Thanks for your patience. The first thing that sprang to mind reading your question was actually a condition called narcolepsy - you've probably heard of this in people where there is a sudden and unexpected need for the patient to go to sleep and they often recover within a few minutes. If a horses collapses and lies down to go to sleep, this suggests narcolpesy rather than a seizure or cardiac problem. Narcolepsy is the classic cause of a horse flopping to the floor, as opposed to a fit or seizure. With a fit, the affected animal is likely to be stiff and rigid with involuntary twitches and paddling of the limbs.

It seems to be more common in Shetland and miniature foals (called the fainting disease) and Welsh ponies, as well as the Thoroughbred, Quarter Horse, Morgan, Appaloosa, and Standardbreds - which as a dressage horse she may have some of these breed components?

With narcolepsy, the horse will often quickly lie down or even collapse, or sometimes excessive drowsiness and head droop. Horses may buckle at the fetlock and knees and stumble. Sometimes they will sink to the ground, but others will appear to wake up and regain their feet. They may have difficulty moving and are hard to lead because they are unco-ordinated, as if they are sleepwalking.

These episodes can occur rarely or frequently; indeed, with some horses they are seen several times a day and the horse often seems to need to lie still and sleep before they can get up again. Episodes may last from a few seconds to about 10minutes and a lot of horses that have it generally have episodes of narcolepsy following routine daily events, such as grooming, tacking up or even exercise - which could be the specific trigger with your horse - the lunging. It is rare for it to happen while a horse is being ridden, but the risk is there and I would not want to ride a known narcoleptic.

The prognosis for narcolepsy varies. Horses which develop the disease as adults are likely to continue having collapsing episodes. There are medical treatment options, but any medication is a double-edged sword that may have damaging side effects as well as possible benefits and I'm not sure what these options are in the US. I would get your vet to come and look at her first of all, check her heart and circulation, and also coordination when moving etc. You then might hopefully be able to induce an episode for your vet to see by doing some lunging with her. Checking blood levels for chemistry and also red cell numbers should help to rule out other possible causes too.

If this is narcolepsy, it may be that it can be managed by taking away the trigger, so actually never lunging - but there is still a risk of accidents if she does it whilst being ridden or even collapses in the stable or while you are leading her.

I hope this helps,

Jen