Letter: Effect of Aqueous Tears on Topical Fluorescein Tracer Emission Signal from Cornea and Anterior Chamber

Dear Editor:

A key determinant of intraocular pressure is aqueous humor flow rate, which is measured by determining the clearance rate of fluorescein applied topically to the eye. A fluorophotometer excites the fluorescein with blue light and a photodetector measures the emitted green fluorescence in the cornea and anterior chamber (AC). Beyond a certain fluorescein concentration, attenuation of the excitation beam intensity can occur due to light absorption by fluorescein in the more superficial layers of the eye, leading to invalid measurements. ¹ The amount of fluorescein applied to and retained by the eye is essential for minimizing this inner filter effect and for accurate assessment of flow rate. The purpose of this study was to determine if fluorescein uptake in the cornea and AC is related to basal tear production.

We measured tears by Schirmer’s test (ST) and assessed corneal staining and central cornea thickness, calculated cornea volume, AC depth and volume, and fluorescence in the cornea (K_fl), and AC (AC_fl) in 39 participants with ocular hypertension or open-angle glaucoma (NCT04412096). Fluorescence was measured using the Fluorotron^TM Master (Ocumetrics, Inc., Mountain View, CA, USA). The evening before the flow measurement, study participants self-administered 5–10 drops of topical 2% sodium fluorescein at 5-min intervals. Dosing was based on subject age,^2,3 timing of dosing, and history of prior dosing to ensure that each subject had an adequate fluorescein concentration for the measurements. Demographics included 51% females, mean age 60.7 ± 9.8 years, and self-reported Asian (6%), Black (26%), and White (68%). Other characteristics included diabetes (31%), hypertension (46%), smoking history (31%), and glaucoma medications (67%).

Significant differences in ST, K_fl, and AC_fl were found between the self-reported White and Black racial groups. By mixed model analyses, Black participants had 7.6 mm higher ST, 662 U lower K_fl, and 106 U lower AC_fl compared to the overall mean of White participants (Fig. 1). ST, K_fl, and AC_fl were not associated with diabetes, hypertension, sex, age, glaucoma medications, or ocular biometry. Using principal component (PC) analysis, race and corneal staining were inversely associated with ST and explained most of the variance (PC1, 27%), while age, K_fl, AC_fl, and the number of fluorescein drops had a mixed relationship that explained the second component of variance (PC2, 16%).

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FIG. 1.

(A) Corneal fluorescence (K_fl) (R² = 0.08, P = 0.02) and (B) anterior chamber fluorescence (AC_fl) (R² = 0.07, P = 0.03) by Schirmer’s test (ST) among self-reported racial groups. Black participants had higher mean ST associated with lower mean K_fl and mean AC_fl than White and Asian participants.

Based on these results, it appears that those with lower basal tear production, assessed by low ST values, have higher K_fl and AC_fl. Two prior fluorophotometric studies^4,5 of fluorescein and dry eye disease support this finding. Thus, individuals with low ST may require fewer fluorescein drops to achieve proper threshold K_fl and AC_fl compared to individuals with near-normal ST. In this cohort, there is an inverse relationship between ST and fluorescence signal in the cornea and AC (Fig. 1). This observation has implications regarding the delivery of fluorescein drops for aqueous humor flow studies and perhaps absorption of topical medications. Such medications also might affect fluorescein absorption. These ideas based on our results are worthy of further investigation.