To compare tear film thickness between normal subjects and aqueous tear deficiency dry eye patients by tear interferometry.
Prospective case-control study.
Central precorneal tear film thickness was measured noninvasively using an interference thin-film thickness measurement device (Quore MSPA1100; Mamiya-OP). Tear film thickness of 14 eyes from 14 normal subjects and of 28 eyes from 28 aqueous tear deficiency dry eye patients were compared along with noninvasively measured tear meniscus height, DR-1 (Kowa) dry eye severity grading, fluorescein and rose bengal staining scores, tear film break-up time, and Schirmer test results. Among dry eye patients, 13 eyes underwent punctal occlusion, and tear film thickness was compared before and after the surgery.
Tear film was significantly thinner in dry eye patients (2.0 ± 1.5 μm) than normal subjects (6.0 ± 2.4 μm; P < .0001). Tear film thickness showed good correlation with other dry eye examinations. After punctal occlusion, tear film thickness increased significantly from 1.7 ± 1.5 μm to 4.9 ± 2.8 μm ( P = .001) with the improvement of tear meniscus height, fluorescein and rose bengal staining scores, tear film break-up time, and Schirmer test values.
Interferometric tear film thickness measurement revealed impaired precorneal tear film formation in aqueous tear deficiency dry eyes and was useful for showing the reconstruction of tear film after punctal occlusion surgery. Interferometry of precorneal tear film may be helpful for the evaluation of aqueous tear deficiency in conjunction with other dry eye examinations.
Tears are secreted from the lacrimal gland and distributed by blinking to form the tear film. Tear film is responsible for wetting the ocular surface, which is the first line of defense, and is also essential for clear visual imaging. Tears are distributed to the cul de sac as well as to the exposed ocular surface area, including precorneal and preconjunctival tear film and tear menisci.
Evaluation of aqueous tear volume on the ocular surface or diagnosis of dry eye mainly has been performed with the Schirmer test. Although it has been considered to be a standard test, the accurate evaluation of tear volume was limited by its invasive nature, and there is a need for less invasive measurement methods. Several methods have been tried to evaluate aqueous tear status noninvasively. Danjo and associates and Yokoi and associates used tear interferometry to grade the severity of aqueous tear deficiency dry eyes. Tear meniscus also has been investigated noninvasively as an important indicator of aqueous tear volume by Yokoi and associates, Savini and associates, and Uchida and associates.
Another method to quantify the lucent tear film is to use interferometry based on wavelength-dependent fringes (WDF). The optical path difference from the reflection at the surface of tear film and at the interface of tear film and cornea causes an interference wave, which could indicate precorneal tear film thickness. Using this method, Danjo and associates and King-Smith and associates reported human precorneal tear film thickness of normal subjects.
Using an interference thin-film thickness measurement device by the above principle of WDF, we investigated in the present study central precorneal tear film thickness in normal subjects and aqueous tear deficiency dry eye patients. For several cases, change of tear film thickness after punctal occlusion surgery also was assessed.
Interference Thin-Film Thickness Measurement Device Based on Wavelength-Dependent Fringes
An interference thin-film thickness measurement device (Quore MSPA1100; Mamiya-OP, Saitama, Japan) was used to measure tear film thickness noninvasively. This device consisted of light source (halogen lamp), probe (glass fiber), and detector (256 light-sensitive elements photodiode array). The diameter of the glass fiber in the probe is 1.0 mm. Exposure time for measurement was 0.1 second. Intensity of the light source of the device measured using the luminance power meter (ADCE 8230E; Advantest, Tokyo, Japan) was 43 μW, showing enough low energy and less invasiveness compared with the light source of usual ophthalmic examination instruments.
According to Danjo and associate’s or King-Smith and associate’s methods, measurement of interference WDF was applied to the precorneal tear film in this study using the above device. The device detects the optical thickness of aqueous tear layer by the reflection from the 2 surfaces: surface of the tear film and interface between tear film and cornea.
Reflectance of interference (R) is shown in the following equation:
R = 1 − 4 n a n t 2 n c n t 2 ( n a + n c ) 2 − ( n c 2 − n t 2 ) ( n t 2 − n a 2 ) sin 2 ( 2 π n t d/ λ )