Impaction in leopard geckos, or other lizards, generally refers to gut Impaction. There are other kinds of impactions, hemipene, and mouth impactions being a couple. Gut impaction, however, is serious and all too common in leopard geckos, though it can be easily treated when caught in time, and easily prevented with the right husbandry and care for the animal. Many times people think "gut impaction", and immediately think that sand or other loose substrates are the only thing to blame. Though it is a big part of it in many cases, loose substrates are not the only thing that causes this issue. I will be discussing a couple more large issues involving impaction, along with loose substrates.
Typically, a leopard gecko should not be fed a food item larger than the space between it's eyes. Worms can be slightly larger, however, because their thin or soft bodies. Some also insist on feeding their leopard geckos pinky, fuzzy, or even hopper mice to their leo. Prey size is an issue with impaction. A prey item that is too large can get lodged in their system while digesting, much less multiple prey items of a similar size. Mice should be avoided entirely, due to inability to digest bone and cartilage matter. They are insectivores and made to digest/pass chitin and bug exoskeleton, their diet should never consist of small mammals. Mice are also very dangerous and unhealthy in other ways that I will not discuss here, as this topic is not the place to do so.
Temperatures are a HUGE issue with impaction, and is one of the main reaons why loose substrates can be so dangerous. Remember, leopard geckos are reptiles and are ectothermic; they are cold-blooded. They rely on heat to maintain a stable metabolic rate, and to properly digest food. Not only do they rely on heat, they rely on belly heat, not air temperatures, not overhead lighting. They are nocturnal and even overhead night bulbs are not reccommended for primary heat, as they would normally lie on rock beds to obtain heat the rock had absorbed throughout the day. An under tank heater (preferrably not zoo med brand, which are unreliable and tend to vary in temoerature despite being thermally controlled) is best at achieving this. Make sure it is on a thermostat, rheostat, or normal household light dimmer to control the temperatures. Belly heat is the best at aiding in digestion. Exact temperatures are important, DO NOT GUESS!!! They need to be read with a temperature gun, or a digital thermometer. 83-85*F on the cool side, 93-95*F on the warm. Again, temperatures that are set properly will aid in digestion and help reduce the risk of impaction.
Proper hydration is also key in achieving healthy bowel movements as well. If your gecko is not properly hydrated, the body will attempt to get the moisture it needs from the poo that's in it's system. As the moisture gets sucked from the poo, it becomes drier, and tougher to pass. As odd as it sounds, you will need to check it's poo often. The urates (white part) should be at least 95% white. 100% white is even better! If it is yellow, your gecko needs water. If it is orange, your gecko is very dehydrated and the issue needs to be resolved ASAP.
The loose substrate issue is a very touchy subject, even still, as it has been argued for years now. We used to think sand, or other loose substrates were actually good for them. Then, an entire anti-loose substrate epidemic had spread throughout the web. Now, we have come to a compromise. Only those very experienced keepers who knows the risks and can take the major extra precaution in using loose substrate should ever use it. Even the people who use loose substrates only typically use one or two kinds; Washed, sifted play sand or a washed/sifted play sand soil mix. Even people who have had years of loose substrate experience knows there are many other substrates out there than sand that would prove much more risky (examples: CALCI/VITA-SAND, CRUSHED WALNUT SHELL). There are many other issues with loose substrates, to be discussed elsewhere.
For the beginner-moderate owners, I would suggest Slate or ceramic tiles (very natural looking, especially because they don't live on sand in the wild, anyway) repti-carpet, non stick shelf liner, or unprinted newspaper works fine. The most natural looking would be the slate/tiles, or a brown colored repticarpet.A few Signs your gecko may have gut impaction:
Distended, hard belly
Abnormally large, dark spots on belly (do not mistake for other organs)
Twitching, low usage of, or total paralysis of back legs
Little to no appetite
Difficulty passing stool
Abnormally small, hard, dry poos
Poos with nothing but substrate in them
No poo at all.
Here are a few things you can do if your gecko has Impaction:
Remove all loose substrates, replace with acceptable ones
Baths in lukewarm water 15-20 minutes daily + belly massage
2-3 drops of Olive oil (NOT mineral oil) on nose, let them lick it off
Raising warm side temps to 95-97*F
And if the case is severe, a vet visit is in order.
Metabolic Bone disease (MBD)