Simon R. Platt, BVM&S, MRCVS, DACVIM (Neurology), DECVN
Small Animal Medicine & Surgery, College of Veterinary Medicine, University of Georgia, Athens, GA, USA
There are several broad categories of disease or abnormality that should be considered when determining whether a paroxysmal event is a manifestation of a seizure disorder or not. These include neuromuscular disorders leading to collapse (e.g., myasthenia gravis), cardiovascular disease causing syncope, sleep related events such as R.E.M. sleep disorder and narcolepsy/cataplexy as well a new defined group of disorders of involuntary movement that are predominantly breed related. Obsessive compulsive disorders will also be mentioned based on their stereotypical presentations and similarities to the focal sensory seizures described in veterinary medicine. These disorders will be discussed in terms of the classical presenting signs and how they may be considered differently from epileptic disorders. Absolute confirmation of the epileptic nature can only be obtained by observing simultaneously the characteristic EEG changes and physical manifestation of the seizures.
1. Neuromuscular Collapse
Activity associated weakness is the most typical clinical sign of neuromuscular disease. The interpretation of the neurological examination may be challenging in these patients. At the time of examination, they may appear normal or only mildly affected; additionally, if weakness is exhibited, it is rarely specifically indicative of nerve, neuromuscular junction or muscle disease.
In a patient with a neuromuscular disorder, observation and gait analysis may detect ventroflexion of neck, shortstrided gait with overflexion of joints (often more evident in the pelvic limbs), a plantigrade and/or palmigrade stance at rest, and generalised decreased muscle tone.
2. Involuntary Movement Abnormalities
Dyskinesia is defined as impairment of the power of voluntary movements resulting in fragmented or incomplete movements. Dogs reported with these abnormalities may exhibit abnormal postures such as holding up a limb in an attempt to move or adopting a kyphotic posture of the spine without being able to initiate movement. The pathophysiologic mechanisms underlying these movements are poorly understood, but may represent a central neurotransmitter or pathway abnormality, or possibly a local muscular abnormality. The impaired movement can appear as and have been termed muscle 'cramps' which are defined as paroxysmal, prolonged and severe contraction of muscles that may be painful and can be either focal or generalized. Examples of diseases associated with cramps which may be dyskinesias include Scotty Cramp, episodic falling of Cavalier King Charles spaniels, 'epileptoid cramping' of border terriers, and extreme generalized muscular stiffness in male Labrador retrievers. Muscle cramps have also been described secondary to systemic diseases such as hypoadrenocorticism.
Dyskinesias are movement disorders that occur spontaneously during activity or at rest causing involuntary contractions of groups of muscles in a conscious animal. The descriptions of these conditions indicate that the most common clinical sign is that of dystonia causing increased muscle tone in one or several limbs, possibly leading to collapse. The movements can be triggered by excitement or exercise. The localization of the purported functional neurotransmitter based abnormalities responsible for these disorders may be central or peripheral nervous system. In general, movement disorders may have origins in the cerebrocortical neurons, basal nuclei or peripheral nervous system.
Idiopathic Head Tremor/Head Bob
This head tremor syndrome appears to occur without definable cause in some breeds such as the Doberman pinschers (especially dogs less than 1 year of age), boxers and bulldogs; however, a variety of breeds can be affected. These dogs have no other clinical abnormalities and are usually young. Head tremors may be either in an up down or in a side-to-side plane. Sometimes this is referred to as a head bob. Head tremors are usually more prominent when the dog is less active. Also, dogs seem to be able to stop this movement if they desire, are conscious, can walk and can respond to verbal commands. This is almost the opposite of an intention tremor, as the tremor can be stopped when the dog is focused on a goal-orientated task such as eating. The pathogenesis of this disease is not known. In human beings, a nodding of the head can occur with lesions of the thalamus and one of the authors has seen this in a dog with a thalamic lesion. A 'yes' head tremor also may accompany midline cerebellar lesions. Full diagnostic workup (blood work, CSF analysis and imaging of the brain) is normal with the idiopathic condition. There is little information on the most appropriate treatment: although there may be a partial response to antiepileptic drugs, usually they are ineffective. Fortunately, these tremors rarely impact the animal's quality of life.
