Phenobarbitone (Phenobarbital) Toxicity

Mechanism of Toxicity

• Sustained repetitive neuronal firing
• Autonomic ganglia transmission
• Myocardial activity
• GI smooth muscle tone
• Acetylcholine response at the neuromuscular junction

Clinical Features

Dose-Related CNS Depression (Mild → Severe)

Respiratory

• Respiratory depression is the primary life threat — direct action on medullary respiratory centres
• Respirations may be rapid but shallow; hypoventilation can be missed on examination → use pulse oximetry/capnography
• Significantly potentiated by concurrent opioids or other CNS depressants

Cardiovascular

• Hypotension: direct myocardial depression + venous pooling in a dilated venous system
• Peripheral vascular resistance often normal or increased, but autonomic reflexes cannot compensate
• Particularly dangerous in patients with pre-existing heart failure or hypovolaemia
• Decreased cerebral blood flow and intracranial pressure

Temperature

• Hypothermia (30–36°C) is common; requires continuous core temperature monitoring

GI

• High levels depress GI motility → delayed drug absorption; as levels fall, peristalsis resumes and absorption may resume, causing secondary rise in plasma levels

Chronic Therapy Toxicity

• Sedation (most frequent) — tolerance develops
• Irritability and hyperactivity in children; agitation and confusion in the elderly
• Nystagmus and ataxia at excessive doses
• Rash (scarlatiniform/morbilliform) in 1–2%; rarely exfoliative dermatitis
• Megaloblastic anaemia (responds to folate)
• Osteomalacia (responds to high-dose vitamin D)
• Hypoprothrombinaemia with haemorrhage in neonates born to mothers on phenobarbitone → treat/prevent with vitamin K
• Behavioural toxicity (particularly in children) even without overt signs — serum levels should guide dosing

Therapeutic vs. Toxic Levels

A quantitative serum level can document toxicity but rarely guides moment-to-moment management. Because the EEG may be isoelectric from phenobarbitone alone, brain death must not be declared if phenobarbitone is present at therapeutic levels or higher.

Diagnosis

• Qualitative urine screen: confirms exposure only, does not indicate toxic amounts
• Quantitative serum level: useful for phenobarbitone specifically
• Other barbiturates have high V_d — serum levels don't reflect CNS concentrations
• CXR: non-cardiogenic pulmonary oedema, aspiration pneumonia
• CT head: if coma + trauma, focal signs, papilloedema, or unknown cause

Management

1. Supportive Care (Cornerstone — No Specific Antidote)

• Airway: Severely intoxicated patients cannot protect their airway → intubation + mechanical ventilation often required
• Circulation: IV crystalloids for hypotension; target SBP >90 mmHg; avoid excess fluids (pulmonary oedema risk)
• Vasopressors: Dopamine or norepinephrine if fluid resuscitation fails
• Hypothermia: Active rewarming

2. Gastrointestinal Decontamination

• Gastric lavage: not indicated
• Single-dose activated charcoal (50–100 g) within 1 hour if patient is cooperative and clinically stable
• Multidose activated charcoal (MDAC): increases clearance via hepato-enteric circulation — consider if life-threatening ingestion; typical regimen: 50–100 g initial, then 12.5–25 g every 4 hours
  • More effective than urinary alkalisation
  • No convincing evidence of improved outcomes vs. supportive care alone in Rosen's; Tintinalli's supports use in severe cases
• Concurrent cathartics: not recommended

3. Enhanced Elimination

Withdrawal Syndrome

• Mild: anxiety, restlessness, tremor, insomnia
• Severe: hallucinations, delirium, generalised seizures (life-threatening) Treatment: benzodiazepines (or phenobarbitone itself) with gradual taper

Key Drug Interactions (Chronic Use)

• Phenobarbitone is a potent inducer of CYP2C, CYP3A subfamilies, and UGTs
• Accelerates metabolism of: oral contraceptives (CYP3A4), warfarin, other anticonvulsants, corticosteroids
• Additive CNS depression with: alcohol, opioids, antihistamines, antihypertensives, TCAs, phenothiazines

Disposition

• Mild/moderate: supportive care ± charcoal; observe 6–8 hours; serial levels
• Severe toxicity: ICU admission
• Brain death not to be declared while phenobarbitone levels are therapeutic or above

Investigations in Phenobarbitone Toxicity

1. Drug Levels

Serum Phenobarbitone Level (Quantitative)

• Therapeutic range: 10–40 mcg/mL
• Toxic: >40 mcg/mL
• Severe/consider haemodialysis: persistent levels >100 mcg/mL
• Purpose: establishes diagnosis, guides decision on extracorporeal elimination
• Limitations:
  • Acute treatment decisions should be clinically based, not solely on levels
  • Does not reliably reflect brain concentrations
  • May underestimate clinical severity in polydrug overdose
  • Invalid in chronic abusers (physiological tolerance) and in renal/hepatic disease (reduced clearance)
  • Serial levels are mandatory — phenobarbitone has a long t½ (24–96 hours); peak plasma levels typically occur 3–12 hours after a single oral dose; levels may rise again as gut motility recovers

For other barbiturates (short/intermediate-acting): serum levels are not useful — high volume of distribution means levels don't reflect CNS concentrations and are not rapidly available.

Urine Drug Screen (Qualitative Immunoassay)

• Most commonly detects a class of barbiturates (immunoassay cross-reactivity varies)
• Confirms exposure only — does not prove toxic amounts
• Cannot reliably explain decreased mental status on its own
• Useful as part of a broader toxicology screen for co-ingestants

2. Haematological Investigations

3. Biochemistry / Metabolic Panel

4. Blood Gas Analysis

Pulse oximetry alone is insufficient to detect hypoventilation — SpO₂ may be normal on supplemental O₂ while CO₂ rises.

5. Cardiac Investigations

6. Imaging

Chest X-Ray (CXR)

• Detects:
  • Non-cardiogenic pulmonary oedema (NCPE)
  • Acute lung injury (ALI)
  • Aspiration pneumonia (common complication in comatose patients)
• Mandatory in moderate–severe toxicity

CT Head (Non-contrast)

• Indicated in comatose patients when:
  • Evidence of head trauma (patient found unconscious)
  • Focal neurological signs
  • Papilloedema
  • No otherwise identifiable cause of stupor/coma
• Purpose: exclude structural cause (haemorrhage, raised ICP) before attributing coma to drug

7. Electroencephalogram (EEG)

• Phenobarbitone overdose can produce a flat/isoelectric EEG
• Critical warning: Brain death must NOT be declared based on an isoelectric EEG if phenobarbitone is present in serum at therapeutic levels or above — the EEG silence is pharmacological, not structural
• Role in monitoring barbiturate coma therapy (if used for refractory status epilepticus or raised ICP): titrate to burst suppression

8. Skin Findings (Clinical Investigation)

• "Barb blisters" / Coma blisters: skin bullae at pressure points
  • Uncommon
  • Not specific to barbiturates (seen with other causes of coma)
  • Possibly related to local pressure ischaemia or hypoxia
  • Their presence can support the diagnosis in an unresponsive patient with unknown cause

Summary Table