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: A group of 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 (fR), 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.

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, fR, and temperature remained within normal limits across treatments (p > 0.05), and no adverse effects were observed during the 3-month follow-up.

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.