New study provides further evidence that problems with sensory gating may contribute to fibromyalgia pain and other symptoms.
It’s a Jungle Out There
The key to survival in the animal world is to be able to detect threats in the environment and react appropriately. From the simplest sea creature dependent on temperature or saline gradients for survival to wildebeest on the savannah on the lookout for predatory lions – knowledge is god and fast and efficient processing of sensory information can make the difference between life and death.
Too little information can be fatal if you don’t spot that lion behind the tree. Too much information can be fatal too. Which of those three potential predators is closest and poses the greatest threat? Should I run or lay low? Too many choices and too much deliberation and you’re someone’s lunch.
Even on Main Street
Relatively few modern humans face similar threats, yet the early evolutionary mechanisms that enable animals to survive in a hostile environment are still very much active in the human brain, although they are overlaid by more sophisticated concepts of ‘threat’, attraction, hunger, etc.
Minute to minute, hour to hour, day to day we are bombarded with sensory information from all five senses – from the outside environment and from our awareness of our own bodily sensations. This constant bombardment would easily overload the conscious brain if mechanisms didn’t exist which filtered out ongoing and trivial sensory input and allowed us to focus instead on information important to survival or other behavioral goals.
Sensorimotor or sensory gating is the neurological mechanism that enables our brain to filter out and selectively attend to what’s important in the constant stream of incoming sensory information. Way back I suggested that a deficit in sensory gating might offer a key insight into the mechanisms underlying the symptoms of ME/CFS and fibromyalgia (FM) as well as a wide range of other neurological and neuropsychiatric disorders.
Everyday examples of effective sensory gating include the ‘cocktail party effect’ – the ability to hold a conversation in a crowded room while filtering out the other conversations around you, and the ‘seat of the pants effect’ where, for example, over time you no longer feel the discomfort of sitting on a hard chair
I don’t know about you, but I have major difficulties with both these examples of sensory gating.
Chronic widespread pain is the primary symptom of fibromyalgia, and abnormal perception of pain – whether it be hyperalgesia, allodynia, abnormal sensitivity to heat or skin pressure, or diminished activation of pain inhibitory mechanisms – appears to be a consistent FM research finding.
Pain, of course, is a sensation, and abnormal sensitivity to pain would seem to point to abnormal sensory gating, especially in the apparent absence of peripheral tissue damage. Besides pain, though, many of the other symptoms common to fibromyalgia (and chronic fatigue syndrome) – fatigue, sleep disturbances, cognitive dysfunction (aka fibro-fog or brain-fog), noise, light and scent sensitivities, anxiety/depression, IBS, restless legs, etc,, could all conceivably be linked to dysfunctions in properly processing sensory information.
When the ‘Gates’ Break…
Several studies directly or indirectly suggest sensory gating problems exist in ME/CFS. Now a small but potentially significant study – “Alterations in excitatory and inhibitory brainstem interneuronal circuits in fibromyalgia: Evidence of brainstem dysfunction” – suggests that sensory gating is ‘broken’ in fibromyalgia, as well, and that the dysfunction may be located in a key area of the brainstem – the oldest and smallest region of the human brain often referred to as the ‘reptilian brain’.
This of course makes sense as a location for the mechanisms controlling something as fundamental as survival. The reptilian brain is an archaic and relatively small region of the brain that is necessary for survival but doesn’t include the additional neural baggage of cortical structures normally associated with higher level cognitive processes.
Sensory gating can be measured in the lab by recording the brain’s electrical reaction to a sensory stimulus (usually auditory, visual or tactile involving painless pressure or mild pain). One lab measure of sensory gating is called pre-pulse inhibition (PPI) which measures the reduction in reaction in the brain that should occur when the same stimulus is presented again and again. Ideally, an initial stimulus should cause a strong reaction which should fade in speed (latency) and strength (amplitude) as the brain understands how to deal with the input.
It’s easy to see how this mechanism protects the brain from getting overloaded from the ongoing streams of sensory information it receives.
A ‘Hyperexcitable’ Brainstem?
The finding that FM patients (10 compared with 26 healthy controls) had a normal ‘blink response’ (analogous to the acoustic startle response) suggests that their nervous systems were not ‘jumpy’ and surprisingly they had a slightly higher sensory threshold than controls.
In contrast FM patients reported a greater perception of ‘discomfort’ in response to the stimulus suggesting an “apparent dissociation between peripheral nociceptive (unpleasant stimulus) input and subjective pain sensation”. A recent study, in fact, suggests that brain regions associated with increasing discomfort in association with pain, i.e., in making pain really ‘painful,’ are over-activated in fibromyalgia.
On repeated measures the FM patients blink reflex returned to a ‘hyperexcitable’ state rather than becoming habituated to the stimulus (a finding curiously also seen in the movement disorder Parkinson’s disease). Similarly, when the blink response was ‘conditioned’ by a weak pre-pulse, pre-pulse inhibition (sensorymotor gating) was significantly reduced in the FMS patients, and unlike controls the perception of ‘discomfort’ did not reduce significantly over time.
