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The initial clinical manifestations of generalized-onset seizures indicate more than minimal involvement of both cerebral hemispheres. The motor manifestations begin bilaterally and are often symmetrical. An alteration in consciousness frequently occurs at some stage during ictus. The term ‘primary generalized’ has sometimes been used in the veterinary literature to describe generalized-onset seizures and to differentiate them from secondarily generalized focal (partial) seizures. In this case the term primary does not refer to seizure aetiology, but to the fact that the initial clinical manifestations reflect involvement of both cerebral hemispheres simultaneously from the onset of the seizure.

GENERALISED-ONSET TONIC-CLONIC SEIZURE. The most common type of generalized-onset seizure in dogs is the tonic-clonic seizure (formerly called grand mal seizure) (Berendt and Gram, 1999; Licht et al., 2002). A prodromal phase and aura are not always recognized by the owner. Their duration and clinical manifestations are variable. The prodromal phase can last hours to days and it is commonly characterized by restlessness, anxiety or reluctance to perform normal activities. The aura may manifest as increased or decreased attention seeking, stereotypical sensory or motor behaviour (e.g. licking, pacing) or autonomic manifestations (e.g. salivating, vomiting, urinating). The ictal phase of a tonic-clonic seizure is characterized by sustained contraction of all muscles resulting in rigid extension of the limbs and opisthotonos, usually lasting 10 to 60 s (tonic phase). The animal falls on its side and often loses consciousness (defined as attentiveness or responsiveness to the owner and other external stimuli). Breathing is often irregular or absent, and cyanosis is common. The tonic phase is followed by the clonic phase, which is characterized by rhythmic muscular contractions resulting in uncoordinated, purposeless, jerking movements of the limbs and chewing movements. Automatisms such as running or paddling movements of the limbs commonly occur. The clonic phase may alternate with tonic activity. Autonomic manifestations, such as hypersalivation, urination, defecation and mydriasis are common in dogs with generalized-onset tonic-clonic seizures although not a constant feature. The ictus usually lasts 1 to 2 min. The post-ictal phase duration and clinical manifestations are variable. The dog may rest and rapidly return to normal activity or may show confusion, disorientation, aggressive behaviour, restlessness, pacing, lethargy, deep sleep, hunger, thirst, defecation, urination, ataxia, proprioceptive deficits, and decreased or absent menace response with or without actual blindness, which can persist for 24 h or longer. The post-ictal phase duration and severity of clinical manifestations may be unrelated to the duration and severity of the ictus.

Table 3.2. Seizure classification based on clinical manifestations.

Generalized-onset seizures	Tonic-clonic
	Tonic
	Clonic
	Myoclonic
	Atonic
	Absence
Focal-onset seizures	Motor
	Autonomic
	Sensory
	Simple (without impairment of consciousness/awareness)
	Complex (with impairment of consciousness/awareness)
	Secondarily generalized (evolving to a bilateral tonic, clonic or tonic-clonic seizure)

Tonic-clonic seizures also have been reported as the most common type of generalized-onset seizures in cats (Schwartz-Porsche and Kaiser, 1989; Schriefl et al., 2008). During the pre-ictal phase cats may display aggressiveness (hissing and growling), vocalization (including growling and crying), restlessness (running around erratically), anxiety, hiding or increased affection (seeking refuge with the owner), or may act as fearful. The clinical manifestations of the ictus can be violent and sometimes result in self-inflicted trauma such as excoriations, contusions, avulsion of nails and biting of the tongue (Schwartz-Porsche and Kaiser, 1989; Quesnel et al., 1997). Cats may be propelled forward, up in the air and from side to side. Facial muscle twitching is often observed before or after the tonic phase of the seizure. Autonomic manifestations such as mydriasis, salivation, piloerection and urination and occasionally defecation can also occur (Quesnel et al., 1997). The ictal phase usually lasts 30 s to 2 min. The post-ictal phase is similar to dogs, lasting minutes to several hours or days and is characterized by restlessness, disorientation, aimless wandering, thirst and hunger. Both cats and dogs may sleep for a few hours after the termination of the seizure.

