Sometimes, being able to spot the differences in something is not only elusive to lay people, but to professionals as well. This is the case with abnormalities in the disc, the cushion between the bones of the back, called the vertebrae.. Being able to identify disc abnormalities is important because they can be painful. While doctors can easily identify a large or obvious abnormality of the disc, being able to define more subtle abnormalities can be difficult. This difficulty led several spine related professional medical societies, the American Society of Neuroradiology, the American Society of Spine Radiology and the North American Spine Society to develop recommendations for how to name certain conditions of the disc. The first set of recommendations was published in 2001.[1] These recommendations were later revised in 2014. Since first published, these recommendations have been adopted by major orthopedic, neurosurgical, radiologic and rehabilitation organizations in the United States.

Firstly, one must be able to define what a normal disc is. In 2014, a normal disc is described as one that has a normal shape and does not have evidence of degeneration [2]. The recommendations further write that the normal disc is maintained wholly within the boundaries of the disc space, and defines what those boundaries are (see Figure 1). Therefore a normal disc is not only normally-shaped, but also does not extend beyond its usual boundaries and does not have evidence of degeneration. It is important to note that, in this context, “normal” simply is meant to describe the size and shape of the disc. The 2014 guidelines note that the discs’ size and shape often does not match-up very well with pain or other types of disease.[2] In other words, disc degeneration and other wear-tear changes are commonly seen and cannot be used to diagnose the source of pain or other disease.

Once we can define normal, we can move on to what abnormal is. One of the most commonly-discussed abnormalities are disc herniations. As of 2014, a herniation was defined as a localized displacement of disc material involving 25% or less of the disc circumference (Figure 2a and 2b) [2]. Disc herniations can be further subdivided into protrusions and extrusions. In a protrusion the herniated disc material possesses a sizable connection to the disc space (Fig 2b). In an extrusion a large amount of disc material has extended beyond the disc space, connected only by a thin stalk (Figure 3). So, in summary, if an MRI shows that the disc displacement beyond the normal confines of the disc involves 25% or less of the disc circumference, then that disc may be herniated and may be further classified as a protrusion or an extrusion. Importantly, these are descriptions of disc-shape and do not match-up with pain or severity levels. However, disc herniations can certainly cause back or leg pain. Disc herniations can compress nerves traveling into the leg, leading to nerve pain, numbness or muscle weakness. Even if a disc herniation is not compressing a nerve, they can cause pain by generating a strong inflammatory reaction.

A disc bulge is another type of abnormal disc shape fully described in the 2014 recommendations (2). Unlike herniations however, disc bulges are caused by normal wear and tear to the disc and are commonly seen in normal people who have no pain. The difference between a bulge and a herniation is that, in a bulge, a larger part of the disc displaces beyond its normal boundaries. In a disc herniation, there is a less than 25% focal displacement of the disc beyond its normal boundaries. In a disc bulge, there is a greater than or equal to 25% generalized displacement of the disc (Figure 4 (2). Interestingly, even though disc bulges show larger portions of the disc abnormally displaced, they are less likely to be painful when compared to disc herniations.

Disc herniations can usually be distinguished from disc bulges. However, there are some situations where a disc herniation can look like a disc bulge. For example, one disc may have multiple herniations. These herniations may blend together to give the appearance of a bulge when they are in fact a multi-focal herniation (Figure 5). When a patient has significant back or nerve pain and an MRI is initially interpreted as showing bulging discs (which are often not painful), it is certainly worth considering if they have a disc that has a multi-focal herniation.

The 2001 and 2014 recommendations are an example of medicine’s earnest effort to assist providers in making accurate diagnoses. If your condition has reached a point where advanced studies are necessary, the multi-specialty guidelines have given your spine professional sound advice in precisely describing what they are seeing when evaluating your back.

Fardon, D.F., et al., Nomenclature and classification of lumbar disc pathology. Recommendations of the Combined task Forces of the North American Spine Society, American Society of Spine Radiology, and American Society of Neuroradiology. Spine (Phila Pa 1976), 2001. 26(5): p. E93-E113.

Fardon, D.F., et al., Lumbar disc nomenclature: version 2.0: Recommendations of the combined task forces of the North American Spine Society, the American Society of Spine Radiology and the American Society of Neuroradiology. Spine J, 2014. 14(11): p. 2525-45.