Birth of a bulge

[ | The Normal Disc | Three face of Disc Bulge | MRI Presentations | ]

Introduction:

Of all the e-mail questions that I received, the question "What is a disc bulge and can it cause pain?", is by far the most common.

The answer to this question is not as 'black & white' as many doctors will have you believe.

Although about 80% of completely asymptomatic people will demonstrate a disc bulging or worse on MRI (1-4) (MRI False Positives), about 40% of chronic back and/or leg pain patients will have 'disruptions' (tears) within the substance of their discs that are often invisible on MRI. (5) Fig.#1 depicts such an 'internal disruption'. Note that the posterior of the disc has bulged into and contacted the anterior portion of the thecal sac (dura & arachnoid mater) - your typical disc bulge. What the MRI may not demonstrate is the wide full thickness 'Radial Anular Tear' that connects the Nucleus with the outer layers of the anulus. Note that the Sinuvertebral Nerves, which have been exposed to the irritating nuclear material, are irritated (red dots) and generating chronic back pain.

Although I've covered IDD ad nauseum (here), I think it best that we touch on this important subject here as well:

Internal Disc Disruption (AKA: IDD): IDD, which is believed to be a major cause of 'discogenic back pain' (5,6), occurs when the intervertebral disc develops a 'rent' or 'tear' through its substance. This tear, because of the high pressure within the nucleus, allows the irritating nuclear material to escape the confines of the nucleus pulposus and enter the nerve infested outer 1/3 of the anulus fibrosus. It is believed that, in some patients, an inflammatory reaction can occurs within that outer 1/3 of the anulus, hence causing chronic and debilitating back pain and/or leg pain (5-10).

The Birth of a Disc Bulge: The Normal Disc.

Lets do a quick refresher course on normal disc anatomy: (click 'here' for a more thorough lesion.)

We shall study the evolution of the 'disc bulge' from an over-head vantage point (aka: axial views): The most important thing to keep in mind is that in a normal, non-bulging, disc, the Ring Apophysis is visible and NOT over-shadowed by a bulging anulus. Fig.#3 demonstrates this nicely: note that the posterior of the disc has a concave configuration. and no disc material has bulged past the posterior ring apophysis. I will now 'briefly' discuss the main components of the disc. For a full discussion on disc anatomy see my 'Disc Anatomy' page.

The Nucleus Pulposus (pink) is the gelatinous-like center of the disc. Normally, is has a very high water content, as it is predominantly composed of proteoglycan molecules produced by the cells of the nucleus. It's job is to absorb the tremendous downward weight of the body (like the shock absorber of a car) and act as a pivot point to allow motion between the bones of the spine (vertebrae).

The Anulus Fibrosus (green) is a fibrous structure that surrounds the nucleus and shields the delicate nerve roots and thecal sac from the irritative nucleus pulposus. It's made up mostly of collagen which is generated from fibroblast like cells. It has a much lower water content than the nucleus. The anulus is a layered structure, in that it contains 15 to 25 sheets of collagen; these sheets are called Lamellae and are 'glued' together with proteoglycan molecules. The job of the anulus is to corral the highly pressurized nucleus and protect the highly nerve-infested outer 1/3 of the anulus and posterior epidural neural structures.

The Sinuvertebral Nerves (yellow balls) are tiny nerve fibers and endings that live in the posterior 1/3 of the anulus fibrosus. Irritation of these pain-carrying fibers are thought to be one of the causes of discogenic back and/or leg pain.

The Posterior Neural Structures (bigger yellow balls): Within the anterior epidural space (black) lives the delicate spinal nerve roots (NR), thecal sac, and exiting nerve roots (spinal nerve). It's important to understand that at each disc level, there are two nerve roots that are vulnerable to irritation: the 'Exiting Spinal Nerve Root', and the 'Traversing Spinal Nerve'.

THE THREE FACES OF DISC BULGE: Asymptomatic, Grade 3, and Grade5.

The Disc Bulge: Asymptomatic - (grade 1 IDD)

Figure. #4 depicts a 'Bulging' lumbar disc. Note that the posterior of the disc has bulged backwards past the ring apophysis (gray) and into the anterior epidural space. This bulge has slightly contacted the left Traversing Nerve Root without pushing it out of place (#2). This is quite a typical presentation for a disc bulge; one that your doctor might call completely normal!

