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Q1: What is the abnormality on the 12 lead electrocardiogram?
The electrocardiogram (see p 721) shows a prolonged QT interval and a prolonged QTc.
Q2: What are the possible causes of the abnormality of the electrocardiogram?
The various causes of a prolonged QTc are listed in box 1.
Box 1: Causes of long QT
Long QT syndromes: Jervell and Lange-Nielsen syndrome (autosomal recessive, congenital deafness, mutism, and sudden death); Romano-Ward syndrome (autosomal dominant transmission with genetic heterogeneity is commoner in females and sudden death); congenital long QT syndrome
Drugs: antiarrhythmics (quinidine, procainamide, amiodarone and sotalol); psychiatric drugs (pimozide, chlorpromazine, tricyclic antidepressants, lithium); others (cisapride, tacrolimus, terfenedine, quinine, and chloroquinine)
Q3: What is the likely diagnosis?
The diagnosis is hypocalcaemia. The combination of the electrocardiogram with prolonged QT and QTc associated with muscle cramps, weakness, and cataracts is supportive of a diagnosis of hypocalcaemia. Further investigations revealed a serum corrected calcium of 1.13 mmol/l (normal range 2.20–2.60 mmol/l) and a serum phosphate of 1.50 mmol/l (0.70–1.40 mmol/l) and normal renal function, with undetectable serum parathyroid hormone (PTH), suggesting a diagnosis of hypoparathyroidism.
Hypocalcaemia can be caused by a number of conditions (box 2). The symptoms of hypocalcaemia include neuromuscular irritability, paraethesia, muscle cramps, and seizures. The symptoms do not correlate with the absolute degree of hypocalcaemia.1 Decreased total serum calcium, with a normal ionised fraction can exist with hypoproteinaemia or acidosis. These possibilities should be excluded by measuring serum albumin concentration and calculating the corrected serum calcium concentration or by directly measuring serum ionised calcium concentration. Alkalosis decreases ionised serum calcium concentration and may precipitate tetany.
Box 2: Causes of hypocalcaemia
Primary hypoparathyroidism: idiopathic, postsurgical
Hypocalcaemia associated with malignant disease
Toxic shock syndrome
Vitamin D deficiency: dietary, malabsorption, anticonvulsant therapy, chronic liver disease, chronic renal disease, vitamin D dependent rickets
Electrocardiographic manifestations of hypocalcaemia include prolongation of the QT interval particularly the QTc and T wave changes such as peaking and inversion. The QT interval returns to normal rapidly after correction of hypocalcaemia, but the T wave abnormality may be slower to regress.1 A prolonged QT interval should theoretically predispose to ventricular tachycardia, but this arrhythmia is rare in association with hypocalcaemia.1
Hypocalcaemia with hyperphosphataemia and normal renal function strongly suggest a diagnosis of hypoparathyroidism. This commonly may occur after previous neck surgery suggesting postsurgical hypoparathyroidism. Idiopathic hypoparathyroidism may be associated with Addison's disease, primary hypothyroidism, type 1 diabetes, and primary hypogonadism or associated with Addison's disease, chronic mucocutaneous candidiasis as part of autoimmune polyglandular syndrome type II (Schmitd's syndrome).2
Pseudohyperparathyroidism is associated with similar biochemical features but with raised serum PTH because of target organ resistance. There are various subtypes and is characterised by short stature, round face, shortening of the metacarpals, and reduced intelligence.3 Hypocalcaemia is frequently associated with hypomagnesaemia. Hypomagnesaemia may be caused by chronic alcoholism, malabsorption, prolonged parental nutrition or the use of cisplatin, aminoglycosides and diuretics.4 Hypomagnesaemia may cause hypocalcaemia, both by impaired secretion of PTH from the parathyroid glands and by end organ resistance to the effects of PTH.5
Thus in patients with tetany or convulsions and low serum calcium with low serum PTH unresponsive to calcium supplements hypomagnesaemia should be considered.