The primary acute effects of alcohol are on the CNS-causing depression of the nervous system as shown in Figure 1. The effect of alcohol on the CNS is influenced by the degree of tolerance from chronic use and the rate of rise in the blood and brain alcohol levels. With regular consumption of ethanol considerable tolerance develops to its CNS effects. A chronic alcoholic might tolerate an alcohol level that would be lethal to a nonalcoholic.
Figure 1. Clinical Effects of Ethanol at Various Serum Levels
| Blood Alcohol Concentration (mg/dl) | Clinical Effects |
| 30-100 | Mild euphoria, increased speech. Decreased inhibition. Impaired attention, judgement. Mild incoordination, nystagmus. |
| 100-200 | Emotional instability (excitement or withdrawal). Impaired memory, reaction time. Loss of critical judgement. Conmjunctival hyperemia. Ataxia, nystagmus, dysarthria. Hypalgesia. |
| 200-300 | Confusion, disorientation, dizziness. Disturbed perception, sensation. Diplopia, pupils dilated. Marked ataxia, dysarthria. |
| 300-400 | Apathy, stupor. Decreased response seen to stimuli. Vomiting, incontinence. Unable to stand or walk. |
| over 400 | Unconsciousness, coma. Anesthesia. Decreased or abolished reflexes. Hypothermia. Hypoventilation. Hypotension. Death from respiratory arrest. |
Chronic alcohol use is also associated with neurological and mental disorders, such as brain damage, memory loss, sleep disturbances and psychosis. Associated nutritional and vitamin deficiencies can cause nerve damage and neuropsychiatric syndromes, such as Wernicke’s encepthalopathy, Korsakoff’s psychosis, cerebellar atrophy, polyneuritis and nicotinic acid deficiency encephalopathy. Almost every psychiatric syndrome can be seen during heavy drinking or subsequent withdrawal.
Chronic intake of high doses of ethanol causes peripheral neuropathy (thought to be related to thiamine deficiency) in 5 to 15% of alcoholics.
Patients under the influence of alcohol can be regarded as partially anesthetized, decreasing the dose requirement for an induction agent. On the other hand, chronic exposure to alcohol also results in tolerance to anesthetic agents, often increasing maintenance anesthesia requirements and the chance of recall.