Theophylline toxicity

What it is

Theophylline has a narrow therapeutic window. Inhibition of the cytochrome oxidase system by other medications, diet, or underlying disease can alter that window, and ciprofloxacine is one of those medications.

How it presents

Theophylline toxicity can manifest as central nervous system stimulation, such as headache, insomnia and seizures; gastrointestinal disturbances, such as nausea and vomiting; and cardiac toxicity, that is arrhythmia.

Theophylline, together with beta-adrenergic agents, diuretics and insulin, is also a cause of hypokalemia, which is found on electrolytes; and theophyllines, together with organophosphate or MAOIs, are a cause of hyperglycemia, which is found on glucose. The toxicity table sets the row out in four features: seizure, tachycardia, GI upset and arrhythmia. The clue beside it is asthma with a narrow therapeutic window. So an asthmatic who has a seizure, with tachycardia, gastrointestinal upset and an arrhythmia, is theophylline and not aspirin, isoniazid or lithium. Those three read differently on the same table: salicylate gives tinnitus and tachypnea with a mixed acid-base picture, first alkalosis and then an anion gap acidosis; isoniazid gives seizure with metabolic acidosis and hepatotoxicity in a patient on TB treatment with no pyridoxine; and lithium gives tremor, ataxia, nephrogenic DI and seizures in a bipolar patient, often elderly or newly started on the drug.