What is a herniated disc?
Between the vertebrae, soft discs cushion the movements of the spine. According to the UK's NHS, a herniated (slipped or prolapsed) disc is when part of this disc bulges outwards; it becomes painful when it presses on a nerve.
In the lower back, a lumbar herniated disc can irritate a root of the sciatic nerve: it is one of the causes of sciatica. Doctors name the herniation after the vertebrae on either side of the disc: an L5-S1 herniation, for example, affects the disc between the last lumbar vertebra and the sacrum (the triangular bone at the base of the spine). The part of the leg affected depends on which nerve is irritated. In the neck, a cervical herniated disc can cause pain that travels down the arm.
The NHS points out that not all herniated discs cause symptoms, and the Swiss League against Rheumatism notes that a herniation seen on imaging is not necessarily linked to the pain you feel. To understand the terms in a report, read Lumbar MRI: disc problems explained.

