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

Epidural diphenhydramine (2–4 mg/kg) provides prolonged, predominantly sensory blockade with minimal motor impairment in sheep.

This randomized, blinded, crossover study evaluated the sensory-motor profile and physiological effects of epidural diphenhydramine (DPH) in eight healthy adult male Zel sheep, comparing it to lidocaine 2%. Animals received lumbosacral epidural injections of DPH at doses of 1, 2, 3, or 4 mg/kg (0.2 mL/kg) or lidocaine at 4 mg/kg (0.2 mL/kg), with at least two-week washout intervals between treatments. Sensory blockade was assessed via needle-prick testing, and motor blockade was graded by ataxia and ability to stand. Epidural DPH produced rapid-onset (3–4 minutes) dose-dependent sensory and motor blockade. At 2–3 mg/kg, DPH yielded predominantly sensory blockade with minimal motor impairment, while 1 mg/kg produced incomplete and inconsistent effects. At 4 mg/kg, DPH provided significantly longer sensory blockade than lidocaine (median 126 vs. 86 minutes; p < 0.001) with considerably shorter motor blockade (58 vs. 94 minutes; p < 0.001), resulting in a favorable sensory-to-motor ratio of 2.2. Heart rate, respiratory rate, and temperature remained within normal physiological limits across all treatments, and no adverse effects were observed during a 3-month follow-up period. These findings suggest epidural DPH at 2–4 mg/kg offers clinically relevant sensory-selective analgesia with motor-sparing properties in sheep, potentially making it a useful alternative or adjunct to conventional local anesthetics in ruminant regional anesthesia.

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