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

Objective To evaluate the sensory–motor profile and physiological effects of epidural diphenhydramine (DPH) in sheep and to compare it with lidocaine. Study design Randomized, blinded, crossover trial with ≥ 2-week intervals between treatments. Animals Eight healthy adult male Zel sheep [24.0 ± 4.6 kg (mean ± standard deviation)]. Methods 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 analysed using Kruskal-Wallis and Wilcoxon signed-rank tests with significance set at p < 0.05. Results 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 follow-up at 3 months. Conclusions and clinical relevance Epidural DPH (2–4 mg kg -1 ) produces dose-dependent, predominantly sensory blockade with minimal motor impairment in sheep, while maintaining stable physiological variables.