ACTA 0 P H T H A L M 0 L O G I C A 70 (1992) 108-110

The central corneal thickness in keratoconjunctivitis sicca

Gunnar Hclvding

Department of Ophthalmology, University of Bergen, Bergen, Norway

Abstract. The central corneal thickness of 17 patients with keratoconjunctivitis sicca was compared with the thickness recorded in a control group of 105 subjects with healthy eyes. A moderate, but highly statistically sig- nificant central corneal thinning was found in the kera- toconjunctivitis sicca group. The possible etiology and clinical importance of this finding is briefly discussed.

Key words: central corneal thickness - keratoconjunctivitis sicca - tear fluid hyperosmolarity.

Due to its structural characteristics, the cornea swells only in the direction of its thickness. Corneal thickness and hydration are thus linearly related (Dohlman 1983). Corneal hydration is mainly regu- lated by the stromal swelling pressure, the barrier function of the epithelium and endothelium and the water pumping mechanism located in the en- dothelium (Dohlman 1983). In addition, evapora- tion from the corneal surface as well as changes of the intraocular pressure may influence the corneal thickness (Mishima 1968; Hansen 1971; Ehlers et al. 1975). The tear fluid osmolarity in keratocon- junctivitis sicca is significantly increased (Gilbard et al. 1978; Farris et al. 1981), and it has been sug- gested that this increased osmolarity plays an im- portant role in the corneal and conjunctival changes seen in this disease (Balik 1952; Roland0 et al. 1983). Although it seems likely that these al- terations of tear fluid osmolarity may induce changes of the corneal thickness in dry eye disease, the magnitude of such possible thickness changes has to the best of my knowledge not previously been reported. This paper therefore presents a

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comparison between the central corneal thickness measured in patients with keratoconjunctivitis sicca and that recorded in a group of subjects with healthy eyes.

Material and Methods

The present study group included 17 patients aged 22-75 years (mean 55.5 years, SD 14.78 years) (12 fe- males and 5 males) with keratoconjunctivitis sicca. Further patient data are presented in Table 1. All the patients had symptoms characteristic of dry eye disease, as well as 5 nun or less wetting of the Schirmer strip, a reduced tear film break up time (BUT) and some degree of rose bengal staining of the cornea. None of the eyes showed signs of severe inflammation at the time of the measurements. No eye drops had been used during the last 30 min be- fore the measurements were made.

The normal control group consisted of 105 sub- jects (77 females and 28 males) aged 14-48 years (mean 23.9 years, SD 5.78 years) (Hovding 1983). Apart from refractive errors, they all had healthy eyes. None had worn contact lenses or undergone eye surgery. No eye drops were used.

The central corneal thickness was measured with a Haag-Streit pachometer No. 1 mounted on a Haag-Streit 900 slit-lamp. The correct cornea-pa- chometer position was ensured by a method de- scribed by Stone (1974). The mean value of 3 measurements were recorded as the central cor- neal thickness. All the measurements were made by the author.

Table 1. Patient data in the keratoconjunctivitis sicca study group.

Patient No. Age

Duration Use Central of of corneal

symptoms artificial thickness

Schirmer Sex test

(mm) (years) tears

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17

47 69 75 65 29 71 22 62 55 54 53 66 53 55 38 60 70

F M F M F F F F M F F M F F F M F

5 5 5 4 4 1 2 2 1 1 1 1 1 1 0 1 2

1 > 1 > 1

5 > 1

> 15 1

20 20

> 10 > 5 > 3 < 1 > 1

3 > 5

3

0.500 0.440 0.475 0.500 0.505 0.445 0.505 0.498 0.495 0.530 0.540 0.520 0.490 0.490 0.530 0.500 0.512

F = Female. M = Male.

Statistical methods The central corneal thickness measured in the group of patients with dry eyes was compared to that recorded in the normal control group by means of the zM-test and the Student’s t-test. Only the measurements obtained in the right eye of both the patients with keratoconjunctivitis sicca and the subjects in the normal control group were included.

Results

The mean central corneal thickness measured in the group of patients with dry eye disease was 0.499 mm (range 0.440-0.540 mm) (Table 2). This value was significantly less than that record- ed in the normal control group (mean central corneal thickness 0.532 mm, range 0.470-0.623 mm) (P<0.002). This difference was statistically signiGcant even if the thickness of a normal tear film (0.007 mm) (Holly & Lemp 1977) was added to the thickness recorded in the sicca patients (P < 0.002).

Discussion

The present results show that the central corneal thickness is reduced in patients with keratocon- junctivitis sicca, at least in the absence of marked inflammation. The higher mean age of our dry eye patients than in our normal control group is not believed to have significantly biased our results. Contradicting views regarding the influence of age on the central corneal thickness have been pub- lished (Lowe 1969; Hansen 1971; Olsen & Ehlers 1984). However, the thickness difference between our dry eye group and our control group still reached statistical significance when the central corneal thinning of 0.0045 mm per decade re- ported by Olsen & Ehlers (1984) was taken into ac- count. Apart from the well-known corneal thin- ning associated with keratoconus and other ectatic corneal degenerations, and the corneal swelling occurring in all cases of endothelial dysfunction, a study by Ehlers & Bramsen (1978) is to my knowl- edge the only published paper reporting the cen- tral corneal thickness in various corneal disorders. They found that hereditary corneal dystrophies have a normal central corneal thickness, with the

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No. Central corneal thickness (mm)

eyes Mean SD Range right

exception of the macular dystrophy of Groenow (type 11), in which a significantly reduced thickness was found. A reduced central corneal thickness in chronic corneal degenerations of luetic, tubercu- lous, indefinite postkeratitic or marginal (Fuchs- Terrien) nature was also reported, and the possible role of abiotrophic processes in this corneal thin- ning was discussed.

The etiology of the presently demonstrated moderate, but statistically significant corneal thin- ning in keratoconjunctivitis sicca remains specula- tive. A normal tear film is essential for the preser- vation of corneal integrity, and abiotrophic pro- cesses may therefore play a role in the corneal thin- ning also in these disorders. The aqueous layer of the normal tear film contains in dissolved forms in- organic salts, glucose and urea, as well as surface active biopolymers, proteins and glycoproteins (Holly & Lemp 1977). The fluid volume of this layer is reduced in keratoconjunctivitis sicca (Holly & Lemp 1977), while the rate of tear fluid evapora- tion seems to be increased (Rolando et al. 1983). The resulting hyperosmolarity of the precorneal tear film may well be the main cause of the moder- ate, but statistically significant central corneal thinning in this disease. Regardless of the etiology of the demonstrated corneal thinning in kerato- conjunctivitis sicca, measurements of the central corneal thickness may aid in the early diagnosis of this disease.

P

Ehlers N & Bramsen T (1978): Central thickness in cor- neal disorders. Acta Ophthalmol (Copenh) 5 6 412-416.

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Farris R L, Stuchell R N & Mandel I D (1981): Basal and reflex human tear analysis. I. Physical measurements: Osmolarity, basal volumes and reflex flow rate. Oph- thalmology 88: 852-857.

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References Received on May 21st, 1991.

Author’s address:

Gunnar H~vding, MD, Department of Ophthalmology, N-5021 Haukeland sykehus, Norway.

Balik J (1952): The lacrimal fluid in keratoconjunctivitis sicca. A quantitative and qualitative investigation. Am J Ophthalmol35: 773-782.

Dohlman C H (1983): The cornea. Scientific foundations and clinical practice. In: Smolin G & Thoft R A (eds), pp 3-17. Little, Brown and Company, BostodToronto.

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