What is a Pinched Nerve?

Pinched Nerve

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).

lumbar HNP

A fragment of herniated disk pinching nerves in the lumbar spine. The nerves are the small gray dots amid the white background in the center.

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.


The best 11 minutes on LBP

Doc Mike Evans on Low Back Pain

This is an incredible 11 minute video on the etiology of chronic low back pain, as well as the common treatments and some myths. I have found this video to be useful to many of my patients, especially those whom I cannot help with surgery.

Believe it or not, it is considerably easier for me, as a surgeon, to tell somebody they need an operation than to tell them I can’t help them with surgery. This is because surgery usually implies an endpoint to their suffering, while non-operative conservative management (which the video discusses in detail) seems to imply their suffering will never end. This is WRONG, but explaining that to a person who is in the midst of suffering is fraught with pitfalls. Their disappointment is often palpable.

This and other videos by Doc Mike Evans are available on YouTube. Highly recommended.SkeletonBlogVideo


9 Reasons Back Surgery Might Help

9 Reasons Your Back Pain Might Improve With Surgery—Or Not

back is killing me

Back pain is a fact of life—if you are a human being (and presumably if you are reading this you are) sooner or later you will have back pain. That’s the bad news.

The good news is that for most of us, it will run its course and we will be none the worse for it. Most of us will have a single bout, perhaps two, lasting a few hours to several days, and that will be it. Some of us will have more, and sometimes that will be recurring pain (every few months or once every few years as an example), or chronic low level discomfort that’s just enough to interfere with life’s enjoyment but not at all incapacitating.

Others will be saddled with incapacitating pain, though even then most folks will find a happy medium and be able to function with certain accommodations, as with any other chronic illness (diabetes, heart disease, kidney failure) or condition (rheumatoid or osteoarthritis, fibromyalgia, emphysema).

But for an unlucky few, and unfortunately the masses of people on this earth mean the numbers work out to hundreds of thousands each year, back pain just might be an indicator for back surgery. This sort of back pain typically doesn’t get better without surgery—or at least doesn’t get better quickly enough for we busy humans. That’s the bad news.

The good news is that for these few, back surgery is overwhelmingly successful.

Here are nine reasons your back pain might need to be treated with surgery.

1. If it’s associated with leg pain.

Back pain that’s associated with leg pain may indicate a pinched nerve. Although even then most of these won’t need surgery, if the pain becomes unrelenting or lasts more than two months, see a physician. This kind of pain generally responds well to surgery, especially if the leg pain is worse than the back pain.

In the overall scheme of things, back pain that is purely and truly back pain is rarely an indication for surgery, though there are always exceptions. There are hundreds of ligaments, bones, and nerves in the back and any one (or group) of them can be a pain generator. This sort of pain might be from a stress fracture, a pulled ligament, a small muscle tear, or any of a thousand other sources—most of little consequence in the long run and most of which will heal and resolve on their own, though it will almost always take longer than you would like or expect.

2. If it’s off the midline to one side or the other.

Back pain that’s off the midline may indicate a pinched nerve or arthritic joint on one side of the back. If it’s always in the same place, it might respond to fusing the joint, deadening the nerve, or decompressing (unpinching) a pinched nerve.

3. If it’s always in the same place and does not roam around.

Pain that will respond to surgery does not roam around. The intensity may vary, and frequently does, but the location of the pain is a constant. Usually it is one sided and off the midline of the spine.

Roaming back pain rarely responds to back surgery and seems more likely related to muscle strain or spasm, or perhaps has no relation to the back at all. 

4. If it’s associated with leg weakness as opposed to pain or numbness.

One of the most frequent back problems is a pinched nerve, and while these don’t always require surgery they certainly can. Leg weakness with back pain is one indication of a pinched nerve and generally indicates that surgery should be strongly considered, especially if it is severe or not getting better with time. In general, if weakness is present, see a physician sooner rather than later.

5. If it’s been present for more than 6-8 weeks and doesn’t seem to be getting better.

Most back pain is self-limited and improves even without therapy, though it takes time since something is wrong and that something takes time to heal. Sometimes pain itself is a diagnostic tool though, and pain that isn’t obviously improving after two months is such a tool and should be thoroughly investigated. 

By the same token, chronic back pain that has been present for more than 2-3 years and is stable is generally unlikely to have a surgically treatable cause, even if severe at times. However, such pain should be thoroughly investigated at some point (at least once) since exceptions do exist.

