Day 3 (1): Anatomy of the Cornea, Sclera, Conjunctiva and Tear Film Flashcards
What are the outer coats of the eyeball?
- Cornea
- Sclera
- Conjunctiva
- Limbus
Functions of the cornea.
1/3 of the ocular tunic; anterior surface of the globe
- Refract and transmit light
- Mechanical and biologic barrier
Transparency = AVASCULARITY + DETURGESCENCE (only 78% water)
What are the normal dimensions of the cornea?
- 11-12 x 10 mm (H x V)
- Central thickness: 550 um
- Peripheral thickness: 700 - 1000 um
Note:
- Thinner centrally, thicker with AGING (>65 yo: 570 um centrally)
- Megalocornea: HD > / = 13 mm
- Microcornea: HD < / = 10 mm
Physical properties of the cornea
Strength: 5 kg/sq. cm before rupture
Elasticity: stretch by 0.25% in normal IOP
Outer surface RoC: 7.8 mm (flatter, less power)
Inner surface RoC: 6.7 mm (more curved, more power)
Shape:
- CONVEX looking anteriorly, CONCAVE looking posteriorly
- PROLATE: steep centrally, flattens peripherally
- Central 3-4 mm ~ spherical
- With age:
- infant: spherical
- childhood/adolescence: steeper vertically (football on it’s side)
- middle age: spherical
- elderly: flatter vertically (upright football)
What optical properties enable the cornea to properly function?
CRuST
- Contour
- Refractive Index
- Smoothness of surface
- Transparency = AVASCULAR + DETURGESCENT
Why is the cornea transparent?
- UNIFORM collagen fibril SIZE
- EQUIDISTANT from each other
- Both diameter and distance < 1/2 the wavelength of visible light (400 - 700 nm)
- Incident ray scattered by each fibril is cancelled by interference of other scattered rays –> LIGHT PASSES THROUGH
Characteristics of an edematous cornea
- Increased distance between fibrils
- (+) Fibril aggregation
- Loss of GAGs
2 structures responsible for corneal smoothness
- Corneal epithelium
- Tear film
What best describes the corneal contour?
Spherocylindrical with a minor and major axis
What are refractive indices of the different structures that light must pass through when entering the eye?
Air: 1.000
Tears: 1.336
Cornea: 1.376 (1.338 if combined A and P surface)
AH: 1.336
DIFFERENCES IN THE REFRACTIVE INDICES OF THE STRUCTURES: responsible for the refractive power of the cornea
Air - Tear interface: (+) 44 D (Diff: 0.336)
Tear - Cornea interface: (+) 5 D (Diff: 0.040)
Corneal - AH interface: (-) 6 D (Diff: 0.040)
Total Power: (+) 43 D - 2/3 or 70% of total refractive power of eye
What are the layers of the cornea?
- Epithelium
- Bowman’s Layer
- Stroma
- Descemet’s Membrane
- Endothelium
What are the cells present in the cornea?
- Epithelial Cells: ectodermal
- Endothelial Cells: neural crest
- Keratocytes (fibroblasts): neural crest
- Dendritic Langerhans Cells : tissue macrophages present only at the PERIPHERAL EPITHELIUM and LIMBUS
Describe the corneal epithelium
- Non-keratinized stratified squamous
- Constant turnover
- 50 um thick (10% of cornea)
- 5-6 layers of 3 different cells:
- S (Superficial) Cells: flat; 3 layers
- (+) microvilli and (+) microplicae
- (+) tight junctions: prevents entry of tears
- (+) desmosomes: anchors cells to each other - W (Wing) Cells: wing-like; 2 layers
- (+) gap junctions: passage of small molecules between cells
- (+) desmosomes: anchorage - B (Basal) Cells: columnar/elongated; 1 layer
- MITOTIC
- (+) gap junctions, (+) desmosomes
- (+) hemidesmosomes: anchors cells to BM
Describe the corneal epithelial BM
- Secreted by the corneal epithelium
- 40-60 um thick
- Type 4 Collagen + Laminin + Fibronectin + Anchoring fibrils and plaque
- Function: epithelial organization and wound healing
- 2 Layers:
1. Lamina Lucida: light band; anchoring fibrils only; superficial
2. Lamina Densa: dark band; deeper
What is the corneal adhesion complex composed of?
