Tonic-clonic seizures (also known as Grand Mal Seizures, though this term is now discouraged and rarely used in a clinical setting) are a type of generalised seizure affecting the whole brain. Tonic-clonic seizures are the seizure type most commonly associated with epilepsy and seizures in general, though it is a misconception that they are the only type. (See seizure types)
Tonic-clonic seizures are induced deliberately in Electroconvulsive therapy.
The vast majority of generalised seizures are idiopathic.  However, some generalised seizures start as a smaller seizure such as a simple partial seizure or a complex partial seizure and then spread to both hemispheres of the brain. This is called a secondary generalisation.  In the case of idiopathic epilepsy, it is not certain what is the cause of the seizures. However, it is believed that factors could include chemical and neurotransmitter imbalances, and a genetically determined seizure threshold, which have both been implicated. The seizure threshold can be altered by fatigue, malnutrition, lack of sleep or rest, hypertension, stress, diabetes, the presence of neon or laser flashes or lights, rapid motion or flight, blood sugar imbalances, anxiety and other factors. 
In the case of symptomatic epilepsy, it is often determined by MRI or other neuroimaging techniques that there is some degree of damage to a large number of neurons. The lesions (scar tissue) caused by the loss of these neurons can result in groups of neurons episodically firing abnormally, creating a seizure.
- In the aura, the person may feel a sense of strong deja vu, lightheadedness and/or dizziness, unusual (and possibly inappropriate) emotions, altered vision and hearing (which may or may not include hallucinations), and sometimes other symptoms. This is actually a simple partial seizure. Sometimes, the person will lose complete awareness and start making odd or pointless movements (such as picking at clothes) towards the end of the aura, at which point the seizure has progressed to become a complex partial seizure. The aura stage is due to the fact that tonic-clonic seizures often start in an isolated area of the brain, and gradually progress to the whole brain, at which point it becomes a tonic-clonic seizure. An aura may last as little as a few seconds up to several minutes, depending on the person, and some people with epilepsy do not experience auras at all. It should also be noted that in many individuals, not all auras result in a tonic-clonic seizure.
- In the tonic phase, the person will fall unconscious, and the person's muscles will suddenly tense up, often causing the extremities to either be pulled towards the body or rigidly pushed away from it, which will cause the person to fall to the ground if they are standing. The tonic phase is usually the shortest part of the seizure, usually lasting only a few seconds. The person may also express vocalisations sounding like a loud moan during the tonic stage, though this is not always the case.
- In the clonic phase, the person's muscles will start to contract and relax rapidly, causing convulsions. These may range from exaggerated twitches of the arms and legs to violent shaking or vibrating of the stiffened extremities. The person may roll and stretch as the seizure spreads. The eyes typically roll back, the tongue is often bitten, and incontinence is seen in some cases. Post-ictal sleep invariably follows a tonic-clonic seizure. Confusion and amnesia upon awakening is usually experienced.
Many people who experience tonic-clonic seizures will be aware of an oncoming seizure for up to several minutes before the full seizure begins. This is called an aura and is typically a simple partial seizure or a complex partial seizure which has spread to the whole brain. However, many people who have epilepsy do not experience auras. If a person reports they believe they are about to have a seizure, their safety should be ensured. This can be done by laying them into the recovery position, and removing any objects which may pose a danger to the person during the seizure. If the person does not experience an aura, and goes directly into a seizure, they should be gently eased to the ground if possible.
Once the convulsions have begun, the seizure must simply run its course. No attempt to restrain the person should be made because this risks injury to either party, instead it should be ensured that they do not injure themselves by placing something soft under their head, and ensuring their limbs and body don't bump into walls or other objects. If the person vomits, the person's head should be placed to the side to allow the vomit to run out of the mouth without blocking the airway. Nothing should ever be placed into the person's mouth, as this can cause the person to bite their tongue or choke, (or injure the one placing the object into their mouth). Despite popular belief, it is not possible for someone having a seizure to swallow their tongue. The frenulum of the tongue prevents this.
Once the seizure ends, the person will stop convulsing, the limbs will go limp, and the person will be completely unconscious for a while. Once they start to come to, they will usually be tired, disoriented, and unaware they have had a seizure. A person having a seizure should never be left unattended until they are fully recovered.
If the person is known to have epilepsy, it is not usually necessary to call an ambulance. However, if the person is not known to have epilepsy, the seizure lasts four to five minutes or longer, the person has a second seizure before regaining consciousness, or the person injures themselves or stops breathing (apnea) during or after the seizure, medical attention is needed.
Template:Skin and subcutaneous tissue symptoms and signs Template:Nervous and musculoskeletal system symptoms and signs Template:Urinary system symptoms and signs Template:Cognition, perception, emotional state and behaviour symptoms and signs Template:Speech and voice symptoms and signs Template:General symptoms and signs