6. If it’s always present in the morning and does not improve as the day advances.

Back pain that is worse in the morning after just getting out of bed and improves with activity rarely responds to surgical intervention. This is often age related. There’s a reason professional athletes retire around age forty. 

On the other hand, if you have back pain from the time you get up and it consistently does not improve (or even worsens) with activity, you should see a physician if it persists over weeks or months.

7. If the pain increases with walking—until you MUST sit down or else.

Pain that consistently worsens with activity to the point of intolerability, especially if just sitting down for a few minutes improves or alleviates it (sitting for five or ten minutes, not an hour or two), may indicate narrowing in the spinal canal—a condition which responds very well to surgery.

8. If the pain is associated with trouble walking—and you can improve the walking by leaning over a shopping cart.

This is known as a positive shopping cart sign and is so suggestive of a problem that can be fixed that if you have noticed this you should discuss it with your physician. Note that the trouble walking may be pain or weakness or both. Sometimes patients complain of “rubbery legs.”

Typically a person notices these symptoms when grocery shopping—they have to both push the cart and lean forward over it in order to shop. Just hanging on to the cart has no effect, the uncomfortable pain persists. Typically, those with a surgically treatable problem get great relief leaning over the cart as they move around the store. In fact, leaning over the cart becomes a must and is the difference between being able to do their own shopping or not being able to shop at all.

As the severity of the condition progresses, even leaning over the cart becomes ineffective and patients stop doing their own shopping. They become homebound.

9. If the “back pain” is really buttocks and leg pain.

In general, the more leg pain one has, the more likely their problem is to be treatable with surgery. Surgery for back pain alone is, in general, a disappointing experience (except in cases of fracture, tumor, infection, or perhaps scoliosis), but back pain associated with leg pain is a different story.

To understand why, one has to consider the generalized nature of back pain, which makes it difficult to narrow down an exact cause and thus limits what surgery can do. However, the occurrence of leg pain often indicates a pinched nerve and since each nerve has a stereotypic course, the location of the leg pain often marks which nerve is pinched—and hence the location of the back problem—with more or less precision. Surgery can then unpinch, or decompress, the nerve.


Brain Squeeze


Brain Squeeze: The Intracranial Pressure Monster
by Edison McDaniels, MD

The skull is a closed box. Think about it. They don’t call it the cranial vault for nothing. Once past the age of a toddler, the skull is fused—it continues to grow towards the normal adult size, but the bones of the skull are knitted together and there is no mechanism whereby they can open again (short of the surgeon’s knife, or a terrible fracture) to rapidly expand the volume of the vault.

The box is closed and the volume within is, for all intents and purposes, fixed.

This, of course, presents a curious problem. There’s only so much room inside a closed box. What happens when that room is exhausted, that is, over subscribed? What happens when there’s more stuff filling the space than it was designed for?

Huh? What?

To understand this further, imagine a shoe box with an expanding balloon inside. With the lid off the box, the balloon simply expands outward, eventually expanding out of the box entirely. But with the lid on, the expanding balloon eventually destroys the box. Now imagine the box is not empty to begin with, that when the balloon is placed inside it, the box is already 90% full. What happens then? In that case, it’s the stuff filling the box that gets damaged first as the ballon expands to squeeze the contents of the box.

Now, imagine that box is the skull, already filled to near capacity by the brain. Picture the expanding balloon to be a tumor, or perhaps trapped spinal fluid (hydrocephalus), or maybe even a brain swelling out of control after a head injury.

Ouch. Not a pretty picture. Let’s look at this box and its contents more closely.

For starters, it turns out that for all practical purposes there are only three things inside this skull box: blood, brain, and spinal fluid (CSF).

CSF is a non-compressible, usually straw colored fluid that is largely water. It contains no cells under normal circumstances. It bathes the brain and spinal cord, providing a cushion in which these elements float.

Yes, the brain floats.

This is good and bad. It’s good in that under normal circumstances there is nothing pushing against the brain. It’s bad in that with a rapid acceleration or deceleration, like a fall or a blow to the head (think of a baseball bat, or a fist), the skull stops moving before the floating brain does. Or the skull starts moving while the brain is still standing still. Either way, the brain slams into the skull. As they say, it’s not the fall that gets you, nor even the sudden stop. It’s that you don’t stop all at once. Ouch again. 