- Basal cells (posterior surface) joined to the epithelial BM and stroma by anchoring fibrils
Anchors:
1. Desmosomes: joins basal cells together
2. Hemidesmosomes: joins anchoring fibrils to basal cells
3. Anchoring plaque: joins anchoring fibrils to the stroma
4. Anchoring fibrils: secreted by basal cells
Collagen composition:
BM - Type 4 + Laminin + Fibronectin
Anchoring fibrils - Type 7
Anchoring plaque - Type 1
What are corneal epithelial stem cells?
- Source of new stratified squamous epithelial cells
- Repository: Corneoscleral Limbus
- Requires good vascularity as nutrition source; since cornea is avascular, this can only be found in the LIMBUS or corneal PERIPHERY with stromal vasculature
What are the similarities and differences in the structures of the cornea and the limbus? What is its function?
Similarities:
- Epithelial cell layers are similar
Differences:
- (+) Melanocytes and (+) Langerhans cells in epithelium
- Thicker: 7-10 layers
- Basal cells: smaller and less columnar, (+) undulations, less hemidesmosomes
- (+) Palisades of Vogt
Function:
1. Repository of corneal epithelial stem cells
2. Prevents vascularization of the cornea by the conjunctiva
What are Palisades of Vogt?
- Pigmented crypts formed by radial folds/undulations of the bulbar conjunctiva in the basal cell layer of the limbus
- Repository of corneal epithelial stem cells
- Pigmentation (melanocytes) protects stem cells against UV damage
Describe the process of migration, proliferation and maturation of corneal epithelial stem cells.
- Happens CENTRIPETALLY (from the Palisades of Vogt at the limbus to the central cornea) and from BASAL TO SUPERFICIAL LAYER
- Duration: 7-14 days
Steps:
1. Migration
2. Proliferation
3. Maturation: basal cells –> superficially –> superficial cells –> coated with microvilli
4. Desquamation: into tear film
X + Y = Z, where
X: PROLIFERATION of basal cells and movement from basal to superficial layer
Y: MIGRATION of limbal stem cells from the Palisades of Vogt to the basal layer
Z: DESQUAMATION and apoptosis of superficial cells
What happens if the process of migration, proliferation and maturation of corneal epithelial stem cells is disturbed?
- Poor corneal epithelial wound healing
- Neovascularization and conjunctivalization of the cornea
What happens in limbal stem cell deficiency?
- Cause: Loss of integrity of the limbus
Effects:
- Conjunctival cells invade the cornea –> neovascularization and conjunctivalization
- Deficient stem cells cannot repopulate the corneal epithelium –> cornea cannot repair itself –> epithelial layer breakdown –> decreased barrier function
Acellular, membrane-like layer representing the ANTERIOR surface of the stroma.
