled by medications, surgery may be needed to remove epileptogenic zones - abnormal brain tissues that generate seizures. Prior to performing such a delicate surgery, the epileptogenic zones, language and memory areas should be accurately located to ensure the removal of abnormal tissue without causing further neurological damage. Until recently, the Wada test, which involves injecting sodium amobarbital into the carotid artery, has been the most widely used technique for presurgical evaluation of language and memory functions. Since it is an invasive test, other methods have been tested, one of which is TMS.
After reviewing several studies in 2007, Pelletier et al concluded that although rTMS correlate well with Wada test in terms of locating the language area of the brain, it only assesses expressive speech and not a patient’s language comprehension. Furthermore, it has not been shown to localize the memory area as accurately as the Wada test.
For patients with medication-refractory seizures and who are poor candidates for surgery, TMS offers hope as it may also have therapeutic value for epilepsy. In general, low-frequency repetitive TMS reduces brain excitability. Tergau et al studied the effects of rTMS in 9 patients with refractory epilepsy. Patients received TMS on five consecutive days. They demonstrated a 38.6% decrease in seizure frequency four weeks after treatment. However, the treatment effect disappeared after 6 to 8 weeks. Subsequent studies had mixed results. In 2007, Joo et al reported that patients receiving longer stimulation tended to have fewer seizures. Cantello et al failed to demonstrate a decrease in seizure frequency but EEG abnormalities were decreased in one third of patients.
TMS is generally safe with headache being the most common adverse effect. However, it does carry a small risk of causing seizures in epileptic patients. Although research has yet to fully establish TMS as a treatment modality for epilepsy, it has generated much interest in the medical community as it is inexpensive, non-invasive and relatively safe.