Dry eye syndrome is a highly prevalent ocular disease with an increasing incidence in the elderly population. Topically administered lubricants are the basis for treatment of this disease. However, exact information about the tear film thickness and the corneal residence time of topical lubricants is still sparse, therefore no ideal treatment regimen has been found.

Recently a new method for assessment of tear film thickness based on ultra high resolution optical coherence tomography (OCT) has become available. The aim of the present study is to assess tear film thickness and corneal residence time of three different formulations of topical lubricants, in particular Thealoz Duo® Eye Drops, Hylo® Gel Eye Drops and Systane® Gel Eye Drops in patients with moderate to severe dry eye disease. In addition, standard tests for dry eye syndrome, such as the ocular surface disease index (OSDI©), Schirmer I test and determination of tear break up time (BUT) will be performed.

Inclusion Criteria:

1. Men and women aged over 18 years
2. Signed and dated written informed consent
3. History of dry eye syndrome for at least 3 months
4. Tear Break Up Time (BUT) ≤ 10 seconds or Schirmer I test ≤ 5 mm and ≥ 2mm
5. OSDI ≥ 22
6. Normal ophthalmic findings except dry eye syndrome, ametropia < 6 Dpt.
7. No administration of topical lubricants 12-24 hours before the screening examination

Exclusion Criteria:

1. Presence of an ocular pathology judged by the investigator as incompatible with the study.
2. Any other clinical relevant ocular abnormality except DES.
3. History of allergy, known hypersensitivity to one of the components: the study medications or Fluorescein.
4. History of known clinically relevant allergy.
5. Medical or surgical history judged by the investigator to be incompatible with the study participation (hepatic or renal insufficiency; all chronic severe organic disease: metabolic, endocrine, neoplastic, haematological disease; severe psychiatric illness, etc.).
6. History of a recent acute illness with a recovery period within the 2 weeks before the inclusion visit (Day 0).
7. Pregnancy, lactation.
8. Pre-menopausal woman who is not using a reliable birth control method (oral contraceptives or coil) or is not surgically sterilised.
9. Participation in any high-speed or water-sports during the study without ocular protection (goggles or glasses).
10. Subject unable to understand the study instructions or unlikely to comply with the study schedule and treatment.
11. Participation in another clinical study in the 4 weeks before the start of the present study or at the same time as the present study.
12. Subject is a ward of court.