4 Alteration in mental state – an introduction
This is the most common reason for animals to be presented to the veterinarian. Owners remark that the animal is ‘not himself’, ‘not right’. When pressed for details, the next observation is just as non-specific: lethargy, depression. This should be no surprise; many diseases result in a less reactive mental state, drowsiness or apparent indifference to stimuli.
Alertness is promoted by the ascending reticular activating system (ARAS), a network of neurons projecting from the brainstem to the cerebral cortex. Damage to the ARAS alters the level of consciousness.
Some animals are described as having a ‘far-off’ gaze, or staring, vacant expression. This may signal a depressed state of alertness. Mydriasis, blindness, a lack of blinking, and a rigid neck posture can create the same impression. Deafness and exercise intolerance will alter an animal’s responsiveness to commands.
Consciousness is the state of awareness of the self and the environment. The former is anthropomorphized by owners, while the latter is used by veterinarians to judge the level of mental responsiveness. This state of awareness relies on the animal being alert.
Consciousness is graded by the decreasing level of function from alert to comatose. The term semi-comatose has been used in texts, but as coma is defined as a total lack of responsiveness to stimuli, using the prefix ‘semi-’ can confuse communication.
In the human this is characterized by disorientation, fear, irritability, agitation and misperception of sensory stimuli which may be episodic and interspersed with lucid intervals. It results from a generalized impairment of brain function and occurs in metabolic or toxic disorders or multifocal cerebral disease. It may precede or follow stupor/coma.
A lesion in the compact brainstem damages a larger proportion of the ARAS with a more profound effect on consciousness than would the same sized lesion in the cerebrum which only picks off a small fraction of the now-dispersed fibres.