The use of topical anesthesia as well as corneal vital staining with fluorescein is an inevitable part of various ophthalmological examinations and surgical treatments. However, eye drops that don't come in single dose packages are required to contain preservatives such as chlorhexidine diacetate. An increasing number of surveys proves the partly severe side effects that preservative-containing eye drops may induce. The aim of the present study therefore is to investigate the effects of four different topical diagnostic eye drops (Novain®, Minims Oxybuprocaine Hydrochloride®, Thilorbin® and Minims®) on tear film thickness in healthy subjects. Tear film thickness will be measured at baseline and at defined time points after single instillation. The course of tear film thickness during the study day will provide information about the influence on tear film stability of the four different eye drops. Healthy subjects will receive Novain®, Minims Oxybuprocaine Hydrochloride®, Thilorbin® and Minims® eye drops on 4 different study days in a randomized order. Assessment of lipid layer thickness will be performed before and at pre-specified time points after instillation as secondary outcome. Other clinical measures such as determination of tear film break up time (TFBUT), corneal sensation, and Schirmer I test will be performed. The study will be conducted in a randomized, single masked, observer blinded four-way cross-over design. Subjects will receive all four diagnostic eye drops on 4 different study days in a randomized order.

Inclusion Criteria:

• Men and women aged at least 18 years
• Written informed consent prior to study-related procedures
• Normal ophthalmic findings
• No use of eye drops including topical lubricants in the 4 weeks before screening

Exclusion Criteria:

• Participation in a clinical trial in the 3 weeks preceding the study
• Symptoms of a clinically relevant illness in the 3 weeks before the first study day
• Presence or history of a severe medical condition as judged by the clinical investigator
• Intake of parasympathomimetic or anti-psychotic drugs
• Wearing of contact lenses
• Dry eye syndrome (Schirmer I test ≤10mm or TFBUT <10 sec.)
• Glaucoma in the medical history
• Treatment with corticosteroids in the 4 weeks preceding the study
• Topical treatment with any ophthalmic drug in the 4 weeks preceding the study
• Ocular infection or clinically significant inflammation
• Ocular surgery in the 3 months preceding the study
• Sjögren's syndrome
• Stevens-Johnson syndrome
• History of allergic conjunctivitis
• Pregnancy, planned pregnancy or lactating
• Known hypersensitivity to any component of the study medication

The data that support the findings of this study are available from the corresponding author upon reasonable request.