Serum magnesium levels and urinary magnesium excretion in patients with chronic constipation

Abstract

Numerous etiologies for constipation are well recognized including abnormalities of serum electrolytes, most notably potassium and calcium. The dynamics of magnesium balance in patients with constipation is poorly understood. The index patient for this study was a 49 year old woman admitted to the hospital for abdominal pain associated with severe, life-long constipation. No other medical problems were identified. Her serum magnesium on admission was 1.6 mg/dl (nl 1.8-2.4). Serum bicarbonate was 20 mEq/1 suggesting renal tubular acidosis, possibly accounting for her hypomagnesemia through renal magnesium wasting. Urine magnesium was 37.4 mg/24hr (nl 12-293). Constipation improved with oral magnesium replacement. Five other patients with life-long constipation have also been found to have low serum magnesium (1.6 _+ 0.5 mg/dl; n=6, mean-SD). Only one patient had an appropriate urinary magnesium (less than 12 mg/24hrs) with the mean urinary magnesium of all six patients being 61.3 +-43.3 mg/24hr (range 10 to 102.2). All patients had early improvement in constipation symptoms with normalization of serum magnesium levels; however, long-term follow-up is not available at this time. In order to determine the prevalence of hypomagnesemia in the patient population with idiopathic constipation, we performed serum magnesium measurements on 25 consecutive patients with moderate to severe constipation (fewer than two spontaneous bowel movements a week). The mean magnesium level was 1.85+0.16 mg/dl. Seven patients were below the lower limit of normal in our laboratory (1.8). Hypomagnesemia associated with renal magnesium wasting may adversely influence colonic motility in a proportion of patients with life-long constipation.