Key points

• MRS is principally used as an adjunct diagnostic technique for evaluating patients with medically intractable epilepsy (in order to identify the seizure focus).

• Most commonly, NAA is reduced in epileptogenic tissue; metabolic abnormalities are often subtle.

• Metabolic abnormalities may be more widespread than seen on MRI, and present in the contralateral hemisphere.

• MRS may occasionally be helpful when other techniques (e.g. MRI) are either normal or non-specific.

• MRS measures of the inhibitory neurotransmitter GABA using spectral editing may help determine optimal drug regimen.

• MRS may also be a useful research tool for determining epileptogenic networks in the brain.

Introduction

Epilepsy, the condition of recurrent seizures, is a relatively common neurological disorder, estimated to affect between 1 and 2 million people in the US alone. A multitude of etiologies cause epilepsy, including tumors, developmental abnormalities, febrile illness, trauma, or infection. However, not infrequently, the cause is unknown. Many patients with epilepsy can be successfully treated pharmacologically, but when medical management fails to adequately control seizure activity, surgical resection of the epileptogenic tissue may be considered. For surgery to be successful, seizures must be of focal onset from a well-defined location. It has been estimated that up to 10% of patients with epilepsy are medically intractable, of whom approximately 20% may be candidates for surgical treatment. Traditionally, scalp electroencephalography (EEG) and often invasive (subdural grid or depth electrode) EEG are used to identify the epileptogenic regions of the brain, but increasingly magnetic resonance imaging (MRI), positron emission tomography (PET), ictal single photon emission computed tomography (SPECT), and, more recently, magnetoencephalography (MEG) are also used.