It can be a very frightening experience if you are out with your dog and it suddenly collapses. Fainting (also known as syncope) does occur in dogs but is less common than in people. Just like in people when a dog faints, it briefly loses consciousness and falls to the ground motionless but in most cases recovers within a few moments without treatment. Fainting is due to a temporary cut off of oxygen supply to the brain and can be due to problems with the airways or the heart. There are also lots of other causes of collapse in animals and it is essential to give your vet as good a description as you can of the event to help them to get an idea of what might be causing the problem.
It is important, but often difficult, to differentiate between collapse and fitting because the causes and treatments for the two conditions are very different. In addition, some other medical problems (for example, reduced blood levels of glucose, or certain diseases of the nerves and muscles) can cause episodes of weakness or collapse. If your dog collapses for whatever reason you should contact your vet immediately for further advice.