A spinal disc herniation (prolapsus disci intervertebralis), informally and misleadingly called a "slipped disc", is a medical condition affecting the spine, in which a tear in the outer, fibrous ring (annulus fibrosus) of an intervertebral disc (discus intervertebralis) allows the soft, central portion (nucleus pulposus) to bulge out. Tears are almost always postero-lateral in nature owing to the presence of the posterior longitudinal ligament in the spinal canal. This tear in the disc ring may result in the release of inflammatory chemical mediators which may directly cause severe pain, even in the absence of nerve root compression (see "chemical radiculitis" below). This is the rationale for the use of anti-inflammatory treatments for pain associated with disc herniation, protrusion, bulge, or disc tear.

 It is normally a further development of a previously existing disc protrusion, a condition in which the outermost layers of the annulus fibrosus are still intact, but can bulge when the disc is under pressure.

 Most English language publications use the spelling disc more often than disk. Nomina Anatomica designates the structures as disci intervertebrales [plural form] and Terminologia Anatomica as discus intervertebralis/intervertebral disc, [singular form].

 Some of the terms commonly used to describe the condition include herniated disc, prolapsed disc, ruptured disc and the misleading expression slipped disc. Other terms that are closely related include disc protrusion, bulging disc, pinched nerve, sciatica, disc disease, disc degeneration, degenerative disc disease, and black disc.

The popular term slipped disc is misleading, as an intervertebral disc, being tightly sandwiched between two vertebrae to which the disc is attached, cannot actually "slip", "slide", or even get "out of place". The disc is actually grown together with the adjacent vertebrae and can be squeezed, stretched and twisted, all in small degrees. It can also be torn, ripped, herniated, and degenerated, but it cannot "slip".  "The term 'slipped disc' may be harmful as it leads to a false idea of what is happening and therefore of the likely outcome." However, one vertebral body can slip relative to an adjacent vertebral body. This is called spondylolisthesis and can damage the disc between the two vertebrae.

 Signs and symptoms

Symptoms of a herniated disc can vary depending on the location of the herniation and the types of soft tissue that become involved. They can range from little or no pain if the disc is the only tissue injured, to severe and unrelenting neck or low back pain that will radiate into the regions served by affected nerve roots that are irritated or impinged by the herniated material. Often, herniated discs are not diagnosed immediately, as the patients come with undefined pains in the thighs, knees, or feet. Other symptoms may include sensory changes such as numbness, tingling, muscular weakness, paralysis, paresthesia, and affection of reflexes. If the herniated disc is in the lumbar region the patient may also experience sciatica due to irritation of one of the nerve roots of the sciatic nerve. Unlike a pulsating pain or pain that comes and goes, which can be caused by muscle spasm, pain from a herniated disc is usually continuous or at least is continuous in a specific position of the body.

It is possible to have a herniated disc without any pain or noticeable symptoms, depending on its location. If the extruded nucleus pulposus material doesn't press on soft tissues or nerves, it may not cause any symptoms. A small-sample study examining the cervical spine in symptom-free volunteers has found focal disc protrusions in 50% of participants, which shows that a considerable part of the population can have focal herniated discs in their cervical region that do not cause noticeable symptoms.

Typically, symptoms are experienced only on one side of the body. If the prolapse is very large and presses on the spinal cord or the cauda equina in the lumbar region, affection of both sides of the body may occur, often with serious consequences. Compression of the cauda equina can cause permanent nerve damage or paralysis. The nerve damage can result in loss of bowel and bladder control as well as sexual dysfunction.

 Cervical

Cervical disc herniations occur in the neck, most often between the fifth & sixth (C5/6) and the sixth and seventh (C6/7) cervical vertebral bodies. Symptoms can affect the back of the skull, the neck, shoulder girdle, scapula, shoulder, arm, and hand. The nerves of the cervical plexus and brachial plexus can be affected.

Thoracic

Thoracic discs are very stable and herniations in this region are quite rare. Herniation of the uppermost thoracic discs can mimic cervical disc herniations, while herniation of the other discs can mimic lumbar herniations.

 Lumbar

 Lumbar disc herniations occur in the lower back, most often between the fourth and fifth lumbar vertebral bodies or between the fifth and the sacrum. Symptoms can affect the lower back, buttocks, thigh, anal/genital region (via the Perineal nerve), and may radiate into the foot and/or toe. The sciatic nerve is the most commonly affected nerve, causing symptoms of sciatica. The femoral nerve can also be affected and cause the patient to experience a numb, tingling feeling throughout one or both legs and even feet or even a burning feeling in the hips and legs.