Abstract
Background
Pseudoexfoliative material is being implicated in causing tear film abnormalities, and the aim of the present study was to prove it and to compare with the normal healthy participants.
Methods
A total of 398 eyes of 200 participants were studied, and two groups were formed. The first, pseudoexfoliation syndrome (PEXS) group, included 198 eyes of 100 patients, and the second, age-matched control group, included 200 eyes of 100 participants. Tear film abnormalities were compared in the two groups by tear meniscus height (TMH), Schirmer test I, Schirmer test II and tear film break-up time (TBUT).
Results
On comparing the two groups using independent t-test, a significant difference was found in Schirmer test I, Schirmer test II, and TBUT between the PEXS group (Schirmer test I: 23.98 ± 10.68 mm, Schirmer test II: 17.11 ± 8.78 mm, and TBUT: 9.778 ± 5.54 s) and the age-matched control group (Schirmer test I: 27.08 ± 9.58 mm, Schirmer test II: 19.98 ± 8.48 mm, and TBUT: 13.495 ± 5.65 s) (p = 0.003 [Schirmer test I]; p = 0.001 [Schirmer test II]; and p < 0.001 [TBUT]). However, an insignificant difference was found in terms of TMH (p = 0.195) between the two groups.
Conclusion
PEXS affects tear production and leads to unstable tear film.
Keywords: Pseudoexfoliation syndrome, Tear meniscus height, Schirmer test, Tear film break-up time, Dry eye
Introduction
Pseudoexfoliation syndrome (PEXS) is clinically a generalised fibrillopathy and is characterised by deposition of an abnormal extracellular material in intraocular and extraocular tissues.1 Intraocularly, its deposition on the corneal endothelium, trabecular meshwork, iris, zonules and lens capsule may manifest as corneal endotheliopathy, glaucoma, iris atrophy, poor dilatation of the pupil and zonular weakness.2, 3 In addition, the deposition of PEX material in the goblet cells and accessory lacrimal glands may lead to the abnormalities of the tear film, and the primary aim of the present study is to evaluate tear film abnormalities in patients with PEXS and to compare with the normal healthy participants.4
Material and methods
This study adheres to the tenets of the Declaration of Helsinki, and prior approval of the institutional ethical committee was obtained. In this cross-sectional comparative study, a total of 398 eyes of 200 participants were studied, and two groups were formed. The first, the PEXS group, included 198 eyes of 100 patients, and the second, age-matched control group, included 200 eyes of 100 participants. In the PEXS group, PEX material was present in both the eyes in 98 patients whereas the exfoliative material was present in only the right eye in the remaining two patients. The healthy control participants were recruited from the sampling frame that was created from the attendants of patients with PEXS considering the inclusion and exclusion criteria.
The PEXS group comprised patients with PEX material either on the lens surface or at the pupillary margin or on both the aforementioned surfaces. Conditions such as ocular surface disorder; previous ocular surgery; acquired nasolacrimal duct obstruction; prolonged contact lens wearer; lagophthalmos; entropion; ectropion; placement of punctal plugs or punctal cauterisation; ocular allergy; use of systemic or topical drugs, such as antidepressants, diuretics, corticosteroids or immunomodulators; and systemic diseases, such as diabetes mellitus, thyroid diseases and Sjogren's syndrome affecting the tear film have been excluded from the study.
The sample size has been calculated after considering an effect size of clinical interest (d), that is, difference in means of tear film break-up time (TBUT) as 3.5 s (from previous study)5; combined standard deviation as 8 (from previous study)5; Z alpha (Za) as 1.96 (corresponding to type I error of 5%, i.e., 0.05) and Z beta (Zb) as 0.84 (corresponding to power of 80%). The number of eyes in each group was calculated as follows:
N = 2(Za + Zb)2 × SD2/d2 = 85.33(∼ 85 patients in each group). However, we have included 100 patients in each group in this study.