Back to the CSF. The liquor cerebri, as it has been called, is produced at a constant rate of about 500 cc per day come hell or high water. Under normal circumstances it is reabsorbed into the venous system through the large vein at the top of the brain. The production and reabsorption of CSF are independent of each other however, and uncoupling of this fine balance can lead to life threatening problems within a matter of hours. This buildup of CSF is called hydrocephalus.

You might think the brain itself is a fixed volume, but you’d be wrong. The substance of the brain is composed of cells interspersed in a watery milieu, the so-called interstitial fluid that exists between the tightly packed cells. Both the cells themselves and the interstitial space they sit in are capable of changes in volume relative to a given physiological stress.

What kind of stresses? Simple breathing for one. The volume of the brain is reduced slightly with hyperventilation, i.e., rapid breathing. As a caveat, hypoventilation (very slow breathing) can cause an increase in the volume of the brain. This happens every night when we sleep (sleeping produces a relative hypoventilation—shallow breathing—leading to a slight increase in brain volume. Not enough to cause a problem in the normal circumstance, but if there is something else taking up space inside the head (hydrocephalus, a tumor, or even a slowly expanding hemorrhage—the chronic subdural of the aged), this slight change in brain volume while sleeping can cause symptoms (this is partly why patients with brain tumors tend to have a headache in the morning when they first get up—later in the process they often vomit in the mornings as well, after which they feel better because the act of vomiting has pushed a good deal of CSF out of the head and into the spine, relieving pressure within the skull).

Gravity is another stress. The volume of the brain (as well as the amount of CSF within the head itself) tends to be lower during the day when we are upright. Most of us sleep lying flat, however, and lying flat takes gravity out of the picture and fluid tends to flow back into the head during the sleeping hours. Patients with untreated brain tumors or chronic hydrocephalus learn that sleeping upright feels better and causes less trouble. Neurosurgeons often elevate the head of their patients after surgery for the same reason.

A much more significant stress is head injury. Physicians often refer to closed head injury, CHI. CHI is generally a blunt force trauma, like a fall or striking the head in an auto accident. Being hit by a thrown ball is another example. Gunshot wounds and stabbings are examples of penetrating head injuries. All of these cause swelling in various degrees, i.e., an increase in the amount of water in the brain tissue (not to mention hemorrhage—this is the definition of contusion, which is bruising).

Uncontrolled brain swelling can cause shifts of brain tissue within the skull. It turns out the intracranial space is not a simple box with a brain within. Rather, it is a complicated 3-D space with various shelves, nooks, and crannies. These divide the space into various compartments, each called a fossa (anterior, middle, and posterior—front, middle, and back), as well a right and left half with a large shelf of tissue between. Neurosurgeons spend years learning how to get in and out of these various fossas with as little injury as possible.

Swelling represents an increase in the local pressure within the brain. Simple physics dictates that material moves from an area of higher pressure to an area of lower pressure. The brain is no different. With brain swelling, injured tissue has a higher pressure than surrounding normal brain. These gradients are generally gradual and so the shift is small at first. But when the gradient increases (the swelling rises), shifts of significant magnitude can occur between and across the various shelves and compartments. This not only squeezes and distorts good brain, it also squeezes blood vessels, which may lead to stroke. All of these things can potentially turn good brain into bad brain. Bad brain swells, and the situation is potentiated until either the surgeon intervenes successfully or the patient dies. This is malignant cerebral edema (AKA malignant brain swelling).

So that’s CSF and brain. What about the third substance normally present within the skull, blood? Does the volume of blood change? Yes. In fact, the hyperventilation/hypoventilation discussed above also affects the amount of blood in the cerebral vessels. Hyperventilation reduces any pooling of blood, which is a very quick and effective way of reducing increased pressure in the head related either to brain swelling or tumors. This effect is short lived, hours to a day or so, but is quite potent and is used often in emergencies to temporize on the way to the operating room. It is one of the reasons that head injured patients are intubated so quickly.

The caveat of the above paragraph is that hypoventilation (under ventilation or shallow breathing) can kill. Hypoventilation allows pooling of blood in the head and, in the setting of brain swelling, that is very dangerous. One place this can occur is in the CT scanner in the first moments evaluating a head injured patient. This is another reason such patients are intubated early in their care, not so much because hyperventilation is necessary, but because hypoventilation is so bad.

