Neurotoxicity refers to adverse effects of chemical agents on the structure or function of the central nervous system, peripheral nervous system, or both, and represents a significant dose-limiting toxicity for many therapeutic agents. The nervous system is particularly vulnerable to toxic injury due to its high metabolic demand, limited regenerative capacity, and the presence of specialized barriers that can paradoxically concentrate certain toxicants. Neurotoxic effects range from mild, reversible symptoms such as headache and dizziness to permanent, disabling conditions including peripheral neuropathy, encephalopathy, and sensorineural hearing loss.
Mechanisms of injury differ across neurotoxic agents but commonly involve mitochondrial dysfunction, oxidative stress, axonal transport disruption, demyelination, and excitotoxicity. Mitochondrial damage impairs energy production in neurons, which are heavily dependent on oxidative phosphorylation. Axonal degeneration results from disruption of microtubule-based transport, leading to dying-back neuropathy. Excitotoxicity occurs when excessive glutamate receptor activation causes calcium overload and neuronal death. Some agents produce neurotoxicity through immune-mediated mechanisms, while others interfere with neurotransmitter synthesis, release, or receptor binding.
Vinca alkaloids — vincristine, vinblastine, and vinorelbine — cause a dose-dependent peripheral neuropathy that is the most common dose-limiting toxicity of this drug class. Vincristine binds to tubulin and disrupts microtubule polymerization, impairing axonal transport and leading to axonal degeneration. Patients develop symmetric sensory and motor deficits beginning in the distal extremities, with loss of deep tendon reflexes, paresthesias, and motor weakness. The “coasting” phenomenon, where neuropathy continues to progress for weeks after discontinuation, is characteristic. Platinum-based drugs produce distinct neurotoxic profiles: cisplatin causes a predominantly sensory neuropathy with large-fiber involvement, while oxaliplatin causes acute cold-induced paresthesias and a chronic sensory neuropathy.
Isoniazid causes peripheral neuropathy through interference with pyridoxine (vitamin B6) metabolism, leading to deficiency of this essential cofactor for neurotransmitter synthesis. The risk is increased in slow acetylators, malnourished patients, and those with predisposing conditions such as diabetes or alcoholism. Prophylactic pyridoxine supplementation effectively prevents this complication. Antiepileptic drugs can produce both central and peripheral neurotoxic effects. Phenytoin causes cerebellar degeneration with chronic use, presenting with ataxia, nystagmus, and dysarthria. Vigabatrin is associated with irreversible visual field defects due to retinal toxicity.
CNS toxicity manifests as encephalopathy, seizures, movement disorders, or cognitive impairment. Cephalosporins and penicillin at high doses can cause myoclonus and seizures, particularly in patients with renal impairment. Metronidazole produces cerebellar dysfunction with dysarthria and ataxia. Methotrexate, particularly when administered intrathecally, can cause chemical arachnoiditis, transverse myelopathy, and leukoencephalopathy. Immunosuppressive agents such as cyclosporine and tacrolimus cause posterior reversible encephalopathy syndrome (PRES), presenting with headache, visual disturbances, seizures, and characteristic neuroimaging findings.
Ototoxicity is a form of neurotoxicity affecting the eighth cranial nerve. Aminoglycoside antibiotics accumulate in hair cells of the inner ear, causing irreversible high-frequency hearing loss and vestibular dysfunction. Cisplatin produces dose-dependent ototoxicity through oxidative stress and apoptosis of cochlear hair cells. Genetic susceptibility to aminoglycoside ototoxicity is associated with a mitochondrial DNA mutation (A1555G) that increases sensitivity. Audiologic monitoring is recommended for patients receiving prolonged courses of ototoxic medications.
Diagnosis and monitoring rely on clinical examination, nerve conduction studies, and electromyography. Quantitative sensory testing and patient-reported outcome measures help track progression. For CNS toxicity, neuroimaging, electroencephalography, and cognitive assessment may be indicated. Preventive strategies include dose limitation, substitution with less neurotoxic alternatives, and prophylactic supplementation where evidence supports it, such as pyridoxine with isoniazid and amifostine with cisplatin.
