What is fatigue?

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Fatigue as presence of sensation or absence?

This is another one of those posts where I try to describe some highly subjective symptom in the hopes of improving our understanding of the illness.

I think that everyone knows the kind of fatigue where there is an intense fatigue sensation. This is more the kind that you get after sufficiently intense activities or at the end of a day of work. It makes you want to rest.

There is another kind which I suspect is much less common and it's the kind of fatigue that makes someone say they have no energy, or more accurately, no energy reserves. Because that is what it feels like: it is an absence of energy reserves. You know in that you're in a state with little stamina and reduced strength. It makes you want to limit and avoid activities that aren't necessary.

The second kind seems to be more common with delayed PEM and with disrupted sleep. I am not sure that fatigue is a good name for it.

Does any of this make sense?

 

I have the ‘concrete’ symptoms (pain, headache, tiredness, sore throat, poor balance, etc), but I also have a subjective sensation of emptiness when my symptoms are bad or in a crash. It is a sense of absence in all the larger muscles (arms, legs, torso, head/neck), a sense of discomfort that generally falls short of pain.

It is only over the last four or five years, when my ME has been more severe and involved more marked orthostatic intolerance, that I have been conscious of this sensation and felt the need to find a way of verbalising it. It is both an aspect of my PEM and an indicator that the activity threshold for triggering PEM is considerably lower.

 

There is the sudden loss of stamina when I go over my limit, and this vestibular dysfunction with OI distress that is amplified during PEM. It does not feel like fatigue in the general sense. I feel sick.

 

I am worse these past few days, so will just say a bit. The fatigue feels different at various times. Sometimes I come to a halt, and just can't put one foot in front of the other. It's almost a paralysis.

Other times it feels like all my insides have been taken out - completely lacking any oomph at all - weak muscles, feel limp, and more flu-like.

The OI causes weak feelings too, but more: dizziness, faintness, become uncoordinated, confused, must lay down.

OMG, I wish medicine could FIX THIS!!!

 

Just to throw something else in: to me, fatigue and pain are to some extent the same thing.

I'm unwell with a virus at the moment, but as my friend had been out and fetched me a Christmas tree, I decided to decorate it. I carried on until the Stop That Right Now fatigue kicked in, and crashed down onto the sofa. But then the really nasty neck pain that I always get with a cold started coming through as well.

I couldn't deal with both, so I took a couple of tramadol tablets. The pain is reduced to some extent, but the drug has helped more with the fatigue. I actually sat up again and opened my laptop 40 minutes after taking the tablets, something I couldn't have done otherwise.

I don't have the clarity to think it through any more than that, but the fatigue/pain continuum is something I've always been very aware of. It's different to the feeling that all your muscles have been surgically removed and replaced with bags of water, though – I've got that to look forward to for the next 48 hours!

 

I am not getting the feedback that indicates others understand what I'm trying to say.

I want to see if others can distinguish between feeling fatigued and a feeling of lacking energy reserves. I think they can be quite distinct, but describing that seems very difficult.

Something in the body seems to have a way of knowing in advance when we have very little stamina. It communicates this with a distinct sensation that we instinctively recognize as meaning "you have very little energy reserves". I also have other days where the forecast (for lack of a better term) indicates good energy levels and I begin some activity, only to unexpectly tire quickly.

 

@strategist yes I can distinguish the difference. With lack of stamina I feel out of breath, shaky, off balance, slurred speech and need to stop everything- can't do anymore.

Fatigue I feel I need to lie down and rest, no distress and can recover within an hour. If I had some things to do I can finish them. I don't need to immediately stop everything.

 

I think this is actually best described in a medical setting as somnolence. What a lay person would describe as drowsiness or sleepiness.

Fatigue is different from somnolence and would be better related to as tiredness or a lack of energy.

 

This is quite interesting to me because very early in my illness I experienced that feeling of non-restorative sleep and feeling like death on waking, but now it is a very rare occurrence indeed. Now I almost always wake up feeling 'normal' and then hit a wall after activity. How little activity is required to hit that wall depends on how much I have overdone it the day before. I had wondered if it was because I was more severe at that time due to not pacing, or if it might've had more to do with EBV infection and PVF back then.

Going a little off-topic, I can distinguish 3x types of 'fatigue', each with slightly different characteristics.

1 - CFS fatigue: I'd call this malaise. Primarily lack of energy/motivation, sore throat, swollen nodes, achy, feeling ill, low stamina. Triggered by prolonged exertion over my baseline, delayed by ~24hrs and attenuates slowly over days.

2 - POTS fatigue: I'd call this exhaustion. Primarily somnolence and weakness. Hits an hour or 2x after demanding upright activity, the crash after the adrenaline rush from being on my feet to be simplistic. Responds well to rest and I can function a little better later the same day, but remain tired. Perhaps just an exaggerated form of a healthy person's fatigue?

