What Is It In Your Tear That Prevents Pain?

We all know that if you do not blink --"on purpose," you will have terrible eye pain that will force you to close your eyes. The combination of oil, mucin, and aqueous work in concert to help prevent eye pain. What it is exactly about each tear layer that prevents pain has been of interest for years.

Eicosanoids may be a key component.

Eicosanoids are locally lipids derived from polyunsaturated fatty acids (PUFAs). A complex reaction involving cyclooxygenases (COX), lipoxygenases (LOX), cytochrome P450 monooxygenases (CYP450), Arachidonic acid (AA) (20:4n-6) all contribute to a well balanced tear. It is known that higher eicosanoids in the tears are associated with more severe MGD. 

The composition free fatty acids in the meibum and subjective symptoms or objective signs of MGD have not been correlated yet in a randomized, controlled study. Still, researchers think tear eicosanoid levels are age-dependent and specific eicosanoids may be indicators of clinical obstruction of MG or the severity of ocular surface damage.

Hopefully this study can be reproduced and we will someday be able to quickly measure eicosanoid levels in the tear film as a way to calculate a patient's Dry eye Disease & MGD risk score.

SLC

Sci Rep. 2018 Jul 26;8(1):11296. doi: 10.1038/s41598-018-29568-3.

Tear eicosanoids in healthy people and ocular surface disease.

Ambaw YA1,2, Chao C3,4,5, Ji S2, Raida M2, Torta F1,2, Wenk MR1,2,6, Tong L7,8,9,10.

Author information

1

Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore.

2

Singapore Lipidomics Incubator, Life Sciences Institute, National University of Singapore, Singapore, Singapore.

3

Ocular Surface Research Group, Singapore Eye Research Institute, Singapore, Singapore.

4

The School of Optometry and Vision Science, University of New South Wales, New South Wales, Australia.

5

Center For Translational Ocular Immunology, Department of Ophthalmology and Cornea Service, Tufts Medical Center, Boston, MA, USA.

6

Department of Biological Sciences, National University of Singapore, Singapore, Singapore.

7

Ocular Surface Research Group, Singapore Eye Research Institute, Singapore, Singapore. louis.tong.h.t@singhealth.com.sg.

8

Department of Ophthalmology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore. louis.tong.h.t@singhealth.com.sg.

9

Department of Cornea and External Eye Disease, Singapore National Eye Center, Singapore, Singapore. louis.tong.h.t@singhealth.com.sg.

10

Duke-NUS Medical School, Singapore, Singapore. louis.tong.h.t@singhealth.com.sg.

Abstract

Meibomian gland (MG) dysfunction is the leading cause of evaporative dry eye and it leads to inflammation of the ocular surface. Eicosanoids may be involved in inflammation of dry eye. This study aimed to profile tear eicosanoid levels in healthy individuals and those with MG dysfunction, and to examine if these levels are associated with clinical factors and expressibility of MG. Forty participants with MG dysfunction and 30 healthy controls were recruited in this study. Clinical signs of MG dysfunction were assessed, and tear lactoferrin concentration was evaluated. Tear eicosanoids were extracted from Schirmer's strips and analyzed using mass spectrometry. We were able to quantify 38 tear eicosanoids and levels were increased in older individuals. In participants with MG dysfunction, higher 5-HETE, LTB4, 18-HEPE, 12-HEPE and 14-HDoHE were associated with poorer MG expressibility. The eicosanoids PGF2α, 18-HEPE, 20-HDoHE and 17-HDoHE were elevated with increased corneal staining; higher 5-HETE, LTB4 were associated with lower tear lactoferrin levels. The receiver-operating-characteristics analysis shows higher levels of 5-HETE, LTB4 and 18-HEPE were able to predict poor expressibility of MGs. In conclusion, tear eicosanoid levels are age-dependent and specific eicosanoids may be indicators of clinical obstruction of MG or the severity of ocular surface damage.