These findings suggest that instead of calming down in response to repeated sensory signals as it should, the brainstem remained in an excited state. Taken together, these findings support the concept of altered sensory gating (i.e., pain processing) in FM in common with other chronic pain states.
They’re intriguing findings, particularly given recent research suggesting that when Lyrica works it does so in part by reducing brainstem activity
Digging Deeper: The Pedunculopontine Nucleus
Now there’s a mouthful! PPI appears to be mediated by a brainstem area called the pedunculopontine nucleus (PPN) which connects to various other brain regions including the thalamus and somatosensory cortices, both of which have recently been associated with pain processing in FM.
In addition to the vital function of ‘gating’ sensory information, the PPN is implicated in a host of other functions that can have a serious impact on quality of life relevant to FM symptomology. These include ‘arousal’, attention, learning and maintenance of REM sleep:
“The PPN is involved in many functions, including arousal, attention, learning, reward, and voluntary limb movements and locomotion. While once thought important to the initiation of movement, recent research suggests a role in providing sensory feedback to the cerebral cortex. It is also implicated in the generation and maintenance of REM sleep.”
The authors concluded that alterations in excitatory and inhibitory circuits located in the deepest part of the brain, the brainstem, cause altered pain processing in fibromyalgia.
Is This Central Pain Sensitization Again?
The concept of central (brain) pain sensitization in FM is, of course, nothing new and some have suggested that a dysfunction in ‘top-down’ cortical inhibitory mechanisms may be to blame.
Recent findings of small fibre neuropathy and ischemia in FM patients, however, suggest that, at least in some patients, ‘bottom up’ processes that provide a steady stream of pain signals to the brain may be a key driver.
But why not both? It’s long been recognized that an injury can lead to chronic and generalised central pain sensitizations that persist long after the original injury has healed.
The current study, though, suggests that whether top down or bottom up processes are involved, the brain mechanisms responsible for the increased pain in FM may be situated deep in the brain indeed, well below the higher brain regions involved with ‘emotions’ or ‘cognition’.
Pain Plus… Again!
It’s important to note that sensory gating is not just about pain. The same sensory gating mechanisms that filter pain signals have to filter sensory input from all five of the senses including input from the external environment and from bodily sensations. What happens when the entire sensory processing system, i.e., the ‘sensory gates’ that protect the brain from being overwhelmed with sensory data, break down?
The authors suggest that:
“…. PPI may serve to protect the brain from experiencing sensory overload …. Reduced PPI therefore concurs with less filtering of information flow from the periphery to the brain, and as a consequence may be associated with more information arriving simultaneously at higher centers. Altered cortical information processing may then be a further consequence, which may contribute to characteristic findings of abnormal perception and allodynia in FM patients”
The implications for FM/ME/CFS patients are obvious. Symptoms such as pain, fatigue, sleep disturbances, cognitive dysfunction (fibro/brain-fog), noise, light and scent sensitivities, anxiety/depression, IBS, restless legs, may all result from the relentless unfiltered bombardment of sensory information. Even mental fatigue and reduced cognitive performance can be tied to cognitive overload resulting from having to deal with too many stimuli at once. (Do FM and ME/CFS patients have to devote more brain regions than normal to perform a task because they’re always, in fact, performing more tasks than normal?) Could this be from cognitive loading (constant mental processing without a break) and performing more than one mental task simultaneously?)
“Diminished performance during dual tasks, compared to performance of separate single tasks, is attributed to the allocation of limited resources to attend to and perform competing task requirements.”
Those of us old enough may remember when personal computers were much less powerful, and that running more than one program at a time would inevitably result in the central processor slowing to a crawl. Similarly the unavoidable need to process ongoing unfiltered sensory input may easily result in FM ‘fibro-fog’, ME/CFS ‘brain fog’ and the reduced processing speed.
seen in both conditions. Reduced information processing is, in fact, found in both disorders and its likely that similar overload may explain reported difficulties with higher ‘executive functions’ such as decision making.
I think so. If a sensory gating deficit plays a role in pain and other FM symptoms then ‘fixing’ the deficit could bring significant global relief.
Several studies have examined sensory gating deficits in fibromylagia (although not in recent years) using the related ‘event related potential’ (ERP) measure, but this is the first to study the PPI startle reflex that implicates brain stem regions. One ME/CFS sensory gating study has been published. Elsewhere, sensory gating has been extensively studied in a wide range of neurological and neuropsychiatric disorders, and an extensive literature on drugs/compounds that impact positively and negatively on sensory gating exists. Perhaps this literature contains an alternative to the three FDA approved FM drugs?
- Dig Deeper: JAPANESE SENSORY GATING STUDY REVEALS PROFOUND COGNITIVE DEFICITS PRESENT IN CHRONIC FATIGUE SYNDROME
- Dig Deeper – Marco’s Neuroinflammatory Blog Series - check out Marco’s blog series here.