Large Animal Neurology. Joe Mayhew

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

Figure 6.1 Most cases of seizures and epilepsy in large animals are acquired. Some accompany inherited neurologic disorders (Chapter 31) and others are associated with metabolic problems including electrolyte disturbances and liver disease, and with some specific toxicities such as Tutu (Coriaria arborea) and metaldehyde poisoning. In neonates, seizures can accompany birth asphyxia in calves (A) and the hypoxic–ischemic encephalopathy syndrome in foals (B). The seizures in these two cases were heralded by tonic movements of the head and neck that were symmetric in the calf (A) and asymmetric in the foal (B), prior to generalizing to tonic and clonic contractions of the whole body. Focal seizures, and mild generalized seizures, where the patient does not lose consciousness to become recumbent, may be restricted only to the face and head as shown by the early ictal movements in two other foals (C and D). The consequences of generalized seizures are seen in a horse (E) that had epilepsy and focal seizures with secondary generalization causing self‐inflicted head trauma.

      Neonatal animals, particularly foals, convulse more readily than adults, and foals frequently demonstrate mild generalized seizures seen as periods of jaw chomping (“chewing‐gum fit”),¹² tachypnea, tremor and grimacing of facial muscles, and jerky head movements without evolving from a focal seizure. Epilepsy may occur in conjunction with other signs of forebrain disease persistent during the interictal period. These may be quite subtle and consist of degrees of partial blindness seen as an asymmetric menace response, partial facial hypalgesia seen as asymmetric reactions to touching the sides of the nasal septum, an asymmetric hopping response on the thoracic limbs, and a tendency to drift to one side when blindfolded and verbally coaxed, but not led, to walk straight forward.

      Evidence of prior self‐inflicted trauma over bony prominences or on the tongue or gums as seen here can herald the onset of epilepsy as seen by an upper lip lesion in this foal.

      Large animals, especially horses, often become violent in response to painful processes and when attempting to regain their footing. These intermittent and sometimes continuous episodes of struggling and thrashing are difficult to distinguish from convulsions. Secondary head trauma often ensues, usually adding to the delirious state. Primary processes that fall in this category include colic, myopathy, vertebral injury, long bone fractures, and acute spinal cord, brainstem, and particularly vestibular disease.

      Because suspected epilepsy does appear to be more frequently a presenting problem in horses, and some attempts have been made to classify these,^13,14 a few specific comments are warranted for that species. Repeated generalized seizures that persist, with no active underlying disease process identified, and occurring in families of purebred animals, are true or idiopathic epilepsy. This does not appear to have been demonstrated in adult horses, although it may have been seen in some breeds of cattle.¹⁵ However, a patient that has had one seizure from any cause is more likely to have another—a process referred to as kindling. Thus, epilepsy beginning in adulthood in horses can be regarded as acquired until proven otherwise. Any morbid or biochemical forebrain lesion potentially can act as a seizure focus, and the resulting epileptic syndrome may begin days to years after the initial lesion. Also, a nonprogressive cerebral lesion such as an old glial scar may result in epilepsy that resolves, progresses, or remains stable.

Repeated seizures in neonatal foals most frequently indicate sepsis or neonatal encephalopathy,^16–18 and a similar syndrome has been seen in calves suffering birth asphyxia with resuscitation (Figure 6.1). Equine familial isolated hypocalcemia (EFIH) in Thoroughbred foals, hypoglycemia, and possibly hyperbilirubinemic encephalopathy (kernicterus) should also be considered in young patients,^19,20 and in adults under appropriate situations.²¹ Adolescent foals, especially of the Arabian breed, can suffer from epilepsy that is of genetic origin^22,23 and can also suffer from severe seizures associated with the lethal Lavender foal syndrome.^22,24 In the genetic adolescent epilepsy, seizures are usually symmetrical and generalized, and the first seizure may accompany an episode of sepsis or other acquired diseases. Control of the seizures with medication is usually successful, and the patients appear to grow out of the condition. Finally, although not true epilepsy, foals and calves with severe cerebellar disease have been seen to have convulsive episodes regarded as cerebellar fits, but they are aware of their environment during these attacks.

      On a practical note, the recommendation to proceed with straightforward epilepsy cases having no interictal signs to advanced diagnostic imaging is not really helpful unless everything possible, including heroic intracranial surgery, is to be entertained. The utility of electroencephalography and MR imaging in improving the outcome of such cases has been discussed,^25,26 but is not widely accepted.

      Signs that may relate to a focal forebrain lesion that is a potential seizure focus must be searched for during an interictal—not postictal—neurologic