So, the three most frequent occasions when there is a problem with the volume of brain tissue itself are tumors, brain swelling from head injury, and aging.

A brain tumor may be thought of in terms of an expansion of brain volume or brain tissue. As the tumor enlarges, it takes up valuable space inside the so-called cranial vault, space normally occupied by the normal brain. As the tumor grows, it displaces normal brain (and spinal fluid). If the tumor gets large enough, this displacement leads to shifts in brain matter (for the same reasons as above). These shifts are known as herniation. Herniation is a situation in which brain tissue is displaced out of it’s normal position. Sometimes this herniation is relatively benign and serves as a marker that there is a problem. Though the herniation itself might be relatively benign, if allowed to continue it may lead to either stroke or death—so not so benign afterall. Other types of herniation are even more significant, leading to pressure on the centers of the brain controlling heart rate, blood pressure, and breathing. Obviously, such herniations are immediately life threatening. These are sometimes irreversible.

Another situation in which blood becomes a problem inside the head is with hemorrhage (intracranial bleeding). An expanding intracranial hematoma can rapidly become lethal. This may take the form of a post-traumatic hemorrhage (epidural or sudural hematoma, hemorrhages outside the brain proper) or a spontaneous hemorrhage, which is usually within the substance of the brain itself. This latter is a form of stroke and is caused by the rupture of a small vessel within the brain substance. Sometimes such a hematoma will be amenable to removal, sometimes it occurs in an area where it cannot be reached without leaving a person devastated from a brain function perspective (in such a case, surgery is usually deferred).

Not only is the size of an expanding mass inside the skull important, but the rate of expansion of the mass is equally important. I have seen a slow growing benign tumor the size of a softball cause little in the way of problems (the young woman presented with a lump on her head she was curious about, no headaches or other symptoms!). Benign tumors can grow very, very slowly, taking many years to achieve a clinically significant size. For this reason, when such a tumor is discovered in an elderly person (whose brains have lost volume due to aging and so have extra room already), neurosurgeons often chose not to remove the tumor unless it shows evidence of significant growth over time. Depending on a person’s age, a slow growing tumor may not grow fast enough to become a problem during a person’s life.

On the other hand, an expanding epidural hematoma after a fall or other head injury may cause life-threatening problems at a relatively small size of just 25-30 cc. Sometimes a lucid interval occurs after a head injury, often seemingly minor, during which blood is accumulating in the skull until it reaches a symptomatic size, by which time it may be too late. The classic case is an epidural hematoma (bleeding outside the brain and just under the skull). Natasha Richardson, the actress, was in a lucid interval when she declined medical aid after suffering what was thought to be a minor head injury while skiing. Somewhat later, perhaps an hour or two, she collapsed when the hematoma finally reached a size where it caused one of the above noted herniations.

So, to recap. First, the skull is a closed box. Second, there are only three things inside the box: blood, brain, and CSF. Third, an increase in the volume of any one of these three substances (blood, brain, CSF) leads to a compensatory decrease in the volume of the other two. And finally, once these compensatory decreases are exhausted, the intracranial pressure rises and shift and herniation is the result. If this badness cycle isn’t interrupted at some point, death or stroke is the result.

The last paragraph is a statement of the most fundamental doctrine in all of neurosurgery, the Munroe-Kellie Doctrine. This is learned by every neurosurgery resident during the first week of training and never forgotten. In fact, there is nothing a neurosurgeon does inside the head where he or she does not have to take the Monroe-Kellie Doctrine into account.

Stated another way, as a neurosurgeon, with every intervention I make involving the brain, I have to consider the effect on intracranial pressure. Not to do so invites catastrophe, and preventing catastrophe is what treating brain squeeze is all about.

Oh, one more thing. I am often asked what happens after we lose CSF in surgery. The short answer is, nothing. As I noted above, CSF is produced at a constant rate of about 500 cc a day, which is about the volume of CSF in the head. Thus, anything lost at surgery is replenished in a matter of hours. Until then, the brain sort of sits like a lump inside the head. This might well cause a headache, though it doesn’t seem to be too bad in most patients. Sometimes nausea as well, though it’s difficult to say if this comes from the anesthesia meds, the removal of the tumor itself, or the loss of spinal fluid. Either way, the situation resolves itself before the patient goes home. This is one reason most patients don’t go dancing the night of surgery, though many are able to walk the halls of the hospital by the morning after surgery.