Epilepsy is a disorder confined to the brain.
Terminology is crucial in this question. The International League Against Epilepsy (ILAE) defines epilepsy as a disorder of the brain characterized by an enduring predisposition to generate epileptic seizures and by the neurobiologic, cognitive, psychological, and social consequences of this condition. ILAE defines an epileptic seizure as a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain (Fisher et al., 2005).
Hence, by definition, epilepsy is a disorder of the brain. Note that the ILAE is an authorative organ and their definition overrides whatever your medical textbook on internal medicine says.
An epileptic focus is a commonly used term, but as far as I know not well-defined. It's often used in relation to EEG and other methods such as MRI and PET that attempt to define the hot spot of seizure activity. More strictly defined terms in this regard are eloquent cortex and epileptogenic zone. The eloquent cortex refers to any cortical area in which injury produces symptomatic cognitive or motor deficit. The epileptogenic zone refers to the region of cerebral cortex that is both necessary and sufficient to generate epileptic seizures. Its entire removal can be used as a treatment for intractable epilepsy (Richardson, 2003).
With regard to your question, I expect you are referring to the epileptogenic zone. By definition, this refers to a region in the brain.
- Fisher et al, Epilepsia (2005); 46(4): 470-2
- Richardson, Brit Med Bull (2003); 65: 179–92
I expect this answer may not be entirely satisfactory. If you are looking to the physiological basis of the occurrence of seizures and why they occur in the brain and not in the spinal cord, it may be wise to formulate a more specific separate question. Preferably do not grossly edit this question given it received three answers. vervet's answer on focal epilepsy clearly shows the confusing terminology in epilepsy world. Focal epilepsy is not the same as an epileptic focus. However, epileptic focus is a term often used for focal epilepsy. Long story short, specific answers need specific questions, preferably with the definitions embedded to prevent deleted answers and under-appreciated answers like vervet's.