Measurement of the Resistance to Fluid Flow within the Lacrimal Outflow System

Background: Although the presence of complete obstruction within the human lacrimal drainage system can be easily determined with diagnostic techniques currently available, it is not possible to quantitate degrees of partial obstruction. The authors have developed a new instrument that directly measures pressures associated with fluid flow, allowing the calculation (pressure ÷ flow) of resistance within the tear drainage system. The authors believe that the ability to measure the degree of partial lacrimal stenosis quantitatively will be clinically useful, allowing the diagnosis of lacrimal disease before complete obstruction. Of course, it is first necessary to establish normal values, which was the goal of the study.

Methods: A constant flow rate of distilled water was irrigated into the lacrimal system through the lower canaliculus, and pressure was measured continuously. Resistance then was calculated as the differential of pressure to flow rate (pressure flow). In the first stage of a two-part study, the authors determined the resistance to fluid flow in 43 tear ducts of 24 healthy patients. In this group, the resistance within the entire lacrimal system was obtained. In the second stage of this study, the authors measured the resistance in 28 tear ducts of 26 patients after successful lacrimal bypass surgery (dacryocystorhinostomy). Because dacryocystorhinostomy bypasses the lower drainage system (sac and nasolacrimal duct), the resistance measured in this group of patients was determined only by the upper (canalicular) system.

Results: The mean resistance within the normal lacrimal system is 49.5 ± 17.0 mmHg · seconds/ml. Just more than half of the resistance, 26.8 ± 13.1 mmHg · seconds/ ml (54%), comes from the canaliculi, leaving 22.7 mmHg · seconds/ml (46%) attributable to the lower system. The authors comment on comparisons between these values and theoretical values calculated using information on average dimensions obtained from casts of the lacrimal drainage system.

Conclusion: The authors have developed a new instrument to measure the resistance to fluid flow within the human lacrimal outflow system, and resistance values in control subjects have been documented.