Paroxysmal dyskinesias are episodes of abnormal involuntary hyperkinetic movement or muscle tone. These events are distinguished from seizures by the presence of a normal consciousness, although an EEG would be necessary to definitively determine this. A movement disorder has been described in young bichon frisé dogs with an extreme variability of frequency and random occurrence. A rapid muscular contraction causes hyperflexion and/or extension of an individual limb. The thoracolumbar spinal column can be affected by altered muscle tone during the event causing a kyphotic posture. A similar condition has also been described in young boxer pups provoked by excitement causing abnormal facial, truncal and limb movements with sustained hyperflexion.
No successful treatment regimens have been described. It remains to be seen whether a genetic disorder confirms these as truly breed-related disorders, as documented below. Several drugs have been reported to cause similar dyskinesias and include phenobarbitone and propofol in dogs. These disorders are usually reversible with drug tapering or withdrawal.
During a syncopal event, the animal usually collapses into lateral recumbency. Stiffening of the limbs, opisthotonic posture, micturition, and vocalization are common, but facial 'spasms,' persistent tonic/clonic motion, defaecation, a prodromal aura, (postictal) dementia, and neurologic deficits are not usually associated with cardiovascular syncope; however, profound hypotension or asystole can cause hypoxic 'convulsive syncope,' with seizure-like activity or twitching. Convulsive syncopal episodes are preceded by loss of muscle tone; however, seizure activity caused by underlying neurologic disease is usually preceded by atypical limb or facial movement or staring spells before the loss of postural tone. 'Presyncope,' where reduced brain perfusion, or substrate delivery, is not severe enough to cause unconsciousness, may appear as transient 'wobbliness' or weakness, especially affecting the pelvic limbs.
4. Narcolepsy/Cataplexy and Sleep Disorders
Narcolepsy is a disorder of sleep/wake control characterized by a tendency to fall asleep during the day, disturbed night-time sleep patterns and cataplexy.
Cataplexy refers to sudden loss of motor tone ranging in severity from a dropped jaw to complete collapse without loss of consciousness and it represents a disorder of rapid eye movement (REM) sleep. Narcolepsy has been reported in many canine breeds, including Doberman pinscher, Labrador retriever, miniature poodle, beagle and dachshund.
The predominant sign in dogs and cats is cataplexy, but excessive daytime sleepiness and fragmented sleep patterns have also been reported. Cataplexy is characterized by paroxysmal attacks of flaccid paralysis without loss of consciousness and may last up to 20 minutes, with a sudden return to normality. The event is not accompanied by faecal or urinary incontinence, salivation or rigidity of muscle groups. The episodes, which may occur multiple times a day, are frequently induced by excitement, such as eating or playing, and they can be reversed by verbal or tactile stimuli. Cataplexy has been recorded in puppies and adult dogs, but usually begins in the first 6 months of life with the establishment of REM sleep.
R.E.M. Sleep Disorders
Normal sleep is divided into 2 stages called non-rapid eye movement, the first stage of sleep lasting 20 minutes and rapid eye movement (REM) sleep. During non-REM sleep, there is a decrease in body temperature, heart rate and respiratory rate and the animals are immobile but retain muscle tone. REM sleep lasts for about 15 minutes during which animals have an increase in body temperature, heart rate and respiratory rate coincident with the eye movements and atonia of the postural muscles. Normal movements seen during this phase can include twitching of the eyelids, face, larynx and paws with occasional rhythmic paddling of all four limbs and yelping.
5. Compulsive Behavioural Disorders
In dogs and cats, behaviors such as “fly biting” and tail chasing have commonly been considered symptomatic for seizure disorders, although treatment with antiepileptic medications may not be successful. These abnormal behaviors in companion animals have also been considered homologous to the stereotypic behavior of livestock and zoo animals. Such behaviors share similarities with human obsessive compulsive disorder (OCD), and have been referred to as OCD or compulsive disorders (CD). Obsessive compulsive behaviors in people include repetitive behaviors, such as hand washing, rituals, checking, arranging/ordering, counting and hoarding, and are accompanied by intrusive thoughts, such as concern of contamination; concern for symmetry; fear of harm; aggressive, religious, or sexual thoughts; or pathologic doubt. Interestingly, the intrusive thoughts (obsessions) and the associated behaviors (compulsions) do not necessarily correspond.