Yes, that's exactly the correct term. The sweat patch itself is not the myotome, but it occurs within a myotome.
A myotome is a developmental division in the embryo. Vertebrate embryos are segmented. A myotome is one of these segments, a developmental unit with its own innervation, embryonic 'stem' cells which will become muscles, as well as other tissues.
It is also correct to think that the localized sweating is due to hyperstimulation of the nerves in that myotome. Sweat glands are innervated; when the nerve branchlet that serves a given sweat gland is firing, the gland is stimulated to produce sweat.
There is also an endocrine component to this. The interaction between the nervous and endocrine systems is complex. On a practical basis, your osteopath is right on -- a good way to explain it as well as being technically correct.
To see myotomes in the horse, you can look 'em up in Goody or Sisson and Grossmann -- there are good illustrations of what the myotome divisions look like in the adult animal. The boundaries between these divisions in the adult are completely invisible but nonetheless, stimulation of individual nerve-roots coming out of the spine shows where they are, and it is on that basis that the illustration was produced. -- Dr. Deb
I too have a TB gelding with a localized sweat spot on his near side hip. I have had him for 8 years and it only started about 6 months ago. It is 90% there all the time. When I get him in to work, take his rug off and it is always there, rest of the body fine, no sweat, even in cold weather. Have tried chiro and Contact Care with no change. I've been told "it's just one of those things." But you get paranoid. He is a show jumper and also hunts. He will hump in the canter especially when in my arena at the canter. Work him in slowly. Haven't been able to find any sort of result yet.