This is in contrast to meningitis which means inflammation of the meninges (which are the linings around the brain; for those of you soon to play on Jeopardy, there are three linings surrounding the brain, in order from outside to inside, Dura Mater, Arachnoid Process, Pia Mater).

These two terms simply describe what areas are inflammed and mean nothing by themselves. What is always key is what is causing these areas to be inflammed and that determines the symptoms and their severity.

For example, encephalitis can be caused by West Nile Virus, Herpes Simplex and so on. Meningitis can be bacterial or viral.

As for imaging studies, CT scans are less sensitive than MRIs in general. Depending on the degree of inflammation, the time course of the illness, and the cause, you may or may not see changes in the images. In encephalitis cause by Herpes Simplex, there is a VERY characteristic MRI image you usually see. In West Nile Virus encephalitis, you also can see changes on the MRI but usually later in the course of the disease.

As for Lyme, the inflammation is USUALLY not so severe or focal (that is, in a specific spot) that it is seen on either CT scans or MRIs. So, in Lyme, because you usually don't see image changes and because there is rarely changes in the cerebral spinal fluid (via spinal tap), one usually describes the problem as "Lyme encephalopathy" which simply means that there is a disease process in the brain due to Lyme.

Lyme encephalopathy is usually diagnosed when the person has changes in memory and cognitive ability, subtle personality changes, inability to concentrate, lethargy, and, if severe, progressive loss of consciousness. Other neurological symptoms MAY include myoclonus (involuntary twitching of a muscle or group of muscles), nystagmus (rapid, involuntary eye movement), tremor, muscle atrophy and weakness, dementia, seizures, and loss of ability to swallow or speak.

Hope this helps!