Brain waves of horses in study reveal presence of chronic back pain

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The brain waves of horses show telltale signs of chronic back pain, researchers in France report.

The assessment of pain, and in particular chronic pain, is a major challenge that remains difficult to solve since there is a subjective emotional and cognitive dimension to it, Mathilde Stomp and her colleagues at the University of Rennes 1 reported in the open-access journal, PLOS ONE.

Since pain reflects neural activity in the brain, there has been a growing interest in exploring resting-state electroencephalograms (EEGs) for evidence of its presence. An EEG is a test that detects electrical activity in the brain using electrodes placed on the head.

In humans, EEGs have proven to be a promising tool in this regard, although the results of various studies are contradictory.

Riding horses show a very high prevalence of back disorders thought to be associated with chronic pain, the study team noted.

“Moreover, horses with known back problems show cognitive alterations, such as a lower attentional engagement.”

Therefore, they hypothesized that the EEGs of horses in a resting state could reflect the state of their back.

The researchers, all with the animal and human ethology unit at the university, took EEGs of 18 adult riding horses who were fitted with a headset and a telemetric recorder to obtain the readings. The tests were undertaken while the animals stood quietly in a familiar covered arena.

Each horse also underwent a back evaluation, in which shape and muscular tension were assessed along the spine. Precise measurements of neck shape were also taken, as it has been shown in previous research that a hollow or flat neck reflects muscular tensions in different parts of the spine, while a round neck characterizes healthier backs.

The horses were classified as having either a round or a hollow/flat neck.

Thirteen of the horses then underwent surface electromyography (sEMG) examinations to measure muscle activity along the spine. It is a tool promoted to assess lower back pain in human patients.

Sixty minutes was also spent monitoring each horse in its stall for stereotypic behaviours (addictive-like repetitive behaviours such as head weaving and cribbing), calculated for each horse as the number of stereotypies shown per hour.

The EEG headset used in the study. At right is an example of 8 sec. electroencephalography (EEG) recordings of left (LH) and right (RH) hemispheres obtained with the EEG headset on two horses.
The EEG headset used in the study. At right is an example of 8 sec. electroencephalography (EEG) recordings of left (LH) and right (RH) hemispheres obtained with the EEG headset on two horses.

The researchers reported that the horses had highly consistent individual EEG profiles over time.

Horses assessed as having elevated back tension showed resting-state EEG profiles characterized by more fast (beta and gamma) waves and fewer slow (theta and alpha) waves.

“The proportion of theta waves was particularly negatively correlated with muscular tension along the spine,” they reported.

Elevated back tension was also linked with the frequency of stereotypic behaviours.

Resting-state EEGs appear to be a very promising tool to assess individual subjective reactions to chronic pain, beyond more objective measures of tension.

These results open new lines of research for a multi-species comparative approach and might reveal very important matters in the context of animal welfare, they said.

“The clear correlations observed here between muscular activity along the spine, its asymmetry, neck shape alterations and increased fast (beta and gamma) waves, suggest strongly the involvement of fast waves in the processing of spontaneous chronic pain in this species.

“This involvement of fast waves is concordant with the finding that massage therapy suppresses the beta rhythm in low back pain human patients and with the association between ongoing pain intensity and amplitude of beta and gamma waves in patients suffering chronic back pain.”

The fact that horses showing higher frequency bands in their resting state EEGs were also performing more stereotypic behaviours indirectly confirms that these horses were experiencing pain, they said.

It was possible, they said, that EEG readings could be used for assessing an animal’s subjective state beyond the objective measure of back tension, but more work is needed.

The study team comprised Stomp, Serenella d’Ingeo, Séverine Henry, Clémence Lesimple, Hugo Cousillas and Martine Hausberger.

Stomp M, d’Ingeo S, Henry S, Lesimple C, Cousillas H, Hausberger M (2020) EEG individual power profiles correlate with tension along spine in horses. PLoS ONE 15(12): e0243970.

The study, published under a Creative Commons License, can be read here

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