Also note that the disc has greatly degenerated (dark green). DDD is usually a precursor to bulging. You can learn more about DDD (degenerative disc disease) on my DDD page, but to recap, DDD begins from discal trauma and/or nutritional compromise. What ever the cause, once it begins a vicious cycle is perpetuated that leads to discal drying, weakening, and tearing. In fact, in our model to the left, the disc has indeed began steps toward pathological degeneration. A grade 1 radial anular tear has developed and is allowing nuclear material into the inner region of the anulus. This would not be expected to be painful, for it has not progressed into the outer 1/3 of the anulus - which is where the sensitive sinuvertebral nerves are. This would NOT be visible on MRI; only CT discography would denote the above tear.

The Painful Disc Bulge: Internal Disc Disruption - (grade 3 IDD).

Now, our above model has taken a turn for the worse! The grade 1 anular tear has progressed into a grade 3 full thickness radial anular tear. Note that the sinuvertebral nerve is now 'activated' (red) and transmitting pain signals to the brain! This is called 'Discogenic Pain' and usually presents as lower back pain, but it may also present as a referred pain down the lower limb - we call the latter phenomenon 'Discogenic Sciatica'.

Note that the bulging has NOT worsen, yet the disc has become painful. MRI probably would NOT demonstrate this anular tear! Only a test called 'Provocation Discography' can confirm the presents of IDD. Please go to my IDD tutorial for more information on Internal Disc Disruptions.

As noted above, about 40% of the chronically disabled will have IDD. Please read the research paper that demonstrated this commonly used number (here: Schwarzer AC, Aprill CN, Derby R, Bogduk N )

The Painful Disc Bulge: The Leaking Anular Tear - (grade 5 IDD)

Another condition, which again is somewhat controversial, is the leaking anular tear. Some believe that grade 5 anular tears can leak biochemicals and nuclear particle from the disc and actually cause true radicular like symptoms (sciatica) WITHOUT the physical compression of the nerve roots. This has been termed 'Chemical Radiculitis' (11,12). Again, I've got this more thoroughly covered on my 'Sciatica page', but lets recap: It is known that nucleus pulposus, in vitro, can cause severe neurological damage to the nerve roots (13-17). Complete full thickness, leaking, anular tears are commonly seen during discography. Therefore, some anular tears that leak, will soak the adjacent nerve roots with nucleus pulposus and other biochemicals. This may MAY induce severe damage to the nerve roots and result in true radicular pain (sciatica). I'm convinced this happened to me as well... how else can you explain a three level radiculopathy (L4, L5, and S1) with no compression of the L4 or L5 nerve roots?

Our disc bulge, in Fig.#6, is now 'hiding' a leaking grade 5 anular disc tear. This patient will not only will have lower back pain but may have full blown root-pain (sciatica) and neurological deficit as well. I think the picture speaks for itself. Note that the dura, traversing nerve root and exiting nerve root are extremely inflamed and irritated.

MRI EXAMPLES:

The key for identifying a disc bulge is that the entire posterior of the disc has bulged backwards, in a subtle, general manner, as noted above in figure #4. This greatly differs from a protrusion or herniation, in that the protrusion is a more "focal" or "eccentric" or a 'concentrated out-pouching' of a portion of the posterior of the disc.

Figure #5 is a perfect example of how a disc herniation presents itself on MRI. Here we have a 4 millimeter central contained disc herniation (#2). Note the severely dehydrated disc (#1) where you can not even see much of a nucleus. Also note the S1 roots (#4), and the cauda equina (#3) are free and clear of the herniation. Luckily this patient had a large neural canal which allow for this herniation.

To make things still more complicated, you can have bulges that give rise to herniation's. My MRI is a perfect example of this. In Figure #6 you can see my axial MRI. Note the broad based bulge (#2) that extends below a line that I've drawn represents where the back of the vertebra should be. The disc should NOT extend beyond this line! If you 'hallucinate' enough you can see where the small 3 or 4 mm herniation (#3) 'eccentrically' disrupts the smooth out-pouching of the bulge. These are very hard for the untrained eye to see, but you can surely see the difference between the clear cut 4 mm herniation above (Fig. 5) and the 4mm disguised herniation to the left (Fig. 6). Also note that unlike the S1 roots in Figure #5, my left S1 root has contacted by the herniation and pushed it ever so slightly backwards.

