A Case of Central Oculomotor Disorder Associated with Lithium Toxicity

Herein, we present the case of a patient with dizziness who presented with various clinical findings associated with lithium toxicity. The patient, a man in his 40s, had been receiving treatment for bipolar disorder, depression, and delusional disorder for four years. Shortly after he started receiving lithium carbonate (200 mg/day) for bipolar disorder, he developed dysarthria and an unsteady gait. Two weeks thereafter, he developed consciousness disturbance and was transported to the emergency room of a nearby hospital. Based on the high blood concentration of lithium (2.19 mEq/L), the patient was diagnosed as having lithium toxicity. His systemic symptoms improved with hemodialysis; however, he continued to suffer from dizziness episodes, prompting referral to our department for further examination. At our department, gaze testing with the patient sitting upright revealed spontaneous nystagmus on lateral gaze. Positional testing in the supine position showed a downbeat nystagmus. An eye tracking test demonstrated a saccadic pattern, and optokinetic nystagmus testing indicated poor nystagmus development. Additionally, caloric testing showed reduced vestibular response and impaired visual suppression. A brain MRI showed no abnormalities. Based on these findings, we concluded that the patient’s dizziness could be attributable to cerebellar dysfunction caused by lithium toxicity. Despite treatment, however, the dizziness failed to improve. Lithium carbonate has a narrow therapeutic index, necessitating careful monitoring during administration. Notably, in patients receiving diuretics, ACE inhibitors, NSAIDs, or metronidazole, drug-drug interactions are associated with a higher risk of lithium toxicity. Furthermore, patients taking psychotropic medications, phenothiazines, or butyrophenones are at a higher risk of developing irreversible cerebellar or extrapyramidal symptoms associated with lithium toxicity. Therefore, frequent monitoring of serum lithium levels should be ensured in patients receiving treatment with lithium carbonate.