Neurotoxicity in Pharmacotherapy
Drug-induced neurotoxicity ranges from peripheral neuropathy to seizures and encephalopathy. Awareness of risk agents and early symptoms is critical.
## Overview
The central and peripheral nervous systems are susceptible to drug toxicity because neurons have high metabolic demands, limited regenerative capacity, and the blood-brain barrier can be breached by lipophilic compounds. Drug-induced neurotoxicity affects millions of patients annually, particularly those receiving chemotherapy.
## Peripheral Neuropathy
Chemotherapy-induced peripheral neuropathy (CIPN) affects 30-70% of patients receiving neurotoxic chemotherapy. Platinum agents (cisplatin, oxaliplatin) damage dorsal root ganglia through DNA adduct formation. Vinca alkaloids (vincristine) disrupt microtubule-dependent axonal transport. Taxanes (paclitaxel) stabilize microtubules excessively, impairing normal axonal function.
Non-chemotherapy agents also cause neuropathy. Isoniazid depletes pyridoxine (vitamin B6), causing sensory neuropathy preventable with B6 supplementation. Metronidazole and nitrofurantoin cause dose-dependent neuropathy with prolonged use. Colchicine causes neuromyopathy, especially with concurrent statin use.
## Central Neurotoxicity
**Seizures** can be provoked by fluoroquinolones (which antagonize GABA-A receptors), imipenem, tramadol, bupropion, and theophylline. Risk increases with renal impairment due to drug accumulation.
**Encephalopathy** presents as confusion, altered consciousness, and cognitive dysfunction. Methotrexate causes leukoencephalopathy, particularly with intrathecal administration. Ifosfamide encephalopathy occurs in 10-30% of patients due to the neurotoxic metabolite chloroacetaldehyde. Valproic acid can cause hyperammonemic encephalopathy even at therapeutic levels.
**Serotonin syndrome** results from excessive serotonergic activity, typically from drug combinations (SSRIs + MAOIs, tramadol + SSRIs). It presents with altered mental status, autonomic instability, and neuromuscular hyperactivity.
## Ototoxicity
Aminoglycosides damage cochlear and vestibular hair cells irreversibly. Risk increases with duration, cumulative dose, and concurrent loop diuretic use. Cisplatin causes bilateral, high-frequency hearing loss in up to 80% of patients. Salicylate ototoxicity (tinnitus) is typically reversible upon dose reduction.
## Risk Factors
Advanced age, pre-existing neurological disease, renal or hepatic impairment, diabetes (baseline neuropathy risk), and blood-brain barrier disruption (meningitis, radiation) increase susceptibility.
## Monitoring and Prevention
Baseline neurological examination and audiometry should precede therapy with known neurotoxins. Dose modification or discontinuation at early symptom onset is essential, as neuropathy may progress after drug cessation ("coasting"). Pyridoxine co-administration prevents isoniazid neuropathy.
## Key Takeaways
- CIPN affects up to 70% of patients on platinum, vinca, or taxane agents
- Fluoroquinolones and imipenem lower the seizure threshold
- Serotonin syndrome requires rapid recognition and discontinuation of serotonergic agents
- Aminoglycoside ototoxicity is irreversible; monitor with audiometry
- Early detection and dose adjustment are the best neuroprotective strategies