Mechanistic Interplay among Peripheral, Spinal and Brain Adaptions to Chronic Visceral Pain

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By Britt Van Hees

 

For those of you following our journey through the 3rd World Congress on Abdominal and Pelvic Pain (#WCAPP17), here is another light-bulb lecture explaining the interconnections of chronic visceral pain. The presenter, Melissa A. Farmer, PhD, is a researcher at the Feinberg School of Medicine at Northwestern University with a long-standing relationship with the International Pelvic Pain Society (the organizer for #WCAPP17). So, needless to say, she has an impressive resume and a passion for understanding pelvic pain. Just check out her wrap sheet. I’ve chosen a few interesting nuggets from her lecture.

 

Three Camps

 

Pain, pain, pain.  There are three camps of pain chronification:

 

Cortical reorganization: occurring in the brain

Spinal central sensitization: occurring at the spinal cord

Nociceptive sensitization: occurring at the sensory nerve level

 

Each of these camps have their own combination of modifiers that can lead to sensitization.

 

Pause- so this is where one can get overloaded with information, so I took some creative liberties and decided to draw a picture.

 

 

As you see from the drawing, there are several areas where our brain and our body can modify signals related to pain. At the “nociceptor sensitization camp” the sensory nerves can become extra excited, and maybe throw too much input into the nervous system. For example, do you remember the last sunburn you had? Remember how sensitive your skin was- even the slightest touch was immensely painful. Well, now imagine that you don’t have a sunburn but your nerves are sending that same intense signal, as if your tissue was still burned. This nervous system mix up is called allodynia, when normally non-painful stimulus is extremely painful.

 

At the “stream of spinal central sensitization” the spinal cord can send information upstream, to the brain, or downstream, to the peripheral nerve, to modify signals related to pain. For example, #1 on the list, prolonged dorsal horn firing, is similar to an echo. This is when your hand-nerve yells “watch out” to your spinal cord, and your spinal cord yells to your brain “watch out….watch out….watch out….watch out….watch out… watch out…watch out….” well you get the idea.  A single signal potentiates into multiple.

 

Finally we have the “lake of cortical reorganization”, or in layman’s terms, brain change.  Here you can have tidepools of emotional distress or reefs of prior history or insert another water-based metaphor here.  Essentially, upcoming signals are then interpreted within the framework of the brain where it decides if it’s enough to produce a pain response. How this all works is a bit murky, pardon the pun, but we do know that our interpretation of nociceptive signals greatly impacts how the brain choose to deal with them.

 

Somatic vs. Visceral: Crosstalk

 

Dr. Farmer also touched on the differences between somatic and visceral pain and the crosstalk that can occur between the two.

 

When comparing sensations from somatic (aka your body) and visceral structures (aka your organs), there are several differences. Somatic pain has a distinct quality, like a sharp or pinching sensation. It correlates specifically to the intensity, duration and location of the stimulus and is caused by tissue injury and inflammation. One easy example of somatic pain is the ankle pain one might experience following a sprain. Conversely, visceral pain is diffuse in quality and generally is found along the midline of the body. It doesn’t have the same on/off switch and tends to correlate very poorly with its stimulus. Just think how you felt the last time you had food poisoning…yah not fun.

 

Okay, that seems reasonable, now we have some ways to separate out different types of pain. However, sometimes messages can get muddled when somatic and visceral structures start to talk to one another – called crosstalk.  This can happen when sensory fibers of pelvic organs synapse close by where other somatic or visceral structures synapse at the spinal cord. You can see how it can get confusing when a problem in one organ might feel like a problem in a completely separate organ or body part.

 

One case of this is viscero-somatic pain, when a visceral structure creates body pain. A well-known example of viscero-somatic pain is when during a heart attack someone might report significant left arm pain. In the pelvis however, this would look like a patient complaining of low back pain when the stimulus is actually coming from uterine contractions.

 

This crosstalk can also go the opposite direction, somato-visceral pain. This could happen with vulvar inflammation. A patient may have multiple yeast infections leading to inflammation of the vulva (somato-), but then also experience bladder pain (visceral). And it doesn’t stop there.

 

Viscero-visceral pain, yup that happens. Stimulus from one organ can be confused with stimulus from another organ. This can even lead to changes in how the organs function. This type of viscero-visceral pain is one explanation for the high prevalence of irritable bowel syndrome in women with endometriosis.

 

What does this mean for patients?

 

So now what? All of this can be quite overwhelming. It’s important for both patient and practitioners to educate themselves regarding the different facets of pain. Great resources including Dr. Farmer’s research and books like Explain Pain or Pelvic Pain Explained can be very helpful in help navigating these uncertain waters. Since knowledge is power, the more we understand the cause, the better we can treat pain together.

 

Thank you again for the enlightening presentation Dr. Farmer! If you want to check out the whole presentation, which we recommend, look here.


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