Caused by the posterior or posterior-lateral migration of the nucleus polposus due to a weakened fibrous ring
The intervertebral discs act as shock-absorbing cushions interposed between two consecutive vertebra. They consist of a central ‘nucleus polposus’, rich in water and held in place by an external fibrous cover. Movements of the spine change the position of the nucleus polposus inside the vertebral disc; an anterior flexion of the spine causes the nucleus polposus to move backward, while an extension of the spine will result in it moving forward. These physiological movements are prone to stress during the repetition of incorrect posture and during sudden, large impacts. In these cases, the nucleus polposus may protrude (bulge), pushing it to the side without treating the fibrous ring that retains the nucleus inside the intervertebral disc. These fibrous rings are supplied with rich supplies of nerves; therefore the pressure exerted by displaced nuclei can result in pain. In more advanced phases, where the fibrous ring has torn, the material contained inside the ring leaks out (slipped disc) pushing downwards or sideways. Slipped discs and bulging of the nuclei vary according to their location and direction in which they have migrated. This is important to establish as it will dictate both the urgency, and the type of treatment required. Clinical examination are used to determine whether or not there are signs of a compressed nerve. This may causing an aching, shooting type of pain as well as a loss of strength in the muscle supplied (myotome) and/or an alteration in cutaneous sensations in the area around the compressed nerve root (dermatome). These alterations may include hyperalgesia (an increase in sensation) or hypoalgesia (a decrease/ loss of sensation). Diagnostic tests require here include MRI or CT scans. It may also be necessary to undergo an eletromyography (EMG) or nerve conduction study (NCS), in order to evaluate any neurological deficits. These examinations can be quite invasive as they require the introduction of needles into muscles or nerves to test their functioning. 

Conservative (non operative) methods can treat the vast majority of cases of disc injuries, especially when there is minimal muscle weakness, since even the most severely affected discs tend to correct themselves in a few months. Surgical treatment is reserved for more severe cases where the disc has slipped into the intraforaminal area (the space in which the spinal cord travels), where the nucleus is dehydrated, when muscle and sensory limitation do not improve, cases where conservative treatment has been unsuccessful and in urgent cases where irreversible neurologic deficits are possible.