Recently, a new case definition for ME has been introduced by the Myalgic Encephalopmyelitis International Consensus Panel (ME-ICC) (2012). Within their primer, PEM is referred to as post-exertional neuroimmune exhaustion (PENE). The authors indicate that PENE can be assessed with a two consecutive day exercise test involving expired gas exchange, but as indicated above, cardiopulmonary devices are expensive and not always available in clinic and research settings, so self-report questionnaires will need to be used in most settings to assess this symptom cluster. For the self-report questionnaire that is included in the primer, PENE is characterized by 1) marked, rapid physical or cognitive fatigability in response to exertion, 2) symptoms that worsen with exertion, 3) post-exertional exhaustion that may be immediate or delayed, 4) exhaustion is not relieved by rest, and 5) substantial reduction in pre-illness activity level due to low threshold physical and mental fatigability. There are a number of problems with this level of specification. First, it is unclear whether all five characteristics must be present for PENE to occur. If a person meets all but one characteristic, it is unclear whether they would be counted as having this symptom. For example, in a separate dataset utilized by Jason, Sunnquist, Brown, & Evans (2013), it was found that out of 122 participants who met the Canadian ME criteria (Carruthers, 2011), only 77 participants would have met five differently worded statements that are characteristics of PENE. Therefore, whether all characteristics or just some are required for the ME-ICC case definition of PENE is of considerable importance.
Furthermore, the precise operationalization of each of these characteristics is still ambiguous. For example, the requirements for the onset and duration of PENE are vague and therefore reliability problems might occur. Additionally, physical and mental fatigability is supposed to result in a substantial reduction in activity level that can be mild, moderate, severe, or very severe. However, the meaning of substantial reduction or significantly reduced activity level is not clearly defined. As an example, one characteristic of PENE involves having a “Low threshold of physical and mental fatigability (lack of stamina) results in a substantial (approximately 50%) reduction in pre-illness activity level.” Directions are then provided in the primer indicating that symptom severity must result in a significant reduction in a patient’s premorbid status, which can be rated as mild, moderate or severe. Moderate is defined as an approximate 50% reduction in pre-illness activity level, but there is also an option for mild, which “meets criteria” and involves a significant reduction; but as moderate is defined as a 50% reduction, mild must be less, and this contradicts the prior statement regarding the need to have a 50% reduction. Similar issues arise with the other required symptoms, thus, there will likely be reliability problems with this new case definition.
