Do people with anorexia nervosa show less pain on their faces?

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People with anorexia nervosa (AN) are known to show reduced pain sensitivity when assessed by thresholds and subjective ratings, as well as diminished facial expressions of emotion. The researchers therefore investigated their facial responses to pain.

Seventeen patients with AN and 18 age- and sex-matched healthy controls were assessed using a thermode to induce heat pain. Subjective measures included pain threshold, pain tolerance and ratings of stimuli above the pain threshold, using a numerical rating scale (NRS). Facial responses to the above-threshold stimuli were analyzed using the Facial Action Coding System (FACS). Eating pathology was assessed using the Eating Attitudes Test (EAT-26), Eating Disorder Inventory-2 (EDI-2) and body mass index (BMI); depression was measured using the Beck Depression Inventory-II (BDI-II).

Compared with healthy controls, patients with AN showed significantly reduced facial expressions of pain overall, with particularly pronounced reductions in Action Units AU 6_7 and AU 9_10. By contrast, subjective pain measures showed only marginal differences between groups. The reduction in facial expression could not be accounted for by differences in pain thresholds or ratings, or by levels of eating pathology or depression.

The researchers described the markedly reduced facial expression of pain in AN as a first observation, consistent with similar findings for facial expressions of emotion. Because subjective pain reports could not explain the reduction, they suggested that pain communication is impaired at two levels in AN: verbal and nonverbal signaling. This may make it harder for others to recognize and respond appropriately to their pain.