Neurological emergencies (Proceedings)
May 01, 2011
Neurological emergencies are common in small animal practice. As with other body systems, infections, neoplasms, trauma, inflammatory processes, congenital and hereditary diseases all affect the central and peripheral nervous systems causing a range of mild to severe conditions. Advanced forms of imaging provided by computed tomography (CT) and magnetic resonance imaging (MRI) in veterinary medicine are allowing improved diagnostic plans and management options. The assessment, stabilization and treatment of the neurology emergency, however, still relies on the ability of the emergency clinician to consider a complete history, perform a complete physical examination, and analyze test results readily available on an emergency basis. The following section provides an overview of common neurology emergencies: changes in mental alteration, seizures, paresis and paralysis, vestibular syndrome and traumatic brain injury. Basic knowledge of neurological disease processes in animals allows the emergency clinician to consider a variety of differential diagnoses quickly and efficiently. Conditions causing hypoxia such as pulmonary disease, airway obstruction and anemia, or hypovolemia can be manifested as mental alteration. Metabolic disorders such as hypoglycemia or electrolyte derangements can cause seizures. Cardiovascular and metabolic instability must be recognized and corrected prior to neurological evaluation. Once satisfied that other underlying causes of central nervous signs have been ruled out, the clinician can consider a primary neurological disorder.
1. Neurological abnormalities should be localized to help identify the underlying cause.
Seizures are the physical manifestation of an abnormal balance between excitatory and inhibitory tone in the central nervous system. Seizures are categorized as either generalized or partial. Generalized seizures include those that, in the past, have been referred to as convulsions, or grand mal seizures. Partial seizures are further classified as either simple partial or complex seizures. Partial seizures include seizure activity that is atypical. Both generalized and partial seizures represent a potentially life-threatening neurological emergency usually requiring treatment.
The central nervous system relies on a balance of excitatory tone and inhibitory tone for normal function of neurons. Generally, excitatory tone is mediated by the neurotransmitter glutamate, and inhibitory tone is mediated by gaba aminobutyric acid (GABA). Depolarization of the cell membrane results with binding of glutamate to its receptors. Hyperpolarization of the cell membrane occurs when GABA binds to its receptors. Uncontrolled, perpetuated neuronal activity may derail normal mechanisms that act to prevent random excitation, resulting in seizures.
Pathophysiology and clinical signs
Seizures may occur with a wide variety of conditions. These conditions are grouped according to intracranial causes (structural epileptic seizures or SES), extracranial causes (reactive epileptic seizures or RES), and idiopathic epilepsy (primary epileptic seizures or PES). Although each of these categories should be considered with animals presenting with acute or recent onset of seizures, the age of the animal may suggest one group over another. Animals less than 1 year or older than 5 years of age are more commonly represented in the SES group. Recurrent SES typically occur within 2-4 weeks of the acute onset (first seizure). Dogs aged 1 to 5 years old are more likely to have PES, and recurrent seizures may not recur for greater than 4-6 weeks. Cats develop PES very uncommonly without a prior inciting cause (eg, trauma or inflammation).
Generalized seizures (commonly referred to as grand mal or convulsive seizures) are characterized by loss of consciousness and bilateral muscle activity. Generalized seizures may have increased tone (tone) and or muscle flexation (clonic). Loss of autonomic tone is common, so the animal may vomit, urinate or defecate. Generalized seizures are the most commonly recognized seizures in small animals.
Partial seizures (both simple and complex) are clinically manifested by unilateral, bizarre behavior. Partial seizures are commonly referred to as focal seizures. Dogs with simple seizures remain alert and responsive, where dogs with complex seizures may have some loss or alteration in consciousness. The seizure activity itself is stereotypical of the region of the brain that is involved, such as the temporal or frontal lobes. Commonly recognized partial seizures include biting at imaginary flies, tail chasing, circling, ear or whisker twitching, head turning, or pupil dilation/constriction.
The seizure or ictal state is can be followed by a post ictal phase, during which the animal may seem confused, tired, pant, pace, and have visual impairment. A pre ictal phase, if recognized, is commonly referred to as the aura. During the pre ictal phase, the animal may seem confused or seek the attention of the owner. A partial seizure that progresses to a generalized seizure, is also referred to as an aura.
A cluster of seizures is a series of isolated seizures lasting over minutes to days. Status epilepticus is prolonged single seizure lasting more than 3 minutes without recovery. Status epilepticus can be life threatening, resulting in brain anoxia and residual brain impairment or death.