Oliver Findl 1,2, Nino Hirnschall 1,2, Alja Crnej 2

1 VIROS Vienna Institute for Research in Ocular Surgery, Hanusch Hospital, Vienna, Austria2 Moorfields Eye Hospital, London, UK

Effect of Fluorescein dye staining of the tear film on Scheimpflug measurements of

central corneal thickness

Financial interest: noneOF is scientific advisor to Alcon, AMO, Croma & Carl Zeiss Meditec AG

Background

• Fluorescein dye is commonly used in routine ophthalmic examination

– staining epithelial defects of the cornea

– break up time of the tear film

– Goldmann applanation tonometry

• Correct CCT measurement needed for:

– corneal refractive surgery

– diagnosis of corneal diseases (keratoconus)

– to correct for systematic errors of IOP measurements using Goldmann applanation.

CCT measurement devices

• Pentacam: rotating Scheimpflug photographic

– measures the thickness from the image, which is created from the back-scattered light from the corneal tissue

=> Influenced by fluorescein eye drops?

• ACMaster: partial coherence interferometry (PCI)

– gold standard for CCT measurement

– measures distances between surface interfaces with a different refractive index

Methods

• Prospective randomised controlled study

• 30 eyes of 15 patients (8 female, 7 male)

Methods3 x rotating Scheimpflug imaging + 3 x PCI measurement

Instilling 0.25% fluorescein + 0.5% proxymetacaine hydrochloride

Rotating Scheimpflug imaging + PCI measurement

Irrigation with 2ml BSS

Rotating Scheimpflug imaging + PCI

Rotating Scheimpflug imaging + PCIafter 5min, 10min, 20min, 40min, 60min, (120min)

Without Fluorescein With Fluorescein

Results

Continuous measurement group

CCTScheimpflug (SD) in µm

CCTPCI (SD) in µm

Baseline 539.1 (32.2) 519.0 (33.1)

1min 585.7 (29.7) 520.6 (33.2)

5min 570.1 (31.6) 519.7 (33.0)

10min 560.8 (35.0) 519.8 (32.8)

20min 550.1 (33.4) 520.0 (32.7)

40min (10 patients)

545.0 (33.8) 523.2 (38.7)

Irrigation group

CCTScheimpflug (SD) in µm

CCTPCI (SD) in µm

Baseline 536.5 (34.7) 519.4 (33.9)

1min 586.5 (44.3) 521.7 (33.8)

After irrigating 561.7 (39.5) 520.7 (34.0)

Rotating Scheimpflug: 46.6µm difference

PCI: 1.6µm difference

Results

CC

T d

iffer

ence

(P

CI)

in µ

m

CCT difference (Scheimpflug) in µm

Results

time after applying Fluorescein (min)

Sub

ject

s w

ith F

luor

esce

in la

yer

(%)

Fluorescein layer -> more than 5µm difference to the baseline measurement

Number of patients (%) with a fluorescein layer +5µm compared to baseline

Results

time after applying Fluorescein (min)

Sch

irmer

2 r

esul

ts (

sec)

Conclusion

• Scheimpflug imaging

• Fluorescein dye causes intense back-scatter of light resulting in a measurement error

• Irrigating the eye reduces this effect only insignificantly

• PCI: is not influenced by altered or higher reflectivity

=> Fluorescein eye drops MUST NOT be used prior to Scheimpflug imaging measurements