Classification of myalgic encephalomyelitis/chronic fatigue syndrome by types of fatigue.

Persons with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) often complain of fatigue states (eg, postexertional malaise, brain fog) that are qualitatively different than normal, daily fatigue. Given the heterogeneous nature of ME/CFS, it is likely that individuals with this illness experience these fatigue types differently in terms of severity and frequency. It is also possible that meaningful subgroups of patients exist that exhibit different patterns of the fatigue experience. The purpose of this study was to investigate whether individuals with ME/CFS can be classified in a meaningful way according to the different types of fatigue they experience. One hundred individuals with ME/CFS participated in the study. Individuals that met inclusion criteria were administered the Multiple Fatigue Types Questionnaire (MFTQ), a 5-factor instrument that distinguishes between different types of fatigue. A cluster analysis was used to classify patients into various clusters based on factor subscale scores. Using a 3-factor solution, individuals were classified according to illness severity (low, moderate, severe) across the different fatigue factors. However, a 5-cluster solution enabled participants with moderate to severe fatigue levels to fall into more differentiated clusters and demonstrate distinct fatigue state patterns. These results suggest that fatigue patterns of individuals with ME/CFS are heterogeneous, and that patients may be classified into meaningful subgroups.