Approximately fifteen percent of dogs are naturally able to predict seizures before they occur. On average, these predictions or alerts are made 10-20 minutes before the seizure, giving the person with the seizure disorder an opportunity to move to a safe place, take medication, call for help, or notify friends or family of the impending seizure so they can be checked later.
Other dogs, called seizure response dogs, are trained to perform tasks during or following a seizure to assist the owner. These tasks may include rolling the person to create an open airway, clearing vomitus from the mouth, getting help, operating a call button or k9 phone, blocking the person with postictal disorientation from stairs and intersections, helping the person to rise, helping with postictal balance issues, guiding the disoriented person to a preset location or person, et cetera. Because these response tasks are so useful, most seizure alert dogs are also trained in response work. These dual seizure dogs are called "seizure alert/response dogs" or simply "seizure dogs."
No one knows for certain how these dogs are able to detect when a seizure is about to occur. Two popular theories are (1) that the dog is able to smell minute changes in the person's biochemistry, possibly in their blood chemistry or (2) that they are able to detect fine motor changes imperceptible to human eyes. Regardless of how precisely they do it, this behavior is a natural one that occurs in some dogs and not others. It is not possible to train a dog to predict seizures without being able to accurately predict or create the seizures for training. While some attempt to "train" alerts by exposing the dog to sweat from a person who had a seizure or by focusing the dog's attention on a person during or after a seizure, these are not actually training. The event that you want the dog to respond to has already happened 10-20 minutes previously and we know that for conditioning reinforcement to be effective, it must be delivered within seconds of the desired behavior. While some report success with these methods, what is probably happening is that they are simply identifying dogs who already had it in themselves to alert.
It may take a dog with this innate ability up to six months living with a person with a seizure disorder to begin displaying the ability. Programs that place seizure response dogs have reported that about half of the response dogs they place go on to alert as well within six months of placement.