Post-Exertional Malaise in Patients with ME and CFS with Comorbid Fibromyalgia.

Background: Myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS) share some similar symptoms with fibromyalgia (FM). Prior research has found increased illness severity when patients have FM that is comorbid with ME and CFS. For example, post-exertional malaise (PEM) has been shown to be more severe in those with comorbid FM. However, PEM can be separated into two factors, Muscle and General PEM. It is unknown if the more severe PEM findings in comorbid FM are due to the Muscle or General PEM factor.

Purpose: The purpose of this study was to determine if the PEM differences seen between patients with and without comorbid FM exist for the Muscle or General PEM factors.

Method: An international convenience sample was collected via an online questionnaire. The questionnaire assessed the frequency and severity of several PEM-related symptoms. Additionally, participants provided information regarding the course and characteristics of their illness.

Results: Participants that indicated a comorbid diagnosis of FM displayed significantly more frequent and severe PEM symptoms in the Muscle and General PEM factors. The FM group also indicated significantly worse physical functioning compared to the group without comorbid FM.

Discussion: The secondary diagnosis of FM in addition to ME and CFS appears to amplify the PEM symptomatology and worsen patients' physical functioning. The findings of this study have notable implications on the inclusion of patients with comorbid FM in ME and CFS research studies.