Electrodiagnostic evaluation of hypomagnesemia in sheep.

Experiments were conducted to permit electrophysiological characterization of hypomagnesemia in sheep. For 6 weeks, adult wethers were fed: (1) a diet deficient in magnesium and calcium, (2) a diet deficient in magnesium, (3) a diet deficient in calcium and (4) a control diet. Weekly blood samples were collected to determine plasma concentrations of calcium (CA++), magnesium (Mg++) and potassium (K+). During the 6-week feeding period, electromyography (EMG), nerve conduction velocity (NCV) and strength-duration (S-D) determinations were conducted. At the end of a 6-week period, cerebrospinal fluid (CSF) samples also were taken from the cisterna magna and analyzed for Ca++, Mg++ and K+. The control plasma Mg++ concentration was 2.2 mg/100 ml. There was a depression (P less than .05) in plasma Mg++ in sheep that were fed diets deficient in Mg++ or deficient in Mg++ or Ca++. Plasma Mg++ concentrations in these two groups were .79 and .95 mg/100 ml, respectively, by the last week of diet feeding. EMGs performed during magnesium depletion were normal. In hypomagnesemic sheep, neither motor unit potentials (MUP) resulting from induced voluntary movement nor muscle-evoked potentials (EP) resulting from motor nerve stimulation showed differences (P less than .05) from those in control animals. NCV and muscle excitability (as evaluated by S-D curves) were normal in all sheep. CSF Ca++, Mg++ and K+ were not altered (P less than .05) in sheep fed any of the four diets.