[1] A mildly depressed level of consciousness or alertness may be classed as lethargy; someone in this state can be aroused with little difficulty.

An altered level of consciousness can result from a variety of factors, including alterations in the chemical environment of the brain (e.g. exposure to poisons or intoxicants), insufficient oxygen or blood flow in the brain, and excessive pressure within the skull.

[3] A deficit in the level of consciousness suggests that both of the cerebral hemispheres or the reticular activating system have been injured.

[9] Although the neural science behind alertness, wakefulness, and arousal are not fully known, the reticular formation is known to play a role in these.

[9] Since this system is thought to modulate wakefulness and sleep, interference with it, such as injury, illness, or metabolic disturbances, could alter the level of consciousness.

[9] Mass lesions in the brain stem normally cause coma due to their effects on the reticular formation.

[11] Mass lesions that occur above the tentorium cerebelli normally do not significantly alter the level of consciousness unless they are very large or affect both cerebral hemispheres.

[9] Usually the first tests in the ER are pulse oximetry to determine if there is hypoxia, serum glucose levels to rule out hypoglycemia.

Checking serum ammonia is particularly advised in neonatal coma to discern inborn errors of metabolism.

[12] Hypo- or hypernatremia (decreased and elevated levels of sodium, respectively) as well as dehydration can also produce an altered LOC.

[9] Exposure to drugs (e.g. alcohol) or toxins may also lower LOC,[3] as may a core temperature that is too high or too low (hyperthermia or hypothermia).