Sat27November0556PM 48

An otherwise healthy 48-year-old woman with thyroid cancer undergoes a thyroidectomy lasting three hours.

The following day she complains of tingling in her hands and mouth before having a self terminating seizure. Her ECG shows sinus rhythm with a QTc interval of 510 ms.

Which of the following deranged investigations is most likely to account for her clinical presentation?

(Please select 1 option)

Potassium 2.1 mmol/L

Calcium 1.9 mmol/L This is the correct answer

PaO2 8.5 kPa (FiO2 0.4)

Sodium 114 mmol/L Incorrect answer selected

Glucose 2.1 mmol/L

Explanation

Temporary hypocalcaemia may affect up to 20% of patients after thyroidectomy as a result of trauma to the parathyroid glands or their inadvertent excision in theatre.

Acute hypocalcaemia usually presents 24-48 hours postoperatively and may present with perioral tingling, twitching or tetany. This may progress to seizures and ventricular arrhythmias. The ECG may show a prolonged QT interval (QTc is prolonged if >440 ms in men or >460 ms in women.) is associated with increased risk of torsades de pointes and treatment for serum calcium levels <2 mmol/L is urgent intravenous calcium (10 ml of 10% calcium gluconate) followed by an infusion if necessary.

Hypoglycaemia can, in extremis, cause a seizure but this would usually be preceded by neuroglycopenic symptoms such as altered mentation, lethargy as well as hunger and sweating which are absent in the patient. The patient is not diabetic.

Acute severe hyponatraemia is rare in an otherwise healthy patient. It is associated initially with abdominal symptoms, followed by neuropsychiatric symptoms not present in this case. Hyponatraemic patients complain of muscle weakness but not perioral tingling. Ventricular ectopics are not a characteristic feature of hyponatraemia.

Such a severe hypokalaemia in the absence of urinary or GI losses of potassium would be extremely rare in an otherwise healthy patient. In addition, hypokalaemia presents more commonly with skeletal muscle weakness and myalgia leading to flaccid paralysis in extremis. Although cardiac arrhythmias are a feature of hypokalaemia, perioral tingling is not common, and seizures are rare.

There is nothing in the clinical presentation that suggests the patient is hypoxic. Paraesthesia and ventricular ectopics are not features of hypoxia and a hypoxic seizure would be extremely unlikely at a PaO2 of 8.5 kPa.

Answer Statistics

1

6%

2

89%

3

6%

4

1%

Times answered: 309