Better methods of clinically assessing mucus functions are required

Aqueous deficiency is a contributor to evaporation-related dry eye disease

Dry eye disease aetiologies can be classified dichotomously into aqueous deficient and evaporative types although many cases involve combinations of both. Differential diagnosis can be confounded by some features of dry eye disease being common to both aetiologies. For example, short tear break-up times are prime diagnostic findings of tear instability due to lipid and/or mucin deficiencies, but thin tear layers in aqueous deficient eyes also shorten tear break-up times, even at normal range rates of evaporation in eyes without lipid and/or mucin deficiencies. Because tear instability and short tear film break-up times due to thin tear layers can be independent of lipid and/or mucin deficiency, aqueous deficiency can be another form of evaporation-related dry eye. Conversely, tear layers which are thickened by punctal occlusion can be less susceptible to tear break-up. An inflamed lacrimal gland producing reduced quantities of warmer tears can be a basis for thin tear layers and tear instability demonstrated by shorter tear break-up times. Commonly used clinical tests for aqueous deficiency can be unreliable and less sensitive. Consequently, failure to detect or confirm aqueous deficiency as a contributor to short tear break-up times could result in too much weight being given to a diagnosis of meibomian gland deficiency. Less successful treatment outcomes may be a consequence of failing to detect aqueous deficiency. Refining disease classification by considering aqueous deficiency as a contributor to, or even a form of evaporation-related dry eye, could be the basis for more comprehensive and appropriate treatment strategies. For example, some treatment methods for evaporation-related dry eye might be appropriate for aqueous and mucin-deficient as well as lipid-deficient dry eyes. Anti-inflammatory treatment for the lacrimal gland as well as the conjunctiva, may result in increased aqueous production, reduced tear temperature, tear instability and evaporation rates as well as lower osmolarity.

Clinical subtypes of dry eye in youthful clinical sample in Ghana

PURPOSE

To evaluate in a cohort of patients with dry eye disease (DED) the distribution of patients with asymptomatic and symptomatic dry eye involving aqueous deficient or the evaporative subtype of DED.

METHODS

This was a clinic-based cross-sectional study of consecutive patients visiting the University of Cape Coast eye clinic for a comprehensive eye examination. Eligible participants completed the Ocular Surface Disease Index (OSDI), ocular surface staining and tear break up time. Subjects with dry eye (symptomatic or asymptomatic) underwent clinical examination comprising lid margin assessments, meibomian gland expressibility and quality assessments, and Schirmer 1 test.

RESULTS

A total of 172 patients were included in the study. The mean age of the eligible participants was 22.7 (±3.7) years, with a range of 17-35 years. There were 83 males and 89 females in the sample. There was an association between the form of the dry eye whether symptomatic or asymptomatic and sex, (X² = 6.51 df = 1 p = 0.011). The frequency of symptomatic aqueous-deficient dry eye was 9(5.2%) whiles that of the symptomatic evaporative dry eye was 20(11.6%). The frequencies of the asymptomatic aqueous-deficient dry eye and asymptomatic evaporative dry eye were 9(5.2%) and 12(7.0%) respectively. The frequency of symptomatic mixed dry eye was 30(17.4%) and the asymptomatic mixed dry eye was 8(4.7%). The distribution of unclassifiable dry eye among the symptomatic dry eye group was 41(23.8%) and 43(25%) among the asymptomatic dry eye group.

CONCLUSION

The number of patients exhibiting signs of the evaporative dry eye resulting from meibomian gland dysfunction far outnumbers those of aqueous deficient dry eye but there remains a high unclassifiable group with an unknown etiology. Further, studies are needed to explore the other etiology or etiologies of dry eye apart from meibomian gland dysfunction and reduced aqueous production.

Tear instability importance, mechanisms, validity and reliability of assessment

PURPOSE

To examine the factors which contribute to tear stability and the validity and reliability of methods used for assessing tear break up time which is a core part of an examination of tear stability in dry eye patients.

METHODS

A review of publications which are relevant to tear stability and its assessment.

RESULTS

Tear break up time may be more invasive than intended if difficulty avoiding blinking during assessment results in reflex tearing. Notwithstanding control of instilled volume and concentration of fluorescein, on-eye dilution is highly variable according to resident tear volume. Blinking to evenly distribute fluorescein may improve tear and lipid layer thickness so habitual tear function is not assessed. Emphasis on a role for Meibomian gland dysfunction as a cause of tear instability may be appropriate in many cases but ignores the roles for other sources of tear lipid and other non-lipid contributions to tear instability such as aqueous or mucus deficiency, desiccated epitheliopathy or anomalous blinking. Objective less-invasive methods eliminate problems of inter-observer variability and can reliably 'maintain vigilance' over wide areas of the tear layer. However less-invasive results to date include mean tear break up findings which are both shorter and longer than expected for normal controls.

CONCLUSIONS

Fluorescein tear break up time assessments cannot be standardised and less-invasive methods are not yet standardised. Objective less-invasive and subjective fluorescein break up time tests do not appear to be measuring the same tear phenomena although both should be performed before other invasive procedures.