Difficult to Classify

Fibromyalgia (FMS) and chronic fatigue syndrome (ME/CFS) are difficult illnesses to classify. Both have a broad range of physical symptoms that span multiple systems, and they're associated with multiple psychological symptoms as well. In addition, they're often accompanied by a slew of other illnesses — many of which are also hard to classify.

As scientists are getting more of a handle on FMS, ME/CFS and other related illness, an umbrella term that's more frequently used to describe them is central sensitivity syndromes, or CSS.

Some researchers argue that this term should replace other terms, such as functional somatic syndrome, medically unexplained syndrome, and somatoform disorders, because they believe CSS is more accurate.

What is a Central Sensitivity Syndrome?

An illness described as a CSS involves something called central sensitization. "Central" means the central nervous system, which is made up of your brain and spinal cord. "Sensitization" is the end result of something that has made you sensitive.

Allergies are the type of sensitivity people are generally the most familiar with. In allergies, your body has an inappropriate physical reaction to something that doesn't bother other people at all. In fact, While the sensitivities of a CSS aren't exactly allergies, they do involve an inappropriate physical reaction.

In a CSS, we become sensitive to things that are processed by the central nervous system, which can include bright lights, loud noises, strong smells, rough textures, and pressure on the body.

It may also involve certain foods or chemicals. Especially in FMS, the body is sensitized to anything unpleasant, i.e., cold, heat, a tickle or an itch.

Aside from FMS and ME/CFS, the following conditions have been proposed to be part of the CSS family:

chronic pelvic pain, including vulvodynia
headache and migraine

Psychiatric disorders are common in CSS as well. Research suggests that's because they all involve dysregulation of the same neurotransmitters, with the dysregulation in CSS in different regions of the brain than in psychiatric disorders.

Psychiatric conditions that commonly overlap with CSS include:

major depression
obsessive-compulsive disorder
bipolar disorder
post-traumatic stress disorder
generalized anxiety disorder
panic attack

Features of CSS

Neurotransmitters that are involved in at least some of the CSS include:

The pain of CSS comes from a couple of different abnormal pain types: hyperalgesia and allodynia.

Hyperalgesia takes normal pain from things that everyone considers painful (a broken limb, an infected tooth, etc.) and makes it worse. It's often referred to as "turning up the volume" of pain. This makes things like injuries, surgeries, and chronic sources of pain especially debilitating.

Allodynia makes you feel pain from things that shouldn't hurt, such as the brush of fabric against your skin, or your arm resting against your side when you sleep.

Allodynia can make your clothes painful even when they're not too tight, or make you unable to enjoy a hug. It turns all manner of ordinary experiences into painful ones, which often means making significant changes to your life to minimize it.

Other proposed mechanisms of CSS include:

inflammation in or originating in the nervous system
autonomic nervous system dysfunction
dysfunction of the HPA axis, which is part of the body's stress-response system

Treating Central Sensitivity Syndromes

The different individual symptoms and mechanisms of each CSS require a customized treatment approach, but in general, most CSS tend to respond to some of the same types of treatment, especially antidepressants (which help correct neurotransmitter dysregulation), exercise, and cognitive behavioral therapy (CBT).

However, it should be noted that people with ME/CFS have special considerations when it comes to exercise, and CBT is a highly controversial treatment for this illness, especially when it's paired with graded exercise.

While these conditions are believed to be related, it's important to get each one diagnosed and treated properly. Be sure to talk to your doctor about any symptoms that don't seem to be related to your current diagnoses.

A Word from Verywell

The CSS classification is good news for all of us with these conditions. It signifies a positive change in how these illnesses are understood and viewed by the medical community. That's what we need when it comes to getting research attention, which leads to better diagnostics and treatments.

Sources:

_{Chong YY, Ng BY. Annals of the academy of medicine, Singapore. 2009 Nov;38(11):967-73. Clincal aspects and management of fibromyalgia syndrome.}

_{Mayer TG, et al. Pain practice. 2012 Apr;12(4):276-85. The development and psychometric validation of the central sensitization inventory.}

_{Smith HS, Barkin RL. American journal of therapeutics. 2010 Jul-Aug;17(4):418-39. Fibromyalgia syndrome: a discussion of the syndrome and pharmacotherapy.}

_{Smith HS, Harris R, Clauw D. Pain physician. 2011 Mar-Apr;14(2):E217-45. Fibromyalgia: an afferent processing disorder leading to a complex pain generalized syndrome.}

_{Yunus MB. Seminars in arthritis and rheumatism. 2008 Jun;37(6):339-52. Central sensitivity syndrome: a new paradigm and group nosology for fibromyalgia and overlapping conditions, and the related issue of disease versus illness.}