Case
A 16-year-old girl, who recently emigrated from Haiti, was brought to the pediatric ED by her mother for evaluation of a 2-hour history of gastric discomfort. Upon arrival at the ED waiting area, the patient experienced a sudden onset of generalized tonic-clonic movement with altered sensorium, though she did not fall to the ground and was not injured. Vital signs from triage were: blood pressure, 110/76 mm Hg; heart rate, 112 beats/min; respiratory rate, 22 breaths/min; and temperature, 97°F. Oxygen saturation was 98% on room air.
The patient was immediately attached to a cardiac monitor, given oxygen via a face mask, and received airway suctioning. Despite receiving a total of 4 mg of lorazepam, the seizure continued. Physical examination revealed no signs of external injury, but the ongoing generalized status epilepticus made the examination difficult.
What are the causes of refractory seizures in an adolescent patient?
The differential diagnosis for pediatric patients presenting with refractory seizure is the same as that for adult patients and should include treatment noncompliance, infection, vascular event (eg, stroke, hemorrhage), trauma (eg, cerebral contusions), metabolic and electrolyte disturbances, anticonvulsant toxicity, and exposure to a convulsant toxin.
While certain drugs (eg, cocaine) may cause status epilepticus through a secondary effect such as ischemia or a bleed, some drugs can directly cause refractory seizures. A few drugs and toxins are responsible for the majority of such seizures: bupropion; carbon monoxide; diphenhydramine; ethanol (withdrawal); hypoglycemics; lead; theophylline; tramadol; and certain antibiotics, including cephalosporins, penicillins, quinolones, and, in particular, isoniazid (INH).1
Case Continuation
Upon further history-taking, the patient’s mother informed the ED staff that during a recent visit to a local clinic, her daughter tested positive on routine screening for tuberculosis and was given “some medications.” The patient’s mother further noted that her daughter was scheduled for a follow-up appointment at the same clinic later this morning. She believed the patient had taken “a few” of the prescribed pills at once to “catch-up” on missed doses prior to that appointment, and provided the ED staff with an empty bottle of INH that she had found in her daughter’s purse.
What are the signs and symptoms of acute isoniazid toxicity?
Isoniazid toxicity should be suspected in any patient who has access to INH—even if the drug was prescribed for someone other than the patient. Acute toxicity develops rapidly after the ingestion of supratherapeutic doses of INH and includes nausea, abdominal discomfort, vomiting, dizziness, and excessive fatigue or lethargy. Patients can present with tachycardia, stupor, agitation, mydriasis, increased anion gap metabolic acidosis, and encephalopathy.
Seizures occur due to an INH-induced functional pyridoxine deficiency. Isoniazid inhibits pyridoxine phosphokinase, the enzyme that converts pyridoxine (vitamin B6) to its physiologically active form, pyridoxal 5’-phosphate (PLP). Because the conversion of glutamate (an excitatory neurotransmitter) to gamma-aminobutyric acid (GABA; the body’s main inhibitory neurotransmitter) is dependent on PLP, an excess of glutamate and a deficiency of GABA occurs following INH overdose. The result is neuroexcitation, which manifests as generalized seizures in affected patients.
The most consequential effect of INH overdose, however, is the development of seizure refractory to conventional therapy, such as benzodiazepines. This occurs because benzodiazepines are indirect-acting GABA agonists, and require the presence of GABA to elicit their effect. Therefore, due to the impairment of GABA synthesis, benzodiazepines are limited or ineffective as anticonvulsants. Although INH doses in excess of 20 mg/kg may result in neuroexcitation, refractory seizures are uncommon with doses <70 mg/kg.
Complications of chronic INH use include hepatotoxicity, and patients will present with jaundice, hepatomegaly, and right upper quadrant pain and tenderness. Isoniazid must be discontinued rapidly in patients demonstrating hepatotoxicity, and the risk/benefit of treatment reconsidered. Patients can also develop peripheral neuropathy while using INH therapeutically, and prophylactic vitamin B6 supplementation is recommended for certain at-risk patients, such as those who are pregnant or breastfeeding.2 Supplementation, however, neither reduces the risk of hepatotoxicity nor prevents onset of seizure following an INH overdose (the B6 dose is not sufficient enough to exert this effect).