Purpose: The aim of this study was to explore the use of the 'drop test' as a method of assessing maximal lacrimal outflow capacity, and to measure the relative contribution of the superior and inferior canaliculi to the drainage capacity in normal subjects.
Method: The drop test involves instilling measured aliquots of normal saline over 3-min periods to raise the tear lake medially. Both eyes were assessed; however, for the right lacrimal drainage system only, sequential insertion and then removal of silicone punctal plugs was performed. After each stage the maximal lacrimal drainage capacity was measured. no punctal plugs were placed in the puncta on the left side.
Results: Complete data were collected from 20 subjects with a mean age of 35.6 years. The intraclass correlation coefficient for the five left eye readings was 0.98 (CI 0.96-0.99) and the limits of agreement of a single reading were -22.6 to +93.0 micro L/3 min. Without intervention, no statistically significant difference was found in the mean lacrimal outflow between the left and right eyes (P = 0.16). A statistically significant reduction in outflow resulted from punctal occlusion (P < 0.05). Presenting the proportion of lacrimal outflow as a percentage of the combined values of the superior and inferior canaliculi, 59.9% of outflow occurred through the inferior canaliculus.
Conclusion: The drop test was found to provide a simple and repeatable method of assessing lacrimal drainage in a minimally invasive manner in the clinical setting. In healthy volunteers in the supine position 60% of maximal lacrimal outflow capacity occurs through the inferior canaliculus.