Several forms of injury affect the vertebral discs. Disc bulges, for instance, are more broad-based injuries, while disc herniations tend to be more focal. Complete tears of the annulus result in disc extensions, while free fragments of the annulus cause disc sequestrations. All of these forms of disc injury can cause pain in both the back and extremities, and the symptoms worsen as the injuries progress from bulges to sequestrations. Most commonly, pain is due to nerve root compression by the protruding disc. However, 5-10% of cases of pain associated with disc protrusions are not due to nervous impingement.
Several studies have been conducted to explain the cause of back and leg pain in the absence of nerve root compression. Most notably are the studies that demonstrate the stimulation of sympathetics innervating the structures of the vertebral column. Bogduk, Jenkins, and others have shown the annulus to be well innervated by both sensory and sympathetic fibers. Injury to the disc can thus cause localized pain in the back as well as referred pain to the legs. The sinuvertebral nerve is most commonly affected and may be irritated by the bulging disc. Jenkins also determined that referred pain zones in the back and extremities were not dermatomal (relating to nerve root impingement) but rather the result of irritation of the sympathetic nerves of the sinuvertebral nerve.
In addition, studies have shown that mechanoreceptors, nociceptors, and chemoreceptors all detect changes in the disc due to distension of the disc. These conditions may all further heighten the sense of pain. Phospholipase A, bradykinin, histamine, and other chemicals and enzymes present during disc injuries influence the chemoreceptors that may cause the pain of disc protrusions. Other studies have also correlated inflammation and edema to the pain associated with disc bulges and herniations.