I have had a client who questioned the appearance of blisters after riding, which then disappear again after about 20 minutes. I suggest the following might be happening. The spinal vertebrae have three processes. Between the spinal processes, you will find the spinal dorsal ligament system . There should never be any pressure on the spinal vertebral processes, however, if you have a saddle which has a too narrow chamber and has been lying on the spine itself, probably at least one of the 7 layers of skin has been sheared / separated, . When the skin tries to heal itself it creates a fluid which results in a blister. If you continue to ride with this saddle which doesn’t fit properly, the fluid will continue to grow the blister, and actually ‘bleed’ into the spinal area, which can be dangerous. In its worst case, this can lead to spondylosis, which is defined as a term referring to degenerative arthrosis of the joints between the centres of the spinal vertebrae and/or neural foraminae. (taken from wikipedia). In other words, the neural canal can be severely impacted to the point of complete breakdown, resulting in gait dysfunction and loss of balance.
Once you start riding in a properly fitted saddle with a wide enough gullet chamber (3-4 fingers width) which is supported by the muscles along the side of the spinal column instead of being directly on the vertebra; the skin irritation can develop and release in the form of a visible blister to the outside, rather than to the inside of the horse’s back. This is of course somewhat disconcerting to the rider, however, the healing process is working and can take from 4-6 weeks to complete itself. While my client was dismayed to think that she was causing her horse such pain, she recognized that this was something they would both have to go through until he was healed enough to really enjoy working under his new saddle!
Jochen Schleese, CMS