Bowman’s Layer
- Condensation of woven RANDOMLY arranged collagen fibers and proteoglycans
- Fibrils THINNER in diameter than stromal collagen
- DOES NOT regenerate
- 12 um thick
- Collagen Type 1 and 2
- Inelastic
Thickest layer of the cornea
Stroma
- 90% of the cornea
- 480 um thick
- 78% water (deturgescent)
- 97% extracellular matrix (collagen + proteoglycans): Collagen Type 1 most common
- 3% cellular (keratocytes/fibroblasts)
- Fibrils THICKER compared to Bowman’s Layer
- Fibrils –> Fibers –> Lamella (sheets)
- Lamella runs parallel to the surface from limbus to limbus in an ORDERLY arrangement
Division:
1. Anterior 1/3
- more compact
- woven, unidirectional lamellae
- Posterior 2/3
- looser
- non-woven, unidirectional lamellae
Penetrating Keratoplasty (PKP) vs
Deep Anterior Lamellar Keratoplasty (DALK) vs
Descemet Stripping Automated Endothelial Keratoplasty (DSAEK)
PKP: full-thickness transplant of ALL LAYERS of the cornea
- corneal diseases involving all layers
DALK: partial-thickness transplant of the entire STROMA
- diseased stroma BUT NORMAL Descemet’s and endothelium
DSAEK: partial-thickness transplant of the ENDOTHELIUM, Descemet’s Membrane and some posterior stroma
- for corneal endothelial dystrophies, dysfunction and failure BUT NORMAL stroma
Main extracellular matrix of the cornea
Glycosaminoglycans
- Keratan sulfate: primary (65%)
- Chondroitin sulfate
- Dermatan sulfate
- Hyaluronan sulfate: embryonic and injured cornea
- Keeps collagen fibrils equidistant (fills spaces in between)
- Absorbs and retains large amounts of water
- Binds to core proteins
Component of the ECM that functions as RECEPTORS
Integrins
- a6b4 heterodimer: hemidesmosomes
- a5b1 heterodimer: fibronectin (stimulated by IL6 in inflammation)
Main cellular component of the corneal stroma
Keratocytes
- 3% of the stroma
- flattened fibroblasts in between lamellae
- responsible for maintaining the ECM by synthesizing collagen, GAGs and MMP
- normally QUIESCENT; activated by inflammation and injury
Basement membrane of the corneal endothelium
Descemet’s Membrane
- Collagen Type 4 + Laminin + Fibronectin
- Tough and resistant to MMP BUT easily torn by shearing
- Shows folds when stroma is edematous
- Gets thicker with age:
+ birth: 3 um (only Anterior Banded Zone)
+ adult: 8-20 um (increasing Posterior Non-Banded Zone)
Zones:
- Anterior Banded Zone
- 2-4 um, present at BIRTH - Posterior Amorphous Non-Banded Zone
- thicker; laid down OVER TIME
SINGLE layer of cells covering the posterior surface of the Descemet’s Membrane
Endothelium
- hexagonal shape, interdigitated laterally
- 5 um thick, 20 um wide
- large nucleus, abundant organelles
- (+) Microvilli posteriorly facing the AH
- (+) Na-K ATPase pump at basolateral border facing DM
- DO NOT PROLIFERATE in vivo
- Endothelial cell density decreases with age:
Birth: 350,000 cells/cornea
Normal attrition rate: 0.5-0.6%/year
Children: 3000 - 4000 cells/sq. mm
Adult: 2000 - 3000 cells/sq. mm (Ave: 2500)
Elderly: < 2000 cells/sq. mm
Minimum ECD for normal function: 1500 cells/sq. mm
Possible corneal edema: < 800 cells/sq. mm
Other factors that decrease ECD:
1. Trauma
2. Intraocular surgery
3. Elevated IOP
4. Decreased aqueous (source of nutrition)
5. Inflammation
- maintains corneal transparency and clarity by:
1. Barrier to aqueous humor
2. Metabolic pump which maintains dehydrated state of the cornea (deturgescence)
Normal Parameters
Coefficient of Variation: < /= 0.30
HEX: >/= 60%
Polymegathism vs Pleomorphism
Polymegathism: different cell sizes
Pleomorphism: different cell shapes
Mechanisms that compensate for the decrease in ECD: remaining cells enlarge and alter their shapes to cover the occupied area of lost cell.
How is corneal endothelial cell morphology determined and imaged?
Specular Microscopy
Polymegathism
- Coefficient of Variation > 0.30
Pleomorphism
- HEX < 60
Innervation of the cornea?
Terminal Superficial Plexus <– Perilimbal Nerve Ring <– Long Ciliary Nerve <– Ophthalmic Nerve (CN5A) <– Trigeminal Nerve
- Penetrate the deep peripheral stroma radially –> course anteriorly –> exits the Bowman’s layer –> terminate at W cell layer as Free Nerve Endings
- Highly innervated and sensitive tissue
- More nerve endings per unit than the epidermis
Vascular supply of cornea?
