Evaluation of 0.1% and 1% atropine eyedrops in cats: A comparative study of tolerance, stability, and efficacy

Objective: Investigate the tolerance, stability, and efficacy of topical 0.1% and 1% atropine in cats. Procedures: Six cats underwent two trials separated by a 2-week washout period. One drop of artificial tears was placed in one randomly selected eye (control), and one drop of either 0.1% atropine (Trial I) or 1% atropine (Trial II) was placed in the other eye. Immediate adverse effects were recorded for severity (0–3) and duration (seconds). Horizontal pupil diameter (HPD), pupillary light reflexes (PLRs), intraocular pressure (IOP), Schirmer tear test-1 (STT-1), and heart rate (HR) were monitored at baseline then 8 h post-administration. PLRs were assessed for a total of 72 h. Stability was assessed weekly for 1 month in room temperature and refrigerated conditions, evaluating solution clarity, pH, and drug concentrations. Results: Adverse effects had a significantly lower severity score and shorter duration with 0.1% versus 1% atropine (severity 1.2 ± 0.4 vs. 2.5 ± 0.5, p =.010; duration 107.5 ± 53.3 vs. 293.3 ± 106.5 s, p =.009). HPD was significantly greater than baseline measurements as early as 40 min for both atropine formulations. Pupils were non-responsive for a significantly shorter duration with 0.1% versus 1% atropine (median 7 h vs. 47.5 h, p =.031). Compared with control eyes, IOP was significantly elevated by 1% atropine (p =.021) but not 0.1% atropine (p =.502). No significant differences were noted in STT-1 and HR measurements. Both solutions were stable in room temperature and refrigerated conditions for 1 month. Conclusions: Diluted 0.1% atropine was stable and better tolerated by cats, offering a potential alternative to feline patients that experience adverse effects from topical 1% atropine.