Magnetic stimulation during a somatosensory-precipitated epileptic seizure

Abstract

Sirs: Epilepsy with somatosensory evoked seizures is a form of reflex epilepsy and the mechanisms involved are poorly understood [1]. It has been hypothesised that extensive sensory input plays an important role in the recruitment of a critical number of neurons, culminating in an attack [2]. Painless transcranial magnetic stimulation (TMS) is an important clinical and research tool for examining excitatory and inhibitory neuronal mechanisms within the motor cortex. The unpredictable timing of epileptic seizures, however, makes direct observation of cortical excitability during seizures difficult. Patients with epilepsy have been studied with TMS between seizures, but there are no data on excitability changes immediately before, during or after seizures. We report a case of a man without anticonvulsant medication who could induce seizures by rubbing and kneading of the lateral part of his right shoulder.The 30-year-old engineer had experienced spontaneous seizures since the age of 15 years that started in the right arm with pain and tingling followed by muscle stiffening. He had learned that stimulating his lateral right shoulder with his left hand induced a seizure, which was followed by a postictal refractory period that made spontaneous seizures during the following hours unlikely. He was admitted for diagnostic evaluation because two additional, generalized tonic-clonic seizures had occurred. The patient was studied after we obtained informed consent and ethics committee approval.Magnetic resonance imaging studies were unremarkable. Videoelectroencephalographic recordings revealed a seizure beginning with a brief stiffening of the right arm that was followed by a secondarily generalized tonic-clonic seizure. At clinical onset of the seizure there were sharp slow-wave complexes with phase reversal over the left centro-parietal region, followed by irregular, bilateral frontal discharges and postictal electroencephalographic depression.Rubbing and kneading of the right shoulder with his left hand for about 2 min induced a sensation of pain and tingling in the right arm, followed by dystonic posturing lasting for 15-20 s without loss of consciousness. Ictal perfusion single photon emission computed tomography revealed a relative increase in perfusion by 9-10 % (i. e. more than 2 SD) in the left parietal cortex. TMS (stimulation frequency: 0.33 Hz, 51 % of maximum output) was performed with the Magstim 200 stimulator (2-T version; Magstim, Dyfed, UK) using a round coil (outer diameter 12 cm) centred over the vertex. Surface electromyographic recordings were taken from the right first dorsal interosseous muscle (FDI), the right biceps brachii (BB) and the right extensor carpi radialis muscle (ECR). Continuous TMS over the left sensorimotor cortex for 4 min did not elicit a seizure nor did magnetic stimulation over the right brachial plexus (28 stimuli, 35 % max.) or over the BB (14 stimuli, 50 % max.). On stimulation of the motor cortex and rubbing the shoulder to elicit a seizure, the amplitudes of the cortically ...