As discussed in the spinal anatomy paper, the intervertebral discs are located between the vertebral bodies, anterior to the spinal cord. In the cervical spine, there are 6, between C2 and T1. There is no disc between C1 and C2 due to the unique shape of those vertebrae and their articulation to each other. There are 12 discs in the thoracic spine (between T1 and L1), and 5 in the lumbar spine (between L1 and S1, the sacrum or posterior pelvis). We all agree on the numbering of the vertebrae (C1-7, T1-12, T1-5, S1, etc.) and to be clear, we will generally refer to discs by naming them according to the vertebra above and below. So, for example, the disc between Cervical 5 and 6 vertebrae is called the C5-6 disc; that between Lumbar 2 and 3 the L2-3 disc. The lowest disc, usually L5-S1, is sometimes called the lumbosacral disc. This is because some people have 4 or 6 lumbar vertebrae and it’s important to recognize this variance (commonly referred to as a "transitional vertebra") when comparing x-rays to MRI scans and correlating imaging findings with symptoms and physical findings. Confusion here is common and can lead to wrong level surgery.
The discs are made up of an outer thick collagen later called the annulus fibrosus, or annulus, and a central portion (called the nucleus pulposus or nucleus) made up of less collagen but more complex biochemical structures (like glucosamine and chondroitin sulfate, among others).This makes the inner part of the disc softer. Additionally the biochemical in the nucleus are hydrophilic – or water attracting, which causes them to appear white on T2 MRI images. In early childhood the nucleus has a gelatinous quality, and this turns more fibrous with skeletal maturity. Unfortunately many internet sites don’t make this clear, so people with disc herniations have a misconception of the disc leaking jelly or fluid which is incorrect. More on that later.
Please understand that the term disc herniation is often used interchangeably with disc bulge, slipped disc, disc prolapse, disc protrusion, and probably others. These terms all refer to the situation where a portion of the disc balloons out or is spit out beyond its outer wall, usually posteriorly toward the spinal cord or nerve roots. The important distinction is not so much the adjective used to describe the condition as it is the site of the bulge, or more importantly, the degree to which it is putting pressure on adjacent neural structures. Another common term is disc extrusion or sequestration, implying a part of the disc having been “spit out,” near the nerves or cord, and no longer in contact with the main part of the disc. Since there are disc from head to pelvis, this condition of disc herniation can occur in the cervical, thoracic, or lumbar areas. More often this occurs in the cervical and lumbar regions since these parts of the spine are more mobile. The consequences of disc herniations (or HNP, for herniated nucleus pulposus) vary from spinal cord compression (in the cervical or thoracic areas) to nerve root compression in those areas as well as the lumbar spine.
Disc herniations (HNP) generally occur when the annulus tears and the softer nucleus protrudes through the hole much like an innertube coming out of a tear in a bicycle tire. The protruding part of the disc has a very fibrous consistency (think crab meat) rather than the jelly-like consistency commonly depicted on the web. If it weren’t for the fibrous nature of the herniation, the disc material wouldn’t be able to exert pressure on the nerve structure(s).
The consequence of HNP varies with its location. When up against the cord there is (surprisingly) little to no pain, since the cord has no sensory nerve supply. In those cases, the result is akin to a spinal cord injury with loss of motor or sensory function or both, below the level of the herniation. Patients can present with gait (walking) abnormalities or varying degrees of numbness/weakness in what is referred to as a clinical picture of "myelopathy," referring to spinal cord dysfunction.
More commonly, HNP affects nerve roots. Unlike the spinal cord, nerve roots are very sensitive to pain. Pressure on a nerve root results in pain in the distribution of that nerve (or the path it travels to the arm, leg, or chest). When severe, there can also be loss of motor strength and sensation in the same distribution. The diagnosis is made by MRI, CT, myelogram/CT, sometimes augmented by EMG studies. By observing the pattern of numbness, weakness, and the anatomic path of the pain we can frequently identify the nerve affected by the HNP even without an MRI. In fact, prior to having MRI and CT scans this neurologic evaluation was the only way to make a diagnosis of which disc was causing the symptoms.
