Chapter 8 Deteriorating Mental Status
Altered mentation in patients, whether rapidly or slowly progressive, is of particular concern to the clinician in the intensive care unit. It is seen with primary neurologic disease, neurologic complications of other diseases, many systemic diseases, and with some drugs.1 A decline in mental status is characterized by decreasing responsiveness and interaction with the environment, although agitation and hyperreactivity can also indicate neurologic dysfunction. Rapid neurologic assessment of the declining patient, coupled with knowledge of underlying disease and medication, will permit the formulation of a list of possible causes, diagnostic and therapeutic plans, as well as an estimation of prognosis.
The animal has a normal demeanor and interaction with its environment. “Normal” varies among animals, and the clinician relies on the client’s knowledge of the pet’s behavior, as well as the initial neurologic evaluation. For example, docile behavior in a cat, which most clinicians would consider desirable, may be very abnormal if the owner reports the cat is typically fearful and aggressive.
Obtundation is a state of decreased responsiveness or alertness and is graded as mild, moderate, or severe. Lethargy is similar, reflecting decreased level of consciousness with listlessness and drowsiness. Other terms commonly used to describe altered mentation in humans, such as confusion, delirium, and dementia, are difficult to extrapolate to veterinary medicine because these states are characterized by disorientation to time and place, loss of memory, and disorganized speech, which are difficult to impossible to evaluate in the veterinary patient.2,3
The cerebrum is the region of the brain responsible for the integration of sensory information from the entire body, planning of motor activity, and appropriate responses to this information, emotion, and memory. Functionally distinct regions are present in the cerebral cortex (e.g., occipital lobe associated with vision; temporal lobe associated with auditory function).5
The ascending reticular activating system (RAS), or reticular formation, is a network of anatomically and physiologically distinct nuclei in the brain stem that function to “activate” the cerebral cortex and maintain consciousness.6 Experimentally, stimulation of the RAS in anesthetized cats produced electroencephalogram patterns consistent with the conscious state.7
Numerous nuclei in the reticular activating system have projections to the cerebrum, but those in the midbrain, rostral pons, and thalamus are the most important for maintaining consciousness.2,8 In anesthetized cats, transection of the brain stem at the level of the pons and midbrain produced coma, but transection at the junction of the medulla and cervical spinal cord did not.9
Lesions causing changes in mentation will be structural, metabolic, or toxic in origin. Clinical signs of diffuse cerebral disease with normal brain stem function are most common with metabolic disease, toxins, or drugs affecting the cerebrum globally (Boxes 8-1 and 8-2). Seizures indicate cerebral cortical dysfunction caused by either extracranial or intracranial disease (see Chapter 98, Seizures and Status Epilepticus). Impairment of the brain stem or thalamus, as well as lateralized cerebral dysfunction (e.g., compulsive circling in one direction, unilateral cortical blindness), is more likely the result of structural disease or injury (Box 8-3).