Effect of orally administered melatonin on intraocular pressure of ophthalmologically normal dogs

Effects of long-term oral administration of melatonin on tear production, intraocular pressure, and tear and serum melatonin concentrations in healthy dogs

OBJECTIVE

To evaluate the effects of long-term (30-day) oral administration of melatonin on tear production, intraocular pressure (IOP), and concentration of melatonin in the tears and serum of healthy dogs.

ANIMALS

20 healthy sexually intact adult male dogs.

PROCEDURES

10 dogs were given melatonin (0.3 mg/kg, PO, q 24 h, administered in food at 9 am), and 10 dogs were given a placebo. Tear and serum melatonin concentrations, IOP, and tear production (determined with a Schirmer tear test) were recorded before (baseline) and 30 minutes, 3 hours, and 5 hours after administration of melatonin or the placebo on day 1 and 30 minutes after administration of melatonin or the placebo on days 8, 15, and 30.

RESULTS

Data collection time had significant effects on tear production, IOP, and tear melatonin concentration but not on serum melatonin concentration. Treatment (melatonin vs placebo) had a significant effect on tear melatonin concentration, but not on tear production, IOP, or serum melatonin concentration; however, tear melatonin concentration was significantly different between groups only 30 minutes after administration on day 1 and not at other times.

CLINICAL RELEVANCE

In healthy dogs, long-term administration of melatonin at a dosage of 0.3 mg/kg, PO, every 24 hours did not have any clinically important effects on tear production, IOP, or serum or tear melatonin concentrations.

Does the order of intraocular pressure measurement affect tonometry results?

PURPOSE

To determine whether the order of intraocular pressure (IOP) measurement affects readings, regardless of which eye is measured first.

METHODS

Intraocular pressure was measured in 31 and 41 dogs using applanation and rebound tonometry, respectively. Initially, IOP was measured in the first (randomly chosen) eye (reading A), followed by measurement in the fellow eye (reading B), and a third (repeated) measurement in the first eye (reading C). After 15 minutes, measurements were repeated in reverse order (readings D - F).

RESULTS

Applanation tonometry revealed significant differences between readings A & B (15.6 ± 2.3 and 14.8 ± 2.7 mm Hg, respectively, p = .02), A & C (15.6 ± 2.3 and 14.5 ± 2.4 mm Hg, respectively, p = .002), D & E (14.5 ± 2.3 and 13.7 ± 2.1 mm Hg, respectively, p = .02), D & F (14.5 ± 2.3 and 13.9 ± 1.9 mm Hg, respectively, p = .05), and A & E (15.6 ± 2.3 and 13.7 ± 2.1 mm Hg, respectively, p = .001). Rebound tonometry yielded similar results, with additional differences between B & C (19.1 ± 3.0 and 18.2 ± 2.4 mm Hg, respectively, p = .002) and E & F (18.7 ± 3.3 and 18.2 ± 3.3 mm Hg, respectively, p = .02).

CONCLUSIONS

Intraocular pressure measured in the first eye, whether right or left, is higher than in the fellow eye. Repeated tonometry in the same visit could result in a significant IOP decrease, though the magnitude may not be clinically appreciable.