The horse's expression of pain

A horse can express pain through its facial expressions, and recognizing these signs is essential. If your horse shows signs of pain in its expression, contact a veterinarian. However, the absence of a pain expression does not rule out pain. In such cases, conduct a more detailed assessment of pain-related behavior using the Equine Pain Finder tool.

When evaluating a horse's expression of pain, attention is paid to six different points on the horse's face. Each item is scored on a scale of 0–2. The value 0 means that the part of the facial expression telling about pain is not visible, the value 1 means that the part of the facial expression is partially visible, and the value 2 means that the part of the facial expression is clearly visible.

Back-turned ears:

0 = Ears are relaxed and point forward.
1 = The ears are slightly stiff and slightly turned back. The distance between the ears has increased. The ears can also point in different directions.
2 = The ears are stiff and turned back. The distance between the ears has clearly increased.

Narrowing of the eye:

0 = Eye is fully open.
1 = The eyelid is partially closed.
2 = The eyelid closes the eye more than half or the eye is completely closed. 

Tension above the eye:

0 = The muscles above the eye are relaxed.
1 = The tension of the muscles above the eye makes the shape of the bones below visible more clearly than usual. A triangular wrinkle forms above the eye.
2 = The tension of the muscles above the eye makes the shape of the bones below visible more clearly than usual and the ridge of the temporal bone is clearly visible. A triangular wrinkle forms above the eye. 

Clearly visible masticatory muscles : 

0 = Cheek and masticatory muscles are relaxed.
1 = Tense muscles of mastication are partially visible and increase the tightness visible above the mouth.
2 = Tense masticatory muscles are clearly visible. There is a definite tightness above the mouth.

Tense stubble and a clearly visible chin:

0 = The muzzle is relaxed, the jawline is not accentuated.
1 = The tension of the turva is somewhat visible, the upper lip has pulled back a little and the lower lip, which is a little tense, causes the jaw line to be accentuated.
2 = The tension of the tongue is clearly visible, the upper lip is clearly drawn back and the tense lower lip causes a clear accentuation of the jaw line.  

Flared nostrils and a flattened safety profile:

0 = Nostrils are relaxed, nose shape is normal and lips are Relaxed and round.
1 = Nostrils are tense and slightly dilated. Turva's profile is slightly flattened and the lips look flatter and stretched.
2 = Nostrils are tense and dilated. Turva's profile is flattened and the lips look clearly elongated.

Sources:
Costa, ED, Minero, M., Lebelt, D., Stucke, D., Canali, E., & Leach, MC (2014). Development of the Horse Grimace Scale (HGS) as a Pain Assessment Tool in Horses Undergoing Routine Castration. PLoS ONE, 9(3). https://doi.org/10.1371/journal.pone.0092281

Gleerup, KB, Forkman, B., Lindegaard, C., & Andersen, PH (2014). An equine pain face. Veterinary Anesthesia and Analgesia, 42(1), 103-114. https://doi.org/10.1111/vaa.12212