Chronic fatigue syndrome: Is it one discrete syndrome or many? Implications for the "one vs. many" functional somatic syndromes debate.

There is a current debate as to whether "functional somatic syndromes" (FSSs) are more similar to or different from each other. While at the same time, there is evidence of heterogeneity within single syndromes. So, it could be that these syndromes are all part of one big process/illness, are discrete in their own right, or that they are heterogeneous collections of different illnesses lumped together by common symptoms but separated by uncommon pathophysiologies. The example of chronic fatigue syndrome (CFS) is instructive. There is evidence to support all three models of understanding. Three recent large studies have suggested that FSSs are both similar and dissimilar at the same time. The solution to the debate is that we need to both "lump" and "split." We need to study both the similarities between syndromes and their dissimilarities to better understand what we currently call the FSSs.