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

To determine tear volume, turnover rate, and flow rate in ophthalmologically normal horses by use of fluorophotometry.

12 mares free of ophthalmic disease.

2 microL of 10% sodium fluorescein was instilled onto 1 eye of each horse, and tear samples were collected via microcapillary tubes from the inferonasal conjunctival culde-sac at 0, 2, 4, 6, 10, 15, and 20 minutes after instillation. Collected tear samples were then measured for fluorescein concentrations with a computerized scanning ocular fluorophotometer. A decay curve plot of concentration changes over time was used to determine tear flow rate and volume through 2 different mathematical treatments of the data (the including method and the excluding method).

Fluorescein concentration in tears decreased in a first-order manner. The including method yielded a mean tear volume of 360.09 microL, a turnover rate of 12.22%/min, and a flow rate of 47.77 microL/min. The excluding method yielded values of 233.74 microL, 13.21%/min, and 33.62 microL/min, respectively. Mean +/- SD correlation coefficients for the natural logarithm of the fluorescein concentration versus time were 0.93 +/- 0.12 for the including method and 0.98 +/- 0.03 for the excluding method.

The excluding method yielded more accurate results. A tear flow rate of 33.62 microL/min and a tear volume of 233.74 microL imply a complete recycling of the tear volume in approximately 7 minutes and suggest that increased dosing regimens or constant infusion methods for topical administration of ophthalmic drugs may be indicated when treating horses for corneal disease in which high ocular surface concentrations are needed.