Limbal Vascular Arcade <– Anterior Ciliary Artery <– Ophthalmic Artery
- Cornea itself is AVASCULAR
- Blood supply limited only to the LIMBUS and PERIPHERAL cornea
- Arcade anastomoses with branches of Facial Artery <– ECA
Dense white fibrous connective tissue ensheathing the eye from the corneal limbus to the dural sheath of the ON
Sclera
- Mesenchymal origin
- Structural support to the contents of the eye
- Limits the size and shape of the eye during development
- 90% of the surface area of the eye
- Thinnest posterior to the insertions of the rectus muscles (0.3 mm)
- Thickest at the posterior pole near the ON (1.0 mm)
- At the equator: 0.5 mm
- Weakest point: Lamina Cribrosa
- 50-70% collagen: fibrils are THICKER than that of cornea
1. Adult - Type 1
2. Fetal - Type 4 (same as BM) - most common GAGs:
1. Chondroitin sulfate
2. Dermatan sulfate
Special structures:
1. Scleral Sulcus and Scleral Spur
2. Corneoscleral Junction or Limbus
3. Schlemm’s Canal
4. Lamina Cribrosa
Parts and layers of the sclera
Apertures:
1. Anterior Scleral Apertures
- Anterior Scleral Foramen: attached to cornea
- Anterior Ciliary Arteries
- Posterior Scleral Apertures
- Posterior Sclera Foramen: where ON passes
- Circle of Zinn-Haller: formed by Short Posterior Ciliary Arteries around the ON
- Long Posterior Ciliary Arteries (2) - Middle Scleral Apertures
- 4 mm posterior to equator
- exit points of the 4 - 5 Vortex Veins
Layers:
1. Episclera
2. Stroma/Sclera Proper/Substantia Propria
3. Lamina Fusca
Outermost layer of the sclera
Episclera
- loose fibrous elastic tissue attached to the Tenon’s capsule anteriorly
- highly vascular anteriorly (vascular arcades)
- collagen fibers FINER but IRREGULARLY arranged
Middle layer of sclera
Stroma Proper
- DENSE mass of collagen fibers arranged concentrically around cornea and ON
- (+) fibroblasts: flattened, numerous superficially, forms syncytia
- (+) melanocytes
- collagen fibers THICKER, of VARIABLE SIZES and IRREGULARLY arranged compared to the cornea hence its OPACITY
- deeper layers run from limbus to optic foramen
- structure allows adjustment to strain and changes in IOP
- relatively AVASCULAR: receives nutrition from the episclera and the choroidal vasculature
- emissary canals: channels through which the arteries, veins and nerves pass
Innermost layer of the sclera
Lamina Fusca
- collagen fibers are smaller and THINNER
- more elastic fibers
- adjacent to the uvea, thus:
1. more melanocytes and pigmented macrophages
2. continuous with SUPRACHOROIDAL space
Projection of the scleral stroma into the anterior chamber angle adjacent to the trabecular meshwork
Scleral Spur
- posterior boundary of the scleral sulcus which forms the Schlemm’s Canal
- ORIGIN of the longitudinal and circular fibers of the ciliary muscle
Scleral Spur cells:
- similar to contractile myofibroblasts that may influence the rate of aqueous outflow
- (+) mechanoreceptors: monitor ciliary muscle tone or IOP
Demarcation of the peripheral termination of the Descemet’s Membrane and corneal endothelium
Schwalbe’s Line
Vascular supply of sclera
A. Anterior Ciliary Artery
- supplies the anterior (epi)sclera
- scleritis occurs more commonly anterior to the equator because of the more abundant anterior vascular supply
- penetrates sclera posterior to the limbus and anterior to the recti muscle insertions to become the:
Anterior Episcleral Arterial Circle (AEAC)
- supplies episclera
- branches out in the limbal arcades into the:
- Conjunctival Plexus
- most superficial
- located at the conjunctiva
- arteries: tortuous
- veins: straight
- freely mobile
- bright red
- blanched with topical vasoconstrictors
- inflammation: Conjunctivitis - Superficial Episcleral Plexus
- middle plexus
- between Tenon’s capsule and episclera
- vessels are straight and radially-arranged
- less mobile
- salmon-pink
- blanched with topical vasoconstrictors
- inflammation: Episcleritis - Deep Episcleral Plexus
- deepest plexus
- between the episclera and sclera proper
- vessels are criss-crossed
- immobile
- violaceous
- does not blanch
- inflammation: Scleritis
B. Posterior Ciliary Arteries
- supplies posterior (epi)sclera and choroid
Differentiate Episcleritis from Scleritis.
