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    Tear volume – a neglected issue?

    We report a new tool for measuring tear volume, which is essential for selecting the appropriate treatment for a patient with symptoms of dry eye.

     

    Saleh et al.16 suggest that the poor correlation between tests results from the fact that each test utilises a different mechanism to assess the ocular surface. Therefore, owing to the multifactorial nature of dry eye, many mechanisms may not apply to an individual dry-eye patient. Saleh et al. further demonstrated that, in a cataract population being screened for surgery, neither Schirmer nor PRT results agreed with symptoms (28 of 103 patients were symptomatic of dry eye based on questionnaire) and that PRT results showed no correlation with Schirmer results.

    It has been suggested that the ability of the PRT to differentiate aqueous dry-eyed from non-dry-eyed subjects is because of its ability to first absorb the tears naturally present in the lacrimal lake and then to measure the replenishment of fluid into the lake as a result of basal flow and/or mild stimulation. Subjects with aqueous deficiency cannot replenish their fluid as quickly, so less wetting of the thread occurs.18

    Blades and Patel19 have argued that the PRT test should take longer than 15 s as they and others have demonstrated that wetting is not linear: the threads they employed reached equilibrium in 120 s. Further adding complexity to our understanding of what is being measured is the possibility that the composition of the tear fluid can influence wetting length. Both lipids and mucins can influence flow and it has been demonstrated that, unlike simple migration of saline through a thread, variable tear composition likely increases the variability of both PRT and Schirmer measurements.19

    It is argued that leaving the thread in for a longer time will increase the accuracy of the test. For example, the measuring scale for PRT has a resolution of 1 mm; thus, if mean migration in 15 s is 9.2 mm, then the resolving power = 100 × 1/9.2 ,  11%. If the test runs for 60 s and mean wetting is 18 mm, then resolving power is reduced to 5.6%, increasing the ability to detect change or differences among test patients. Perhaps finding the time that provides the optimal balance between sufficient time for wetting and minimal irritation (which causes reflex tearing) is key.22

    In summary, although rarely used at present, the PRT and the lacrymeter offer advantages over the Schirmer test with respect to increased measurement reproducibility. To this end, they might provide both better diagnostic utility as well as serving as a more meaningful tool for therapeutic drug evaluation. The test, however, still demonstrates variability and compromised reproducibility because of patient variation in the volume, depth and shape of the lacrimal lake, the temperature of the environment in which the test is performed and the variation in tear composition between patients. The threads might be somewhat difficult to handle and insert, thus necessitating appropriate training. The testing time is vital and should not be less than 30 seconds.

    With the lacrymeter, the ophthalmologist and the general practitioner have a new tool to measure tear volume, which is essential for selecting the appropriate treatment for a patient with symptoms of dry eye.

     

    Claes Feinbaum
    Claes Feinbaum is Professor Emeritus Optometry at the University of Rostock, Germany. He is an Optometric Consultant for Specspavers ...

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