A seizure can last from a few seconds to more than five minutes, at which point it is known as status epilepticus. Most tonic-clonic seizures last less than two or three minutes. Absence seizures are usually around 10 seconds in duration.

There are six main types of generalized seizures: tonic-clonic, tonic, clonic, myoclonic, absence, and atonic seizures. They all involve a loss of consciousness and typically happen without warning.

- Tonic-clonic seizures present with a contraction of the limbs followed by their extension, along with arching of the back for 10–30 seconds. A cry may be heard due to contraction of the chest muscles. The limbs then begin to shake in unison. After the shaking has stopped it may take 10–30 minutes for the person to return to normal.

- Tonic seizures produce constant contractions of the muscles. The person may turn blue if breathing is impaired.

- Clonic seizures involve shaking of the limbs in unison.

- Myoclonic seizures involve spasms of muscles in either a few areas or generalized through the body.

- Absence seizures can be subtle, with only a slight turn of the head or eye blinking. The person often does not fall over and may return to normal right after the seizure ends, though there may also be a period of post-ictal disorientation.

- Atonic seizures involve the loss of muscle activity for greater than one second. This typically occurs bilaterally (on both sides of the body).

The clinical manifestations of absence seizures vary significantly among patients. Impairment of consciousness is the essential symptom, and may be the only clinical symptom, but this can be combined with other manifestations. The hallmark of the absence seizures is abrupt and sudden-onset impairment of consciousness, interruption of ongoing activities, a blank stare, possibly a brief upward rotation of the eyes. If the patient is speaking, speech is slowed or interrupted; if walking, they stand transfixed; if eating, the food will stop on its way to the mouth. Usually, the patient will be unresponsive when addressed. In some cases, attacks are aborted when the patient is called. The attack lasts from a few seconds to half a minute, and evaporates as rapidly as it commenced. Absence seizures generally are not followed by a period of disorientation or lethargy (post-ictal state), in contrast to the majority of seizure disorders.

1. Absence with impairment of consciousness only as per the above description.

2. Absence with mild clonic components. Here the onset of the attack is indistinguishable from the above, but clonic components may occur in the eyelids, at the corner of the mouth, or in other muscle groups which may vary in severity from almost imperceptible movements to generalised myoclonic jerks. Objects held in the hand may be dropped.

3. Absence with atonic components. Here there may be a diminution in tone of muscles subserving posture as well as in the limbs leading to dropping of the head, occasionally slumping of the trunk, dropping of the arms, and relaxation of the grip. Rarely tone is sufficiently diminished to cause this person to fall.

4. Absence with tonic components. Here during the attack tonic muscular contraction may occur, leading to increase in muscle tone which may affect the extensor muscles or the flexor muscles symmetrically or asymmetrically. If the patient is standing, the head may be drawn backward and the trunk may arch. This may lead to retropulsion, which may cause eyelids to twitch rapidly, eyes may jerk upwards or the patients head may rock back and forth slowly, as if nodding. The head may tonically draw to one or another side.

5. Absence with automatisms. Purposeful or quasipurposeful movements occurring in the absence of awareness during an absence attack are frequent and may range from lip licking and swallowing to clothes fumbling or aimless walking. If spoken to, the patient may grunt, and when touched or tickled may rub the site. Automatisms are quite elaborate and may consist of combinations of the above described movements or may be so simple as to be missed by casual observation.

6. Absence with autonomic components. These may be pallor, and less frequently flushing, sweating, dilatation of pupils and incontinence of urine.

Mixed forms of absence frequently occur.

These seizures can happen a few times a day or in some cases hundreds of times a day, to the point that the person cannot concentrate in school or in other situations requiring sustained, concentrated attention.

The most common type (60%) of seizures are convulsive. Of these, one-third begin as generalized seizures from the start, affecting both hemispheres of the brain. Two-thirds begin as focal seizures (which affect one hemisphere of the brain) which may then progress to generalized seizures. The remaining 40% of seizures are non-convulsive. An example of this type is the absence seizure, which presents as a decreased level of consciousness and usually lasts about 10 seconds.

