Hypothyroxinemia and phenytoin toxicity: a vicious circle

Acute phenytoin intoxication in a 4-year-old mimicking viral meningoencephalitis

We report here the case of a 4-year-old female preschooler who presented to the emergency department with generalised tonic-clonic convulsions and history of vomiting, irritability and dysarthria of short duration. On examination she was found to be responsive only to painful stimulus, had terminal neck stiffness and bilateral extensor plantars. In view of her clinical presentation, an initial diagnosis of viral meningoencephalitis was made in the emergency room and the child treated accordingly. On subsequent transfer to the intensive care unit (ICU), the parents revealed additional history of an elder sibling taking phenytoin for seizures. Therefore, a suspicion of acute phenytoin toxicity was made and phenytoin levels sent for confirmation. Her serum phenytoin level was 80 μgm/mL (normal: 10-20). The child was managed conservatively and discharged after 5 days of hospitalisation. We chose to report this case to highlight the unusual presentation of this rare intoxication.

Elevated thyroid-stimulating hormone in a 65-year-old patient with phenytoin toxicity

Phenytoin has a narrow therapeutic range and exhibits concentration-dependent kinetics within this therapeutic window. Therefore, small changes in dose or metabolism can have a large effect on serum concentrations. Therapeutic drug monitoring may be helpful in identifying drug toxicity. A 65-year-old male with a history of hydrocephalus was admitted for generalized weakness and altered mental status. Laboratory testing revealed an elevated phenytoin level, and the patient's phenytoin was discontinued. Further laboratory tests led to the diagnosis of hypothyroidism, and treatment was initiated. The complicated interaction between phenytoin and thyroid hormones is described. Pharmacists can play a role in therapeutic drug monitoring and the identification of drug-disease interactions. Pharmacist drug counseling can help patients and family members identify adverse effects and toxicity and know when to seek medical assistance.

Unusual presentation of iatrogenic phenytoin toxicity in a newborn

BACKGROUND

Medication errors may produce severe toxicity resulting in hospitalization. This can be compounded if the physician obtains the wrong concentration from a reference manual and a pharmacy miscalculates the conversion. We report a child presenting with ileus, hypothermia and lethargy after receiving supratherapeutic dosing of phenytoin after a concentration miscalculation.

CASE REPORT

A one-month-old infant presented to the Emergency Department with progressive worsening of abdominal distension, diminished activity, and a one day history of difficulty feeding secondary to a decreased level of consciousness. The past medical history was significant for neonatal Group B Strep meningitis with seizures. Among the child's discharge medications was a prescription for phenytoin (30 mg/5 mL) 2.5 cc by mouth three times daily. On exam, the child was hypothermic with pink mottled skin, poor responsiveness, prolonged capillary refill, abdominal distension with hypoactive bowel sounds, and a dysconjugate gaze. The Initial phenytoin serum concentration was 91.8 mcg/mL. She was admitted to the PICU and was started on ampicillin and cefotaxime for R/O sepsis. Phenytoin was withheld and subsequent serum concentrations revealed an extremely slow elimination (mcg/mL vs. time pair coordinates were 78.2/13.3h; 74.3/62.3h; 43.7/109.6h; 10.8/160.9h) reflecting zero-order kinetics. Post discontinuing antibiotics, phenytoin levels decreased at rates expected. She was discharged after resolution of symptoms. The MD who had written the phenytoin prescription had based it on the Harriet Lane Handbook, 2000 Ed. The 30 mg/5 mL formulation has been unavailable in the US for several years. A community pharmacy substituted the 125 mg/5 mL formulation, but miscalculated the dosage to be 1.6 cc (40 mg) tid.

CONCLUSIONS

Abdominal distension and ileus may be presenting symptoms in children at toxic phenytoin levels. Ampicillin and cefotaxime may effect the elimination rate of phenytoin at such levels. We report one of the highest phenytoin levels recorded after therapeutic misadventure. Physicians must be aware of inaccuracies in reference manuals that may result in dosing errors.

Hypothyroidism

Hypothyroidism is common, potentially serious, often clinically overlooked, readily diagnosed by laboratory testing, and eminently treatable. The condition is particularly prevalent in older women, in whom autoimmune thyroiditis is common. Other important causes include congenital thyroid disorders, previous thyroid surgery and irradiation, drugs such as lithium carbonate and amiodarone, and pituitary and hypothalamic disorders. Worldwide, dietary iodine deficiency remains an important cause. Hypothyroidism can present with nonspecific constitutional and neuropsychiatric complaints, or with hypercholesterolaemia, hyponatraemia, hyperprolactinaemia, or hyperhomocysteinaemia. Severe untreated hypothyroidism can lead to heart failure, psychosis, and coma. Although these manifestations are neither specific nor sensitive, the diagnosis is confirmed or excluded by measurements of serum thyrotropin and free thyroxine. Thyroxine replacement therapy is highly effective and safe, but suboptimal dosing is common in clinical practice. Patient noncompliance, drug interactions, and pregnancy can lead to inadequate treatment. Iatrogenic thyrotoxicosis can cause symptoms, and, even when mild, provoke atrial fibrillation and osteoporosis. We summarise present understanding of the history, epidemiology, pathophysiology, and clinical diagnosis and management of hypothyroidism.