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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