Can antitubercular drugs cause delirium?

Can Antitubercular Drugs Cause Delirium?

Yes, certain antitubercular drugs can definitively cause delirium, particularly the second-line agents cycloserine and ethionamide, which have well-documented central nervous system toxicity including psychosis that can manifest as delirium.

Primary Culprits: Second-Line Agents

Cycloserine

Cycloserine poses the highest risk for delirium-related neuropsychiatric effects among antitubercular medications. 1 The drug causes a spectrum of CNS effects ranging from mild symptoms (headache, restlessness) to severe reactions including psychosis and seizures. The risk is dose-dependent:

• Seizures occur in up to 16% of patients receiving 500 mg twice daily
• Only 3% risk when receiving 500 mg once daily 1
• The drug may exacerbate underlying seizure disorders or mental illness 1

Critical monitoring requirement: Neuropsychiatric status must be assessed at least monthly, and more frequently if symptoms develop 1

Risk mitigation: Pyridoxine 100-200 mg/day may help prevent and treat neurotoxic side effects 1

Ethionamide

Ethionamide causes neurotoxicity including anxiety, depression, and psychosis in 1-2% of patients taking shorter courses, with higher rates reported with prolonged treatment 1. The neuropsychiatric effects can present as delirium, particularly in vulnerable populations.

First-Line Agents

Isoniazid (INH)

While isoniazid is primarily known for hepatotoxicity and peripheral neuropathy, it can cause delirium in susceptible patients. 2 A 2022 case report documented isoniazid-associated delirium in an elderly female with spinal tuberculosis, with remarkable improvement following drug discontinuation and pyridoxine augmentation 2.

The European guidelines note that isoniazid can cause drowsiness and mood changes 3, which may progress to delirium in vulnerable populations, particularly the elderly.

Clinical Algorithm for Management

When delirium develops in a patient on antitubercular therapy:

1. Immediately assess which drugs the patient is receiving:

   • If on cycloserine: Consider this the primary suspect, especially if dose >500 mg/day
   • If on ethionamide: High suspicion for drug-induced neurotoxicity
   • If on isoniazid alone: Consider in elderly or those with risk factors

2. Check pyridoxine supplementation status:

   • If not receiving pyridoxine, start 100-200 mg/day immediately 1
   • May prevent progression and treat existing neurotoxicity

3. For cycloserine-induced delirium:

   • Measure serum concentrations (target peak 20-35 mg/ml) 1
   • Reduce dose or discontinue based on severity
   • Consider switching to alternative second-line agent

4. For ethionamide-induced delirium:

   • Discontinue drug if psychosis or severe neuropsychiatric symptoms present 1
   • Replace with alternative second-line agent based on susceptibility testing

5. For isoniazid-induced delirium:

   • Augment with high-dose pyridoxine 2
   • If no improvement, discontinue and substitute with alternative first-line agent

Critical Caveats

Age-related vulnerability: Elderly patients are at substantially higher risk for antitubercular drug-induced delirium 2. The case report specifically highlighted this in an elderly female, and general delirium literature confirms older adults have increased susceptibility 4.

Alcohol-related hepatitis: Patients with alcohol-related hepatitis on cycloserine have an increased risk of seizures 1, which can present with or precipitate delirium.

Renal impairment: Cycloserine accumulates in renal insufficiency, increasing neurotoxicity risk. The drug should not be used in patients with creatinine clearance <50 ml/minute unless receiving hemodialysis 1.

Drug interactions: The neuropsychiatric effects may be potentiated by other CNS-active medications or underlying psychiatric conditions 1.

Distinguishing Drug-Induced from Other Causes

While antitubercular drugs can cause delirium, tuberculosis itself, particularly CNS tuberculosis, can present with altered mental status. The temporal relationship between drug initiation/dose escalation and symptom onset is key. Drug-induced delirium typically:

• Develops within days to weeks of starting the offending agent
• Improves with dose reduction or drug discontinuation
• Responds to pyridoxine supplementation (for cycloserine and isoniazid)
• Occurs without fever or other signs of CNS infection progression

Bottom line: Monitor all patients on second-line antitubercular therapy monthly for neuropsychiatric changes, ensure adequate pyridoxine supplementation, maintain cycloserine doses ≤500 mg/day when possible, and have a low threshold for drug discontinuation in elderly patients who develop delirium 1, 2.