Epidural diphenhydramine in sheep: an exploratory study of sensory-selective blockade and motor-sparing effects

To evaluate the sensory-motor profile and physiological effects of epidural diphenhydramine (DPH) in sheep and to compare it with lidocaine.

Randomized, blinded, crossover trial with ≥ 2-week intervals between treatments.

A group of eight healthy adult male Zel sheep [24.0 ± 4.6 kg (mean ± standard deviation)].

Lumbosacral epidural injections of DPH at doses of 1 (0.5%), 2 (1.0%), 3 (1.5%), or 4 (2.0%) mg kg^-1 (0.2 mL kg^-1) or lidocaine 2% at 4 mg kg^-1 (0.2 mL kg^-1) were performed. Sensory blockade was assessed by needle-prick testing, and motor blockade was assessed by the degree of ataxia and ability to stand. Heart rate (HR), respiratory rate (f_R), and temperature were monitored. Sensory and motor blockade outcomes were analyzed using Kruskal-Wallis and Wilcoxon signed-rank tests with significance set at p < 0.05.

Epidural DPH produced rapid-onset sensory and motor blockade in sheep (3-4 minutes) with dose-dependent effects; 2-3 mg kg^-1 yielded predominantly sensory blockade with minimal motor impairment, whereas 1 mg kg^-1 resulted in incomplete and inconsistent effects. At 4 mg kg^-1, DPH produced significantly prolonged sensory blockade compared with lidocaine [median (minimum-maximum): 126 (103-162) versus 86 (69-104) minutes; p < 0.001], while producing a shorter motor blockade [58 (48-63) versus 94 (71-129) minutes; p < 0.001], yielding a sensory-to-motor ratio of 2.2. HR, f_R, and temperature remained within normal limits across treatments (p > 0.05), and no adverse effects were observed during the 3-month follow-up.

Epidural DPH (2-4 mg kg^-1) produces dose-dependent, predominantly sensory blockade with minimal motor impairment in sheep while maintaining stable physiological variables.