It is life threatening. My horse was tested for it a few months ago, and the vet came the same day as the symptoms appeared. It's that serious.
Here's what we have on another site that I belong to.
Hyperlipidemia
Hypertriglyceridemia can occur in any pony, at any time. Ponies that have their diet drastically restricted can develop it, as can ponies that go off feed for any reason at all and pregnant ponies (probably because of the worsening insulin resistsance with pregnancy).
Many old type pony breeds are inherently already insulin resistant. One of insulin's functions is to control the release of fat from fat stores. As a survival mechanism though, the pony is "programmed" by nature and evolution to easily mobilize fats for energy during times of poor food supply and is less sensitive to the suppressive effects of insulin on fat mobilization. This serves the pony well in the wild.
Problems come up when we domesticate the pony and feed it a diet higher in simple carbs, including sugary grasses, than it would normally eat roaming around on a Scottish mountainside. In the research setting, feeding high levels of simple carbs to an animal with insulin resistance makes the insulin resistance much worse. This is probably what happens with domesticated ponies. With specific
reference to the hypertriglyceridemia, you now have a situation where the fat is even less responsive to insulin that its normally poorly responsive state so anything that further lowers the sensitivity to insulin puts the pony at very high risk of hypertriglyceridemia.
The liver damage you are worried about comes after the hypertriglyceridemia, not before, although the liver can be slowly loading up with fat over a long period of time before any obvious crisis situation occurs. This may or may not elevate liver enzymes.
Carefully controlled glucose infusions are sometimes effective in emergency situations because it can reverse the "starvation" triggers somehow. Getting the pony to eat again is also effective (and safer) and since your pony has started eating you shouldn't give up on her yet.
Management is the same as for any case but the pony that tends toward hypertriglyceridemia is very fragile. Critically important that she never go long without eating and always have plenty of palatable high fiber, low NFC feeds to choose from. Also important to make sure she is able to efficiently chew, and therefore digest, everything she is getting.