The most common symptoms of dry eye disease (DED) are having a scratchy feeling or foreign body sensation, stinging or burning, red eyes, sensitivity to light, and blurry vision. If these symptoms or a case of DED is severe, it can lead to damage to the cornea, the clear outer layer at the front of the eye. DED is a very common disease of the eye, affecting over 16 million Americans.
DED occurs when you don’t make enough tears, or when your tears evaporate too quickly. Aqueous deficient dry eye appears when the lacrimal glands do not produce enough of the aqueous, watery component of tears. The more prevalent primary subtype of dry eye is evaporative, which is a common consequence of meibomian gland dysfunction (MGD). Evaporative DED occurs when the meibomian glands, small modified sebaceous glands in the upper eyelid do not produce enough of the oily component of the tear film. This outermost layer typically slows evaporation of the aqueous component of tears.
Historically, the precorneal tear film (PCTF) was thought to consist of three layers—an outer tear film lipid layer (TFLL), a middle aqueous layer, and an inner mucin layer that interacts with the corneal surface. More recently, though, the aqueous and mucin layers are thought of as a gel gradient and considered together as the mucoaqueous layer. Consisting of water, salts, proteins, and carbohydrates, the muco-aqueous gradient adjacent to the corneal surface is the thicker of the two layers at up to 4000nm. The outer TFLL is 40-90nm thick and has two sublayers. The outermost non-polar lipid sublayer consists mostly of cholesterol ester, wax ester, triacylglycerol, diacylglycerol, and free cholesterol. The inner amphiphilic lipid sublayer contains (O)-acyl-Ѡ-hydroxy fatty acids, cholesteryl sulfate, phospholipids, and sphingolipids.
Find more information in the below diagram, published in 2023 in the American Journal of Managed Care by Karpecki PM, Nichols KK, & Sheppard JD, titled, Addressing Excessive Evaporation: An Unmet Need in Dry Eye Disease.
The oily TFLL serves as the barrier against evaporative aqueous loss, and stabilizes it by facilitating the spread of its aqueous component and reducing surface tension. However, the exact mechanism by which the TFLL retards tear evaporation and promotes PCTF stability remains poorly understood. For instance, while most would agree that a more uniform and thicker TFLL would be more protective against evaporation, and therefore prevent DED, this relationship remains controversial in published literature. Some of our work aims to resolve this controversy, which can be found by visiting our RESEARCH and PUBLICATIONS & PATENTS pages.
Some risk factors of DED, such as frequent contact lens usage or prolonged screen usage are preventable. As is the case with certainly dietary deficiencies, such as not getting enough vitamin A, which is found in foods like carrots, broccoli, and liver, or omega-3 fatty acids, which are found in foods like fish, walnuts, and vegetable oils. Spending time in a dry, smoky, or windy environment can increase a person’s risk of developing DED. Some risk factors, though, are not modifiable. DED is more common in women, those over the age of 50 or of east Asian ethnicity, and patients with autoimmune disorders such as lupus or Sjögren syndrome. Other medical conditions such as diabetes or thyroid conditions increase a person’s risk of developing DED, as do certain medications such as antihistamines, decongestants, antidepressants, blood pressure medications, and hormone replacement therapy.
For more information on dry eye disease, visit the National Eye Institute’s website.