GENERALIZED-ONSET TONIC SEIZURES. Tonic seizures are characterized by sustained increase in muscle contraction without clonic motor activity. Impairment of consciousness and autonomic manifestations can occur. The pre- and post-ictal phases are similar to those described for generalized-onset tonic-clonic seizure.

GENERALIZED-ONSET CLONIC SEIZURES. Clonic seizures are characterized by regularly repetitive, sudden, brief, involuntary contractions, which involve the same muscle groups, and are prolonged. Impairment of consciousness and autonomic manifestations can occur. The pre- and post-ictal phases are similar to those described for generalized-onset tonic-clonic seizure.

Tonic or clonic seizures alone have been reported uncommonly in dogs and cats (Schwartz-Porsche and Kaiser, 1989; Heynold et al., 1997; Licht et al., 2002, 2007).

GENERALIZED-ONSET MYOCLONIC SEIZURES. Myoclonic seizures are characterized by sudden, brief, involuntary, shock-like contractions that can be generalized or confined to individual muscles or muscle groups (e.g. face, trunk, one extremity).

This type of seizure has been reported in the miniature wire-haired dachshund, beagle and basset hound dogs in association with Lafora disease (Jian et al., 1990; Fitzmaurice et al., 2001; Gredal et al., 2003). The clinical presentation is characterized by repetitive, brief myoclonic jerking of the head, neck and thoracic limbs that are frequently strong enough to cause the animal to fall backward into a sitting or lying position (Davis et al., 1990; Fitzmaurice et al., 2001; Schoeman et al., 2002). Myoclonic seizures may occur spontaneously or in response to visual (including light), tactile or auditory stimuli (Fitzmaurice et al., 2001; Webb et al., 2009). Myoclonic seizures have been reported also in cats (Schwartz-Porsche, 1989). Not all myoclonic jerks are seizures, as they can result from other causes.

GENERALIZED-ONSET ATONIC SEIZURES. Atonic seizures are characterized by sudden loss of postural tone of the head, one limb, or of the entire body usually lasting 1, 2 or more seconds. Consciousness may be lost. This type of seizure has been reported in dogs (Podell, 2004) and needs to be differentiated from narcolepsy/cataplexy and syncope (see Table 9.1, Chapter 9). Generalized seizures characterized by collapse, loss of consciousness and minimal limb movements have been reported in a cat with structural epilepsy (Barnes et al., 2004) and may represent a form of atonic seizure activity.

GENERALIZED-ONSET ABSENCE SEIZURES. Absence seizures (formerly called petit mal seizures) are characterized by a transient and brief impairment of consciousness associated with a characteristic EEG pattern (2.5- to 4-Hz spike-and-wave complexes) (see Fig. 11.12, Chapter 11). Absence seizures have been clinically described as a transient cessation of activity with staring, unresponsiveness and ‘blanking out’ episodes. Absence seizures with myoclonic features have been reported in an 8-month-old male Chihuahua that presented with recurrent episodes of head and nose twitching associated with intermittent hind-limb jerking and suspected staring for a few seconds (Poma et al., 2010). Video-EEG documented multiple staring episodes either alone or in association with head and/or nose myoclonic jerks associated with generalized bilaterally synchronous 4 Hz spike-and-wave complexes (Poma et al., 2010). Absence seizures may occur also in cats, but they have not been reported as such or supported by EEG findings. One feline study reported a ‘mild’ form of generalized seizure activity characterized by transient cessation of activity, impaired consciousness, and bilateral facial muscle twitching for a few to several seconds (Quesnel et al., 1997). These may have been a form of absence seizures in cats. Unless absence seizures are frequent, associated with some motor manifestations or the pet-owner is very observant, they go unrecognized. Video-EEG monitoring is very helpful in the diagnosis of this type of seizure.