2.2 Pharmacology of Alcohol and Sedative-Hypnotics

2.2 Pharmacology of Alcohol and Sedative-Hypnotics

Central Nervous System (CNS) depressants, including alcohol (ethanol), benzodiazepines, and barbiturates, represent some of the most widely used and clinically challenging substances in addiction medicine. Addiction counselors must understand the pharmacokinetics, neurochemical mechanisms, withdrawal syndromes, and medical management strategies associated with these agents.

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Alcohol Pharmacokinetics and Metabolism

Absorption and Distribution

Alcohol is a small, water-soluble molecule that requires no digestion prior to systemic absorption:

• Absorption Sites: Approximately 20% of ingested ethanol is absorbed directly through the stomach lining into the bloodstream; the remaining 80% is rapidly absorbed in the upper small intestine (duodenum and jejunum).
• Absorption Rates: Delayed by the presence of food (especially fatty meals) in the stomach, which slows gastric emptying. Carbonated beverages accelerate absorption by opening the pyloric sphincter.
• Blood Alcohol Concentration (BAC): Expressed as grams of ethanol per 100 mL of blood (e.g., 0.08 g/dL represents the legal limit for driving in most jurisdictions).

Hepatic Elimination Kinetics

Over 90% of absorbed alcohol is metabolized in the liver via a two-step enzymatic process:

$\text{Ethanol} \xrightarrow{\text{Alcohol Dehydrogenase (ADH)}} \text{Acetaldehyde} \xrightarrow{\text{Acetaldehyde Dehydrogenase (ALDH)}} \text{Acetate} \rightarrow \text{Water + } CO_2$

1. Step 1: Alcohol Dehydrogenase (ADH) converts ethanol into acetaldehyde, a highly toxic intermediate compound responsible for tissue damage and hangover symptoms.
2. Step 2: Acetaldehyde Dehydrogenase (ALDH) rapidly converts acetaldehyde into harmless acetate, which is broken down into water and carbon dioxide.

Zero-Order Kinetics

Unlike most drugs whose elimination rate is proportional to blood concentration (first-order kinetics), alcohol is metabolized via zero-order kinetics. The liver's ADH enzyme system saturates at low BAC levels, resulting in a constant rate of elimination averaging 0.015 g/dL per hour (roughly equivalent to one standard drink per hour), regardless of total blood alcohol level.

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Neurochemical Mechanism of Action

Alcohol exerts widespread depressant effects on the central nervous system through two primary neurotransmitter systems:

• GABA-A Enhancement: Alcohol binds as a positive allosteric modulator at the GABA-A receptor complex, increasing chloride ion influx into postsynaptic neurons. This hyperpolarizes neuronal membranes, reducing neuronal firing and producing sedation, anxiolysis, muscle relaxation, and ataxia.
• Glutamate NMDA Inhibition: Alcohol acts as an antagonist at N-methyl-D-aspartate (NMDA) glutamate receptors, inhibiting excitatory neurotransmission. This contributes to cognitive impairment, memory blackouts, and motor incoordination.

Neuroadaptation and Rebound Hyperexcitability

With chronic, heavy alcohol exposure, the brain attempts to maintain homeostatic balance by down-regulating inhibitory GABA-A receptors and up-regulating excitatory NMDA glutamate receptors. When alcohol is abruptly withdrawn, uninhibited glutamate excitation combined with diminished GABAergic tone creates severe CNS hyperexcitability, driving acute withdrawal symptoms.

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Alcohol Withdrawal Assessment and Delirium Tremens

CIWA-Ar Assessment

The Clinical Institute Withdrawal Assessment for Alcohol, Revised (CIWA-Ar) is a validated 10-item clinical tool used to quantify the severity of alcohol withdrawal:

• Evaluated Categories: Nausea/vomiting, tremor, paroxysmal sweats, anxiety, agitation, tactile disturbances, auditory disturbances, visual disturbances, headache, and orientation/sensorium clouding.
• Scoring and Protocol: Total scores range from 0 to 67. Scores <8 reflect mild withdrawal requiring non-pharmacological monitoring; 8 to 15 indicate moderate withdrawal warranting symptom-triggered medication; scores >15 indicate severe withdrawal requiring aggressive inpatient medical detoxification.

