Muscle Loss: Atrophy Scoring System for Horses Introduced

Horse owners and veterinarians recognize muscle loss as a sign of disease, a harbinger of old age, and an indication of malnutrition. Medical conditions associated with muscle atrophy include pituitary pars intermedia (PPID), equine motor neuron disease, and myosin heavy chain myopathy. Routine monitoring of musculature in horses susceptible to atrophy is valuable, but no easy, inexpensive method for doing so has been available.

Because of this, researchers at the University of Kentucky set out to create a simple scoring system to assess muscle atrophy, to gauge agreement of scores among evaluators, and to determine if atrophy was more common in horses with PPID.

In creating the muscle atrophy scoring system (MASS), the researchers considered four regions of the horse—neck, back, abdomen, and hind (pelvis and hind limb). The scoring scale ranged from 1 (no atrophy) to 4 (severe atrophy). Researchers then generated two tables for each region, one for “lean” horses and one for “adipose” horses. The tables include descriptions for each score to help guide evaluators.

To test the MASS, three evaluators assessed 38 mares and geldings of various ages and breeds. All horses were tested for PPID through measurement of basal plasma adrenocorticotropic hormone levels and diagnosis based on categories proposed by the Equine Endocrinology Group.^**

Agreement among evaluators proved highest in three regions, with neck, back, and hind in the good to excellent range. According to the researchers, this indicates “good reliability of ratings by individual scorers who received MASS training.” Conversely, agreement among evaluators was considered poor for the abdominal region. Low agreement on abdominal scores was believed to be caused by lack of visual appraisal in this region, as abdominal scores are based solely on subjective palpation.

Horses diagnosed with PPID had higher muscle atrophy scores than others, and these scores were positively correlated to age.

“Muscle atrophy is a well-known clinical sign of PPID,” explained Kathleen Crandell, Ph.D., a nutritionist at Kentucky Equine Research. “Because a higher degree of atrophy was identified in PPID-diagnosed horses, the researchers believe their system has merit as an easy-to-use method to monitor gradual muscle mass.”

Several research groups hypothesize that advanced age and the presence of age-related disorders such as PPID could be associated with impaired antioxidant pathways and oxidative stress.⁺

“If the antioxidant-oxidative stress theory of ageing is accurate, or even partially so, then packing the diet full of antioxidants could slow ageing and minimize the development or severity of age-related disorders,” suggested Crandell.

“Natural sources of antioxidants such as vitamin E include fresh forages. Vitamin E levels in fresh green forages can be as high as 120 mg/kg, which would easily meet the horse’s daily requirement if enough of the forage is consumed. Once the forage is conserved and stored, as in haymaking, however, vitamin E levels drop dramatically over time,” she explained.

Because of this, Crandell suggested supplementation with a research-proven vitamin E product. “Nano-E is a natural, water-soluble source of vitamin E, readily available and absorbed following administration,” she said.

Research has shown that muscle atrophy does not slow in horses receiving pergolide, a common medication prescribed to affected horses to offset other clinical signs.

^*Herbst, A.C., M.G. Johnson, H. Gammons, S.E. Reedy, K.L. Urschel, P.A. Harris, and A.A. Adams. 2022. Development and evaluation of a muscle atrophy scoring system (MASS) for horses. Journal of Equine Veterinary Science 110:103771.

^**Equine Endocrinology Group. 2019. The 2019 EEG recommendations on diagnosis and management of pituitary pars intermedia dysfunction (PPID).

⁺Żak, A., N. Siwińska, E. Chełmecka, B. Bazanow, E. Romuk, A. Adams, A. Niedzwiedz, and D. Stygar.  2020. Effects of advanced age, pituitary pars intermedia dysfunction and insulin dysregulation on serum antioxidant markers in horses. Antioxidants (Basel) 9(5):444.