Luteinizing hormone concentrations in healthy horses and horses with trigeminal-mediated headshaking over an 8-hour period.

BACKGROUND: Trigeminal-mediated headshaking results from a low threshold for firing of the trigeminal nerve. A seasonal component has been implicated in onset of clinical signs, which occur during the spring and summer months. Geldings are overrepresented in the affected population and hormonal differences as compared to a healthy control population of geldings might contribute to headshaking. OBJECTIVE/HYPOTHESIS: To assess concentrations of luteinizing hormone (LH) over an 8-hour period in gelded healthy controls and horses affected with headshaking. Our hypothesis was that geldings with seasonal headshaking would have higher concentrations of LH over an 8-hour period compared to control horses during the summer when affected horses manifested headshaking. ANIMALS: Twelve geldings (6 controls and 6 affected). METHODS: Prospective controlled trial. Blood samples were drawn every 15 minutes over an 8-hour time period during summer from all horses to measure circulating LH concentrations by using a radioimmunoassay for equine LH. All affected horses were actively affected by headshaking at the time of sample collection. RESULTS: No statistically significant differences in LH concentrations were found throughout the study period in headshakers as compared to control horses. Time had no significant effect, but a slight decrease in LH concentrations was observed for all horses. The main limitation of the study was the low number of horses. CONCLUSIONS AND CLINICAL IMPORTANCE: Horses affected with headshaking did not have significant differences in circulating LH during the late summer as compared to control horses.

Intravenous infusion of magnesium sulfate and its effect on horses with trigeminal-mediated headshaking.

BACKGROUND: Trigeminal-mediated headshaking results from low-threshold firing of the trigeminal nerve resulting in apparent facial pain. Magnesium may have neuroprotective effects on nerve firing that potentially dampen signs of neuropathic pain. This hypothesis has not been investigated in horses with trigeminal-mediated headshaking. OBJECTIVE: To investigate head-shaking behavior in affected horses after IV magnesium sulfate infusion. ANIMALS: Six geldings with trigeminal-mediated headshaking. METHODS: Prospective randomized crossover study. Horses were controlled for diet and infused IV with 5% dextrose solution (DS; control solution at 2 mL/kg body weight [BW]) and MgSO4 50% solution (MSS at 40 mg/kg BW). Head-shaking behavior was recorded at times T0 (baseline, before infusion) and T15, T30, T60, and T120 minutes post-infusion. Venous blood variables such as pH, HCO3 - , standard base excess (SBE), Na+ , Cl- , K+ , Ca2+ , Mg2+ , total magnesium (tMg), glucose, and lactate were measured; strong ion difference (SID) and anion gap (AG) were calculated for each time point. RESULTS: Blood variables including pH, Na+ , Cl- , K+ , SID, AG, lactate, Ca2+ , tMg, and Mg2+ had significant changes with MSS as compared to DS treatment. Glucose, SBE, and HCO3 - did not have significant changes. A 29% reduction in head-shaking rate occurred after MSS treatment but no change occurred after DS treatment. CONCLUSIONS AND CLINICAL IMPORTANCE: Administration of MSS IV increased plasma total and ionized magnesium concentrations and significantly decreased head-shaking behavior in horses with trigeminal-mediated headshaking.