Delirium Tremens (DTs)

Delirium Tremens is the most severe form of alcohol withdrawal, occurring in approximately 5% of hospitalized withdrawal cases, typically emerging 48 to 96 hours after the last drink.

• Clinical Signs: Acute confusion/delirium, profound autonomic hyperactivity (severe hypertension, tachycardia, hyperthermia, diaphoresis), coarse tremor, visual/tactile hallucinations, and grand mal seizures.
• Medical Management: DTs is a life-threatening medical emergency carrying a high mortality rate if untreated. Management requires intensive care monitoring and parenteral administration of high-dose, long-acting cross-tolerant benzodiazepines (e.g., diazepam or chlordiazepoxide).

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Wernicke-Korsakoff Syndrome

Chronic alcohol misuse frequently leads to severe nutritional deficiencies, specifically Thiamine (Vitamin B1) deficiency, caused by poor dietary intake, impaired intestinal absorption, and reduced hepatic storage.

[Thiamine (Vitamin B1) Deficiency]
               |
               v
[Wernicke's Encephalopathy] --(Untreated/Chronic)--> [Korsakoff's Psychosis]
• Acute & Reversible                                 • Chronic & Irreversible
• Triad: Ataxia, Eye Palsy, Confusion                 • Amnesia & Confabulation

1. Wernicke's Encephalopathy (Acute)

An acute, life-threatening neurological emergency characterized by the classic clinical triad:

1. Ataxia (unsteady, staggering gait)
2. Ophthalmoplegia / Nystagmus (paralysis of ocular muscles or involuntary eye movement)
3. Global Confusion and disorientation

Critical Practice Rule: Parenteral thiamine MUST be administered BEFORE or alongside intravenous glucose. Administering IV glucose alone in a thiamine-depleted patient precipitates acute metabolic crisis and severe neurological deterioration.

2. Korsakoff's Psychosis (Chronic)

If Wernicke's encephalopathy is untreated, it progresses to Korsakoff's Psychosis, a chronic, largely irreversible neurocognitive disorder characterized by:

• Severe anterograde amnesia (inability to form new memories) and retrograde amnesia.
• Confabulation: The fabrication of plausible stories to fill memory gaps, occurring without conscious intent to deceive.

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FDA-Approved Medications for Alcohol Use Disorder (AUD)

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Sedative-Hypnotics: Benzodiazepines and Barbiturates

Sedative-hypnotic drugs act on the central nervous system similarly to alcohol and carry profound risks of tolerance, physical dependence, addiction, and fatal withdrawal.

Pharmacological Differences

• Benzodiazepines (e.g., Diazepam, Lorazepam, Alprazolam): Bind to a distinct allosteric site on the GABA-A receptor and increase the frequency of chloride channel opening. They possess a wider therapeutic window than barbiturates but carry high risks of physical dependence and cross-tolerance with alcohol.
• Barbiturates (e.g., Phenobarbital, Secobarbital): Bind to GABA-A receptors and prolong the duration of chloride channel opening. At higher doses, they act as direct GABA agonists. Barbiturates have a very narrow therapeutic index; accidental overdose frequently causes fatal respiratory depression.

Detoxification and Medical Safety Risks

Abrupt cessation of prolonged, high-dose benzodiazepine or barbiturate use causes severe withdrawal marked by autonomic instability, psychosis, rebound anxiety, and life-threatening status epilepticus (continuous seizures). Detoxification requires slow, medically supervised outpatient or inpatient tapering using long-acting agents such as chlordiazepoxide, diazepam, or phenobarbital.