First Degree Heart Block with Multi-focal Atrial Ectopics in an Adolescent Girl Due to Hypomagnesemia.

Nutritional deficiencies are common in adolescent children and include deficiencies of both micro- and macronutrients. Magnesium is an important mineral that is essential for maintenance of numerous electrophysiological and biochemical processes in the body. We report an adolescent girl who developed an episode of syncope with first degree heart block on electrocardiography and run of multifocal atrial ectopics on 24 h holter monitoring. Serum magnesium was found to be low with decreased urinary magnesium excretion. There were no other electrolyte abnormalities. Structural heart disease was ruled out by a normal echocardiogram. The rhythm changes were attributable to nutritional hypomagnesemia and were promptly reversed on correcting the hypomagnesemia.

INTRODUCTION

Magnesium is the second most abundant cation in the body after potassium.[1] It is chiefly distributed in bone and muscle and a very small amount is present in circulation. It is essential for calcium metabolism and energy production and maintains electrical neutrality in cells. Magnesium deficiency can result in arrhythmias and nervous irritability. We report an adolescent girl who developed first degree heart block with multifocal atrial ectopics due to hypomagnesemia.

CASE REPORT

A 12-year-old adolescent girl presented with history of sudden onset of loss of consciousness (LOC) while walking to school one morning. The LOC lasted for a minute with no memory for the event and was not associated with abnormal movements of limbs, deviation of eyes, abnormal posturing, or loss of bowel or bladder control. She complained of headache during recovery. Past, medication, and family history were unremarkable.

On examination; her heart rate was 119/minute, regular in rhythm, and normal in volume and character. Respiratory rate was 20/min and blood pressure was 100/70 mm Hg with no postural drop. Her anthropometry revealed her weight and height to be on the tenth centile on World Health Organization (WHO) growth charts. General examination was unremarkable. Cardiovascular system examination as well as rest of the systemic examination was essentially normal. An electrocardiopgram (ECG) was obtained in view of resting tachycardia which revealed abnormal P-wave morphology and increased PR interval (220 ms) in most chest leads [Figure 1].

Figure 1

Abnormal P-waves with prolonged PR interval

Biochemical investigations were all normal except for a low serum magnesium level of 0.9 mg/dl (normal value: 1.5-2.5 mg/dl). Fractional magnesium excretion was 0.9% (>2% indicates renal magnesium wasting) and urinary calcium/creatinine ratio was 0.12 (normal < 0.2). The acid-base status, chest X-ray, and echocardiography were all normal. With the above findings, a diagnosis of syncope due to first degree heart block as a result of hypomagnesemia was considered. A 24 h holter recording was also obtained which showed a run of multifocal atrial ectopics with a maximum heart rate of 160/min.

The child was treated with intramuscular magnesium sulfate injections at a dose of 25 mg/kg/day for 3 consecutive days. Following treatment, serum magnesium increased to 2 mg/dl and ECG changes normalized [Figure 2]. The child was discharged home with advice to take oral magnesium supplementation and foods rich in minerals. Serum magnesium levels and ECG were obtained at 2 weeks follow-up and were found to be within normal limits with no further syncopal attacks.

Figure 2

Normal P-waves and PR interval after magnesium correction

DISCUSSION

Hypomagnesemia is known to be associated with both supraventricular as well as ventricular tachyarrhythmia.[2] It is also one of the known causes of first degree heart block. Magnesium therapy has been found to be beneficial in atrial fibrillation, multifocal atrial tachycardia, paroxysmal supraventricular tachycardia, ventricular premature complexes, ventricular tachycardia, polymorphic ventricular tachycardia, and ventricular fibrillation.[3] Magnesium prevents arrhythmia by reduction of automaticity, prolongation of sinus recovery time, reduction of atrioventricular nodal conduction, and blockade of anterograde and retrograde conduction along an accessory pathway.[24567]

In our case, the girl had prolonged PR interval on ECG which was suggestive of first degree heart block along and there was prompt resolution of symptoms and ECG changes soon after the correction of hypomagnesemia. Although prolonged PR interval is generally considered benign, an increased risk of atrial fibrillation, pacemaker implantation, and death was seen in a follow-up study of 7,575 individuals mostly adults with first degree AV block in the Framingham heart study.[8] The cause of hypomagnesemia is attributed to dietary deficiency as she was undernourished and her food intake was largely devoid of nuts, legumes, and green leafy vegetables which are rich sources of magnesium.[9] The normal dietary requirement of magnesium ranges from 65 mg/day for infants and young children up to 3 years of age, 110 mg/day for children between 4 and 8 years of age, and 350 mg/day for children older than 8 years of age.[10] There was no history of laxative or diuretic abuse in the girl and renal parameters were normal. Renal magnesium wasting and familial hypomagnesemic hypercalciuria were ruled out by appropriate tests. The finding of normal potassium levels and the absence of metabolic alkalosis makes the possibility of Gitelman syndrome unlikely in the child.

CONCLUSION

Dietary deficiencies are common among adolescent girls and one should actively seek to find and treat them even in asymptomatic individuals to promote a healthy living. Hypomagnesemia can lead to syncope due to arrhythmias and so evaluation of a child with syncope should also include screening for mineral deficiencies. Prompt recognition and treatment of hypomagnesemia results in rapid reversal of symptoms and may even reduce fatality in life threatening arrhythmias.