Focal seizures are often preceded by certain experiences, known as auras. They include sensory (visual, hearing, or smell), psychic, autonomic, and motor phenomena. Jerking activity may start in a specific muscle group and spread to surrounding muscle groups in which case it is known as a Jacksonian march. Automatisms may occur, which are non-consciously-generated activities and mostly simple repetitive movements like smacking of the lips or more complex activities such as attempts to pick up something.

There are six main types of generalized seizures: tonic-clonic, tonic, clonic, myoclonic, absence, and atonic seizures. They all involve loss of consciousness and typically happen without warning.

Tonic-clonic seizures occur with a contraction of the limbs followed by their extension along with arching of the back which lasts 10–30 seconds (the tonic phase). A cry may be heard due to contraction of the chest muscles, followed by a shaking of the limbs in unison (clonic phase). Tonic seizures produce constant contractions of the muscles. A person often turns blue as breathing is stopped. In clonic seizures there is shaking of the limbs in unison. After the shaking has stopped it may take 10–30 minutes for the person to return to normal; this period is called the "postictal state" or "postictal phase." Loss of bowel or bladder control may occur during a seizure. The tongue may be bitten at either the tip or on the sides during a seizure. In tonic-clonic seizure, bites to the sides are more common. Tongue bites are also relatively common in psychogenic non-epileptic seizures.

Myoclonic seizures involve spasms of muscles in either a few areas or all over. Absence seizures can be subtle with only a slight turn of the head or eye blinking. The person does not fall over and returns to normal right after it ends. Atonic seizures involve the loss of muscle activity for greater than one second. This typically occurs on both sides of the body.

About 6% of those with epilepsy have seizures that are often triggered by specific events and are known as reflex seizures. Those with reflex epilepsy have seizures that are only triggered by specific stimuli. Common triggers include flashing lights and sudden noises. In certain types of epilepsy, seizures happen more often during sleep, and in other types they occur almost only when sleeping.

Juvenile myoclonic epilepsy (JME) is an idiopathic generalized epilepsy that occurs in patients aged 8 to 20 years. Patients have normal cognition and are otherwise neurologically intact. The most common seizure is myoclonic jerks, although generalized tonic-clonic seizures and absence seizures may occur as well. Myoclonic jerks usually cluster in the early morning after awakening. The EEG reveals generalized 4–6 Hz spike wave discharges or multiple spike discharges. These patients are often first diagnosed when they have their first generalized tonic-clonic seizure later in life, when they experience sleep deprivation (e.g., freshman year in college after staying up late to study for exams). Alcohol withdrawal can also be a major contributing factor in breakthrough seizures, as well. The risk of the tendency to have seizures is lifelong; however, the majority have well-controlled seizures with anticonvulsant medication and avoidance of seizure precipitants.

Tonic–clonic Seizures with repetitive sequences of stiffening and jerking of the extremities.

Myoclonic Seizures with rapid, brief contractions of muscles.

Atonic Seizures with a sudden loss of muscle tone, often resulting in sudden collapse. These are also called drop seizures.

Absence A generalized seizure characterized by staring off and occasionally some orofacial automatisms.

Myoclonic astatic Seizures that involve a myoclonic seizure followed immediately by an atonic seizure. This type of seizure is exclusive to MAE and is one of the defining characteristics of this syndrome.

Tonic Muscle stiffening or rigidity. This seizure is rare in this syndrome.

Frontal lobe epilepsy, usually a symptomatic or cryptogenic localization-related epilepsy, arises from lesions causing seizures that occur in the frontal lobes of the brain. These epilepsies can be difficult to diagnose because the symptoms of seizures can easily be confused with nonepileptic spells and, because of limitations of the EEG, be difficult to "see" with standard scalp EEG.

Juvenile absence epilepsy is an idiopathic generalized epilepsy with later onset than CAE, typically in prepubertal adolescence, with the most frequent seizure type being absence seizures. Generalized tonic-clonic seizures can occur. Often, 3 Hz spike-wave or multiple spike discharges can be seen on EEG. The prognosis is mixed, with some patients going on to a syndrome that is poorly distinguishable from JME.