A comprehensive ophthalmic evaluation, including the visual acuity assessment by using the Snellen chart, anterior segment examination by using a slit lamp biomicroscope, posterior segment evaluation by indirect ophthalmoscopy and +90D biomicroscopy, intraocular pressure measurements by using an applanation tonometer and evaluation of tear film abnormalities by tear meniscus height (TMH), Schirmer test I, Schirmer test II and TBUT, was performed in both the PEXS and age-matched control groups after written informed consent, .
Tear meniscus height
The eyes were examined by slit lamp biomicroscopy after normal blinking, and the lower meniscus height was read off the scale on the reticule of the slit lamp. A value of <0.35 mm was considered abnormal.6
Schirmer test I
This test was performed under natural lighting conditions without topical anaesthesia. A standardized commercially available filter paper (Contacare Ophthalmics and Diagnostics, Vadodara, India) for the Schirmer's test was placed in the lower fornix over the junction of the middle and lateral thirds of the lower eyelid. The eyes were left closed for 5 min, and the distance moistened was directly read off the scale on the paper itself. A reading of <10 mm was considered abnormal.7
Schirmer test II
This test was performed after instillation of a drop of topical anaesthesia (proparacaine ophthalmic solution 0.5% [Entod Pharmaceuticals Ltd., Mumbai, India]) in the eyes and waiting for 30 s. About five millimetres of the Schirmer strip was bent and placed as mentioned in Schirmer test I. The eyes were left closed, and the distance moistened was directly read off the scale on the paper after 5 min. The value of <6 mm was considered as abnormal.8
Tear breakup time
A commercially available fluorescein-impregnated strip (Madhu Instruments Pvt. Ltd., New Delhi, India) was wetted with non-preserved saline and placed in the inferior fornix. The patient was then asked to blink three times and then to look straight ahead without blinking. The tear film was observed at the slit lamp biomicroscope using a cobalt blue filter under wide beam illumination. The interval between the last blink and the appearance of the first randomly appeared corneal dry spot was measured as TBUT. A value less than 10 s (s) was regarded as abnormal.9
Statistical analysis
Data analysis was performed by Statistical Package for Social Sciences version 20.0. Owing to non-skewness and normality of data, parametric tests were used. The chi-square test was used to analyse the qualitative data whereas the t-test was used to analyse the quantitative data. A P value < 0.05 was considered as significant.
Results
The mean age of the participants in the PEXS and control groups was 67.99 ± 7.46 years (range, 60–76 years) and 68.32 ± 6.67 years (range, 62–75 years), respectively. On comparison using the unpaired t-test, there was no difference (P = 0.126) between the two groups. In the PEXS group, 55% were women and 45% were men whereas in the control group, 59% were women and 41% were men. There was no difference (p = 0.668) in proportion of gender between the two groups. The best spectacle-corrected visual acuity (BSCVA) in the PEXS group was 0.577 ± 0.730 (range, 0.3–1.08) logarithm of the minimum angle of resolution (logMAR) (Snellen equivalent [SE], 20/74.1) in the right eye and 0.569 ± 0.620 (range, 0.3–1.08) logMAR (SE, 20/70.5) in the left eye. In the control group, BSCVA was 0.552 ± 0.610 (range, 0.2–1.08) logMAR (SE, 20/71.3) in both the eyes.
Tear meniscus height
In Table 1, two independent t-tests were used to compare mean TMH between the PEXS and age-matched control groups. There was no difference (p = 0.195) between the two groups. In the PEXS group, 100 (50.5%) eyes had TMH <0.35 mm whereas it was ≥0.35 mm in 98 (49.49%) eyes. In the control group, TMH was <0.35 mm in 79 (39.5%) eyes whereas it was ≥0.35 mm in 121 (60.5%) eyes.
Table 1.
A comparative analysis of tear film abnormalities between the two groups.
| Tear film abnormalities | Cases (N = 198) | Control (N = 200) | P value |
|---|---|---|---|
| TMH (mm) | 0.478 ± 0.36 | 0.525 ± 0.36 | 0.195 |
| Schirmer test I (mm) | 23.98 ± 10.68 | 27.08 ± 9.58 | 0.003 |
| Schirmer test II (mm) | 17.11 ± 8.78 | 19.98 ± 8.48 | 0.001 |
| TBUT (s) | 9.778 ± 5.54 | 13.495 ± 5.65 | <0.001 |
N, number of eyes; mm, millimetre; s, second; TMH, tear meniscus height; TBUT, tear breakup time.
