There are 31 pairs of spinal nerves along the spine from the upper neck to the lower back and sacrum. Each pair exits at the level of an intervertebral disk, a flexible element situated between two rigid bones (vertebrae).
When a nerve is pinched, it becomes incapable of propagating an electrical charge in the normal fashion. For all intents, the nerve might be seen as short-circuiting. As with any wire that shorts, its function is compromised. There are two major kinds of nerves: sensory and motor. Sensory nerves provide sensation: touch, temperature, pressure, pain, etc. Motor nerves provide for movement, that is, they control muscles.
Pinching a motor nerve produces weakness. The worse the pinching, the greater the weakness. It turns out skeletal muscles are incapable of survival without nerve input in the long run, so that prolonged severe pinching leads to death of muscle cells. As the individual cells die, the muscle shrinks. This is the cause of atrophy, or muscle wasting. In a worst case scenario, a patient comes in with profound weakness and loss of muscle mass.
Pinching a sensory nerve leads to a volley of abnormal sensations. Tingling, numbness, and especially pain. The pain is generally quite disagreeable, described variously as sharp, shooting, lancinating, burning, or even a dull ache. It may be constant or intermittent, and is generally worse with activity or simply standing. Sometimes patients complain of a deep “toothache” type of discomfort. In severe cases, a patient cannot put weight on the affected leg without severe pain. In all of these cases, the pain follows the distribution of the pinched nerve, which in the most common situation is down the back of the thigh and calf, often across the top or bottom of the foot. The pain almost always runs down the leg from the back towards the foot and not vice versa. Pain running up the leg is very unlikely to be a pinched nerve. In addition, pain from a pinched nerve (the medical term is radicular pain or radiculopathy) is rarely focused around a joint, such as the knee or the hip. Joint pain often indicates a problem in the joint itself.
In reality, nerves are not entirely sensory or entirely motor, but a mixture of the two. Thus, pinched nerves often create both sensory and motor symptoms. This means most patients have both some element of radicular pain, numbness, tingling, or achiness, and a sense of weakness. Often times one aspect predominates, usually the sensory symptoms. When sensory symptoms predominate, patients may become very uncomfortable and often seek medical care early.
Cases where the weakness predominates are not rare however. In the extreme case, this is called painless weakness—a dangerous situation. Human beings respond faster to pain than to weakness. If something hurts, we tend to seek medical attention. However, if there is weakness in the absence of pain, many if not most patients ignore the weakness until it is far advanced. Read far advanced as irreversible, even with surgery. Such permanent weakness can be crippling, or at least life-changing.
As an illustration of the above, consider the difference between heart attack and stroke. Both of these are exactly the same problem—lack of blood flow—but they occur in different parts of the body. When the heart is deprived of blood flow, the result is chest pain. Heart attacks hurt and this causes the patient to seek medical attention immediately, which often prevents permanent injury. But when the brain is deprived of blood, there is no pain (the brain is the only organ in the body that does not feel pain!). The result in many cases is painless weakness of an arm or leg—which people tend to ignore, sometimes for days! In any case, delay of just a few hours is generally enough to produce permanent injury. This is, of course, a stroke.
Painless weakness caused by a pinched nerve is less dramatic, but still devastating if ignored. Patients often ignore such weakness until they notice wasting—atrophy of the thigh or calf—or they begin to fall down from the profound muscle weakness.
By far the most common cause of a pinched nerve is a herniated disk. The disk is a soft tissue element that sits between the bones of the spine, not unlike a stack of coins in which quarters (bones) and nickels (disks) alternate. The disk is the flexible element that allows the spine to bend and rotate, which means disks get a lot of wear and tear. This wear eventually catches up with a person, which is why the incidence of herniated disks increases with age. They are vanishingly rare in children (though I have operated on one in a thirteen year-old).
Because of wear and tear, pinched nerves are most common in the more flexible parts of the spine, the lower lumbar and mid to lower cervical spine. They are unusual in the more rigid thoracic spine.
The disk itself is composed of an outer ring of fibers and an inner meat the consistency of crab. Some folks liken this arrangement to a jelly donut, though the filling is not so squishy and does not readily flow out. Nonetheless, when the outer fibers breakdown, the inner material herniates out. Most herniations are inconsequential, since only those that actually pinch a nerve are troublesome. In fact, by some estimates, as many as 80% or more of disk herniations have no clinical significance.
Even when the herniation does pinch a nerve, surgery is not usually necessary. In most cases the pinched nerve resolves (perhaps the herniation goes back in) and the symptoms disappear. Perhaps 80% of disk herniations resolve this way, usually within a few days to six weeks or so.
Surgery for a lumbar herniation is indicated for several reasons.
First, surgery should be considered in any case with more than just mild weakness. Weakness indicates a true insult to the nerve and it is generally impossible to know if the weakness is getting better or worse at the time of initial evaluation. Because of the risk of permanent weakness with prolonged pinching, surgery is generally offered.
Second, surgery should be strongly considered whenever there is objective evidence of bladder dysfunction. The nerves to the urinary bladder are at risk only with very large lumbar herniations. True bladder dysfunction related to lumbar herniation is rare. The average neurosurgeon probably sees it only two or three times a year (out of several hundred patients). The risk of permanent injury to bladder function is high in these instances and surgery should be strongly considered on an urgent basis.
The third reason to consider surgery is for intractable pain. Radicular pain in most patients can be made tolerable with medication, enough to get them through the acute period whereupon the pain resolves on its own. In the occasional patient, the pain is so severe as to warrant surgery early on. This is a subjective call on the part of the patient and surgeon working together.
The final reason to consider surgery is for the convenience of the patient. There are many times when the pain resolves incompletely and after many months folks just get tired of it. Such residual discomfort generally (though not always) resolves with surgery. Another instance of convenience to the patient is for financial reasons, such as when the family bread winner cannot afford to miss work on and off for months waiting for pain to resolve. It is often easier and more certain a cure to operate and so return a person to gainful employment early on.
What exactly is the surgery to fix a lumbar disk herniation?
Although this will be answered more fully in another article, suffice it to say the surgery is generally straight forward, takes less than one hour, is done as an outpatient, and results in more or less immediate relief of pain. There is no risk of paralysis and only a slight risk of injuring a nerve at surgery (well under 1% with an experienced surgeon). It generally does not involve fusing the spine.