Patients are often asking me why nerves become compressed and what causes the compression. Many different structures can cause nerve compression and there are any number of biologic processes that can also result in nerve pressure on a nerve. However, after distilling it down to its basic essence, there are, in my humble opinion, three reasons why nerves can become compressed and I will list those below. The key thing to remember is that all peripheral nerves travel some distance going from the spinal cord to the structure(s) they innervate. Along the course of their travels, they pass along many other structures and through many little spaces. The most common example is the carpal tunnel. Most people have heard the term ‘carpal tunnel syndrome’ and this clinical entity occurs when the median nerve experiences undue pressure as it passes through a normal anatomic canal known as the carpal tunnel in going from the wrist to the hand.
Using the carpal tunnel example, the first basic thing that can cause pressure on a nerve is if the nerve swells within a fixed space. The carpal tunnel is bounded, by bone and ligaments and like the skull, will not expand. Hence when the median nerve is swollen passing through that fixed space, there is pressure on that nerve that didn’t exist when the nerve was not swollen. As the wrist flexes back and forth, that nerve is asked to glide back and forth as well and because it no longer glides as easily, there can be trauma to that nerve that may further exacerbate the damage caused directly by the pressure.
Another basic factor that can cause nerve compression is if the nerve is passing through a fixed space, but that space becomes narrowed. A prime example of this phenomenon would be arthritis. Recall that any ’itis is an inflammation of something. Mastitis is inflammation of the breast, colitis an inflammation of the colon and so forth. Arthritis is an inflammation of the joints and the wrist joint is one prime example. Therefore if a normal sized median nerve is trying to pass through an inflamed wrist joint, there is less room to allow passage and once again the nerve becomes compressed. Similarly as in the above example, as the wrist moves back and forth, the nerve is unable to glide as easily and the same pathologic processes noted above likely occur.
The final basic factor that can result in nerve compression is a combination of swelling of the nerve and narrowing of the space through which it is trying to pass. In this case, the nerve is often very compressed. Putting these three basic principles into play, one can also see that there are many structures that can cause these problems. For example, in the case of the greater occipital nerve, it often passes through a narrow fascial window comprised of the insertion of the trapezius muscle at the base of the skull. At around this level, the GON also passes in close proximity to the occipital artery. If the artery is enlarged (e.g. aneurysmal secondary to trauma) the space through which the nerve passes is by definition narrowed. Moreover, the beating of that larger blood vessel against the GON may be one reason why many people report a “pounding” headache. Alternatively, the supraorbital nerve is theoretically supposed to pass from the back/top of the eye socket to the forehead through a notch in the frontal (i.e. forehead bone) known as the supraorbital notch. In some patients, however, that notch is actually a bony foramen - in other words the nerve is completely surrounded by bone. Bone is therefore the culprit here and the treatment is to convert that foramen into a notch. I have attached two pictures that illustrate this maneuver. The take home message is that the causes of nerve compression are actually simple in their most basic form, but a thorough knowledge of peripheral nerve anatomy and experience in peripheral nerve surgery are key factors in putting these principles into action and in achieving a good outcome from surgical intervention.