Generalized seizures can be either absence seizures, myoclonic seizures, clonic seizures, tonic-clonic seizures or atonic seizures.

Generalized seizures occur in various seizure syndromes, including myoclonic epilepsy, familial neonatal convulsions, childhood absence epilepsy, absence epilepsy, infantile spasms (West's syndrome), Juvenile Myoclonic Epilepsy and Lennox-Gastaut syndrome.

These syndromes are childhood absence epilepsy, epilepsy with myoclonic absences, juvenile absence epilepsy and juvenile myoclonic epilepsy. Other proposed syndromes are Jeavons syndrome (eyelid myoclonia with absences), and genetic generalised epilepsy with phantom absences.

These types of seizures are also known to occur to patients suffering with porphyria and can be triggered by stress or other porphyrin-inducing factors.

As of 2017, focal seizures are split into two main categories, "focal onset aware", and "focal onset impaired awareness". What was previously termed a "secondary generalised seizure" is now termed a "focal to bilateral seizure".

In focal onset aware seizures, a small part of one of the lobes may be affected and the person remains conscious. This can often be a precursor to a larger focal onset impaired awareness seizure. When this is the case, the focal aware seizure is usually called an aura.

A focal impaired awareness seizure affects a larger part of the hemisphere and the person may lose consciousness.

If a focal seizure spreads from one hemisphere to the other side of the brain, this will give rise to a "focal to bilateral seizure". The person will become unconscious and may well have a tonic clonic seizure. When people have multiple focal seizures they generally have a condition known as temporal lobe epilepsy. (A generalized seizure is one that involves both sides of the brain from the onset).

Myoclonus can be described as brief jerks of the body; it can involve any part of the body, but it is mostly seen in limbs or facial muscles. The jerks are usually involuntary and can lead to falls. EEG is used to read brain wave activity. Spike activity produced from the brain is usually correlated with brief jerks seen on EMG or excessive muscle artifact. They usually occur without detectable loss of consciousness and may be generalized, regional or focal on the EEG tracing. Myclonus jerks can be epileptic or not epileptic. Epileptic myoclonus is an elementary electroclinical manifestation of epilepsy involving descending neurons, whose spatial (spread) or temporal (self-sustained repetition) amplification can trigger overt epileptic activity.

Focal aware seizures are seizures which affect only a small region of the brain, often the temporal lobes or structures found there such as the hippocampi. People who have focal aware seizures remain conscious. Focal aware seizures often precede larger focal impaired awareness seizures, where the abnormal electrical activity spreads to a larger area of the brain. This can result in a tonic-clonic seizure.

- Presentation

Focal onset aware seizures are a very subjective experience, and the symptoms vary greatly between people. This is due to the varying locations of the brain the seizures originate in e.g.: Rolandic. A focal aware seizure may go unnoticed by others or shrugged off by the sufferer as merely a "funny turn." Focal aware seizures usually start suddenly and are very brief, typically lasting 60 to 120 seconds.

Some common symptoms of a focal onset aware seizure, when the person is awake, are:

- preserved consciousness

- sudden and inexplicable feelings of fear, anger, sadness, happiness or nausea

- sensations of falling or movement

- experiencing of unusual feelings or sensations

- altered sense of hearing, smelling, tasting, seeing, and tactile perception (sensory illusions or hallucinations), or feeling as though the environment is not real (derealization) or dissociation from the environment or self (depersonalization)

- a sense of spatial distortion—things close by may appear to be at a distance

- déjà vu (familiarity) or jamais vu (unfamiliarity)

- laboured speech or inability to speak at all

- usually the event is remembered in detail

When the seizure occurs during sleep, the person will often become semi-conscious and act out a dream they were having while engaging with the real environment as normal. Objects and people usually appear normal or only slightly distorted to them, and will be able to communicate with them on an otherwise normal level.

However, since the person is still acting in the dream-like state from which they woke, they will assimilate any hallucinations or delusions into their communication, often speaking to a hallucinatory person or speaking of events or thoughts normally pertaining to the dream they were having or other hallucination.

