Seizures
■ BOX 17.2
Guy D. Lester
Recognition
Foals have a lower seizure threshold than adult horses.194 Seizures are generated by abnormal electrical activity within the cerebral cortex.
Depending on the location and extent of the abnormal activity, the seizures may be partial or generalized. Generalized seizure activity is characterized by recumbency, widespread involuntary muscle activity, paddling of the limbs, and extensor rigidity. Partial seizure activity may be more difficult to identify and could be limited to mild movements of the face and jaw, or stretching of the limbs.25Rapid eye movement (REM) sleep can be easily confused with seizure activity. Normal activity is particularly prominent in premature or dysmature foals. Signs can include motor activity, including rhythmic paddling of the limbs, neck extension, chomping of the lips, rapid eye movements, and vocalization.195 Episodes can last several minutes and be distinguished from seizure activity by attempting to arouse the animal. A foal in REM sleep should be easily aroused to full consciousness.
A foal that is simply resisting restraint in lateral recumbency may also appear to be seizuring, and violent paddling of the limbs and occasionally opisthotonos are noted. If confusion exists as to the cause of the activity, the animal should be encouraged to stand and its behavior reevaluated.
Once seizure activity is identified, a complete history should be obtained, including a detailed description of the delivery process, and complete physical and neurologic examinations should be performed. Any signs of trauma, infection, or congenital malformations should be noted. Evaluation of hematologic data and IgG status, combined with historic and physical examination parameters, result in an assessment of the likelihood of sepsis. Blood glucose and serum electrolyte concentrations should be determined promptly. A biochemistry panel, blood gas analysis, bacterial cultures of blood and other body fluids, and possibly CSF analysis complete the database in most cases.
CT or MRI scans are advanced diagnostic procedures for evaluating the anatomic causes of seizures. Premortem diagnosis of agenesis of the corpus callosum and associated malformations was made using CT scanning in a foal that had an abnormally shaped head and seizures refractory to anticonvulsant therapy.196 Identification of the specific abnormalities early in the clinical course allowed the owners to make more of an informed decision regarding the treatment of the foal and the clinicians to acquire valuable information regarding the prognosis of a specific malformation in the horse.
Seizures in foals have numerous causes (Box 17.2). The most common cause of seizure activity in foals is described by the syndrome known as neonatal encephalopathy (NE).