Large Animal Neurology. Joe Mayhew

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Seizures, abnormal sleep patterns, myotonic episodes, and syncopal episodes can be difficult to distinguish apart. The last of these, occurring in the absence of heart failure, is incredibly rare but the first two are quite distinguishable if observed or preferably captured on video. Particularly because they both more often occur at quiet periods when patients are not being observed, 24‐h video recording can be useful to capture suspected episodes of sleep disorders and epilepsy when they are not otherwise overt. Exposure to stray electric currents can be the cause of unusual repetitive recumbency akin to epilepsy.¹ There are also many nonepileptic disorders imitating generalized idiopathic epilepsy that need to be considered when evaluating patients for repeated motor and hypotonic events. These include movement disorders classified as paroxysmal dyskinesia,² benign myoclonus, hypotension, hyperekplexia, and many metabolic and toxic insults.^3,4 Indeed, even severe Psoroptes ovis infestation (sheep scab) has been seen to induce convulsions in sheep⁵ and Otobius megnini (spinose ear tick) will cause fits in horses,^5b and both of these syndromes resolve with antiparasitic therapy.

      Acquired metabolic derangements including hypomagnesemia, hypocalcemia, hyponatremia, hypernatremia, hepatoencephalopathy, hyperammonemia, and uremia all can result in seizures, as also seen with terminal events in many toxicities.

      Dyskinesias, although not apparently confirmed in large animals, do deserve mention here.² These disorders are differentiated from epileptic seizures by there being no impairment to consciousness and no autonomic or postictal signs. Many of these phenotypic syndromes are yet to be specifically defined, but some involve genetic alterations to proteins involved in transmembrane conductance. Interestingly, the paroxysmal, familial ataxia in lambs (see Chapter 31) fits these criteria,⁶ but as with several inherited spinocerebellar ataxias in humans and dogs this disorder is likely a channelopathy.⁷ The onset of a myotonic episode in patients with myotonia (Chapter 31) can be exceedingly abrupt with whole body rigidity and recumbency that mimics a seizure, although again there is no loss of consciousness.

A seizure, fit, ictus, or convulsion is considered abnormal behavior. Seizures are the physical expression of abnormal, synchronous, electrical discharges spreading through forebrain neurons that reach the somatic and visceral motor areas to initiate spontaneous, paroxysmal, involuntary movements. These cerebral dysrhythmias tend to begin and end abruptly, having a finite duration⁸ and may be very brief. There may or may not be a preictal phase or aura for seconds to hours when the animal is distracted from its environment and is usually restless. The beginning of the ictus may be a localizing finding with one part of the body, usually part of the head, involved in motor activity first. If the seizure begins affecting one side of the face, head, or body, it can be referred to as a focal seizure and may or may not progress further (Figure 6.1). These muscle contractions can spread to the whole body, thus resulting in a generalized seizure with the animal falling to the ground. Initially, the motor event is one of tonic muscle contractions that then proceeds to clonic muscle contractions with the patient showing limb paddling while in lateral recumbency. Based on observations of absent responsiveness and extrapolating from human neurology studies, consciousness is lost in generalized seizures.⁹ Autonomic involvement expressed as urination or defecation may intervene at any time. A postictal phase of poor responsiveness, restlessness, and temporary blindness may last for minutes to hours, although central blindness may be apparent in foals for several days following severe generalized seizures. More than one seizure constitutes epilepsy and this, most often, is not an inherited condition in large animals. More than one seizure over several minutes to hours, without full normalcy between, is referred to as a cluster of seizures, and continuous seizure activity with seizures immediately following each other without an interictal period is status epilepticus that is always given a bad prognosis. Notwithstanding this, several cattle, sheep, and horses have been observed to have generalized tonic and clonic seizures with prolonged periods of having a startled expression, hyper‐responsiveness, and repetitive, jerky face, jaw, head, neck and trunk movements that ultimately resolved without therapy between epileptic attacks.

      The onset of a focal or generalized seizure in large animals often begins with an altered facial expression such as grimacing.

Fortunately, large animals have a relatively high seizure threshold as it seems to take a considerable perturbation to forebrain function to precipitate convulsions. Notwithstanding, seizures often accompany many focal and diffuse acquired diseases of the forebrain, often as part of terminal syndromes.^10,11