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Can a smile reduce the pain of an injection?

A recent study finds that both smiling and grimacing could reduce the sensation of pain associated with a “vaccination-like needle injection.” A sincere smile also reduced stress-induced physiological responses in participants.

When humans face acute pain, they tend to close their eyes tightly, raise their cheeks, and bare their teeth. Certain animals use similar facial expressions, which experts often call the grimace response.

As the authors of the recent study explain, “these facial musculature changes can also have a different interpretation: smiling.”

Why these two expressions, which occur for very different reasons, should share so many aspects is unclear. Researchers from the University of California, Irvine School of Ecology recently set out to “test whether these facial movements are beneficial in the context of stress and pain.”

Specifically, they wanted to understand whether manipulating participants’ facial expressions during a needle injection might impact their experience of pain and associated stress levels.

The researchers’ findings appear in the journal Emotion.

For many years, scientists have been interested to understand the impact of facial expressions on pain perception and mood. The facial feedback hypothesis, for instance, states that activating facial muscles can enhance or reduce emotional experiences. These effects on emotion can occur even if researchers manipulate a participant’s facial muscles into an expression.

As the authors of the recent study explain, “feigning a smile, whether conscious or not, may alter emotions in a positive way.”

To investigate possible links between facial expression and pain sensation, the researchers recruited 231 participants. The participants all received a shot of saline solution using a needle similar to those used to deliver a flu vaccine.

The researchers split the participants into four groups. Before and during the shot, the scientists manipulated participant’s faces into the following different expressions using chopsticks held in the mouth:

  • A Duchenne smile: a sincere smile, where the corners of the mouth come up and wrinkles appear around the eyes
  • a non-Duchenne smile
  • a grimace
  • a neutral expression

Example photographs of how the researchers used the chopsticks to elicit these expressions are available here.

Before the injection, participants completed a questionnaire that asked how anxious they were about the needle.

As the participants held their facial expressions, a medical practitioner administered the saline injection. Once the practitioner had applied a bandage, the participant removed the chopsticks from their mouth and completed a questionnaire about how much pain they were experiencing.

After 6 minutes of rest, the participants once again reported their pain levels. The researchers also asked them how stressful the experience was.

Before, during, and after the injection, the participants were linked to an electrocardiogram. Additionally, the researchers measured changes in the electrical resistance of participants’ skin, or electrodermal activity (EDA). EDA is a measure of psychological or physiological arousal.

According to the authors, the effect of the induced facial expression was strongest immediately after the injection. They explain that “the Duchenne smile and grimace groups reported approximately 40% less needle pain versus the neutral group.”

When the researchers examined heart rate data, they found that the Duchenne smile group had significantly lower heart rates than the neutral group. There were no significant differences between the other groups.

As for EDA, they only noted marginal benefits in the Duchenne smile group. Overall, the authors conclude:

“Together, these findings indicate that both smiling and grimacing can improve subjective needle pain experiences, but Duchenne smiling may be better suited for blunting the stress-induced physiological responses of the body versus other facial expressions.”

The study does have some limitations. Firstly, there were only 66 participants in the Duchenne smile group, the largest of the four experimental groups.

Also, 83.5% of the participants were white, meaning that the results might not apply to other groups. As the authors note, the participants were also relatively young and healthy.

It is also worth noting that holding chopsticks in a person’s mouth is relatively unnatural and does not truly replicate a natural facial expression. And, as the authors write, “expression in this study did not meet the intensity of true emotional experience.”

However, principal investigator, Prof. Sarah Pressman, is upbeat about the findings:

“Our study demonstrates a simple, free, and clinically meaningful method of making the needle injection less awful. Given the numerous anxiety- and pain-provoking situations found in medical practice, we hope that an understanding of how and when smiling and grimacing helps will foster effective pain reduction strategies that result in better patient experiences.”

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