Scleral Inflammations
- both are recurrent inflammation
- since sclera is dependent mainly on the episclera for the inflammatory response, scleritis is almost always accompanied with overlying episcleritis; however, episcleritis is usually not associated with scleritis
- Episcleritis
- involvement: Superficial Episcleral Plexus
- benign, self-limiting disease - Scleritis
- involvement: Deep Episcleral Plexus
- severe inflammation of the scleral tissue
- characterized by severe pain, edema and cellular infiltration of the sclera and episclera
- may result to vision loss if not treated promptly
Venous drainage of the sclera?
Episclera: collecting veins –> Anterior Ciliary Veins
Deeper layers: emissary canals –> Scleral Veins
Mucous membrane lining of the anterior surface of eyeball
Conjunctiva
- Borders: Limbus to mucocutaneous junction of eyelids
- Area: 16 sq. cm
- Origin: Ectoderm and mesoderm overlying the optic vesicle
- Functions:
1. Eye mobility
2. Tear reservoir
3. Mucin production
4. Immunologic barrier
What are the parts of the conjunctiva?
- Caruncle
- Plica Semilunaris
- Palpebral conjunctiva
- Superior, Inferior, Temporal Forniceal conjunctival
- Bulbar conjunctiva
Soft fleshy tissue at the most medial aspect of the palpebral fissure?
Caruncle
- attached to the MR
- moves with the plica semilunaris
- (+) fine hairs with sebaceous glands
Crescent-shaped fold of conjunctiva analogous to the nictitating membrane
Plica Semilunaris
- Borders: Junction of the medial 3rd of the fornix to the caruncle
Describe the palpebral conjunctiva.
- Begins at the Mucocutaneous Junction
- Area where reactive pathologies (follicles, papilla) of the conjunctiva may be seen clinically
- Highest density of Langerhans cells: contain rod-shaped or “tennis-racket” shaped cytoplasmic organelles known as Birbeck granule (stains for ATPase)
3 Divisions:
1. Marginal Conjunctiva
- transitional zone between the skin of the eyelid and the conjunctiva proper
- starts at the Mucocutaneous Junction
- transition from keratinized, stratified squamous epithelium of the skin to NON-KERATINIZED, stratified squamous epithelium
- Tarsal Conjunctiva
- begins at the Subtarsal groove
+ 2 mm posterior to the lid margin
+ common site of foreign body lodgment
- transition to STRATIFIED CUBOIDAL to LOW COLUMNAR epithelium
- over superior tarsus: 2 - 3 layers
- over inferior tarsus: 4 - 5 layers
- tightly adherent to tarsus
- vascular: perforating branches of marginal arcade pierce the tarsal plate
- Meibomian glands: visible as yellow lines running parallel to each other vertically - Orbital Conjunctiva
- loose covering between the tarsus and fornix
- lies over the Muller’s muscle
Goblet cell-lined invaginations of the conjunctival epithelium between the lid margin and subtarsal groove
Crypts of Henle
- in this area, conjunctival epithelium is NON-KERATINIZED, STRATIFIED SQUAMOUS
What are the accessory lacrimal glands?
