The term “sciatica” describes a set of symptoms and is not a diagnosis
The symptoms are pain, numbness, weakness, tingling, pins and needles along the sciatic nerve from the lower back to the buttock, all the way to the leg and foot, and involves only one side of the body.
The most common diagnosis is a compression of a lumbar nerve root between the L4 and S1 spinal levels, caused by a herniated disc pressing against it.
Another very common diagnosis is the Piriformis Syndrome, where the piriformis muscle, found deep in the buttock, directly compresses the sciatic nerve that passes under it or, in a small percentage of patients, through it.
Very often the diagnosis for sciatica is made because just part of the symptoms, or very similar ones, are present: for example, low back and buttock pain, or buttock pain that reaches down to the knee, or at times just low back pain. In these instances, the involvement of the sciatic nerve as the cause of the pain is much less likely: sciatic pain reaches down to the foot, has a sharp, searing character, and is usually accompanied by other symptoms like numbness, tingling and pins and needles.