Download THE MIMICS OF SEIZURE ACTIVITY NEUROLOGY Simon Platt

THE MIMICS OF SEIZURE ACTIVITY
Simon Platt, BVM&S, MRCVS, DACVIM (Neurology), DECVN
NEUROLOGY
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. These include neuromuscular disorders leading to collapse
(e.g., myasthenia gravis), cardiovascular disease causing syncope, sleep-related events such as REM sleep disorder
and narcolepsy/cataplexy, and a newly 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.
(A) 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 generalized decreased muscle tone.
(B) Involuntary Movement Abnormalities
Paroxysmal events are characterized by the sudden and reversible onset of neurological dysfunction in an otherwise
normal animal. Some movement disorders can be paroxysmal. The animals do not lose consciousness and rarely
have a structural lesion identifiable within the central nervous system (CNS). The underlying cause of many of these
events may be a functional abnormality related to neurotransmitter imbalances or receptor abnormalities and
dysfunction. Several stereotypical events have been described in specific breeds and are discussed below.
Confirmation of the specific syndrome is difficult or impossible in the clinical setting but depends heavily on the
exclusion of structural CNS abnormalities such as neoplasia, inflammation, and cerebrovascular disease.
Dyskinesia
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 that 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 or 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 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-oriented 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 this has
also been seen in dogs. A “yes” head tremor also may accompany midline cerebellar lesions. Full diagnostic workup
(blood work, cerebrospinal fluid [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
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 frise 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 described in young boxer pups is provoked by excitement and
characterized by 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, including phenobarbitone and propofol in dogs. These disorders are usually reversible with drug
tapering or withdrawal.
(C) Syncope
The term “syncope,” from the Greek for “cutting short,” refers to an abrupt and transient loss of consciousness
accompanied by loss of muscular tone. It is usually caused by a sudden, global reduction in cerebral perfusion, and
clinical recovery occurs with restoration of normal cerebral blood flow. The very transience of this syndrome and
the variety of medical disorders that can cause or mimic it are at the core of the diagnostic problems that the
neurologist faces. The term “fainting” is often used synonymously with “syncope” and captures the essential criteria
of the collapse—loss of consciousness and muscle tone.
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, defecation, 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, whereas 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.
(D) 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 pinschers, Labrador
retrievers, miniature poodles, beagles, and dachshunds.
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
fecal 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 six months of life with
the establishment of REM sleep.
REM Sleep Disorders
Normal sleep is divided into two stages, called non-rapid eye movement (non-REM) sleep, which is the first stage of
sleep and lasts about 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 animal is immobile but retains 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.
(E) Compulsive Behavioral Disorders
In dogs and cats, behaviors such as “fly biting” and tail chasing have commonly been considered symptomatic for
seizure disorders, although treatment with anti-epileptic 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 and ordering, counting, and hoarding and are
accompanied by intrusive thoughts, such as concern about contamination; concern about 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.
The extent of the similarities between the human and canine conditions is not yet known. One similarity is that,
overall, the behaviors in companion animals are amenable to the same pharmacologic treatment as are obsessions
and compulsions in people. However, there are differences between the human and canine conditions.