EDITORIAL

TOMLINSON PhD, DSc, FCOptom¹

Dry eye or keratoconjunctivitis sicca (KCS) is a rubric for a number of clinical diseases characterised by a disturbance in the preocular tear film resulting in ocular surface disease (1). It can occur in many different eye disorders that disturb tear volume, composition and hydrodynamic factors. The consequent tear instability that results from deficiencies of one or more of the three layers of the tear film needs to be identified so that effective therapy, specific for each tear deficiency state can be initiated (2). Therefore accurate determination of the prevalence of dry eye and its successful treatment depend on making an accurate diagnosis of the condition. A number of subjective assessments and objective diagnostic tests are used clinically to measure the integrity and stability of the preocular tear film. The utility of these techniques is hampered by the lack of widely accepted criteria for each type of dry eye and the lack of correlation between the diagnostic tests, the reported symptoms and the conditions.

A number of studies of dry eye prevalence rely only on reports of symptoms for the definition of the condition. It is appropriate to diagnose dry eye from symptoms as the condition rarely progresses to cause damage to the ocular surface in the absence of symptoms (3). But others have argued (2) that symptoms alone are not sufficient for differential diagnosis of dry eye as patients with a range of different tear film disorders have symptoms of ocular irritation, and the same symptoms are often experienced by patients with different types of dry eye (2). Ideally symptoms and objective signs of dry eye are needed for diagnosis and measurement of prevalence rates but this is where the problem arises when comparing prevalence studies. Dependent on whether symptoms or signs or both are used the apparent prevalence of dry eye will vary (fig. 1). As an example in the Salisbury Eye Evaluation (SEE) (3) it was found that of the symptomatic patients only 1 in 6 had a low Schirmer test readings, 1 in 7 had Rose Bengal staining, and only 1 in 4 had both these clinical signs of dry eye. In the group symptomatic of dry eye measured by Linn et al (4), 80% had low tear break up times, 63% low Schirmer readings and 63% meibomian gland dysfunction. In the symptomatic patients observed by Versura (5), only 57% had objective signs of dry eye. But perhaps the best illustrations of the difficulty of applying both symptoms and signs to identify dry eye are given by McCarty (6) who found variable and limited relationships within clinical signs and between signs and the reports of severe symptoms. The poor agreement between objective tests for dry eye is shown by the four clinical tests (Schirmer’s, tear break up time, Rose Bengal and fluorescein staining) carried out on this population study in Melbourne. Out of the 635 patients in the study only 4 showed agreement between three of the tests and only 1 between all four tests. The poorest correlate with severe symptoms was Schirmer test, with only 12% of patients with low Schirmer results complaining of severe symptoms, the best correlate was fluorescein staining with 33% of those patients with staining complaining of severe symptoms (6).

Fig. 1. Shows that the prevalence rates found for dry eye in three general population studies depend on the criteria 
for diagnosis (symptoms or clinical measurements). The studies are de Melbourne Visual Impairment Project (MVIP) (6), 
Salisbury Eye Evaluation (SEE) (3) and the Shihpai Eye Study (Taiwan) (4).

The poor correlation between symptoms and signs of dry eye has been frequently reported in the literature (6,7). There are a number of reasons for this finding, firstly, symptoms may precede signs of dry eye. Secondly, dry eye is a heterogeneous condition differing in aetiology and pathophysiology (1) and different objective tests measure different aspects of tear physiology. Also symptoms of dry eye are not specific to a particular disorder (6), but a single objective test for dry eye is of limited value without a report of symptoms (3,6). The sensitivity and specificity of all tests of dry eye vary with the characteristics and biases of the study population (7). There is also high test/re-test variability in both questionnaires and objective tests for dry eye (8). In addition it should be remembered that the tear system is extremely volatile and the components interdependent, therefore the ideal measurements of tear physiology in dry eye diagnosis would be «simultaneous» (with all measurements can be taken at the same instant in time). This is obviously impossible. Finally as many of the objective tests for tear physiology are to some degree invasive, one measurement may affect subsequent ones and care must be exercised in the order in which measurements are carried out (i.e. least to most invasive).

In view of the difficulties of diagnosis of dry eye in prevalence studies, the use of a test battery (or panel) of measurements for dry eye has been suggested. The importance of establishing diagnostic criteria (based on accepted batteries of tests) was first identified by the NEI/Industry Workshop on clinical trials in dry eye (9). The dry eye workshop proposed four global tests for dry eye:

1. Validated questionnaire of symptoms.
2. Demonstration of ocular surface damage.
3. Demonstration of tear instability.
4. Demonstration of tear hyperosmolality.

The first three tests are readily available, a number of validated questionnaires of symptoms have been devised, ocular surface damage may be measured by Rose Bengal staining and tear instability by break up time. But tear osmolality is much more difficult to measure, currently requiring laboratory techniques with high technical expertise.

A number of different sets of criteria for the definition of dry eye (KCS), usually including Sjögren’s syndrome, have been developed. The need for a comprehensive and agreed sets of tests to define the array of dry eye conditions is clear. Various levels of prevalence have been found for dry eye in different studies. This variability is due, in part, to the different criteria applied in definition. It is anticipated that new criteria based on designated subjective and objective tests for dry eye will emerge from the second NEI/industry workshop on clinical trials of dry eye due to take place in Puerto Rico in November 2004.

¹ Professor of Vision Sciences. Glasgow Caledonian University.

REFERENCES

1. Lemp MA. Epidemiology and classification of dry eye. Adv Exp Med Biol 1998; 438: 791-803.

2. Pflugfelder SC, Tseng SC, Sanabria O, Kell H, Garcia CG, Felix C et al. Evaluation of subjective assessments and objective diagnostic tests for diagnosing tear-film disorders known to cause ocular irritation. Cornea 1998; 17: 38-56.

3. Schein OD, Munoz B, Tielsch JM, Bandeen-Roche K, West S. Prevalence of dry eye among the elderly. Am J Ophthalmol 1997; 124: 723-728.

4. Lin PY, Tsai SY, Cheng CY, Liu JH, Chou P, Hsu WM. Prevalence of dry eye among an elderly Chinese population in Taiwan: the Shihpai Eye Study. Ophthalmology 2003; 110: 1096-1101.

5. Versura P, Cellini M, Torreggiani A, Profazio V, Bernarbini B, Caramazza R. Dryness symptoms, diagnostic protocol and therapeutic management: a report of 1,200 patients. Ophthalmic Res 2001; 33: 221-227.

6. McCarty CA, Bansal AK, Livingstone PM, Stanislavsky YL, Taylor HR. The epidemiology of dry eye in Melbourne, Australia. Ophthalmology 1998; 105: 1114-1119.

7. Schein OD, Tielsch JM, Munoz B, Bandeen-Roche K, West S. Relationship between signs and symptoms of dry eye in the elderly. A population-based perspective. Ophthalmology 1997; 104: 1395-1401.

8. Tomlinson A, Thai LC, Doane MG, McFadyen A. Reliability of measurements of tear physiology. Adv Exp Med Biol 2002; 506: 1097-1105.

9. Lemp MA. Report of the National Eye Institute/Industry workshop on Clinical Trials in Dry Eyes. CLAO J 1995; 21: 221-232.