While asleep symptoms include:

- onset usually in REM sleep

- dream like state

- appearance of full consciousness

- hallucinations or delusions

- behavior or visions typical in dreams

- ability to engage with the environment and other people as in full consciousness, though often behaving abnormally, erratically, or failing to be coherent

- complete amnesia or assimilating the memory as though it was a normal dream on regaining full consciousness

Although hallucinations may occur during focal aware seizures they are differentiated from psychotic symptoms by the fact that the person is usually aware that the hallucinations are not real.

- Jacksonian march

Jacksonian march or Jacksonian seizure is a phenomenon where a focal aware seizure spreads from the distal part of the limb toward the face (on same side of body). They involve a progression of the location of the seizure in the brain, which leads to a "march" of the motor presentation of symptoms.

Jacksonian seizures are initiated with abnormal electrical activity within the primary motor cortex. They are unique in that they travel through the primary motor cortex in succession, affecting the corresponding muscles, often beginning with the fingers. This is felt as a tingling sensation, or a feeling of waves through the fingers when touched together. It then affects the hand and moves on to more proximal areas on the same side of body. Symptoms often associated with a Jacksonian seizure are sudden head and eye movements, tingling, numbness, smacking of the lips, and sudden muscle contractions. Most of the time any one of these actions can be seen as normal movements, without being associated with the seizure occurring. They occur at no particular moment and last only briefly. They may result in secondary generalized seizure involving both hemispheres. They can also start at the feet, manifesting as tingling or pins and needles, and there are painful cramps in the foot muscles, due to the signals from the brain. Because it is a partial seizure, the postictal state is of normal consciousness .

After the active portion of a seizure (the ictal state) there is typically a period of recovery during which there is confusion, referred to as the postictal period before a normal level of consciousness returns. It usually lasts 3 to 15 minutes but may last for hours. Other common symptoms include feeling tired, headache, difficulty speaking, and abnormal behavior. Psychosis after a seizure is relatively common, occurring in 6–10% of people. Often people do not remember what happened during this time. Localized weakness, known as Todd's paralysis, may also occur after a focal seizure. When it occurs it typically lasts for seconds to minutes but may rarely last for a day or two.

"Focal aware" means that the level of consciousness is not altered during the seizure. In temporal lobe epilepsy, a focal seizure usually causes abnormal sensations only.

These may be:

- Sensations such as déjà vu (a feeling of familiarity), jamais vu (a feeling of unfamiliarity)

- Amnesia; or a single memory or set of memories

- A sudden sense of unprovoked fear and anxiety

- Nausea

- Auditory, visual, olfactory, gustatory, or tactile hallucinations.

- Visual distortions such as macropsia and micropsia

- Dissociation or derealisation

- Synesthesia (stimulation of one sense experienced in a second sense) may transpire.

- Dysphoric or euphoric feelings, fear, anger, and other emotions may also occur. Often, the patient cannot describe the sensations.

Olfactory hallucinations often seem indescribable to patients beyond "pleasant" or "unpleasant".

Focal aware seizures are often called "auras" when they serve as a warning sign of a subsequent seizure. Regardless an "aura" is actually a seizure itself, and such a focal seizure may or may not progress to a focal impaired awareness seizure. People who only experience focal aware seizures may not recognize what they are, nor seek medical care.

Myoclonic jerks that are not epileptic may be due to a nervous system disorder or other metabolic abnormalities that may arise in renal (e.g. hyperuraemia) and liver failure (e.g. high ammonia states).

The onset of seizures is between the ages of 2 and 5. EEG shows regular and irregular bilaterally synchronous 2- to 3-Hz spike-waves and polyspike patterns with a 4- to 7-Hz background. 84% of affected children show normal development prior to seizures; the remainder show moderate psychomotor retardation mainly affecting speech. Boys (74%) are more often affected than girls (Doose and Baier 1987a).