3 - Inflammatory fatigue: This has attributes of both malaise and exhaustion. I don't get the sore throat and swollen nodes, but I do very much feel a lack of energy/motivation, low stamina & significant somnolence. Comes with either focal joint inflammation or bowel movements. Hits like a freight train, 'normal' one moment, absolutely wrecked a moment later with no exertion or other explanation required. On the plus side it tends to be short lived, usually it'll last 8hrs max but can repeat for days depending on how bad the flare is.

All types of fatigue tend to disrupt my autonomic function and make my orthostatic symptoms a lot worse.

I'm not sure if many will relate to the inflammatory fatigue description, I consider this part of my inflammatory arthritis diagnosis. But I thought it might be interesting to mention it as its often a bit of a mystery how chronic fatigue in CFS and chronic fatigue in so-called 'organic diseases' differ.

 

Both for me.

 

Yes, this sums it up for me. Fatiguability is when your muscles (or brain equivalent - brain fog, difficulty concentrating) is when your mind sends out the usual level of effort, but the muscles don't respond with the same level of force. I notice this when riding a bike for example and it was particularly noticeable on the 2 day CPET.

The various sensations associated with aren't fatigue, they are not a warning of damage, but they are supposed to be a warning of fatigability.

 

I can feel that I always have a lack of energy reserves even when not 'fatigued'. E.g. when I'm not in a crash, I can feel that the fatigue/sickness feelings are just under the surface ready to break out if I move too much, talk too much, or concentrate too much.

But I don't get a sent of 'emptiness' as such. I feel my lack of stamina/lack of energy reserves. And when I crash I feel really weak.

But my fatigue is an overload of many sensations, not an absence of sensation. My cells feel like they are raging, I get migratory pains, discomfort in all my muscles, sore throat, sound sensitivity, pounding heart, etc. when my fatigue comes on.

It's more like I am ill than fatigued, although fatigue and weakness are big parts of the symptom cluster that comes on when my very minimal stamina runs out.

 

Merged discussion moved from this thread

No. In the case of motor control, fatigue is literally a reduction in performance for a given level of effort in the brain regions upstream of the motor cortex (eg Mesencephalic locomotor region).

The sensations we associate with fatigue (proprioceptive signals, sense of stiffness etc.) is not fatigue itself. But instead functions as warning to our consciousness that performance is reduced. (nor is it a "stop" signal, but that is another debate)

We know that these sensations are mediated by muscle afferents and they have a measurable effect in supressing motor cortex excitability (GABAergic inhibition). The functional result of this "central fatigue" is increased ventilation for a given level of neural drive. (Which we know from TMS studies that utilised lumbar blockade of the aforementioned afferents)

While I cannot prove it, I suggest a similar mechanism for mental exertion - a higher level of upstream mental effort ("concentration" etc) for a functional outcome.

I also speculate that both result from blood flow problems in the capillaries (shunting instead of directing flow to where it is needed), perhaps something along the lines of https://pubmed.ncbi.nlm.nih.gov/33577778/

 

This is not the meaning of fatigue relevant to ME and I doubt you would get many doctors to agree that it is relevant.

 

I have had other doctors agree it is relevant. But what you are really saying is you don't agree, so why not?

What about he frequent patient reports of patients reporting feelings of weakness, or rapid declines in performance when performing motor tasks?

Almost all studies of motor performance in patients do show more rapid decline in performance compared to sedentary controls of similar muscle strength.
At the same time, they also show patients reporting higher levels of effort (compared to controls) as force levels.

(also found on 2 day CPET studies, where participants have a non-linear increase in reported effort, which starts earlier on the second day, coinciding with the earlier ventilatory threshold)

Many studies blamed this on "central fatigue", but we since have learned (using TMS studies and afferent blockade) how muscle afferents are the key cause of central fatigue. This is also a classic example of how "mind and body" are inherently linked and cannot be separated.

Note that (motor/muscle) fatigue is not merely defined as a decline in maximal force output. As submaximal levels of exertion, there can also be a decline and that decline is relative to a constant level of mental effort.

That is a definition of fatigue. Fatigability is the susceptibility to that result.

The other sensations you described are sensations associated with our experience of fatigue, rather than fatigue itself. Then there are the other symptoms of PEM that can be separate from the fatigue-associated sensations.

 

You seem to be missing the point @Snow Leopard that language is messy and complicated. In science terms of have multiple meanings in different language structure contexts. Yes, of course the root fatigue- is used for the sorts of phenomena you describe and as a compound noun like peripheral fatigue it can be used to imply the sort of processes you refer to. (Basically like @Trish said.)

However, just 'fatigue' in the context we are interested in has a completely different meaning - a symptom cluster. People with myasthenia gravis do not suffer from 'fatigue' despite fatiguability of muscle being the origin of their symptoms. People with myasthenia report 'weakness'. Yes, some people with ME report an actual sense of weakness but they report it as weakness or inability to do things.