As a bonus, you can see a nice HIZ sign just to the left (9 o' clock) of number 3 (not marked). It looks like a white bubble. Its even more pronounced on the real T2. (This film is a 'proton density image'.)

MRI Hints for IDDs presents: The HIZ Finding.

IDD, which is not often seen on MRI, can cause extreme and disabling back pain and be very difficult to treat. Occasionally, in about 30% of patients, it can be seen on MRI and presents as a 'high intensity zone' (HIZ) in the outer posterior anular fibers of the lumbar disc. Figure #2 demonstrates this 'HIZ sign (red arrow) which appears as a white bubble in the back of this badly desiccated L5 disc. Pathophysiologically, these are believed to be a combination of at full thickness radial anular disc tear that has merged with a large concentric anular tear. (see my pages on anular disc tears for more information.) When HIZ findings are present in a painful disc, there is a very high chance (over 90%) that the disc is completely torn through. There remains fierce debate over whether or not the HIZ is predictive of a 'concordantly painful' disc on provocation discography, which is the only tool we have to confirm the presents of IDD.

Let's take a look at what a disc bulge is, how it develops, and how IDD may appear within the bulge.

References:

1) Jensen MC, et al. “MRI imaging of the lumbar spine in people without back pain.” N Engl J Med – 1994; 331:369-373

2) Boden SD et al. “Abnormal magnetic resonance scans of the lumbar spine in asymptomatic subjects: A prospective investigation.” J Bone Joint Surg Am 1990; 72A:403-408

3) Weishaupt D et al. “MRI of the lumbar spine: Prevalence of intervertebral disc extrusion and sequestration, nerve root compression and plate abnormalities, and osteoarthritis of the fact joints in Asymptomatic Volunteers.” Radiology – 1998; 209:661-666

4) Boos N, et al. “1995 Volvo Award in clinical science: The diagnostic accuracy of MRI, work perception, and psychosocial factors in identifying symptomatic disc herniations.” Spine – 1995; 20:2613-2625

5) Schwarzer AC, Aprill CN, Derby R, Bogduk N, Kine G. “ The prevalence and clinical features of Internal Disc Disruption in patients with Chronic Low Back Pain. ” Spine 1995; 20(17):1878-1883

6) Crock HV. "Internal disc disruption: A challenge to disc prolapse fifty years on." Spine 1986 ;11:650-3

7) Milette PC, et al. “Radiating Pain to the Lower Extremities Caused by Lumbar Disk Rupture without Spinal Nerve Root Involvement.” AJNR Am J Neuroradiol 1995; 16:1605-1613

8) Ohnmeiss DD, et al "Degree of disc disruption and lower extremity pain" Spine - 1997; 22(14):1600-1665

9) Ohnmeiss DD, et al. "Relationship of pain drawings to invasive tests assessing intervertebral disc pathology." Eur Spine J 1999; 8(2):126-131

10) Ohnmeiss DD, et al. "Relation between pain location and disc pathology: a study of pain drawings and CT/discography." Clin J. Pain 1999; 15(3):210-7

11) Marshall LL, et al. "Chemical radiculitis: A clinical, physiological and immunological study." Clin Orthop - 1977:129:61-7

12) Marshall LL, et al. “Chemical irritation of nerve root in disc prolapse.” Lancet 1973; 2:320

13) Olmarker K et al. “Autologous nucleus pulposus induces neurophysiologic and histologic changes in porcine cauda equina nerve roots [see comments] Spine 1993; 18:1425-32

14) Olmarker K, Larsson K. “Tumor necrosis factor alpha, and nucleus-pulposus-induced nerve root injury.” Spine 1998; 23:2538-2544

15) Kayama S, Olmarker K, et al. “Incision of the anulus fibrosus induces nerve root morphologic, vascular, and functional changes: An experimental study.” Spine 1996;21:2539-43

16) Olmarker K, Brisby H, Rydevik B, et al. “The effects of normal, frozen, and hyaluronidase-digested nucleus pulposus on nerve root structure and function.” Spine 1997; 22:471-5discussion 476

17) Byrod G, Reydevik B, Olmarker K, et al. “Acute increase in endoneural vascular permeability induced by epidural application of nucleus pulposus on spinal nerve roots. Manuscript [550]