The condition may be difficult to diagnose. The subject may be unaware they have a seizure disorder. To others, the involuntary movements made during sleep may appear no different from those typical of normal sleep.People who have nocturnal seizures may notice unusual conditions upon awakening in the morning, such as a headache, having wet the bed, having bitten the tongue, a bone or joint injury, muscle strains or weakness, fatigue, or lightheadedness. Others may notice unusual mental behaviors consistent with the aftermath of a seizure. Objects near the bed may have been knocked to the floor, or the subject may be surprised to find themselves on the floor.

There are many risks associated with nocturnal seizures including concussion, suffocation and sudden unexpected death (SUDEP).

There are various reflex epilepsies, including:

- Photosensitive epilepsy

- Eating epilepsy

- Reading epilepsy

- Hot water epilepsy

- Music induced seizures

Generalised seizures, particularly myoclonic and tonic-clonic, are the most common type of reflex seizures, though other types of seizures may occur.

The International League Against Epilepsy (ILAE) define an epileptic seizure as "a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain." Epileptic seizures can occur in someone who does not have epilepsy – as a consequence of head injury, drug overdose, toxins, eclampsia or febrile convulsions, for example.

Medically, when used on its own, the term seizure implies an epileptic seizure. The lay use of this word can also include sudden attacks of illness, loss of control, spasm or stroke. Where the physician is uncertain as to the diagnosis, the medical term paroxysmal event and the lay terms spells, funny turns or attacks may be used.

Focal impaired awareness seizures are seizures which impair consciousness to some extent: they alter the person's ability to interact normally with their environment. They usually begin with a focal aware seizure, then spread to a larger portion of the temporal lobe, resulting in impaired consciousness. They may include autonomic and psychic features present in focal aware seizures.

Signs may include:

- Motionless staring

- Automatic movements of the hands or mouth

- Confusion and disorientation

- Altered ability to respond to others, unusual speech

- Transient aphasia (losing ability to speak, read, or comprehend spoken word)

These seizures tend to have a warning or aura before they occur, and when they occur they generally tend to last only 1–2 minutes. It is not uncommon for an individual to be tired or confused for up to 15 minutes after a seizure has occurred, although postictal confusion can last for hours or even days. Though they may not seem harmful, due to the fact that the individual does not normally seize, they can be extremely harmful if the individual is left alone around dangerous objects. For example, if a person with complex partial seizures is driving alone, this can cause them to run into the ditch, or worse, cause an accident involving multiple people. With this type, some people do not even realize they are having a seizure and most of the time their memory from right before or after the seizure is wiped. First-aid is only required if there has been an injury or if this is the first time a person has had a seizure.

A wide array of phenomena may resemble epileptic seizures, which may lead to people who do not have epilepsy being misdiagnosed. Indeed, a significant percentage of people initially diagnosed with epilepsy will later have this revised. In one study, the majority of children referred to a secondary clinic with "fits, faints and funny turns" did not have epilepsy, with syncope (fainting) as the most common alternative. In another study, 39% of children referred to a tertiary epilepsy centre did not have epilepsy, with staring episodes in mentally challenged children as the most common alternative. In adults, the figures are similar, with one study reporting a 26% rate of misdiagnosis.

Differentiation of a non-epileptic attack from an epileptic seizure includes the patient keeping their eyes closed and rarely causing themselves harm (both more common in non-epileptic attacks)

Seizures are purely occipital and primarily manifest with elementary visual hallucinations, blindness or both.

They are usually frequent and diurnal, develop rapidly within seconds and are brief, lasting from a few seconds to 1–3 min, and, rarely, longer.

Elementary visual hallucinations are the most common and characteristic ictal symptoms, and are most likely to be the first and often the only clinical manifestation. They consist mainly of small multicoloured circular patterns that often appear in the periphery of a visual field, becoming larger and multiplying during the course of the seizure, frequently moving horizontally towards the other side.