Schirmer test I
On comparing Schirmer test I as shown in Table 1, a statistically significant difference (p = 0.003) was observed between the PEXS group and the age-matched control group. In the PEXS group, 12 (6.06%) eyes had Schirmer I value ≤ 5.5 mm, whereas it was >10 mm in 163 (82.3%) eyes. However, 23 (11.6%) eyes had Schirmer I value between 6 and 10 mm. In the control group, Schirmer I value was ≤5.5 mm in seven (3.5%) eyes, between 6 and 10 mm in 12 (6%) eyes and >10 mm in 181 (90.5%) eyes.
Schirmer test II
In line with Schirmer test I, a statistically significant difference (p = 0.001) was also observed in Schirmer test II (Table 1) between the PEXS and the age-matched control group. In the PEXS group, 16 (8.08%) eyes had Schirmer II value ≤ 5.5 mm, whereas it was >10 mm in 182 (91.91%) eyes. However, none of the eyes had Schirmer II value between 6 and 10 mm. In the control group, Schirmer I value was ≤5.5 mm in two (1%) eyes whereas it was >10 mm in 198 (99%) eyes.
Tear Break-Up Time (TBUT)
Using 2 independent t-test as shown in Table 1, a statistically significant difference (p < 0.001) was observed in TBUT between the PEXS and the age-matched control group. In the PEXS group, 125 (63.1%) eyes had TBUT ≤10s, whereas it was >10s in 73 (36.9%) eyes. In the control group, TBUT was ≤10s in 68 (34%) eyes whereas it was >10S in 132 (66%) eyes.
Discussion
The present study has highlighted the tear film abnormalities in patients with PEXS. Among the various parameters analysed, we found that patients with PEXS exhibit decreased values of both Schirmer I and Schirmer II as well as TBUT as compared with those of the age-matched controls.
The tear film forms an important component of the lacrimal functional unit. Holly et al,10 in 1946, described the classical structure of the tear film comprising an anterior lipid, middle aqueous and deeper mucin layers. The contemporary concept is of a metastable tear film comprising an aqueous gel with mucin concentration decreasing from the ocular surface to the undersurface of the lipid layer. Lipid forms the superficial/outermost layer of the tear film and is derived primarily from the meibomian glands in the eyelids whereas the Moll and Zeis glands partially contribute to it. The aqueous layer of the tear film comprises the bulk of the tear film thickness. It is produced by the main lacrimal gland (reflex secretion) and the accessory glands of Krause and Wolfring (basal secretion). The mucin layer, the innermost one, is primarily produced by the goblet cells of the conjunctiva and the crypts of Henle in the conjunctival fornices. In addition, mucin (secondary source) is also produced from the squamous epithelial cells of the ocular surface (cornea and conjunctiva), with a small contribution from the lacrimal gland. In a routine practice in the outpatient department, dysfunction of the components of the tear film is assessed objectively by TBUT, Schirmer test I, Schirmer test II and TMH.
TBUT, as revealed in our study, is significantly lower in the PEXS group than that in the age-matched controls. Our observation is in line with that of Kaliaperumal et al,11 in which 43 eyes of 30 patients with PEXS was compared with 30 eyes of 15 normal participants and found a significantly (p < 0.001) lower value of TBUT in patients with PEXS (5.6 ± 2.8s) than in the normal participants (14.75 ± 2.5s). Similarly, in 2004, Kozobolis et al12 also observed a significantly lower value of TBUT (p = 0.001) in 40 eyes of 40 patients with PEXS (mean, 8.6s) than in the control group (mean, 12.3s). In 2006, Erdoğan et al,13 in a randomised double-blind prospective study, compared TBUT among the three groups, that is, 45 eyes of 45 patients with PEX glaucoma, 48 eyes of 48 patients with PEXS and 50 eyes of 50 normal participants and found mean values as 5.91 ± 3.27s, 7.39 ± 4.82s and 10.96 ± 3.81s, respectively. It was observed that the values of TBUT were significantly lower in the former two groups than the latter. In line with the above studies, Škegro et al14 also observed a significantly lower vale of TBUT in patients with PEXS.
