Anatomy and Physiology
The cerebral hempispheres are two large areas of white matter surrounded by an outer thin mantel of nerve cell bodies, the grey matter, or cortex. Association areas constitute the majority of the cortex and are involved with higher learning or intelligence. Both cerebral hemispheres must be affected for an altered level of consciousness to occur.
Common causes of bilateral cortical involvement are drugs, and oxygen or glucose deficits (metabolic etiology).
The brainstem consists of the thalamus, hypothalamus, midbrain, pons and medulla, and contains centers for vital functions. The reticular formation, an inner core of neurons in the spinal cord and brainstem, regulates respiration, blood presssure, heart-rate, endocrine secretion, conditioned reflexes, learning and consciousness. Incoming stimuli are integrated by the reticular formation. A portion of it, the reticular activating system (RAS), is responsible for the arousal reaction. Most patients with lesions of the RAS are comatose. Common causes of brainstem involvement are trauma and stroke (structural etiology).
Some drugs affect the brainstem. For example, epinephrine, amphetamines and cocaine stimulate RAS conduction; opiates and barbiturates depress it. In summary, a decrease in level of consciousness occurs if:
- 1. Both cerebral hemispheres are involved, or
- 2. The RAS brainstem is affected.
The vast majority of patients with decreased level of consciousness (about 85%) are in the metabolic category.
Glasgow Coma Scale (GCS) <./strong>
This scale in not only to assess changes in levels of consciousness in brain-damaged patients in prolonged comas, but also to avoid imprecise terms such as lethargy, semicomatose, stupor, etc. when describing the mental status of a patient.
An important point about the GCS is that it can be used as a continuum for changing levels of consciousness. At about a GCS score of 8 the definition of coma is fulfilled. The GCS is an integral part of trauma scales.
The GCS is sometimes bypassed in the field for AVPU, an easier method of evaluating level of consciousness because no numbering system is required.
The GCS is preferred over AVPU since both take about the same amount of time and a tenuous area exists between V and P in terms of airway protection. Sometimes it is difficult to assess a gag reflex. Airway protection (endotracheal intubation) to avoid aspiration of vomitus is required when the gag reflex is lost or the GCS is 8.
- V—responds to verbal stimuli
- P—responds to painful stimuli