WHITHER POST-EXERTIONAL FATIGUE? A Comment on The Empirical Definition and The Georgia Prevalence Study by Cort Johnson (06/18/07)
The Empirical Definition has many positive aspects; better characterization of CFS patients, a way to track treatment efficacy and perhaps identify symptom based subsets and it does appear to identify a very ill population. But does it single out the peculiar condition called CFS. Some aspects of it suggest to me that it does not.
Some researchers have proposed that post-exertional fatigue is a hallmark symptom in CFS. The Canadian Consensus and 1990 Australian definition require post-exertional fatigue to be present for a CFS diagnosis. The Fukuda definition does not; although it is one of eight major symptoms it is not required for a CFS diagnosis. The empirical definition appears to dilute the importance of this symptom further. Some evidence produced by CDC studies and others, however, suggests it is a central component of CFS.
A CDC study examining the symptoms in a wide variety of fatigued groups (prolonged fatigue, chronic fatigue, CFS-like, CFS) found that as the levels of fatigue increased the percentage of people reporting ‘unusual fatigue after exercise’ did as well (Nisenbaum 2006). That only 1.6% of people with no fatigue reported this symptom indicated it is rarely found in healthy people. (Problems with sleep, muscle and joint pain on the other hand were fairly common (10-20% of healthy people)). About 14% of people with prolonged fatigue and 33% with chronic fatigue reported unusual fatigue after exercise but from there the percentages jumped up markedly; 77 and 74% of CFS-like and CFS patients reported this symptom. This suggests there is a big difference between chronic fatigue patients (fatigued but don’t meet 1994 criteria for CFS) on the one hand and CFS-like (met the criteria but didn’t undergo the clinical evaluation) and CFS patients (meet the criteria, visited the clinic) with regard to this symptom.
Similarly CDC studies exploring the question of subsets in CFS (Conna et. al. 2006, Aslakson et. al. 2006) found that post-exertional fatigue – was the first and third most important differentiating variable in the PCA and Latent Class Analyses. Its discriminatory prowess was highlighted by the fact that it and concentration difficulties were the only variables not found at all in the Well Group. The very high levels of post exertional fatigue (78-91%) in the three classes dominated by CFS patients and the low to moderate levels of it (33-41%) in the classes dominated by idiopathic fatigue patients again indicated that this symptom plays a special role in CFS. CFS is often described as being an amalgam of very common symptoms but these studies indicate that post-exertional fatigue is only rarely found even a subset of the population one might expect it to; the BMI matched overweight, obese and even morbidly obese healthy controls in this study who presumably don’t exercise much.
Since the ‘chronic fatigue’ group in the Nisenbaum was the largest fatigued group studied in the Nisenbaum study (PF=575, CF=1085, CFS-like=263, CFS=43) this group will surely account for most of the increase in the prevalence rates under the empirical definition. This indicates that most CFS patients under the empirical definition will not be characterized by ‘unusual fatigue after exercise’. This is not in some ways surprising; Dr. White and Jason have pointed out patients can meet the new criteria for CFS simply by reporting they have low activity levels.
A Personal Response: For the first ten years after I got CFS I, who was formerly an avid exerciser, didn’t try to do anything more than walk. My symptoms at that point, while very disturbing, were nevertheless mostly not that unusual overall; I was very tired, I couldn’t concentrate well, my muscles hurt, I had constant sore throats, I felt out of it, I wasn’t strong. Basically I felt like I was shadow of my former self but I was able to get around, I was able to go to school. It probably could have been argued that I had some strange mental condition. Given the lack of information on CFS around that time – mid-1980’s, in the back of my mind I wondered if something like that had indeed occurred.
About 10 years into the disease I did something very unusual - I began an exercise program and have tried to do so several times since then. Each time my response was to it has been bizarre. The symptom exacerbation has been immense, not just at the beginning of the exercise program but throughout. Over time I was able to increase my strength and duration. In fact I usually felt good while I was exercising – but the aftermath was always devastating. Starting anywhere from ˝ hour to several hours later I could feel that peculiar package of symptoms set in; the stiff, painful muscles, the heart yammering, the uncoordination, the need to lie down, the difficulty thinking or talking, the feelings of irritation. These symptoms would typically peak a day or so later and then slowly subside. It is an utterly strange response to exercise and it is the hallmark symptom of my CFS – it’s what convinced me that there was a physiological basis to my problems.
The idea that this vital part of CFS is being subsumed under this new definition suggests that while researchers may uncover important aspects of unwellness by studying this population they may very well have a very difficult time understanding the condition known as CFS.
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Aslakson, E., Wollmer-Connar, U. and P. White. 2006. The validity of heterogeneity in chronic unexplained fatigue. Pharmacogenomics 7, 365-373
Conna, U., Aslakson, E. and P. White. 2006. An empirical delineation of the heterogeneity of chronic unexplained fatigue in women. Pharmacogenomics 7, 355-364.
Jason, Leonard. 2007. Problems with the new CDC CFS Prevalence Estimates. IACFS Website.
Reeves WC, Jones JF, Maloney E, Heim C, Hoaglin DC, Boneva
RS, Morrissey M, Devlin R,
Prevalence of chronic fatigue syndrome in
metropolitan, urban, and rural Georgia.
Popul Health Metr. 2007 Jun
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Reeves WC, Wagner D, Nisenbaum R, Jones JF, Gurbaxani B, Solomon L, Papanicolaou DA, Unger ER, Vernon SD, Heim C, Chronic fatigue syndrome--a clinically empirical approach to its definition and study. BMC Med. 2005
White, P. 2007. How common is chronic fatigue syndrome; how long is a piece of string? Population Health Metrics 5:6 doi:10.1186/1478-7954-5-6