TBUT assesses the adequacy of the mucin layer of the tear film which is depleted in patients with PEXS. This depletion, what we hypothesise, might be attributed to the deposition of PEX material at the openings of the goblet cells, thereby reducing the secretion of mucin. In addition, the literature has ascribed this mucin depletion secondary to two factors. First, as postulated by Kozobolis et al,12 PEX material alters the basic features of the goblet cell morphology, thus affecting the mean secretion. Second, Erdoğan et al,13 based on impression cytology, reported mucin depletion secondary to the ocular surface changes in the form of variation in shape, quantity and periodic acid-Schiff–positive cytoplasm of the goblet cells.
In our study, although values of Schirmer test I and Schirmer test II in the PEXS group are significantly lower than those in the age-matched control group, their mean values are definitely more than the normal values.6, 7 In terms of Schirmer test I, although we could not find any comparative literature of patients with PEXS with the age-matched controls, however, we ascribe its reduced value to the partial deposition of PEX material in the main lacrimal gland and its ducts. With regard to Schirmer test II, our observation is in line with that of Kaliaperumal et al,11 in which a statistically significant difference (p < 0.001) was observed between the cases (10.6 ± 7 mm) and controls (22.05 ± 4.4 mm). Similarly, Kozobolis et al,12 in 2004, reported a lower value of Schirmer test with anaesthesia for the cases (10.6 ± 4.2 mm) than for the normal individuals (13.4 ± 4.9 mm). Erdoğan et al.,13 in a randomised study, compared basal Schirmer test results among patients with PEX glaucoma, patients with PEXS and normal participants and found a statistically significant lower score for patients with PEX glaucoma and patients with PEXS than the normal participants. In line with the above studies, Škegro et al.14 also found a significantly lower score of Schirmer test II in patients with PEXS.
Schirmer test II assesses the adequacy of basal secretion of the aqueous component of the tear film which is reduced in patients with PEXS. This depletion of the basal aqueous humour might be attributed to the deposition of PEX material and the alteration in the morphology of the glands of Krause and Wolfring. Our hypothesis is in line with that of Kaliaperumal et al,11 who have also attributed the lower score of Schirmer test II to the infiltration of the accessory lacrimal glands by the PEX material.
TMH assesses tear volume which is depleted in all forms of dry eye including mucin deficiency, keratoconjunctivitis sicca and meibomian gland diseases. Because, in our study, TMH of the PEXS group is comparable with that of the age-matched controls, it indicates that despite mucin deficiency (as depicted by lower and significant difference in TBUT score between patients with PEXS and normal individuals), aqueous humour formation is near normal (as depicted by normal mean values of Schirmer test I and Schirmer test II, despite statistically significant score between the cases and controls) in patients with PEXS.
This study has evaluated four important parameters that have the potential to test functionality of almost all three layers of the tear film, which is lacking in previous research. Most previous studies had used only one or two of the parameters, thus missing out on evaluating the tear film in totality. In addition, it needs to highlight the fact that although the aqueous layer tests in patients with PEXS may be normal, they tend to be lower than those of healthy individuals, and this needs to be kept in mind while selecting medication for treating PEX glaucoma. In addition, to the best of our knowledge, this study has compared the largest sample size between the cases and control. The only potential drawback of this study is the lack of histologic correlation of various ocular structures such as conjunctival goblet cells and accessory lacrimal glands with tear film abnormalities. Future studies should investigate the change in distribution, morphology, absolute number and functional aspects of the conjunctival goblet cells which cause tear film instability leading to ocular surface disorders, including dry eye diseases in patients with PEXS.
Conflicts of interest
The authors have none to declare.
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