Something a little different this month. The following is a proposition for a new kind of medical imaging modality.

It is not uncommon to see patients who have pain but cannot, despite spending obscene amounts of time and money, determine any organic cause for this pain. I have thought several times of how useful it might be to scan individual nerve fibers, and see which one(s) is firing an aberrant amount with respect to its baseline.

I will start with some background physiology. The nervous system is divided into central and peripheral portions. The central nervous system includes the brain and spinal cord. Peripheral nervous system is everything else. That includes all the tiny nerve endings in your skin which send “pain” signals. There are also nerves which carry signals to muscle, aka “motor fibers”. The pain and motor fibers are very similar. Both conduct signals by “depolarizing”. That is, neurons (nerve fibers) use energy to maintain an electrochemical gradient. When a fiber is stimulated, the stimulation causes equilibration (depolarization) at the location of the stimulus. Equilibration does not only happen in one area though, because the fiber extends from its origin all the way to the central nervous system. Though simplified, think of the fiber as being divided into 1mm segments. If the point of stimulation is A, and adjacent to that is B, and so and so forth until you get to the spinal cord which we’ll call Z. When A depolarizes, there is suddenly a gradient between A and B, causing B to depolarize, and on down the line to Z. In reality, there is no A, B, etc division, but it is comparable to a domino effect down the nerve. At the nerve’s end, the spinal cord, a more complex method of transferring depolarization occurs onto a new nerve which itself depolarizes just like the first nerve and goes on up toward the brain.

Now, the difference between pain and motor fibers is not much. Like electrical wires, it is mostly where the nerve starts and ends that makes it unique. Pain (and other sensory) fibers travel from peripheral to central, and motor fibers travel from central to peripheral. There are several sizes of nerves. As a general rule, the larger the fiber, the faster the signal travels. Sharp pain is carried by large fibers, and dull, constant pain carried by smaller fibers. Motor control is mostly carried by larger fibers. Also, it is important to add that pain and motor (and again, other sensory fibers such as light touch, vibration, position sense) often are “bundled” together as they travel to and from the central nervous system.

There are two basic organizational systems of “pain” arising from the embryological development pattern of the human. Your “insides” develop from ‘splanchnopleure’, and your body wall (skin, muscle, underlying connective tissue) comes from ‘somatopleure’. The technical differences probably do not matter, but it is important to know that pain from somatopleure (body wall) hurts in a very precise location. This is because the surface of the body is mapped out extensively on the cortex, and some places more detailed than others so that you can tell pain in your fingertip from the back of your hand though pain on to top of your calf may not be discernible from pain in the middle of your calf. Similarly, your organs arise from splanchnopleure and are innervated in such a way that pain from your insides is interpreted as pain all over. Said differently, in your bowels, you might have gas pain or constipation, or a cut of some sort, and as long as there is not stimulation of your body wall pain fibers, your brain interprets this organ pain fiber firing as “abdominal pain”. That is, the organs in the abdomen are not intricately mapped out on the cortex and, as a result, any number of nerves being stimulated from these organs make you feel similarly even though they are different nerves and may be meters apart from one another.

One last bit of information, which is not well understood, but it certainly important, is that pain is not as straightforward as peripheral stimulation and subsequent feeling. There is ‘cortical’ (meaning interconnectedness of the brain’s cortex) influence of pain. This cortical influence explains why getting hit hurts the worst the first time, or why amputees can feel pain in limbs which are no longer there (so called “phantom pain”). Also, there is always the theoretical possibility of functional (aka psychological) pain. There is also gate control theory, which basically says that sensory fibers around a pain fiber will inhibit a pain fiber if stimulated through an inhibitory neuron system. This theory explains why something painful can feel better when you rub it.

With all that background, I am proposing a very crude idea for a “pain scanner”. It should be possible to record depolarization of nerves. This is similar to how an EKG works. Two electrodes can measure depolarization as this movement of charge creates a current through the heart. The same should be true for nerves. The biggest difference of course is that there is only one heart, and it is big, while there are something like 1 trillion nerves in the body, and they are all small. I think this could be dealt with though. The pain fibers need to be isolated from the other types of sensory inputs. This should be achievable by having the patient lie still and comfortably in a quiet, dark room. Any remaining sensory input should be whatever the person is touching, and whatever they are “feeling”. I am, of course, interested in pain.

After this point, my ideas are all theoretical, because I do not know or remember enough about physics to offer more than conjecture. Of the two “feelings” remaining, the feeling of what the person is touching should be fairly constant and should be able to be canceled out as noise with digital processing. In my model then, the only remaining input to the system is pain. This does not account for other cortically-derived sensations such as itching, hunger, etc. I do not know if these cortical sensations would interfere with monitoring of peripheral sensations, so I will ignore this for the moment.

If we could isolate pain fibers when people have no discernible cause for pain, we could determine (perhaps) whether the pain is from the body wall or internal organs, how many pain fibers are included, and what type of fiber. This information could be used for diagnosis, and possibly treatment as well. If we are able to exclude all known causes of pain but could determine with this scanner that pain fibers are being activated, not only could we narrow our focus to the areas innervated by that nerve, but we could also consider cutting the nerve as treatment. If this modality were refined well enough to be reliable, it might even be a cheaper and more readily available diagnostic tool than imaging modalities such as CT or MRI which provide exquisite pictures, but are expensive and only show macroscopic derangement.

Obviously, this is a work in progress, but I am not planning on studying enough physics and/or engineering to figure out if or how this pain scanner might be possible. Any thoughts? Perhaps a name suggestion?