Generally speaking, HNP affecting nerve roots is referred to as sciatica when it affects the legs. This term specifically refers to leg pain caused by nerve root compression. In the upper extremities, this same condition is referred to as radiculopathy, although the term radiculopathy is frequently applied to leg pain from sciatica as well, just to be confusing. Seriously, sciatica refers to leg pain, radiculopathy to motor/sensory dysfunction. So, in essence, there is no term for sciatica in the arms. With rare exception, upper extremity radiculopathy is caused by cervical or neck pathology, and sciatica or lower extremity radiculopathy is caused by lumbar or low back conditions. This is distinct from myelopathy which can affect arms, legs, or both depending on which area of spinal cord is affected.
The treatment of disc herniation is in some ways very straightforward, and in others, controversial. Before thinking about treatment options, it is important to realize that HNP is very common and the symptoms it causes vary from mild to severe. Generally when a nerve root is affected patients present with pain, numbness, or weakness, or some combination, or all three. The situation can seem quite dire. The cause may be obvious (lifting, falling, etc.) or quite minor (coughing, bending over to put on a shoe). Frequently there is no obvious cause. Despite the initial severity there is a roughly 70% chance that the symptoms will resolve over time with no treatment – so unless there are extenuating circumstances (such as progressive nerve deficit, bowel/bladder dysfunction, or unbearable pain) the best approach is basically wait-and- see. The time frame for resolution is typically 3-6 months but can vary.
While waiting and hoping for spontaneous recovery, treatment of symptoms might consist of medications, traction, physical therapy, chiropractic, acupuncture or injections. Obviously none of these actually restore anatomy to normal but can help temporarily with the symptoms. Don’t confuse one or more of the modalities as a cure for the HNP when the symptoms resolve – that’s typically the natural history of the body healing itself. So what happens to this disc if the symptoms get better? I get asked that all the time. We don’t typically follow up with an MRI when the symptoms (it’s not necessary) so we don’t know, except that a study published years ago showed the disc back to normal 50% of the time, and unchanged 50% of the time.
Typical treatment for HNP consists of using these modalities in an algorithmic approach, reserving surgery for those cases that fail to improve with time and conservative (non-surgical) modalities.
HNP affecting the spinal cord may be very different spine injury to the cord can result in quadri/paraplegia. Surgical treatment for HNP varies with the section of the spine affected. The most common area is the lumbar spine. In this area the goal of surgery is to remove the part of the disc putting pressure on the nerve root. Since there is no spinal cord in the lumbar spine, this can be accomplished with a posterior or back exposure. The nerve roots can be (gently) retracted without causing harm. The spinal cord cannot. The disc is in front of the nerves. So in the lumbar spine a posterior approach is safe. Generally a microdiscectomy is done, meaning removal of the part of the disc pressuring the nerve. Some additional disc may be removed to lessen the possibility of the disc re-herniating. Sometimes a small amount of lamina (the bone covering the back of the nerves) is required to get access to the nerves and the HNP. This is called a laminotomy. There is no way to repair the hole in the disc from which the HNP came out – this scars over with time (I believe roughly 3+ months). To do this safely, good visualization is critical. We prefer the use of an operating microscope for illumination and magnification to do this safely. It is typically done through a small incision (one inch or less), and usually in an outpatient surgery center with no overnight stay necessary.
Return to recovery is individualized depending on the patient’s job and daily activities (such as sports) with the goal of preventing recurrence (typically quoted at 6%, most of them occurred in the first three months). Results vary with parameters we can’t always quantify (permanent nerve damage, for example) but there is a typically >90% success rate in resolving symptoms and return to work and sports.
HNP in the cervical and thoracic spine usually requires a different approach since those areas contain the spinal cord, precluding a posterior approach. Rarely a posterior discectomy is possible in the cervical spine if the HNP is lateral to the cord. More typically these cases are managed anteriorly by removing the entire disc and replacing it with bone (fusion) or a total disc replacement device. This is also done with use of microscope and is either an outpatient procedure or done with a one-night stay for observation with similar return to work and sports as with lumbar microdiscectomy.
To summarize, HNP is common and usually self-limited, or resolves spontaneously over time. Conservative care can play a role while waiting for the symptoms to resolve. When the condition persists, minimally invasive surgery is commonly done to treat the pathology and has a high probability of symptom resolution and return to work, sports, and daily activities.