Vanishing, or poorly developed, toplines, is a very common complaint of owners. Successfully addressing it depends on finding the cause.
The obvious sign of a poor topline is prominence of the spinal vertebrae along the back. The tissues along the spine in the back should at least be even with the tips of the bones. The sacrum, at the highest point of the rump, should also be covered, as should the wings of the pelvis behind the flanks.
Muscle forms part of the tissues around these bones, but there is also a generous upper layer of fat both along the back and over the rump. The first cause of diminished coverage to be ruled out is weight loss. This is done by observing the body as a whole for weight loss. In particular, are the ribs less covered by fat? (There is no muscular coverage on the ribs.) Does the neck look thinner? If the horse has obviously lost weight in general, this probably accounts for the changes in the topline as well. A calorie count, evaluation of the digestibility of the diet and, if necessary, check for underlying disease states will locate the problem.
If weight loss in general is not the issue, decrease in the muscular mass is the problem. Reduced exercise may be part of the cause but is usually not enough to cause the bone to protrude unless the horse is very thin to begin with, with one exception.
Older horses can have several reasons for loss of topline. Sarcopenia of aging is loss in muscle mass as part of the aging process. This is one instance where exercise is actually the only treatment and lack of it the main cause. It can prevent and reverse sarcopenia of aging. “Long and low” exercise with the neck gently flexed and back arched is the most effective in rebuilding topline.
Poor topline development, and loss of muscle mass overall, may be related to Cushing’s disease, a benign pituitary growth with over production of hormones, including ACTH which controls cortisol secretion. High cortisol causes poor utilization of glucose and breakdown of muscle tissue. To reverse this process, the horse needs to be started on pergolide.
Poor utilization of protein in the diet can come from poor chewing. This may occur in older horses even if a dental exam doesn’t show any particular problem. The solution here is to provide pellets or cubes rather than coarse hay. Poor utilization can also be related to insufficient intake of L-lysine, a critical amino acid. Supplementing with 10 to 20 grams/day of L-lysine for the average size adult horse will show results in a few weeks if this is part of the problem.
Diseases such as equine motor neuron disease and polysaccharide storage myopathy may have loss of topline definition but this is rarely the only symptom. If you suspect your horse’s loss of topline is part of a larger problem get a veterinary evaluation. Muscle biopsy is the best tool for sorting through these other possibilities.
Eleanor Kellon, VMD