- Gland of Krause: conjunctival fornix
- Gland of Wolfring: orbital border of tarsus
Importance of the bulbar conjunctiva
- Forms the Palisades of Vogt at the limbus
- Smoother and less adherent except near rectus muscle insertions
- NON-KERATINIZED, STRATIFIED SQUAMOUS epithelium
- Marked infoldings
- Apical: (+) Microvilli with glycocalyx
- Basal: (+) Lymphocytes, Melanocytes, Langerhans Cells
- thinnest part of conjunctiva
- eventually fuses with the corneal EPITHELIUM at the limbus
Unicellular, mucin-secreting glands in the conjunctiva
Conjunctival Goblet Cells
- Location: Basal layer of epithelium
- Denser over TARSAL and INFERONASAL BULBAR conjunctiva
- Concentrated in the Crypts of Henle between the lid margin and the subtarsal groove
- Nucleus and organelles basally, mucin packets apically
Fibrovascular connective tissue of the conjunctiva
Conjunctival Substantia Propria
- Limbus, Palpebral: thin, compact
- Fornices: thick, loose
- 2 layers:
- Superficial/LYMPHOID Layer
- NOT present at birth but at 8-12 weeks of age
- prominently lymphoid tissue
- more prominent in fornices
- terminates in the subtarsal groove
- NOT found in the marginal conjunctiva
- mostly T lymphocytes and mast cells
- lymphoid aggregations: CALT - Deep/FIBROUS Layer
- vessels, nerves, lymphatics and glands of Krause
Differentiate conjunctival follicles from papilla.
BOTH seen in the palpebral conjunctiva
Follicles
- identical to lymphoid follicles found elsewhere
- enlarged conjunctival adenoid/lymphoid tissues arising from the substantia propria
- NO blood vessels at the center
- causes: viral and chlamydial infections, toxic conjunctivitis due to medications
Papilla
- chronic inflammatory cells such as lymphocytes and plasma cells
- (+) core blood vessel
- causes: allergic, chronic contact lens use, keratoprosthesis
Vascular supply of the conjunctiva
PALPEBRAL conjunctiva
Palpebral arcades
- formed by the terminal branches of Ophthalmic Artery and Facial Artery
- Peripheral Arterial Arcade
- at the orbital border of the tarsal plate
- also supplies the FORNICEAL conjunctiva - Marginal Arterial Arcade
- in front of the tarsal plate, 2mm away from the margin of the eyelids
BULBAR conjunctiva
Anterior Ciliary Artery –> Superficial Marginal Arcade at limbus (Anterior and Posterior Conjunctival Arteries)
Venous drainage of the conjunctiva
Palpebral:
Posterior Tarsal Veins + Anterior Facial Vein (deep facial branch) –> Pterygoid Plexus
Bulbar:
Episcleral Plexus –> Scleral Plexus
Lymphatic drainage of the conjunctiva
Episcleral Plexus –> Scleral Plexus
Lymphatic channels (1 mm from the limbus) –> Deep Layer of Substantia Propria + Lid Lymphatics –>
Submandibular node: medially
Preauricular node: laterally
Innervation of conjunctiva
CN 5A (Ophthalmic Nerve <– Trigeminal Nerve)
- free nerve endings from the ff:
1. Lacrimal Nerve
2. Supraorbital Nerve (<– Frontal nerve)
3. Supratrochlear Nerve (<– Frontal nerve)
CN 5B (Maxillary Nerve <– Trigeminal Nerve)
1. Infraorbital Nerve
- More sensitive than the cornea
Differences between cornea and conjunctiva
Cornea:
- Clear
- Orderly layers (hence clarity)
- (-) Goblet cells
- AVASCULAR except at limbus and periphery
- Nutrition: aqueous humor posteriorly and tear film anteriorly
- Dependent on Glycogen
Conjunctiva
1. Translucent
2. Less orderly layers
3. (+) Goblet cells
4. Highly vascular
5. Nutrition: Rich vascular supply, tear film
6. NOT dependent on glycogen
Layers of the tear film
2-Layer Model
- Superficial Lipid Layer: Meibomian Glands
- Aqueous Layer: Lacrimal Glands (Main and Accessory)
- Mucin Layer - creates a gel gradient in between (dilute superficially to more concentrated in deeper layers)
- Membrane-Associated Mucins (1, 4): for stabilization
- Gel-Forming Mucins (5AC): for viscosity
- Soluble Mucins
Composition of the tear film
- 98% Water
- Glucose: for nutrition
- Electrolytes: for osmolarity
- Oxygen
- Proteins
- Mucin
- Antimicrobials and immunoglobulins: Lactoferrin, Lysozyme
- Growth factors, cytokines, prostaglandins
Functions of the tear film
- Lubrication: keeps the ocular surface moist and smooth
- Nutrition: for the corneal epithelium
- Regulatory: source of mediators for epithelial maintenance and repair
Describe the lacrimal functional unit
AFFERENT: Free nerve endings in ocular surface –> Lacrimal Nerve –> CN5A –> Superior Salivary Nucleus (Pons)
EFFERENT: CN 7 (via Nervus Intermedius) –> Pterygopalatine Ganglion –> Lacrimal Gland
Blink Reflex:
AFFERENT: CN5A
EFFERENT: CN7 –> OO
How is the Tear Break-Up Time test done?
