MEASUREMENT OF THE RESISTANCE TO FLUID-FLOW WITHIN THE LACRIMAL OUTFLOW SYSTEM

Background: Although the presence of complete obstruction within the h uman lacrimal drainage system can be easily determined with diagnostic techniques currently available, it is not possible to quantitate degr ees of partial obstruction. The authors have developed a new instrumen t that directly measures pressures associated with fluid flow, allowin g the calculation (pressure divided by 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 clinica lly useful, allowing the diagnosis of lacrimal disease before complete obstruction. Of course, it is first necessary to establish normal val ues, which was the goal of the study. Methods: A constant flow rate of distilled water was irrigated into the lacrimal system through the lo wer canaliculus, and pressure was measured continuously. Resistance th en was calculated as the differential of pressure to flow rate (pressu re divided by flow). In the first stage of a two-part study, the autho rs determined the resistance to fluid flow in 43 tear ducts of 24 heal thy patients. In this group, the resistance within the entire lacrimal system was obtained. In the second stage of this study, the authors m easured the resistance in 28 tear ducts of 26 patients after successfu l lacrimal bypass surgery (dacryocystorhinostomy). Because dacryocysto rhinostomy bypasses the lower drainage system (sac and nasolacrimal du ct), the resistance measured in this group of patients was determined only by the upper (canalicular) system. Results: The mean resistance w ithin the normal lacrimal system is 49.5 +/- 17.0 mmHg . seconds/ml. J ust 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 informat ion on average dimensions obtained from casts of the lacrimal drainage system. Conclusion: The authors have developed a new instrument to me asure the resistance to fluid flow within the human lacrimal outflow s ystem, and resistance values in control subjects have been documented.