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Electromyographic changes of motor unit activity in horses with induced hypocalcemia and hypomagnesemia

Inge D. WijnbergDepartment of Equine Sciences, Internal Medicine Section, Faculty of Veterinary Medicine, Utrecht University, 3508 TD Utrecht, The Netherlands.

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Johannes H. van der KolkDepartment of Equine Sciences, Internal Medicine Section, Faculty of Veterinary Medicine, Utrecht University, 3508 TD Utrecht, The Netherlands.

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Hessel FranssenDepartment of Clinical Neurophysiology, Rudolph Magnus Institute, University Medical Center, 3584 CX Utrecht, The Netherlands.

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Henk J. BreukinkDepartment of Equine Sciences, Internal Medicine Section, Faculty of Veterinary Medicine, Utrecht University, 3508 TD Utrecht, The Netherlands.

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Abstract

Objective—To determine whether electromyographic abnormalities are evident in skeletal muscles in horses with induced hypocalcemia and hypomagnesemia.

Animals—7 healthy adult Dutch Warmblood horses.

Procedure—Electromyographic examination was performed in the lateral vastus, triceps, and subclavian muscles before and after IV infusion of EDTA. An initial dose (mean ± SD, 564 ± 48 ml) of a 10% solution of sodium EDTA was administered IV during a period of 21 ± 7.3 minutes to establish a blood concentration of ionized calcium of approximately 0.5 mMol/L. Average rate of EDTA infusion to maintain ionized calcium at this concentration was 6.6 ml/min.

Results—Mean blood concentrations of ionized calcium and magnesium were 1.39 ± 0.06 and 0.84 ± 0.09 mM, respectively before EDTA infusion; after EDTA infusion, concentrations were 0.48 ± 0.05 and 0.44 ± 0.20 mM, respectively. This state induced positive waves; fibrillation potentials; doublets, triplets, and multiplets; complex repetitive discharges; and neuromyotonia. Analysis of motor unit action potentials (MUAP) after EDTA infusion revealed an increase in prevalence of polyphasic and complex MUAP in all muscles.

Conclusion and Clinical Relevance—None of the horses had classical signs of hypocalcemia and hypomagnesemia. In contrast, all horses had spontaneous activity in the measured muscles indicative of nerve hyperirritability. Calcium and magnesium deficits appear to have consequences, which may be subclinical, affecting functions of the neuromuscular system. This is of interest for equestrian sports in which hypocalcemia and hypomagnesemia are expected, such as during endurance rides. (Am J Vet Res 2002;63:849–856)

Abstract

Objective—To determine whether electromyographic abnormalities are evident in skeletal muscles in horses with induced hypocalcemia and hypomagnesemia.

Animals—7 healthy adult Dutch Warmblood horses.

Procedure—Electromyographic examination was performed in the lateral vastus, triceps, and subclavian muscles before and after IV infusion of EDTA. An initial dose (mean ± SD, 564 ± 48 ml) of a 10% solution of sodium EDTA was administered IV during a period of 21 ± 7.3 minutes to establish a blood concentration of ionized calcium of approximately 0.5 mMol/L. Average rate of EDTA infusion to maintain ionized calcium at this concentration was 6.6 ml/min.

Results—Mean blood concentrations of ionized calcium and magnesium were 1.39 ± 0.06 and 0.84 ± 0.09 mM, respectively before EDTA infusion; after EDTA infusion, concentrations were 0.48 ± 0.05 and 0.44 ± 0.20 mM, respectively. This state induced positive waves; fibrillation potentials; doublets, triplets, and multiplets; complex repetitive discharges; and neuromyotonia. Analysis of motor unit action potentials (MUAP) after EDTA infusion revealed an increase in prevalence of polyphasic and complex MUAP in all muscles.

Conclusion and Clinical Relevance—None of the horses had classical signs of hypocalcemia and hypomagnesemia. In contrast, all horses had spontaneous activity in the measured muscles indicative of nerve hyperirritability. Calcium and magnesium deficits appear to have consequences, which may be subclinical, affecting functions of the neuromuscular system. This is of interest for equestrian sports in which hypocalcemia and hypomagnesemia are expected, such as during endurance rides. (Am J Vet Res 2002;63:849–856)