- Test for TEAR STABILITY (QUALITY)
- Measures the time interval between the last blink and appearance of the first randomly-distributed dry (black) spot
- Drop of 2% fluorescein or a moistened fluorescein-impregnated strip is placed at the lower fornix
- Eye is examined under a slit lamp (low magnification, broad beam, cobalt blue filter)
- Ask patient to blink and keep eye open for 10 seconds or more
- Note the formation of black spots indicating dry areas
Result:
Normal - (+) 1-mm tear film forms along the upper and the lower eyelid margin which BREAKS UP (formation of dry black spots) ONLY AFTER 10 SECONDS
5 - 9 seconds: suspicious for TEAR FILM INSTABILITY but can be still normal
< 5 seconds: Short TBUT
- Aqueous tear deficiency
- Mucin deficiency
- Meibomian gland dysfunction
Note:
1. If the patient blinks before 10 seconds have elapsed, RESTART the test.
2. May take two or more measurements and average.
3. Touching the cornea with fluorescein strip can cause excessive tearing and will affect the result.
4. Localized corneal surface abnormalities = dry spots in that location = falsely LOW TBUT. Repeat measurements. Appearance of dry spots in the same location in repeated procedures can indicate a surface abnormality in that area.
5. Benzalkonium chloride, a preservative used in saline solution can PROLONG TBUT. Choose preservative-free saline solution when moistening fluorescein strips.
How is the Schirmer 1 Test done?
- Test for REFLEX + BASAL tear production
- Measures secretion of ALL lacrimal glands
- Tests for TEAR QUANTITY
How it works?
- Capillary Action: water in tears travel along the length of a paper test strip like a horizontal capillary tube
- Rate of travel along the test strip is proportional to the rate of tear production
Steps:
1. Position: upright, looking up with lower eyelid pulled down.
2. A Whatman filter paper measuring 35 mm x 5 mm is folded at a 90 degree angle at the 5 mm end
3. Short end is placed in the inferior fornix between the medial two-thirds and lateral one-third of the lower eyelid, resting between the palpebral and bulbar conjunctiva.
4. Both eyes tested at the SAME TIME. NO anesthetic drops are administered.
5. Pt instructed to close both eyes for 5 minutes.
6. After 5 minutes, test strips are removed and the length of the moistened area is measured.
Result:
>/= 15 mm: NORMAL for < 40 yo
>/= 10 mm: NORMAL for > 40 yo (hypolacrimation normal with aging)
< 5 mm: LOW (decreased total tear production)
How is the Schirmer Test with anesthetic done?
- Test for BASAL tear production ONLY (due to anesthetic use)
- Measures secretion of the ACCESSORY lacrimal glands
- Done AFTER doing Schirmer’s WITHOUT anesthetic
Steps:
1. Drop of topical anesthetic is placed in the fornix, and the eye is wiped after a minute.
2. The filter paper test is repeated as in Schirmer’s 1 Test.
Result:
>/= 10 mm: NORMAL for < 40 yo
>/= 5 mm: NORMAL for > 40 yo
< 5 mm: LOW
Reflex tears : Main Lacrimal Gland :: Basal Tears : Accessory Lacrimal Glands. True or False?
FALSE
Previously it was thought that the main lacrimal gland is responsible for reflex tear secretion and the accessory lacrimal glands of Wolfring and Krause are responsible for the basal secretion.
But recent evidence suggests that ALL TEARING MAY BE REFLEX.
The accessory glands account for approximately 10% of the total lacrimal secretory mass.
