It is tempting for veterinarians to use laboratory reference ranges collected from full-sized horses when treating miniature horses. This is not always appropriate, however, as miniature horses have some breed characteristics that are only now becoming apparent. In fact, researchers at Michigan State University (MSU) have been diagnosing ocular abnormalities in miniature horses more frequently over the past few years. Therefore, Caryn Plummer, DVM, a graduate student at Purdue University, helped design a study to determine normal ranges of ocular characteristics in healthy miniature horses and compare them to full-sized horses. These variables included corneal thickness, horizontal and vertical corneal diameter (HCD, VCD), intraocular pressure (IOP), and others.

There was no difference in corneal thickness between miniature and full-sized horses. Also, there was no effect of aging on corneal thickness of miniature horses, and IOP of miniature horses fell within the normal range for full-sized horses. “It was not a real surprise to us that the values between the two were similar given that both minis and full-sized horses are equids,” Plummer says. “They have very similar anatomy, even though on a different size scale.” This means that diseases relating to corneal thickness are now managed similarly between these two types of animals at MSU.

The only differences that were identified involved HCD and VCD. In full-sized horses, HCD increases with age. This accounts for the fact that young foals have a rounded eye and adult horses have an oval-shaped eye. This eye shape might give adult horses a larger horizontal visual field, allowing them to detect approaching predators and threats to safety. Results from this study indicate that miniature horses have the same age-related increase in HCD. However, maximal HCD is attained by four to five years in full-sized horses, compared to seven years in miniature horses. Plummer speculates that this might relate to the broad size range in minis. Although full-sized light horses also vary quite a bit in size, there are published normal ranges for HCD available in the literature for full-sized light horses that have been based on large-scale samples. “It is difficult to assess at which point the diameter is officially completely mature (in minis),” she explains, “especially since the size of the eye overall varies with the size of the individual animal.” Interestingly, VCD doesn’t reach maximum until at least 22 years in full-sized horses, versus five years in miniature horses. “This may be due to the fact that minis tend to become physically mature overall more quickly than full-sized animals, but this is purely speculation.”

Plummer is continuing her career in equine research at Purdue University. She is currently compiling her research from her time at MSU and hopes to publish it soon