Biochemistry and Metabolism- Tear Film

Question 1

Function of precorneal tear film

Answer 1

• provide a smooth optical surface at the air–cornea interface
• allow diffusion of oxygen and other nutrients
• serve as a medium for removal of debris and protect the ocular surface

Question 2

Components of precorneal tear film

Answer 2

Evidence continues to support a 2­phase model of the tear film, in which a lipid layer overlies a mucoaqueous phase. Components of the tear film (lipids, mucins, proteins, and salts) may interact to prevent tear film evaporation and collapse; however, additional studies are needed to confirm this concept.

Question 3

Normal tear lake

Answer 3

A normal tear lake is approximately 1 mm above the eyelid margin.

Question 4

How thick is the tear film

Answer 4

3.4 μm thick.
Lipid layer 0.015–0.16 μm

Question 5

Function of the lipid layer of tear film

Answer 5

• retards evaporation of the tear film
• contributes to the optical properties of the tear film because of its position at the
  air–tear film interface
• maintains a hydrophobic barrier (lipid strip) that prevents tear overflow by decreasing surface tension
• prevents damage to eyelid margin skin by tears

Question 6

How many meibomian glands are in the upper and lower eyelids

Answer 6

There are approximately 30–40 meibomian glands in the upper eyelid and 20–30 in the lower eyelid

Question 7

What is the melting point of meibomian gland secretions

Answer 7

32°C to 40°C.

Question 8

Oral supplements for dry eye syndrome

Answer 8

omega­3 essential fatty acids (eg, fish oil) has been dem­ onstrated to decrease symptoms associated with dry eye syndrome (DES) in women, pre­ sumably because of its direct effects on tear film fatty acids.

Question 9

Functions of mucoaqueous layer

Answer 9

• transmits oxygen to the avascular corneal epithelium
• maintains a constant electrolyte composition over the ocular surface epithelium
• provides an antibacterial and antiviral defense
• smooths minute irregularities of the anterior corneal surface
• modulates corneal and conjunctival epithelial cell function
• converts the corneal epithelium from a hydrophobic to a hydrophilic layer, which is
  essential for the even and spontaneous distribution of the tear film
• interacts with the tear lipid layer to reduce surface tension, thereby stabilizing the
  tear film
• lubricates the eyelids as they pass over the globe

Question 10

Which gland constitute 2/3 of the accessory lacrimal glands

Answer 10

The glands of Krause located in the lateral part of the upper fornix.

Question 11

What separates the orbital and palpebral lacrimal lobes?

Answer 11

the lateral horn of the levator aponeurosis

Question 12

Concentration of electrolytes in the cornea

Answer 12

The sodium (Na+) concentration of tears parallels that of serum; however, the concentration of potassium (K+) is 5–7 times that of serum. Na+, K+, and chloride (Cl–) regulate the osmotic flow of fluids from the cornea to the tear film and thereby contribute to corneal clarity. Bicarbonate (HCO–3) regulates tear pH. Other tear electrolytes (Fe2+, Cu2+, Mg2+, Ca2+, PO43–) are enzyme cofactors.

Question 13

Tear film solutes

Answer 13

urea, glucose, lactate, citrate, ascorbate, and amino acids. All enter the mucoaqueous layer of the tear film via the systemic circulation, and their con­ centrations parallel those of serum levels

Question 14

Protein in tear film

Answer 14

Proteins in the mucoaqueous layer of the tear film include immunoglobulin (Ig) A and secretory IgA (sIgA). IgA is formed by plasma cells in interstitial tissues of the main and accessory lacrimal glands and by the substantia propria of the conjunctiva

Question 15

What condition is MMP-9 affected in?

Answer 15

Levels of matrix metalloproteinase 9 (MMP­9) in the tear film have been shown to be elevated in patients with severe disorders affecting the ocular surface, including Sjögren syndrome and graft­vs­host disease, as well as in patients after laser in situ keratomileusis (LASIK).

Question 16

Function of MMP-9

Answer 16

MMP­9 cleaves epithelial basement membrane components and tight­junction proteins.

Question 17

Antimicrobial components of the mucoaqueous layer

Answer 17

Lysozyme, lactoferrin, group II phospholipase A2, lipocalins, and defensins are impor­tant antimicrobial constituents of the mucoaqueous layer

Question 18

Function of mucin component of mucoaqueous layer

Answer 18

The mucin component of the mucoaqueous layer coats the microplicae of the superficial corneal epithelial cells and forms a fine network over the conjunctival surface. In addition to mucin, it contains proteins, electrolytes, water, and carbohydrates in a polar glycocalyx.

Question 19

Two types of mucin in mucoaqeous layer

Answer 19

Secreted mucins and Membrane­ spanning mucins (also called membrane-anchored, membrane-bound, membrane-tethered mucins)

Question 20

Dry eye syndrome neural feedback mechanism

Answer 20

The cor­ nea and lacrimal gland are not directly connected; however, corneal damage profoundly affects the lacrimal gland, which, in turn, downregulates tear production. In the vicious circle theory of DES, this downregulation is due to the secretion of inflammatory cyto­kines that block neural signals for tear secretion. The feedback loop, initiated by inflammation on the surface of the eye, further suppresses or downgrades lacrimal gland function, creating a vicious circle that worsens DES.

Question 21

Where are glands of Wolfring located

Answer 21

located along the proximal margin of each tar­sus

Question 22

Diagnostic tests for tear dysfunction

Answer 22

tear breakup time, fluorescein staining, lissamine green staining, rose bengal staining, osmolarity testing, Schirmer test, tear me­niscus evaluation, and MMP­9 testing.

Question 23

Medications for Dry Eye Syndrome (DES)

Answer 23

a variety of anti­inflammatory drugs (includ­ing corticosteroids, cyclosporine, lifitegrast, and doxycycline) have been used as therapy for DES and observed to improve the clinical symptoms of these patients

Question 24

How does cyclosporine work in DES

Answer 24

Cyclosporine, a fungus­ derived peptide emulsion, has been shown to be effective in stimulating aqueous tear production in patients with DES

Question 25

How does Lifitegrast work in DES

Answer 25

Lifitegrast, a lymphocyte function–associated antigen­1 (LFA­1) antagonist that inhibits binding of ICAM­1 to LFA­1, has been shown to reduce inferior corneal staining and provided greater symptom relief in treated patients with DES than in control groups