Large Animal Neurology. Joe Mayhew
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Название: Large Animal Neurology
Автор: Joe Mayhew
Издательство: John Wiley & Sons Limited
Жанр: Биология
isbn: 9781119477198
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8 Coma and other altered states of consciousness
Coma (Gr: deep sleep) is a state of living nonresponsiveness. Stages of increasing levels of obtundation can be referred to as sleepiness (somnolence), semicoma (Figure 8.1) and coma, and represent a progressive lack of awareness of the environment and decreased responsiveness to physiologic and noxious stimuli. It is wise to avoid the anthropomorphic terms depression, delirium, oscitancy, torpor, stupor, lethargy, etc., because of the subjective connotation of these terms in human medicine. It is clearest to simply state what the patient is and is not aware of. Diffuse and focal lesions resulting in coma and other altered states of consciousness involve the forebrain and/or the reticular activating system (RAS) in the brainstem, particularly in the midbrain (Figure 1.7).
An alert state is maintained through multiple sensory inputs to the RAS in the rostral brainstem and subsequently to the thalamus and cerebral cortex where conscious awareness in animals presumably is attained. Diffuse cerebral disease and severe lesions involving just the thalamus, internal capsule, or frontal lobe can result in an inattentive and oblivious mental awareness in animals, often expressed as dummy syndrome. Additional signs of behavioral aberrations, seizures, and visual disturbances frequently accompany this condition. Coma can result from acute damage to these regions of the forebrain, but particularly so from lesions involving the midbrain RAS and especially the thalamus. Almost all of the diffuse inflammatory, metabolic, nutritional, toxic, traumatic, and vascular CNS diseases can ultimately result in coma prior to death.
Mentally obtunded animals do not respond appropriately to noxious stimuli such as loud noises and prodding with a blunt instrument (Figure 8.1). Also, lesions involving the thalamus, internal capsule, or sensory parietal lobe of the cerebrum can be associated with decreased sensation on the contralateral side of the face, most prominent on the nasal septum. This exceeds degrees of nonresponsiveness that can be expected from any associated obtunded mental state.
To witness a large animal suddenly collapse, or to be called urgently to evaluate a patient that has suffered one or more episodes of collapsing, can be confusing for the clinician and certainly distressing for the owner or caregiver. On most occasions, the situation resolves quickly to one of sudden death, recovery with recumbency, a gait abnormality, another neurologic syndrome, recovery with or without evidence of a non‐neurologic problem, or repetitive episodes of collapsing. Poorly informed clients and children must be reminded of their own safety and should be advised to keep clear of a collapsing large animal until the veterinarian arrives. An informed client may be directed to stop any massive bleeding or roll a heavy animal that might be cast. In the event that the animal arises, the client can guide it if it is ambulatory to soft ground, away from potentially harmful objects.
Figure 8.1 This neonatal Limousin cross calf suffering from bacterial meningitis (A) demonstrated opisthotonus and was in a semicoma being nonresponsive to physiologic stimuli and inappropriately responsive to noxious stimuli. He would flail around with his head and neck extended when haemostats were prodded against the nasal septum or they were squeezed across the coronets of a digit. With antibiotic and glucocorticoid therapy, the calf made a complete recovery (B).
Upon arriving at the scene of a collapsed patient, the ABCs of acute care must be attended to concurrently. It must be determined that the patient has a patent airway and is breathing adequately. Oxygen or assisted ventilation may be necessary. Obstructing fluids or objects should be removed from the airway, and 10 mL 50% ethanol can be given intratracheally and/or furosemide 1 mg/kg IV, if pulmonary edema is evident. Hemorrhage should be stopped by packing, suturing, and bandaging. Cardiovascular function should be quickly assessed, and cardiac resuscitation with thoracic massage, if appropriate, and intravenous or intracardiac adrenalin (3–5 mL of 1:1000 in 10 mL H2O for adult horse) should be administered if asystole or anaphylaxis is evident. Intravenous volume expansion with polyionic fluids, plasma, or blood is used as indicated and when available.
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