Other occipital symptoms, such as sensory illusions of ocular movements and ocular pain, tonic deviation of the eyes, eyelid fluttering or repetitive eye closures, may occur at the onset of the seizures or appear after the elementary visual hallucinations. "Deviation of the eyes", often associated with ipsilateral turning of the head, is the most common (in about 70% of cases) nonvisual ictal symptom. It is often associated with ipsilateral turning of the head and usually starts after visual hallucinations, although it may also occur while the hallucinations still persist. It may be mild, but more often it is severe and progresses to hemiconvulsions and secondarily generalised tonic clonic seizures (GTCS). Some children may have seizures of eye deviation from the start without visual hallucinations.

"Forced eyelid closure and eyelid blinking" occur in about 10% of patients, usually at a stage at which consciousness is impaired. They signal an impending secondarily GTCS.

"Ictal blindness", appearing from the start or, less commonly, after other manifestations of occipital seizures, usually lasts for 3–5 min. It can occur alone and be the only ictal event in patients who could, at other times, have visual hallucinations without blindness.

Complex visual hallucinations, visual illusions and other symptoms resulting from more anterior ictal spreading rarely occur from the start. They may terminate in hemiconvulsions or generalised convulsions.

Ictal headache, or mainly orbital pain, may occur and often precedes visual or other ictal occipital symptoms in a small number of patients.

Consciousness is not impaired during the visual symptoms (simple focal seizures), but may be disturbed or lost in the course of the seizure, usually before eye deviation or convulsions.

Occipital seizures of ICOE-G may rarely progress to extra-occipital manifestations, such as hemiparaesthesia. Spread to produce symptoms of temporal lobe involvement is exceptional and may indicate a symptomatic cause.

Post-ictal headache, mainly diffuse, but also severe, unilateral and pulsating, or indistinguishable from migraine headache, occurs in half the patients, in 10% of whom it may be associated with nausea and vomiting.

Circadian distribution: Visual seizures are predominantly diurnal and can occur at any time of the day. Longer seizures, with or without hemi or generalised convulsions, tend to occur either during sleep, causing the patient to wake up, or after awakening. Thus, some children may have numerous diurnal visual seizures and only a few seizures that are exclusively nocturnal or occur on awakening.

Frequency of seizures: If untreated, patients experience frequent and brief visual seizures (often several every day or weekly). However, propagation to other seizure manifestations, such as focal or generalised convulsions, is much less frequent.

A person who suffers from epilepsy regardless of whether it is nocturnal or not, can be categorized into two different types of epilepsy either being generalized, or partial. A generalized epilepsy syndrome is associated with an overall hyperactivity in the brain, where electrical discharges occur all over the brain at once; this syndrome often has a genetic basis. While generalized epilepsy occurs all over the brain, partial epilepsy consists of a regional or localized hyperactivity, which means that the seizures occur conversely in one part of the brain or several parts at once.

Watching an animal have a seizure can be quite frightening. There is not much that can be done during a seizure except to remain calm and not leave the animal alone. If your pet is having a seizure it is important to make sure they are laying down on the floor away from any water, stairs or other animals. When an animal has a seizure, do not try to grab their tongue or clear their mouth as there is a high chance you will be bitten; contrary to popular myth, neither humans nor animals can "swallow their tongue" during a seizure so it is safest to stay well away from their mouth during one. Timing seizures is also crucial. Take notes of seizures - what time they occur, how often and any other specific information which can be passed onto the vet or emergency animal clinic.

All tests apart from the EEG are normal. Video-EEG is the single most important procedure for the diagnosis of eyelid myoclonia with or without absences. It shows frequent high-amplitude 3–6 Hz generalized discharges of mainly polyspikes and waves. These are brief (1–6 s, commonly 2 or 3 s) and they are typically related to eye closure, i.e. they occur immediately (within 0.5–2 s) after closing the eyes in an illuminated recording room.

Eyelid myoclonia of varying severity often occurs during these EEG discharges. Photoparoxysmal discharges induced by photic stimulation occur in all untreated young patients, but may be absent in older patients or those on medication.

Sleep EEG patterns are normal and generalized discharges are more likely to increase during sleep, but may also decrease. The EEG and clinical manifestations deteriorate consistently after awakening. A normal EEG is rare, even in well-controlled patients.