Tear volume, turnover rate, and flow rate in ophthalmologically normal horses.

The research article investigates the tear volume, turnover rate, and flow rate in healthy horses using a technique called fluorophotometry. From their findings, the researchers suggest that increased dosing regimens or constant infusion methods for topical administration of eye medications may be needed when treating horses for ocular diseases.

Methodology

• The study was a lab-based experiment performed on 12 mares without any existing eye diseases.
• 2 microL of 10% sodium fluorescein, a dye which fluoresces under a specific light, was added to one eye of each horse.
• Tear samples were then collected through microcapillary tubes from the inner corner of the eye (inferonasal conjunctival culde-sac) at 7 different time intervals: 0, 2, 4, 6, 10, 15, and 20 minutes after the dye was instilled.
• The fluorescein concentrations in the collected tear samples were assessed using a computerized scanning ocular fluorophotometer, a special device that measures the fluorescein’s intensity.
• Using two different mathematical methodologies (the including method and the excluding method), a decay curve plot was established to determine the tear flow rate and volume by monitoring concentration changes over time.

Findings

• The results showed that fluorescein concentration in tears decreased in a fast, first-order manner.
• The including method yielded an average tear volume of 360.09 microL, a turnover rate of 12.22%/min, and a flow rate of 47.77 microL/min.
• In comparison, the excluding method yielded values of 233.74 microL for tear volume, 13.21%/min for turnover rate, and 33.62 microL/min for flow rate.
• The correlation coefficients for the natural logarithm of the fluorescein concentration against time were 0.93 +/- 0.12 for the including method and 0.98 +/- 0.03 for the excluding method, indicating that excluding method produced more accurate results.

Conclusion

• According to the excluding method (which was considered more precise), the horses had a tear flow rate of 33.62 microL/min and a tear volume of 233.74 microL.
• This implies a complete cycling of the tear volume in about 7 minutes, suggesting increased dosing regimens or constant infusion methods may be required for topical application of eye medications when treating horses for corneal disease where high ocular surface concentrations are paramount.